EGU General Assembly 2024  

The analysis of deformation and failure in many sedimentary settings hinges upon a fundamental understanding of inelastic behaviour, failure mode of porous carbonate rocks and their implications on fluid flow at various scales. The mechanical compaction behaviour of carbonate rock of a broad range of porosity has been investigated in the laboratory over a wide range of stress conditions in the last decades. The phenomenology of brittle failure and inelastic compaction in this rock type with often bimodal pore size distribution was found similar to that of sandstone. Inelastic compaction in limestone involved primarily cataclastic pore collapse and micromechanical analysis showed the strong influence of the micropore size on the yield stress. Compaction experiments on porous limestones also revealed a broad spectrum of complex failure modes. In situ X-ray Computed Tomography imaging combined with Digital Volume Correlation provided the first observations of discrete compaction bands in a high porosity limestone. Permeability variations in carbonates associated with shear-enhanced compaction and these failure modes were found significantly smaller than variations previously reported in porous sandstones of comparable porosities.

In geophysical applications such 4D reservoir monitoring and the production of geothermal reservoirs, an understanding of the mechanical and chemical effects of pore fluid is fundamental. The mechanical influence of pore fluid on different properties is characterized by effective pressure coefficients. For limestone with dual porosity, both effective stress coefficients for permeability and pore volume change were observed to be consistently greater than unity. This implies that microscopic homogeneity is not a valid approximation for a limestone with dual porosity, and a realistic model must explicitly differentiate between the macropores and micropores, as well as account for their interplay in controlling the hydromechanical behaviour. Recent data showed that a significant weakening effect of water could also be expected in most carbonates. 

How to cite: Baud, P.: Inelastic compaction in porous carbonates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6574, https://doi.org/10.5194/egusphere-egu24-6574, 2024.

Due to contrasting results between laboratory tests, geophysical data, and field observations, the strength of the plagioclase-rich lower continental crust remains a topic of debate. It has been shown that its strength highly depends on the presence of fluids as they trigger metamorphic reactions that can result in permanent weakening. An important metamorphic reaction in the lower continental crust is the breakdown or hydration of plagioclase and the associated growth of epidote-group minerals, kyanite, quartz, and jadeite/albite. To investigate the impact of this particular reaction on the strength of the lower continental crust, we combined experimental work, i.e., Griggs-deformation tests, with extensive microstructural observations of the recovered experimental samples. Experimental conditions were 1 to 1.5 GPa confining pressure, 550 to 950 °C, and for the deformation tests, we used strain rates ranging from 10^-6 to 10^-5 s^-1. Our results reveal two main findings. First, deformed plagioclase aggregates as well as deformed granulite drill cores show that deformation-induced features in plagioclase grains, e.g., cleavage cracks and twin boundaries, act as nucleation sites for metamorphic reactions and melting. Consequently, reaction can progress faster in deformed samples as the effective reactive surface area is increased relative to undeformed counterparts. Second, when deformed under identical experimental conditions, pure epidote aggregates are consistently stronger or show equal strengths than pure plagioclase aggregates. Hence, a partial plagioclase breakdown, i.e., the exclusive growth of epidote-group minerals at low reaction progress, is not expected to result in permanent weakening. This result further strengthens the idea that a process akin to Zener pinning is a viable mechanism to cause long-term weakening in rocks.

How to cite: Incel, S., Renner, J., Schubnel, A., Labrousse, L., Baisset, M., and Mohrbach, L. K.: Deformation and reaction of plagioclase-rich rocks at conditions of the lower continental crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10806, https://doi.org/10.5194/egusphere-egu24-10806, 2024.

Subglacial geology remains largely unknowns in Antarctica. Direct geological samples are limited to ice free regions along the coast, high mountain ranges or isolated nunataks, while the origin of geological material transported by glaciers themselves is often ambiguous. 3D singular and joint inversions of airborne gravity and magnetic data recovers subsurface density and susceptibility distribution. The relationship between both inverted petrophysical quantities provide crucial insight for subglacial geology and rock provinces interpretations. Validation of indirect derived subglacial geology models are critical but very challenging in Antarctica due to the sparsity of rock samples.

We present 324 new density and susceptibility measurements on rock samples from Northern Victory Land, East Antarctica. 251 samples have been measured at the National Polar Sample Archive (NAPA) from the Federal Institute for Geosciences and Natural Resources (BGR) in Berlin-Spandau, Germany and additional 73 samples were measured at the BGR in Hannover, Germany. We use the petrophysical measurements to validate our recent regional scale 3D joint inversion model of the Wilkes Subglacial Basin and the Transantarctic Mountains. Furthermore, we validate inversion results on a local scale of singular magnetic inversion based on high resolution airborne magnetic data with a flight line spacing of 500m in the Mesa Range.

We demonstrate that we can provide reliable discrimination between Ferrar Dolerites, Kirkpatrick Basalt and Granite Harbour intrusion rocks based on our local inversion model and that the recovered susceptibilities agree with those measured at rock samples from the study area. Furthermore, we show that regional scale inversion model of gravity and susceptibility distribution agrees for samples of the dominant crustal rock types. However, densities of small-scale dense intrusion bodies like Ferrar Dolerites are underestimated by the regional scale inversion, while the susceptibility range is correctly recovered.

Constraining subglacial geology with joint inversion of airborne potential field data is heavily depended on the resolution of the airborne survey, flight line coverage, the inversion scale, and the scale of the target feature. Regional scale inversion is adequate for large scale geological heterogeneities, which underestimate petrophysical quantities for small scale structures, while local scale inversions are able to resolve such structures but are more computational demanding and in the case of Antarctica lack ultra-high resolution airborne gravity data with a line spacing below 1000 – 500m.

How to cite: Lowe, M., Jordan, T., Moorkamp, M., Ebbing, J., Koglin, N., Ruppel, A., Green, C., Liebsch, J., Ginga, M., and Larter, R.: Verification of susceptibility and density relationship from 3D joint inversion of airborne magnetic and gravity data in northern Victoria Land, East Antarctica, with petrophysical measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-589, https://doi.org/10.5194/egusphere-egu24-589, 2024.

Assessing the thermal influence on the dynamic mechanical properties of Barakar sandstone is crucial, notably in the examination of subsidence phenomena induced by underground coalmine fires in tandem with blast-induced loading. The Jharia region had been affected by underground coalmine fires, resulting in surface fracturing on both small and large scales. So, the objective of the study is to examine the impact of high temperature on the dynamic compressive strength of colliery sandstone subsurface samples, and its correlation with the mineralogical properties. To accomplish this objective, samples were subjected to a 24-hour heat treatment in a furnace at a controlled heating rate of 5°C/min, followed by natural cooling within the furnace. The samples were divided into five groups, each undergoing different thermal treatments at temperatures of 25°C, 200°C, 400°C, 600°C, and 800°C. The dynamic compressive strength was obtained by performing the dynamic compression tests on Split Hopkinson Pressure Bar setup. The results clearly indicate that up to a critical temperature i.e. 400°C, both quasi-static and dynamic compressive strength showing the minor strengthening effect. However, beyond this critical temperature, there is a significant decrease in strength, particularly up to 800°C. Additionally, for each temperature, the dynamic strength also exhibits an increasing trend with increase in strain rate. The study investigated the applicability of the Kimberley Theoretical Universal Scaling Law in predicting the dynamic compressive strength of thermally treated sandstone across different strain rates. Furthermore, it pinpointed the characteristic strain rate at which the dynamic compressive strength of thermally treated sandstone doubled in comparison to its quasi-static compressive strength.

How to cite: Tripathi, A., Khan, M. M., Rai, N., and Pain, A.: Dynamic Compressive Strength of Thermally Treated Barakar Sandstone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-974, https://doi.org/10.5194/egusphere-egu24-974, 2024.

Salt is playing a principal role in the current energy transition. From diapirs to salt walls, salt structures are being proposed for a growing number of non-fossil energy purposes and carbon-neutral projects (i.e., geothermics, hydrogen storage in salt caverns or CCS). However, diapiric structures are not uniform and exhibit significant compositional and structural heterogeneities that prevent an easy characterization and exploration, thus increasing the risk of exploitation. Compositional heterogeneities within salt structures arise by the occurrence of different rocks inherited from the original Layered Evaporite Sequence (LES) and transported during salt flow or created through various diagenetic processes during diapirism (e.g., caprock formation).

One of the most common heterogeneities found in salt bodies is the presence of heterometric rock masses known as stringers. The limestones of the upper Muschelkalk (M3) facies (Middle Triassic) are the primary non-evaporitic lithological unit within the Triassic LES of the South-Central Pyrenees. These limestones are exposed throughout the South-Pyrenean fold-and-thrust belt embedded in mudrocks and gypsums of the Upper Triassic Keuper facies, forming the actual caprock exposures in the region. The Estopanyà salt wall is located in the westernmost part of the South-Central Pyrenean Zone, within the Serres Marginals thrust sheet. This area shows a well-preserved 58 km² caprock exposure formed by Middle and Upper Triassic rocks that surround two adjacent salt-embedded basins known as the Estopanyà and Boix synclines. According to the stratigraphic record of these synclines, from the Upper Cretaceous to the Oligocene, the evolution of the area resulted in salt withdrawal leading to salt inflation and the occurrence of several E-W-oriented salt walls that were exposed during the deformation onset in the Lower Eocene (early Ypresian times).

The M3 stringers in the Estopanyà salt wall are embedded in a caprock matrix formed by the dissolution of halite on the surface, albeit its presence has been proposed at deeper levels based on gravimetric models. The stringers have been identified and mapped, forming decameter-thick and kilometre-long structures showing significant folding and fracturing as well as a well-preserved stratigraphic sequence. The basal part of these stringers is formed by a layered to tabular millimetre-to-centimetre-thick limestones whilst the upper part consists of tabular to massive centimetre to meter-thick limestones. The current contribution presents the petrophysical (i.e., mineral density, connected porosity, permeability and P-wave velocity) and petrothermal (thermal conductivity and specific heat capacity) properties of 40 samples collected throughout the eastern Estopanyà salt wall, covering the basal and upper succession of various stringers. The sample analysis enables us to discuss the factors controlling the studied rock properties and the viability of carbonate stringers as geothermal reservoirs and CCS, which is a novel study that can be replied in similar salt structures worldwide.

How to cite: Ramirez-Perez, P., Cofrade, G., Cruset, D., Cantarero, I., Martín-Martín, J. D., Ferrer, O., Sizun, J.-P., and Travé, A.: Outcrop analogue study of carbonate caprock stringers for CCS and geothermal reservoir: the petrophysical and petrothermal properties of the upper Muschelkalk in the Estopanyà salt wall (South-Central Pyrenees), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1531, https://doi.org/10.5194/egusphere-egu24-1531, 2024.

Tracking CO₂ transport in subsurface rock during geological carbon sequestration has received significant attention. Although X-ray computed tomography (XCT) imaging techniques, visualization and quantification of CO₂ behavior has been undertaken by previous researchers, little attention has been given to the nature of CO₂ transport in a reservoir-caprock interface with well-developed pores in the reservoir and micro-fractures in the caprock. As the transition of two different pore systems, this complex system is crucial in controlling the sequestration safety case. We have used advanced time-lapse synchrotron imaging at in-situ pressure and temperature conditions to image the CO₂ transport process in such an experimental system, for both gaseous and supercritical phases.

Dynamic XCT images of fluids and the pore- and fracture-system in the mudstone and sandstone couplet were acquired at high resolution (effective voxel size 1.625 µm). Strain maps following high-speed gaseous and supercritical CO₂ (ScCO₂) flooding were modelled using a digital volume correlation (DVC) method to reveal the hydro-mechanical effects. The results suggest that under the influence of brine, high-speed gaseous CO₂ will not increase the total pore-throat volume and can even cause a reduction in permeability in sandstone due to fines migration induced by CO₂ flooding. Clay behavior, notably dispersion in brine, migration of fines and swelling induced by CO₂, plays a notable role in the opening and closing of pores/fracture. Fluid breakthrough occurred at 10.6 MPa during high-speed ScCO₂ injection. The significant fracturing effect of high-speed ScCO₂ resulted in the connection of natural fractures in the mudstone with newly developed secondary branches and also created larger pore space in the sandstone, leading to an increase in porosity by approximately 2.8 times in the mudstone and 1.6 times in the sandstone. Additionally, there was an approximate increase of 2.6 times and 8.6 times in the permeability of mudstone and sandstone, respectively, after CO₂ phase transition. The concentrated strains around the main fracture in the mudstone and web-like strains around the boundary of granular minerals in sandstone show the different modes of action of ScCO₂ when passing through a reservoir rock and caprock system. This work is of practical significance in improving understanding of CO₂-fluid-rock interaction in a complex reservoir-caprock system.

How to cite: Taylor, K., Wang, K., and Ma, L.: 4D synchrotron imaging of CO2 transport in reservoir rocks and caprocks: Informing safe CO2 sequestration    , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1866, https://doi.org/10.5194/egusphere-egu24-1866, 2024.

Characterizing accurately rock properties at core scale is critical for reservoir scale modeling. This step is especially complex when dealing with carbonate rocks because of their inherent heterogeneities at several length scales. The deposition and diagenesis processes result in diverse pore-shaped geometries, significantly influencing petrophysical properties. Notably, two core plug samples taken from adjacent locations within a carbonate core may exhibit substantial differences in rock properties. While standard core analysis methods offer precise experimental measurements in the laboratory, they fail to consider the pore-scale variability within core plug samples. Digital rock physics (DRP) emerges as an approach aiming to characterize rock properties at the pore scale through the utilization of X-ray micro-tomography and numerical simulation methods. DRP has been widely employed to estimate various rock properties, including porosity, permeability, and elastic moduli, in both siliciclastics and carbonate rocks. However, a well-defined workflow for numerically characterizing rock properties in carbonates is currently absent. This study introduces a novel multiscale method for simulating permeability and porosity in heterogeneous carbonate samples using 3D X-ray computed tomography images. The unique aspect of our approach lies in incorporating a quantitative description of heterogeneity through texture classification using machine learning. The results of rock texture classification are then utilized to scale up simulations of rock properties from a fine to a coarse scale. The fine-scale properties are examined using the lattice Boltzmann method, while a Darcy-scale flow simulator is applied to estimate coarse-scale properties. In addition, due to the critical role played by petrophysical properties at fine scale, we employ a 3D printing technique to experimentally validate the numerical simulations. Lastly, we demonstrate the application of our proposed approach on two carbonate samples from a Middle East carbonate oilfield reservoir.

How to cite: Jouini, M. S. and Bouchaala, F.: Upscaling porosity and permeability in core plug sample from core plugs simulations using machine learning methods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2786, https://doi.org/10.5194/egusphere-egu24-2786, 2024.

We have employed impedance spectroscopy to investigate the impact of chemical composition and microstructure on the electrical properties of geological samples. Our study focused on a metapelite sample containing graphite, extracted from a depth of 530 meters in borehole DT-1B as part of the DIVE (Drilling the Ivrea-Verbano zonE) project in Ornavasso, Italy. Additionally, we examined the electrical properties of synthetic mineral assemblages. These were created by combining quartz powders with variable amounts of graphite (1% and 5% by weight), muscovite (5% by weight), and biotite (5% by weight). The goal was to identify which conductive or semiconductive phases predominantly influenced the electrical behavior of the metapelite. Measurements were conducted using a Solartron-1260 Impedance/Gain-Phase Analyzer within a piston cylinder apparatus. The experiments were carried out at a pressure of 500 MPa, temperatures ranging from 22 to 1000 °C, nominally dry conditions, and across a frequency range from 0.1 Hz to 200 kHz.

All samples exhibited high electrical resistance (R > 10⁶ Ω.m), low electrical conductivity (< 10^-6 S/m) and behaved as capacitors, with a phase angle magnitude exceeding 70° for most frequency ranges at temperatures below 200 °C. A representative impedance spectrum (Nyquist plot) illustrates this behavior through a partial semicircular arc originating from the origin. An inverse relationship between electrical conductivity and temperature was observed in almost all samples when temperatures increased from 300 to 500 °C. This phenomenon is attributed to the presence of open grain boundaries in the samples, leading to electrical charge scattering. Notable variations in electrical behavior were observed at temperatures exceeding 600 °C, including a linear increase in electrical conductivity, changes in Nyquist plots such as a reduction in prominence of the ‘grain interior arc’ and an increase in significance of the ‘grain boundary arc’, a decrease in sample capacitance, and a significant decline in the phase angle's frequency dependency. Microstructural analysis reveals that these changes were associated with dehydration melting of mica in mica-bearing samples and the growth and interconnection of graphite grains in graphite-bearing samples. Variations in activation enthalpy with temperature suggested that impurity conduction and small polaron hopping played a crucial role at lower temperatures, while the diffusion of H and alkali ions (in mica-bearing samples) or carbon (in graphite-bearing samples) along grain boundaries became significant at higher temperatures.

The natural metapelite sample exhibited electrical conductivities similar to the quartz + 5% graphite sample at high temperatures, reaching 10^-1.5 S/m at 1000°C. This is comparable to the conductivity levels typically measured by magneto-telluric (MT) surveys in Earth's crust.

How to cite: Mansouri, H., Toy, V., Klimm, K., Tholen, S., and Hawemann, F.: Quantifying the influence of the type and arrangement of conductive phases on the electrical properties of rocks using impedance spectroscopy , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3448, https://doi.org/10.5194/egusphere-egu24-3448, 2024.

Featuring a large specific surface area with associated high reactivity, nanomaterials in various morphologies are ideal candidates for improved oil recovery (IOR), enhanced geothermal systems (EGS) and carbon capture utilization and storage (CCUS).  Encapsulation of solid nanomaterials within an oil-in-water (O/W) emulsion (i.e., Pickering emulsion) has been employed to prevent the aggregation and deposition of the nanomaterials in subsurface reservoirs in recent decades. Here, the dispersed phase droplets were decreased to nanoscale through a utilizable procedure. These nanodroplets were stabilized solely by polymer-coated magnetic iron oxide nanoparticles. Low-field NMR and X-ray CT were employed to constantly monitor the stability of Pickering nanoemulsions until phase separation. The polydisperse nanoemulsions are more easily separated due to the severely inhomogeneous chemical potentials of the emulsion droplets. Experimental and theoretical modeling results reveal that the Ostwald ripening is the main instability mechanism for nanoemulsions due to the very small droplets associated with a high surface area. The insolubility of long-chain hydrocarbons in water acts as a kinetic barrier to Ostwald ripening, making those nanoemulsions, both the Pickering and Classical (which is formed only by polymer) ones, inherently stable to Ostwald ripening.

The transport and retention of the Pickering nanodroplets in porous media is examined by X-ray CT imaging. Accordingly, in-situ transport of the nanoemulsions in a water-saturated sandpack was quantified spatiotemporally through X-ray CT. Effluents were collected and analyzed to further comprehend the nanoemulsion displacement and retention in porous media. Experimental results demonstrate that accumulation and retention of the nanodroplets in porous media are stimulated by ionic strength, nanodroplet size distribution, and nanoparticle wettability. Three transport modes in porous media (flow through with minimal retention, migration of accumulated nanodroplets, and retention of accumulated nanodroplets) can be achieved through carefully designing the nanoemulsion system.

These findings shed light on the fundamental understanding of the (nano-)colloidal dispersions transport in porous media and provide implications for IOR, EGS, and CCUS.

How to cite: Ding, B., Ahmadi, S. H., Bryant, S., and Kantzas, A.: Spatio-Temporal Imaging of Instability and Transport of Pickering Nanodroplets in Porous Media, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3734, https://doi.org/10.5194/egusphere-egu24-3734, 2024.

The study of caprock assumes paramount significance, particularly in elucidating alterations in the sealing conditions of petroleum reservoirs. However, it is imperative that corresponding simulation experiments transcend a singular focus on CO₂ and caprock, extending to a comprehensive study of the reservoir-caprock system. Experimental protocols were implemented, entailing the sequential flow of CO₂-rich fluid first through the reservoir and subsequently through the caprock. The chosen samples and conditions were drawn from potential CO₂ utilization and storage blocks in the Subei Basin, China, featuring sandstone reservoirs and mudstone caprocks. The experimental paradigm simulated the interaction when CO₂ migrating along the sandstone and then reaching the mudstone caprock, spanning a duration of 37 days.

Results underscore that the introduction of CO₂-rich fluid predominantly instigates the dissolution of sandstone reservoirs, with notable dissolutions observed in feldspar and clay minerals, while secondary mineral precipitation remains negligible. Upon the fluids traversal through the mudstone caprock, initial dissolution occurs in carbonate minerals, accompanied by continuous precipitation of secondary clay minerals. These secondary minerals not only occupy calcite dissolution pores but also precipitate on the surfaces of rock particles, asserting dominance in the water-rock reaction within the mudstone.

Supported by geological observations and numerical simulations, these experiments illuminate the material adjustment processes ensuing from the introduction of CO₂-rich fluid into the reservoir-caprock system. The infusion of CO₂-rich fluid induces mineral dissolution, augmenting pore space within the reservoir, with ion products subsequently transported to the mudstone caprock. Under conditions characterized by a slower flow rate and a more extensive water-rock reaction surface area in the mudstone caprock, the water-rock interaction accelerates the dissolution and reprecipitation of calcite and other minerals. The reprecipitated minerals effectively occupy caprock pores and fractures, thereby enhancing the caprock's sealing capacity.

How to cite: Zhou, B., Lun, Z., and Wang, B.: An Experimental Study of CO2 Flooding the Reservoir-caprock System: Implication for the Stability of Caprock during CO2 Intrusion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4095, https://doi.org/10.5194/egusphere-egu24-4095, 2024.

Linking the pore structure distributions and velocity-pressure relationship is important for understanding geological processes and in situ stress conditions. During the pressure loading, the pressure-velocity relationship is controlled by variation of pore-aspect-ratio spectrum distribution (pore-aspect-ratio vs. porosity) with pressure: on the one hand, the “flat crack” (aspect ratio) will be squeezed and closed, causing the large velocity variation in the low-pressure regimes; on the other hand, the shape and volume of the “round pore” (aspect ratio) are less sensitive to pressure, controlling the smaller velocity change at high pressures. The above pore-structure variation process and its effect on the velocity-pressure relationship allow us to invert and analyze the distribution characteristics from the laboratory velocity-pressure data.  

The power-law distribution has been universally applied to characterize the length and aperture distribution in natural fractures; however, it is still not well understood whether the pore-aspect-ratio spectrum distribution also follows the power law and how this power-law distribution relates to other power-law distributions of fractures. In this study, we examine the universality of the power-law relationship for the pore aspect ratio spectrum distribution and show that this distribution is the natural outcome of the power-law behavior of the length and aperture distribution in the rock fracture network.

We first collected laboratory ultrasonic velocity-pressure data for 52 rock samples with various porosities and lithologies. We then used a power law assumption to link the pore aspect ratio and porosity. The power-law exponents (hereafter referred to as the E factor) of the power-law distributions were successfully obtained by inverting the compiled velocity-pressure datasets. The inversion results showed a universal relationship between the E factor and porosity. We also demonstrated that the power-law distribution of the pore-aspect-ratio spectrum and the universal E factor-porosity trend can be explained using the length and aperture distribution characteristics of the microcrack system of rocks. This universal E factor-porosity trend provides a link between porosity and sensitivity of seismic velocities to pressure. Hence, our work not only emphasizes the relevance of natural fracture fractal distributions for a broad range of lithologies but also provides the parameterized method for relating the velocity-pressure relationship of any rock to porosity variation.

How to cite: Wang, H.-M., Tang, X.-M., and Doan, M.-L.: The power-law distribution of the rock pore structure: inversion and analysis of laboratory velocity-pressure data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4456, https://doi.org/10.5194/egusphere-egu24-4456, 2024.

Recently, under the umbrella of a public-private collaboration project (CPP2021-009072), Atalaya Riotinto Minera S-L. and the CSIC through its institutes IGEO-Madrid and Geo3BCN-Barcelona, have undertaken an ambitious and innovative initiative to validate the applicability of state-of-the-art monitoring and prospecting systems for better tracking of deformations that may occur in the mine’s environment and to study the petrophysical properties and 3D structure of the mine subsurface. In this work, we present the results of machine learning (ML) models developed to predict various physical properties of rock (PPR) for classifying main lithologies. This analysis is based on over a thousand surface rock samples and nine wells with lithology descriptions and density logs. These data sets have allowed us to characterize the main geological units and formations comprising the subsurface of the Riotinto (RT) mine. A quality control process was applied to the PPR database through lithology and intervals to identify and correct outlier values. Multi-Layer Perceptron neural networks were employed to predict these outliers. Various mathematical and supervised machine learning models were developed to understand and predict PPR associated with different geological units. The models were compared to identify the most efficient and stable one. Additionally, new machine learning models were implemented to predict lithofacies based on PPR. These models were then used to predict PPR and classify lithofacies in wells within a mining site.

The results suggest that machine learning-based PPR prediction reduces uncertainty, providing a clearer understanding of the anisotropic characteristics of geological units. Apparent density, total porosity, and P-wave velocity properties were found to predict lithofacies with an accuracy of approximately 80%. In conclusion, this advancement not only redefines the precision with which lithofacies can be identified in the Riotinto mine but also establishes a new methodology for the lithological characterization of the subsurface, leveraging both well logs and direct measurements on surface samples. This study demonstrates the potential of using new ML techniques in mining and geology, as well as opening the door to the use of these models for 3D characterization of lithological units by integrating geophysical data at the exploratory level.

This work, financed with reference CPP2021 009072, has been funded by MCIN/AEI/10.13039/501100011033 (Ministry of Science, Innovation and Universities/State Innovation Agency) with funds from the European Union Next Generation/PRTR (Recovery, Transformation, and Resilience Plan).

Keywords: Machine Learning, Mining, Petrophysical Properties and Geological Characterization.

How to cite: Balaguera, A., Sánchez-Pastor, P., Du, S., Torné, M., Schimmel, M., Fernández, J., Díaz, J., Vergés, J., Carbonell, R., Rodríguez, S., and Davoise, D.: Prediction of Subsurface Physical Properties Through Machine Learning: The case of the Riotinto Mine., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5512, https://doi.org/10.5194/egusphere-egu24-5512, 2024.

Understanding the coupling between rock permeability, pore-pressure and fluid-flow is crucial, as fluids play an important role in the Earth’s crustal dynamics. Here, we measured the distribution of fluid pressure during fluid-flow experiments on two typical crustal lithologies, a granite and a basalt. Our results demonstrate that the pore-pressure distribution transitions from a linear to a non-linear profile as the imposed pore-pressure gradient is increased (from 2.5 MPa to 60 MPa) across the specimen. This non-linearity results from the effective pressure dependence of permeability, for which two analytical formulations were considered: an empirical exponential or a modified power-law. In both cases, the non-linearity of pore pressure distribution is well predicted. However, using a compilation of permeability vs. effective pressure data for granitic and basaltic rocks, we show that our power-law model, based on crack micromechanics (combining Hertzian contact and cubic law theories), outperforms the exponential formulation at low effective pressures. 

How to cite: lin, G., Chapman, S., Garagash, D., Fortin, J., and Schubnel, A.: Pressure dependence of permeability in cracked rocks: experimental evidence of non-linear pore-pressure gradients from local measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5612, https://doi.org/10.5194/egusphere-egu24-5612, 2024.

Access to top research equipment facilitates top research. However, the research equipment needed may not always be available within individual institutes, while access to external facilities may not in all cases be affordable, or even possible. This restricts the research that any individual can do and hampers scientific breakthroughs, particularly across disciplines. To overcome this limitation, a collaborative infrastructure network was initiated: EPOS-NL (European Plate Observing System- Netherlands). EPOS-NL provides free-of-charge access to geophysical labs at Utrecht University, Delft University of Technology, and the Dutch geological survey of TNO, all in the Netherlands, for research within rock physics, analogue modelling of tectonic processes, X-ray tomography and microscopy. These labs include capabilities for among others: A) Mechanical and transport testing at crustal stress, temperature and chemistry conditions; B) Large-scale experiments, e.g. up to 30 m-scale fluid transport testing and particle tracking, or hydrostatic compression (< 100 MPa) of 6m long samples; C) Analogue tectonic modelling, including dynamic model imaging in 2D and 3D; D) X-ray tomography at sub-µm resolution; E) A correlative workflow for electron microscopy and microchemical mapping, down to nm resolution; and F) Microfluidics: Direct visualization of dynamic, physical and chemical fluid transport processes in the pore networks. As such, these labs can provide you with the means and expertise for your research into the fundamental processes governing the behavior of the Earth’s crust and upper mantle.

Access to EPOS-NL can be requested by applying to a bi-annual call, posted on www.EPOS-NL.nl. This involves submitting a short (1-2 page) research proposal. Research proposals are reviewed on the basis of feasibility and excellence, but generally have a high chance of success (~80% in previous rounds). Interested? Have a look on the EPOS-NL website – and apply!

How to cite: Wessels, R. and Pijnenburg, R.: Access for free: How to get free-of-charge access to Dutch Earth scientific research labs through EPOS-NL , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6120, https://doi.org/10.5194/egusphere-egu24-6120, 2024.

This comprehensive study aimed to analyze pore characteristics and mechanical properties of three carbonate mudrock units (Upper Marrat, Tuwaiq Mountain Limestone, and Hanifa formations) in Central Saudi Arabia. Employing a combination of petrographic thin section analysis, helium porosity measurements, X-ray diffraction, backscattered scanning electron microscopy with energy dispersive scanning, micro-computed tomographic scanning, Total Organic Carbon (TOC) analysis, nuclear magnetic resonance, and N2 adsorption analyses, the research identified various pore types and their abundance. Specific lithofacies were determined: three in Hanifa Formation Hawtah Member (LFH1, LFH2, LFH3), two in Tuwaiq Mountain Limestone mudstone (LFT1, LFT2), and two in Upper Marrat Formation (LFM1, LFM2). These lithofacies exhibited distinct pore types, including intra-skeletal, interparticle, and organic matter-hosted pores, linked to diagenetic processes and burial events.

NMR-relaxation time and pore size distribution curves indicated variable pore abundances across these lithofacies, influencing fluid flow simulations for potential reservoir evaluation. The study also integrated lithofacies analysis with TOC, porosity, and mechanical property measurements to assess rock strength and elasticity. Textural and microstructural analyses suggested different depositional environments: lower shoreface intra-shelf basin for Hanifa and Tuwaiq formations, and peritidal low ramp setting for Upper Marrat Formation.

Correlations between mechanical properties and porosity varied, with tensile strength showing a strong relation to porosity and mineral composition. Hard phase minerals exhibited a stronger correlation with elastic moduli. The study found significant differences in rock strength and stiffness across the formations, influenced by sedimentary layering and anisotropy. These findings are crucial for predicting rock brittleness in these Jurassic intervals and their global equivalents.

How to cite: Abdlmutalib, A. and Abdullatif, O.: Characterization and Correlation of Pore Structures and Mechanical Properties in Carbonate Mudrock Formations of Central Saudi Arabia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7168, https://doi.org/10.5194/egusphere-egu24-7168, 2024.

Water infiltration into crustal rocks, particularly through fractures, significantly impacts seismic wave propagation and the characterization of fracture systems. Our study (Wu et al, 2023a) investigates the acousto-mechanical behavior of fractured granite experiencing gradual water infiltration over 12 days. We reveal an order of magnitude difference in wave amplitudes when compared to intact granite, with a correlation between wave amplitudes and the movement of the wetting front. The laboratory experiments show that fracture stiffness decreases exponentially as the wetting front advances, indicating moisture-induced matrix expansion (Wu et al, 2023b) around the fracture leads to increased asperity mismatch and reduced stiffness. By back-calculating the fracture stiffness and capturing the effects of water infiltration on seismic attenuation through a numerical model, this research illuminates how elastic waves propagate across fractures undergoing moisture-induced expansion, a crucial aspect of fracture characterization and understanding of the near-surface environment's response to hydrological changes. Our research sheds light on an important question in fracture characterization: how elastic waves propagate across a fracture undergoing moisture-induced expansion.

Publications related to this research:

Wu, R., Selvadurai, P. A., Li, Y., Leith, K., Lei, Q., & Loew, S. (2023a). Laboratory acousto-mechanical study into moisture-induced reduction of fracture stiffness in granite. Geophysical Research Letters, 50, e2023GL105725. https://doi.org/10.1029/2023GL105725

Wu, R., Selvadurai, P. A., Li, Y., Sun, Y., Leith, K., & Loew, S. (2023b). Laboratory acousto-mechanical study into moisture-induced changes of elastic properties in intact granite. International Journal of Rock Mechanics and Mining Sciences, 170, 105511. https://doi.org/10.1016/j.ijrmms.2023.105511

How to cite: Wu, R., Selvadurai, P., Li, Y., Leith, K., Lei, Q., and Loew, S.: Time-lapse Acousto-Mechanical Response to Moisture-Induced Reduction of Fracture Stiffness in Granite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7192, https://doi.org/10.5194/egusphere-egu24-7192, 2024.

This study aims to explore the use and effectiveness of the spectral induced polarization (SIP) method for identifying and characterizing iron sulfide scales in oil and gas wells. Laboratory experiments using scale-amended columns aimed to confirm the SIP method's capability to differentiate between iron sulfide scales and other naturally occurring minerals and better understand scale formation. The polarization of different iron sulfide scales was measured in response to an applied electrical field of 0.01-10,000 Hz frequency range, with polarization being dependent on the material's frequency-dependent response to the field. The scale types analyzed in this study mainly consist of iron sulfides such as pyrrhotite, troilite, and pyrite, as well as iron oxides such as goethite and lepidocrocite. To ensure consistency and eliminate any effects of grain heterogeneity, the samples were ground and sieved to a particle size between 425 and 600 μm. Each sample was tested in acrylic columns modified with Ag-AgCl current and potential electrodes. The findings of this study demonstrate that the SIP responses, such as phase shift, impedance, real and imaginary conductivity, and calculated chargeability, are highly sensitive in distinguishing various scale types, including those with minor composition differences. X-ray diffraction and X-ray fluorescence analysis validated the differentiation of the samples' mineral and elemental composition. Although phase and imaginary conductivity responses exhibit similar trends, phase clarified differentiation between the measured samples. The recorded signals present a clear trend dependent on the mineralogical composition and the iron content. Iron sulfide scales have a characteristic electrical response that differs from other natural iron-rich materials tested. This difference in electrical response is due to the unique electronic structure of iron sulfide minerals, which allows them to act as charge carriers. As a result, iron sulfide scales exhibit a high polarization response, which can be detected using the SIP method. SIP is a powerful geophysical technique that can provide valuable information about subsurface properties in micro-scale and fluid content. Unfortunately, due to poor exposure, SIP has not been widely adopted in the Oil & Gas industries. The SIP method offers several advantages over traditional methods of scale identification. With further research and development, the SIP method has the potential to become a standard tool for scale management in the oil and gas industry.

How to cite: Kirmizakis, P., Chen, T., Aljeaban, N., El-Husseiny, A., Mahmoud, M., and Soupios, P.: Iron Sulfide Scale Characterization: Insights from Spectral Induced Polarization Method, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7250, https://doi.org/10.5194/egusphere-egu24-7250, 2024.

       Traditional interpretation of imaging logging data often involves manually importing various data types to calculate the porosity of fractures in the target area. This process becomes challenging due to the lack of gas and oil information in the raw data, especially when dealing with less-than-ideal raw data. The proposed method addresses this challenge by offering a rapid estimation approach for fracture porosity that reduces manual work and enhances process efficiency within an acceptable error limit.  

       The estimation method relies on path morphology [1] and convolutional neural networks for the extraction of fracture and cavity parameters. Initially, a path morphology method is applied to identify inclined fractures, followed by the use of a rotation jamming algorithm [2] to obtain rectangles with the minimum area in each cavity. These rectangles incorporate the angle of the rectangle and the lengths of its short and long sides. Parameters related to horizontal fractures, vertical fractures, and cavities are then utilized for the estimation of porosity.

      The original imaging logging conductivity is processed to distinguish inclined fractures from other fractures during the extraction process. Traditional binarization and denoising methods are not directly applied since cavities on basic binary images are also white. Thus, specific curves need to be extracted from the original conductivity images using a path morphology algorithm. On the other hand, convolutional neural networks (CNNs) are required for the identification of the shape of restored cracks due to the influence of cavities on traditional mathematical fitting processes. LeNet and AlexNet, among various CNN algorithms, are employed for this purpose. Specifically, the modified AlexNet algorithm adopts the maximum pooling method, Softmax function in the output layer, and the Adam optimizer in the learning process to improve efficiency and reduce memory occupation. The related parameters of cavities, horizontal fractures, and vertical fractures are calculated by the rotation jamming algorithm after the extraction of inclined fractures. Traditional Hough transform is considered time-consuming for evaluating a large number of cavities, leading to the adoption of an alternative approach—obtaining circumscribed rectangles with minimum area in a connected domain. This approach improves computation speed by focusing on directions coinciding with the long side of polygons, treating the long and short sides of rectangles as the major and minor axes of ellipses. In a conductivity image, cavities contain both convex and concave closures simultaneously, requiring the filling of concave ones to convex before applying the rotation jamming algorithm. The effective porosity parameters can be obtained using the developed programs.

      The method offers high efficiency and automation, extracting various types of fractures along with cavities within an acceptable error limit, providing valuable information for geologists in evaluating high-potential targets.

[1]Li et al. Estimating Porosity Spectrum of Fracture and Karst Cave from Conductivity Image by Morphological Filtering. JJU, 2017, 47(04): 1295-1307.

[2] Toussaint, G.T. A simple linear algorithm for intersecting convex polygons. The Visual Computer 1, 1985: 118–123. 

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How to cite: Zhang, Z., Liu, L., Xia, Q., Xie, T., and Niu, Y.: Fast quantitative estimation method of fracture cavity porosity based on convolution deep neural network, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7768, https://doi.org/10.5194/egusphere-egu24-7768, 2024.

Abstract: The northern part of the the Junggar Basin is the Siberian plate, and the western part is the Kazakhstan plate, which is an important part of the Central Asian orogenic belt. This study discusses how fluorescence parameters inside the mudstone caprocks of the Mobei Bugle and Mosuowan Bugle respond to the leakage of palaeo-oil zones. It is based on X-ray diffraction analysis, TOC testing, physical property testing, rock pyrolysis experiments, and image observation. First, using quantitative fluorescence technology, it was established that the appropriate particle size range for the study area mudstone is between 100 and 140 mesh by examining the QGF E intensity and sample loss rate of the control group. Then, the influence of retained primary hydrocarbons inside the mudstone on the test results of quantitative fluorescence technology is speculated to be relatively weak based on an analysis of the correlation between TOC value and total hydrocarbon value, TOC value and fluorescence parameters. The QGF index of the 5265-5302m reservoir in PD1 well ranges from 3.9 to 87.2, with an average of 16.18; the QGF index of the 7034-7195m reservoir in MS1 well ranges from 2.2 to 57.5, with an average of 11.6. The reservoirs of the PD1 and MS1 wells contain palaeo-oil zones, based on the QGF index classification criteria. Both image observation and micro resistivity imaging logging analysis demonstrate that the caprock's physical properties in the PD1 well are inferior to those in the MS1 well. The pore types of the PD1 well caprock are filled dissolution pores and residual intergranular pores, whereas the pore types of the MS1 well caprock are dissolution pores and microcracks. There is a discrepancy of approximately 9 times between the development density of fractures in the MS1 well caprock (0.989 pieces/m) and the PD1 well caprock (0.114 pieces/m). The palaeo-oil zone can be effectively sealed due to the poor physical properties of the PD1 well caprock. The oil testing conclusion of the reservoir is oil-water layer, with a daily oil production of 8.52t. Therefore, as the depth decreases, the QGF E intensity value decreases, and the QGF λmax and TSF R₁ values reflect higher hydrocarbon maturity. The MS1 well caprock has good physical properties and cannot effectively seal the palaeo-oil zone. The oil testing conclusion of the reservoir is water layer, with a daily oil production of 4.4t. As the depth decreases, the QGF E intensity value increases, and the QGF λmax and TSF R₁ values reflect lower hydrocarbon maturity. Comprehensive analysis suggests that the fluorescence parameters inside the caprock exhibit characteristics related to the degree and form of palaeo-oil zone leakage.

Keywords: Quantitative fluorescence technology, Fluorescence parameters, Mudstone caprock, Palaeo-oil zones

How to cite: Liu, K., Qu, J., Zha, M., and Ding, X.: Response of the caprock's fluorescence parameters to the leakage of palaeo-oil zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8600, https://doi.org/10.5194/egusphere-egu24-8600, 2024.

Serpentinites result from the hydration of ultramafic rocks (e.g., peridotite and pyroxenite) that transform primary mineralogical assemblages (olivine-pyroxene) into different serpentine species (lizardite-chrysotile-antigorite) depending on the pressure-temperature (P-T) conditions (Schwartz et al, 2013) related to geodynamic context. Serpentinization mainly occurs from oceanic ridges at LP conditions to subduction zones at HP conditions and its extent needs to be quantified.

The Vp/Vs ratio is a tool of choice to distinguish serpentinite from other mafic to ultramafic rocks and thus to investigate the serpentinization extent. The seismic velocity of serpentinites depends on the type and the volume of serpentine species (lizardite/antigorite) present in the rock, as well as the microstructural evolution of the serpentinite, however, these topics remain poorly understood.

We selected blueschist and eclogitic serpentinite samples collected in the internal zone of the western Alps. One eclogitic serpentinite sample was also experimentally dehydrated at 700°C in standard pressure conditions. The petrology and (micro)structures of the samples were characterized using both 2D petrological thin sections and 3D X-ray tomography. We performed velocity measurements on the samples at low frequencies (quasi-static stress-strain method: 0.02-1000 Hz) and ultrasonic frequency (wave travel time method: MHz) at different effective pressures (low-frequency experiments: 2-25 MPa; ultrasonic experiments: 0-70 MPa). The description of experimental equipment and methodology can be found in Borgomano et al. (2020). The studied frequency range covers the field geophysical data frequency band and therefore provides better constraints for interpreting the serpentinization extent. As expected, the Vp/Vs ratio changes depending on the mineralogy: lizardite is characterized by higher Vp/Vs (2-2.1) and lower Vp (5200-5700 m/s) than antigorite with lower Vp/Vs (1.75-2) and higher Vp (6000-7000 m/s). Anisotropic structure due to mineral preferred orientation manifest by velocity variation with sample orientation: overall, the Vp and Vs anisotropy degree (Ap, As) of antigorite (Ap:3%-18%; As:1%-16%) are larger than those of lizardite (Ap:3%-9%; As:3%-8%). Furthermore, the seismic velocity of the serpentinite is almost unchanged with pressure and frequency, but pressure- and frequency-dependence of the velocity arise in the dehydrated antigorite sample: 1) in dry condition, with lower Vp/Vs ratio (1.46-1.6) and lower Vp (3500-4600 m/s) in the effective pressure range of 2-25 MPa and almost no frequency dispersion; 2) in water-saturated condition, with higher Vp/Vs ratio (1.87-1.89) and higher Vp (5060-5190 m/s) at ultrasonic frequency, as well as a significant velocity dispersion band linking the results from seismic frequencies to ultrasonic frequency. This study contributes to the calibration of seismic velocity profiles to track the mineralogical transition between lizardite and antigorite, which is related to increasing degrees of metamorphism.

References:

Schwartz et al. (2013). Pressure–temperature estimates of the lizardite/antigorite transition in high pressure serpentinites. Lithos, 178, 197-210.

Borgomano et al. (2020) An apparatus to measure elastic dispersion and attenuation using hydrostatic-and axial-stress oscillations under undrained conditions. Rev. Sci. Instrument, 91(3).

How to cite: Doan, M.-L., Wang, H., Auzende, A.-L., Schwartz, S., Chapman, S., and Fortin, J.: Seismic properties of serpentinites under increasing pressure and temperature conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8632, https://doi.org/10.5194/egusphere-egu24-8632, 2024.

This study utilized an improved sand column experimental setup to investigate the effects of microplastic on Yangtze River bank sediments. The experiments conducted included Darcy experiments, electrokinetic experiments, and wettability experiments, together for the same sample. By varying the particle size of the microplastics in the samples, we were able to observe different response of the electrical properties as well as hydrogeophysical parameters in the samples.

Results show that increasing the mass fraction of mixed microplastics generally resulted in a significant decrease in sample electrical conductivity associated with an increase in permeability. These were expected to be due to the weak conductivity and strong hydrophobic properties of plastic particles, as well as the small adhesive forces between particles, which increased the pore space of the sediments and ultimately increased permeability. However, an anomalous increase trend was observed when decreasing the particle size of the mixed microplastics. Under this condition, increasing the concentration of same size plastic particles enhanced the electrical conductivity of the sediment sample. This anomaly phenomenon was reflected in both permeability and wettability, resulting in a decrease in sample permeability and a significant increase in sample hydrophobicity. Our observations using optical microscopy revealed two types of microplastic distribution in the sediments: one case was that microplastic particles were distributed within sediment pores and they did not touch each other, the other was that they were adsorbed onto sediment particle surfaces. We hypothesized that changes in the existence form of microplastics altered the double-layer structure of sediments, ultimately changing their hydrogeophysical parameters. This work has significance and relevance for electromagnetic-based characterization of microplastic-filled porous materials; for example, estimation of microplastic abundance in sediments.

How to cite: Li, S., Xu, M., Peng, Y., and Hu, X.: Influence of microplastic occurrence on complex conductivity of river sediments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9014, https://doi.org/10.5194/egusphere-egu24-9014, 2024.

Even though stresses in the crust are triaxial (𝜎1>𝜎2>𝜎3) the overwhelming majority of rock deformation experiments are conducted under axisymmetric (or conventional triaxial) loading (𝜎1>𝜎2=𝜎3). This configuration disregards the effect of 𝜎2 on the physical and deformation properties of rocks, thus complicating and degrading the extrapolation of results to natural crustal conditions. A True Triaxial loading configuration is necessary to overcome this simplification, however, these improvements in addressing real crustal conditions come at a cost, which is the challenging boundary conditions that arise from having six loading rams rather than just two. Two main loading boundary effects can severely impact the stress distribution and failure mechanism of samples deformed in a True Triaxial Apparatus (TTA): 1) the end friction effect caused by the stiffness contrast between the rock sample and the metal loading platens, and 2) the unstressed sample edges resulting from the requirement that loading platens must necessarily be slightly smaller than the rock specimen. Managing and reducing these boundary effects is fundamental for obtaining accurate and representative data from true triaxial experiments, and for the further development of these apparatuses.

A novel TTA developed in the UCL Rock & Ice Physics Laboratory was designed to subject cubic or cuboid rock samples to truly triaxial stresses through the independent control of six loading rams. The apparatus is equipped with a confining and pore pressure system that allows for the deformation of saturated samples whilst simultaneously measuring permeability along the three axes. A suite of Finite Element Method (FEM) models was implemented to evaluate the parameters that minimize loading boundary effects in UCL’s TTA for a 50 mm edge-length cubic sample of sandstone. Our results indicate that using aluminum loading platens (𝐸𝑠𝑎𝑚𝑝𝑙𝑒/𝐸𝑝𝑙𝑎𝑡𝑒𝑛=0.47) reduces the end friction effect by a factor of two compared to using steel platens (𝐸𝑠𝑎𝑚𝑝𝑙𝑒/𝐸𝑝𝑙𝑎𝑡𝑒𝑛=0.17). In addition, we find that elevated confining pressure significantly reduces the stress concentration produced by unstressed edges. Specifically, a confining pressure of 10 MPa eliminates tensile stresses at the sample corners. These results are currently being implemented into the experimental protocol and execution in UCL’s TTA in order to ensure that we obtain reliable true triaxial data. However, these observations are generic and could therefore contribute to improved development and operation of true triaxial loading systems generally.

How to cite: Stanton-Yonge, A., Mitchell, T., Meredith, P., Browning, J., and Healy, D.: Reducing boundary effects during True Triaxial loading of rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9568, https://doi.org/10.5194/egusphere-egu24-9568, 2024.

The detection of metallic particles in the subsurface has been an important field in applied geophysics for several decades, e.g. in the exploration of ore deposits or in monitoring measurements at bioremediation sites. Due to the comparably high conductivity and high polarizability of metallic particles, geophysical methods using the electrical conductivity and especially induced polarization are highly suitable methods for applications of this kind.

To correctly interpret data measured in the field, a deep understanding of the underlying conduction and polarization mechanisms is necessary. Both, laboratory measurements with controlled experimental conditions and theoretical models describing the basic physical processes can help to achieve this kind of understanding. However, theoretical models usually are not suitable to be directly compared to laboratory measurements, because they usually consider idealized situations and exhibit a large number of free parameters that can not be controlled in measurements.

To overcome this gap, we compare experimental data of a cm-sized metallic sphere embedded in water-saturated sand with a corresponding theoretical model. To explain the remaining differences between measured and modeled data, we discuss how different processes, e.g. the geometry of the measurement setup or dissolved metal ions in the fluid, might affect the measurement and adapt our model accordingly. By doing so, we are able to reproduce the experimental results with the predictions by our theoretical model.

How to cite: Kreith, D., Breede, K., Zhang, Z., Weller, A., and Bücker, M.: Modeling of spectral induced-polarization measurements on cm-sized metallic spheres in sand-water-mixtures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9712, https://doi.org/10.5194/egusphere-egu24-9712, 2024.

The porous sandstone formations offer large capacities for the geological storage of clean energy sources like hydrogen and compressed air. This paper aims to investigate the deformation mechanisms in sandstone under the varying loading conditions. Employing a combined experimental and numerical approach, we investigate the mechanical behavior of sandstone under cyclic loading conditions. The results obtained in this study indicate three distinct deformation regimes in sandstone specimens developed under multi-level, multi-frequency cyclic loads, i.e.,1) instantaneous elastic deformation, 2) transient and steady-state strain due to creep, and 3) rapid deformation leading to fatigue failure, mainly driven by micro-crack development. In response to these deformation mechanisms, a viscoelastic-damage model is proposed. This model is based on the standard Burger's viscoelasticity combined with an energy-driven damage model to represent the creep-fatigue behavior in sandstone. The modeling results were verified by comparing the predictions with the experimental data. The experimental and numerical results presented an essential insight for designing and managing geological storage systems in sandstone formations.

How to cite: Tian, D., Song, Z., Khaledi, K., Yang, Z., and Amann, F.: Experimental Analysis and Creep-Fatigue Damage Modeling of Sandstone Deformation related to Energy Storage Systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11028, https://doi.org/10.5194/egusphere-egu24-11028, 2024.

The Emeishan Large Igneous Province (ELIP) at the end of the Middle Permian was a very important geothermal event, which formed a giant intrusive and eruptive magmatic system, and had a great impact on the maturity of Organic matter (OM), hydrocarbon production process, storage capacity and gas content. In order to study the influence of the ELIP on OM characteristics and shale gas development. In this study, the pore structure and kerogen macromolecule structure of OM were characterised using various techniques in several typical wells of Longmaxi Formation shale in the southwestern part of the Sichuan Basin, including laser Raman, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), low-pressure N₂ and CO₂ adsorption, in order to clarify the controlling effect of ELIP on the maturity and carbonation of Longmaxi Formation shale, to reveal the connection between ELIP and the kerogen chemical and pore structure, and to establish a model for the maturity and carbonation of the OM under the influence of volcanic activities model. The results show that the middle zone of the near-mantle column reaches the stage of high-over maturity, and the resistivity is generally lower than about 10 Ω·m, with a negative correlation with TOC. ELIP promotes higher geothermal heat flows, resulting in regular changes of the kerogen molecular structur. In the Middle zone of ELIP, the aliphatic content and oxygenated functional groups decrease, the relative content of sp2 carbon and the degree of OM carbonation increases. The number of stacked layers of aromatic basic structural units in the macromolecular structure of casein increases, the stripe length increases significantly, and the arrangement order gradually increases with consistent orientation. The microporous volume and surface area of the OM in the middle zone of ELIP are smaller than those in the outer zone, which may be attributed to the further condensation of the aromatic compounds caused by the higher paleogeodetic heat flux values, which leads to the increase of π-bonds between carbon atoms, the gradual decrease of layer spacing, and the gradual change of the OM into graphite-like crystals with higher Young's modulus and lower Poisson's ratio. The high degree of OM carbonation reduces the hydrocarbon generating capacity, and in the case of gas dissipation in the area of strong fracture activity, the overall pore fluid pressure is low, which destroys the pore preservation conditions, and the microporosity collapses and closes. Overall, the OM close to the mantle column has a higher degree of graphitization, and the microporous volume and surface area are drastically reduced, while the preservation conditions are destroyed by the fracture activity, resulting in a significant decrease in both the storage capacity of the OM. The present study improves the exploration theory of high overmature organic-rich marine shale, and provides geological basis for the selection of marine shale gas-rich areas in southern China.

How to cite: Liang, Z., Jiang, Z., Muhammad, A., Tang, X., Xue, Z., and Wang, M.: Influence of the Emeishan basalt eruption on organic matter pore structure and chemical characteristics : A case study of shale from Longmaxi shale in southern Sichuan Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11290, https://doi.org/10.5194/egusphere-egu24-11290, 2024.

In geothermal systems the thermo-physical properties of the rocks change as they interact with fluids passing through the volcanic system and during discrete events such as earthquakes and magma intrusion. To characterize a geothermal system and the flow of fluid through a sub-volcanic complex, targeted rock physical tests are needed for the rocks of the area conducted at natural P-T conditions. Here, we present rock property characterization of the main geological units of the active Nevados de Chillan Geothermal System, located in the Southern Volcanic Zone (SVZ), an area with some of the largest geothermal potential in the Andes. 

Six representative blocks of the geothermal host reservoir and overlying strata were collected from the volcanic basement. The main geomechanical units of this system are (from oldest to youngest): 1) andesites, tuffs and breccia of the Cura-Mallin Formation (Miocene country rocks); 2) granodiorites and diorites of the Santa Gertrudis Bullileo Batholith (15.7 Ma and 5.9 Ma, respectively); and 3) hornfels from the contact between the granitoids and country rocks. Cylindrical core samples (26 mm diameter x 65 mm length) from each block were used to quantify density, porosity, and ultrasonic wave velocities at different confining pressures. All tests were carried out at the Rock Deformation Laboratory, University of Manchester. Polished thin sections were prepared from blocks of the same orientation as the cored directions and analyzed using petrographic.

Granodiorite has the lowest porosity at between <1 to 2% and the fastest P- and S-wave velocities (5.5 to 5.9 km/s and 3.1 to 3.5 km/s, respectively). The diorite has a higher porosity of between 4 to 6% which coincides with lower ultrasonic velocities (3.5 to 5.0 km/s and 3.5 to 5.0 km/s, respectively). This can be explained by the higher presence of macro, micro-fractures, and alteration minerals in the diorite. Hornfels has possess similar porosities (>2%) to the granodiorite and 4.8 to 5.7 km/s P-wave velocities and 2.7 to 3.3 km/s S-wave velocities. The andesitic lavas have porosities ranging 3-7%, while the tuffs and breccias have porosities of 12-30%. Elastic waves velocities in the andesitic lavas are around 2 km/s faster than the pyroclastic rocks.

Tests with cycles of increasing and decreasing hydrostatic pressure (up to approximately 150 MPa) show that granodiorite and diorite exhibit sharp increases in P-wave velocity (Vp). This is attributable to the stiffening of the rock from the progressive closure of pre-existing cracks. Above 40 MPa, the rate of increase in Vp with pressure reduces markedly, implying that the remaining porosity is less compliant. This is consistent with the maximum burial depth of the rocks suggesting that those cracks formed because of bringing the rocks to the surface.

Finally, in terms of microstructural observations, the granodiorites, diorites and hornfels have large intragranular and intergranular fractures with very high aspect ratio, which are commonly oriented and therefore impart anisotropy. In contrast, the andesitic, tuffs and breccias porosity is higher than the crystalline rocks and is mainly composed of intergranular pores with low aspect ratios and relatively isotropic.

How to cite: Mura, V., Arancibia, G., Browning, J., Healy, D., Mecklenburgh, J., and Morata, D.: Characterizing Rock Physical Properties in the Nevados de Chillán Geothermal System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11658, https://doi.org/10.5194/egusphere-egu24-11658, 2024.

In petrophysics, characteristic lengths are used to relate fundamental transport properties of porous media. However, these characteristic lengths have mostly been defined and tested in fully saturated conditions, with few exceptions. This contribution revisits the seminal work of Johnson-Koplik-Schwartz (JKS) length, which represents an effective pore size controlling various transport-related properties of porous media, such as permeability and electrical conductivity. A novel closed-form equation is presented to predict the behavior of this characteristic length in partially saturated media for different saturation states. Using previous models in the literature that predict the intrinsic and relative electrical conductivities under partially saturated conditions, we infer the JKS length as functions of water saturation and properties associated with the pore-size distribution of the considered porous medium. The proposed method allows for the direct estimation of effective and relative permeability through electrical conductivity measurements. This creates new opportunities for remotely characterizing partially saturated media. We believe that this new model has potential for various applications in reservoir (CO2 or hydrogen storage) and vadose zone studies.

How to cite: Jougnot, D., Thanh, L. D., Solazzi, S., and Luo, H.: Revisiting the Johnson-Koplik-Schwartz characteristic length to relate transport properties in partially saturated porous media, insights from a fractal-based petrophysical approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12012, https://doi.org/10.5194/egusphere-egu24-12012, 2024.

Grain-size reduction – with amorphization or melting as its extreme forms – plays a crucial role in fault-zones dynamics, e.g., the nucleation or arrest of earthquakes. Previous experiments have been mostly conducted on powdered samples and structural investigations of experimentally generated fault-gouge material provide contrasting results when it comes to the initiation of melting during fault slip. In the present study, we deformed four intact Westerly granite samples, to decipher whether there is a correlation between failure mode, i.e., controlled, self-stabilised, or dynamic, and grain-size reduction within the developing fault gouge. Controlled failure took place over several hours, self-stabilised failure occurred within a few seconds and dynamic failure lasted less than a second. To test the influence of aqueous fluids on the grain-size evolution within fault gouges, two runs were performed on samples, dynamically failing either in the presence or absence of pore fluids. All samples were deformed at the same effective pressure of 40 MPa and displacements along the newly created faults were with 1.2 to 2.0 mm in a similar range. We investigated the microstructures of each sample using a scanning electron microscope (SEM) and cut two focused-ion beam (FIB) sections per sample from selected areas, located within the fault gouges, to analyse their nanostructures using a transmission electron microscope (TEM). At low magnification at the SEM, no striking differences between the different fault gouges are visible. Features resembling “cooling cracks” become apparent at the highest magnification at the SEM and are only found in the samples that failed dynamically. Major differences between the samples are only obvious when comparing their nanostructures using TEM imaging. In the high-resolution TEM images as well as with the aid of selected area electron diffraction (SAED), we observe a clear correlation between failure mode or rupture speed and grain-size reduction, with an increase in amorphous material as rupture speed increases. Regardless of the availability of fluids, the samples that underwent dynamic failure reveal similar nanostructures. Both exhibit flow structures created by amorphous material. We believe that latter is the result of melting as we find numerous structural and chemical evidence for melting, e.g., euhedral magnetite crystals of a few tens of nanometer with adjacent depletion halos. Such indicators for melting are absent in samples that failed in a controlled or self-stabilised manner, highlighting the importance of rupture speed on fault gouge melting.

How to cite: Incel, S., Ohl, M., Aben, F., Plümper, O., and Brantut, N.: Nanostructures as an indicator for deformation dynamics , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12229, https://doi.org/10.5194/egusphere-egu24-12229, 2024.

Understanding the evolution of rock physical properties with changing scale is a critical challenge when characterising spatial subsurface heterogeneities. One of the possible approaches can be using elastic wave velocities at various scales, from laboratory to field, by solely tuning the sensing wavelength to the studied media. Theoretically, in the case of a dry, isotropic, and homogeneous porous medium, at all scales of investigation, the elastic properties are not dependent on the scales of analysis (non-dispersive medium). However, as already pointed out in the literature, carbonate rocks have very heterogeneous pore networks at different scales, which may lead to different Representative Elementary Volumes (REV) with changing scales. In our work, we assume that the elastic wavelength is equal to the upper bound of the REV.

In this study, we investigated marine carbonate rocks of Middle Jurassic age outcropping in the western part of France (Charentes, near Angouleme city) in four different quarries. A total of three REVs were investigated, always in dry conditions: i) the centimeter scale, acquiring P and S wave velocities (Vp, Vs) on 60 cylindrical samples of one inch-diameter using a central frequency of 500 kHz (wavelength ~ 1 cm); ii) the decimeter scale, acquiring more than 1500 measurements of Vp and Vs on outcropping carbonates with a frequency of 40 kHz (wavelength ~ 10 cm); and iii) the decameter scale acquiring seismic wave velocity measurements along a vertical profile (geophones connected to a vertical outcrop wall), where the 6 kg sledgehammer source was situated on the plateau, delivering a frequency of 100 Hz (wavelength ~ 10 m). In parallel, a thorough geological description was done at all the investigated scales, combining i) microscope-driven microstructure analysis of samples under the microscope, ii) sedimentary facies description of outcrops and iii) fracture orientation analysis on photogrammetric models of quarries.

The elastic wave velocity results were interpreted considering facies and diagenetic processes of sedimentary rock fabric. At the centimeter scale (i), for a given sedimentary facies, we show a clear control of diagenesis (cementation and dissolution) on the elastic properties, in agreement with the well-documented literature. At the decimeter scale (ii), horizontal and vertical Vp-Vs data were used to construct 2D acoustic property maps (1 square meter). Two extreme behaviors can be pointed out. On the one hand, velocity data are homogeneous and anisotropic in zones showing evidence of primary stratification and lithostatic compaction. On the other hand, data are heterogeneous and isotropic in zones exhibiting significant early diagenesis heterogeneities (“hardgrounds”). These results are thus linked to facies and diagenesis heterogeneities. Finally, at the decameter scale, seismic velocities clearly show an azimuthal anisotropy, mainly controlled by the occurrence of outcropping open joints from tectonic origin. Our study tends to highlight the crucial need to always characterise sedimentary facies, diagenesis evolution and structural overprint in carbonate reservoir rocks if one wants to interpret and correctly understand the multi-scale elastic properties of carbonates.

How to cite: Bailly, C., Léger, E., Andrieu, S., Regnet, J.-B., Bergogne, M., Monvoisin, G., Saint-Bezar, B., Mas, P., Zeyen, H., and Brigaud, B.: Behavior of elastic properties in carbonates: scale does matter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12400, https://doi.org/10.5194/egusphere-egu24-12400, 2024.

Prolonged hydrocarbon production often leads to subsidence and seismicity in offshore and onshore hydrocarbon fields. In the Netherlands, tens of centimetres of subsidence has occurred above the Groningen Gas Field, with widespread induced seismicity during the 60+ years of its lifetime. These phenomena are driven by reservoir compaction at depth, resulting from gas extraction. Modelling the reversible, elastic component of compaction is straightforward. However, permanent deformation can also occur, the rate and effects of which are very poorly constrained. Furthermore, from smaller fields in the vicinity, it has already become clear that compaction may continue even now that production has stopped in 2023. To be able to confidently forecast the long-term surface impact of fluid production, for fields such as Groningen, and many other fields around the world, models are required that include the physical mechanisms responsible for reservoir compaction. These mechanisms are still poorly known and quantified at true reservoir conditions. Combining microstructural observations, obtained from field material and experimental work, and novel experimental mechanical data, obtained at simulated stress changes relevant to the reservoir, enabled us to identify the main grain-scale deformation mechanisms operating in the reservoir sandstone of the Groningen Gas Field. A key role is played by the thin intragranular clay layers present between the quartz grains making up the load-bearing framework. Compaction of and slip along these thin clay films has accommodated the permanent deformation accumulated during the production stage. After production is halted, experiments suggest that slow, time-dependent grain breakage will start to play a role as well. Microphysical models describing rate-insensitive compaction were implemented in Discrete Element models to assess sandstone compaction behaviour at the cm-dm scale. These numerical models can be used to evaluate reservoir compaction in different locations on the field due to pressure equilibration or repressurisation, with rate-sensitive mechanisms, such as stress corrosion cracking, to be added at a later stage, as their descriptions are still be developed. Eventually such small-scale numerical models should form the basis to upscale the sandstone behaviour to the reservoir scale.

How to cite: Hangx, S., Pijnenburg, R., Shinohara, T., Jefferd, M., Mehranpour, M. H., and Spiers, C.: Reservoir compaction during and after fluid production: A case study of the Groningen Gas Field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12452, https://doi.org/10.5194/egusphere-egu24-12452, 2024.

Carbon capture and storage (CCS) stands as a key technology for mitigating CO₂ emissions, with depleted oil and gas fields being excellent candidates for geological storage. However, injection of relatively cold, high-pressure CO₂ into higher temperature, low-pressure hydrocarbon reservoirs can induce cooling and potential freezing due to the temperature difference between the injected fluid and the reservoir, as well as Joule-Thomson cooling caused by the rapid expansion of the fluid upon entering the reservoir. This may impact wellbore integrity, and near-wellbore stability and injectivity, posing challenges for safe and cost-effective storage. To be able to accurately predict the impact of cooling on storage operations, it is important to quantify the impact of temperature cycling on the mechanical and transport properties of the rock formations in the near-wellbore area.

To address this, we performed thermal cycling experiments under realistic in-situ pressure-temperature conditions on sandstone analogous to typical hydrocarbon reservoir material. We used a novel apparatus comprising a hydrostatic pressure vessel placed inside a climate chamber providing a temperature range of -70°C to +180°C. Bleurswiller sandstone (Vosges, France; 24% porosity) was subjected to temperature changes from 100 °C to +40, +5, or -20°C at constant pore fluid pressure (5 MPa; 0.85 M NaCl brine) and confining pressure (10 or 25 MPa, i.e. similar to reservoirs of up to ~3 km depth). The effect of the rate of temperature change, brine saturation and the number of cycles on the volumetric behaviour of the sandstone were systematically investigated. Thermally treated samples were subsequently subjected to permeability measurements and conventional triaxial compression to evaluate the impact of confined temperature cycling on the transport and mechanical properties.

In all our thermal cycling experiments, we observed permanent volume change (compaction) with each cycle, though the amount of compaction decreased with subsequent cycles. Furthermore, our results showed that confined temperature cycling did not significantly alter the mechanical properties (strength, elastic properties) of Bleurswiller sandstone. This is in contrast to the strength reduction observed in other porous sandstones after unconfined thermal cycling. However, our thermally treated samples did exhibit a significant reduction in permeability by several orders of magnitude (κ = 10^-15 to 10^-16 m² post-treatment) compared to untreated reference samples (initial κ = ~10^-14 m²). Overall, permeability roughly decreased with increasing brine content (i.e. from dry to fully brine saturated), increasing number of thermal cycles, and increased temperature amplitude (i.e. more cooling). Temperature change rate did not affect the permanent volumetric strain or permeability reduction in samples that were only cooled. In experiments achieving sub-zero temperatures, including pore fluid freezing, slower temperature changes resulted in less permeability reduction.

How to cite: Amiri, S., Pijnenburg, R., and Hangx, S.: Effects of Temperature Cycling on the Mechanical and Transport Properties of Porous Sandstone: Implications for CO2 Storage in Depleted Hydrocarbon Reservoirs , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12566, https://doi.org/10.5194/egusphere-egu24-12566, 2024.

As the source material of hydrocarbon and a significant matrix constituent in organic-rich shale, organic matter influences not only the oil/gas generation and accumulation but also the mechanical behavior of shale formations. Previous researches have found that organic matter exhibits different mechanical properties from inorganic minerals, and proved that geochemical features could significantly affect the elastic behavior of organic matter in shale. Here, this work systematically investigates the influence of organic type and/or thermal maturation on mechanical behavior of organic matter. For elastic behavior, in conjunction with vitrinite reflection test, scanning electron microscope (SEM) observation, and micro-Raman analysis, nanoindentation is performed to measure the modulus of different macerals. The results indicate that with the same thermal maturity, inertinite has the highest Young’s modulus, while the modulus of bitumen is the lowest. In addition, with the increase of thermal maturity, the Young’s moduli of all kinds of maceral tend to increase, while the intensity ratio of D peak to G peak measured by micro-Raman analysis shows a decreasing trend, which indicates a higher degree of graphitization. For fracture behavior, maceral identification, focused xenon ion beam fabrication, and in situ fracture test are combined to analyze the deforming and fracturing process of different organic types. Micro cantilever beams are manufactured by using a xenon plasma focused ion beam-SEM (Xe PFIB-SEM), and then loaded in an environmental SEM (ESEM). Organic matter particle is set at the fixed end of cantilever beam, and the load is applied at the free end. Thus, the interaction between micro crack and organic matter can be observed and the corresponding mechanical data can be recorded. Test results indicate that the micro cantilever beams dominated by inertinite and vitrinite show the features of brittle failure, while those dominated by bitumen show the features of ductile failure. These microscale findings can support the upscaling model for precise prediction of mechanical properties at the macro scale, and assist with the understanding and interpretation of macroscopic elastic and fracture behavior in shale reservoirs.

How to cite: Zhao, J., Zhang, P., Zhang, W., and Zhang, D.: Influence of Geochemical Features on Elastic and Fracture Behavior of Organic Matter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13701, https://doi.org/10.5194/egusphere-egu24-13701, 2024.

How to cite: Bernardo, N., Martins-Gomes, V., Bordes, C., and Brito, D.: Exploring Seismoelectric Interface Responses at Poroelastic/Elastic Boundaries: numerical and experimental approaches, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14953, https://doi.org/10.5194/egusphere-egu24-14953, 2024.

Reservoir rock properties play an important role in the overall efficiency of petrothermal systems. Potentially interesting target formations for such systems include igneous and metamorphic rocks. For such lithologies with typically low porosity values, pre-existing fractures and foliation emerge as possible controlling factors of their hydraulic and mechanical behavior, respectively. Understanding the hydromechanical behavior of the reservoir rock is vital for the planning, execution and monitoring of hydraulic stimulation and exploitation, and continued safe operation. We inferred the elastic behavior of Freiberger gneiss from millimetre to tens of meters scale from laboratory and field measurements. Laboratory measurements were performed on samples prepared from blocks collected in the Reiche Zeche mine in Freiberg, Germany, and on cores retrieved from boreholes, drilled to perform hydraulic stimulation experiments as part of the STIMTEC (STIMulation TEChnologies) project. Controlled-source, P- and S-wave ultrasonic measurements were performed on samples and cores at room pressure and temperature conditions, covering a wide range of angles between wave-propagation direction and foliation, to characterize the degree of mechanical anisotropy of the gneiss. Magnitude and anisotropy of P-wave velocities determined from the laboratory measurements on the cores are in broad agreement with velocities calculated from in-situ sonic log measurements and active ultrasonic transmission experiments  (Boese et al., 2022) representing travel path lengths from meter to decameters in the rock mass, with a mean fracture distance of decimeters. At elevated pressures, ultrasonic measurements on cylindrical samples suggest the dominance of foliation over microcracks in determining the elastic behavior. The lack of a substantial reduction in velocities, deduced from in-situ active and passive microseismic analyses (Boese et al., 2022), except in highly deformed volumes, constrains the stiffness of the in-situ fractures.

Reference
Boese, C. M., Kwiatek, G., Fischer, T., Plenkers, K., Starke, J., Blümle, F., Janssen, C., and Dresen, G.: Seismic monitoring of the STIMTEC hydraulic stimulation experiment in anisotropic metamorphic gneiss, Solid Earth, 13, 323–346, https://doi.org/10.5194/se-13-323-2022, 2022.

How to cite: Korkolis, E., Kozlov, E., Adero, B., and Renner, J.: Mechanical characterization of Freiberger Gneiss (Reiche Zeche, Germany) from laboratory to field scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15168, https://doi.org/10.5194/egusphere-egu24-15168, 2024.

Petrophysical rock typing (PRT) is a key integrated workflow in reservoir characterization that utilizes petrophysical properties like permeability and porosity in conjunction with pore size distribution, typically derived from mercury capillary pressure measurements to classify from rich to poor reservoir quality rock units. However, the PRT workflow must encompass additional properties such as mineralogy, surface area, and clay distribution in tight rocks with a high clay mineral content to capture the microstructure and heterogeneity in such formations.

This case study focuses on the Zona Glauconitica (ZG) reservoir in the Magallanes basin, Chile, greensand with permeability ranging from 0.001 to 1 mD and total porosity between 10 and 25%v/v. Its high iron content is due to substantial amounts of chlorite and/or glauconite. The PRT workflow analyses ten petrophysical and mineralogical parameters using principal component analysis and K-means clustering, aiming to identify crucial patterns and correlations between rock properties and their storage potential. Multilinear regression (MLR) analysis was employed to determine the best-fit correlation for predicting pore throat radius at different mercury saturations from capillary pressure curves, using total porosity and gas permeability as input variables.

Four distinct petrofacies were identified as closely associated with clay minerals content, iron levels, permeability, porosity, and pore throat distribution. MLR best correlated the pore throat radius at 25%v/v mercury saturation with Pittman’s (1992) correlation. These findings offer significant promise as they contribute to enhancing and refining the existing petrophysical model. Future work extends this methodological approach to logging data across ten uncored wells, preceded by a validation process involving two cored wells. This iterative process will contribute to developing and validating an effective petrophysical model for the ZG reservoir, facilitating more precise and efficient evaluations of its production potential.

Pittman, E. D. 1992. Relationship of porosity and permeability to various parameters derived from mercury injection-capillary pressure curves for sandstone. AAPG Bulletin, 76, pp.  191-198.

How to cite: Navarro-Perez, D., Fisher, Q., Lorinczi, P., Valderrama Puerto, J., Velasquez Arauna, A., and Verdugo Dobronic, M.: Petrophysical rock typing integrated workflow in a Chilean greensand., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15600, https://doi.org/10.5194/egusphere-egu24-15600, 2024.

Clay-rich rocks play an important role in critical practical applications, particularly as natural barriers in nuclear waste repositories and subsurface caprocks for CO₂ storage. The interaction between electrostatically charged clay minerals and polar fluids (e.g., water) can lead to swelling or, under confined conditions, build-up of swelling stress. Fault closure by swelling in clay-rich rocks has been the focus of many studies. However, it remains unclear how water-clay interactions affect the stability of pre-existing faults, considering that in addition to changes in frictional properties, the stress state may also change due to the build-up of swelling stress.

This study addressed this gap by conducting triaxial friction experiments on oblique saw-cut cylindrical samples. The upper half of the sample consisted of a clay-rich rock (Opalinus claystone) and the lower half of a permeable sandstone (Berea sandstone). The first set of experiments determined the friction slip envelope of the sandstone-claystone interface without fluid injection, at confining pressures ranging from 4 to 25 MPa, and a constant axial loading rate of 0.1 mm/min. These experiments showed a frictional strength well below Byerlee’s law, indicating that the Opalinus claystone dictates the strength of the two-material interface.

Friction experiments with fluid injection were then performed at confining pressures of 10 and 25 MPa with a constant piston position (no axial loading) and an initial differential stress of about 70% of the expected yield stress. The aim was to compare the fluid pressures required to initiate slip in scenarios with and without fluid-clay interactions. For this, the experiments involved stepwise increases in fluid pressure through the injection of either deionized water (a polar fluid) or decan (a non-polar fluid). In one of the decane and one of the water injection experiments, fibre-optic strain sensors were attached to the sample surface. This allowed us to differentiate between poroelastic deformation within the matrix, deformation due to water-clay interaction, and elastic relaxation due to slip along the saw cut.

The friction slip envelope based on decane injection experiments is within the uncertainty of the friction slip envelope based on the experiments with no fluid injection. In contrast, the water injection experiments indicate a weakening of the frictional interface. We interpret this weakening to be due to the transition of the claystone from a solid rock to a mud close to the saw-cut surface. This weakening was evident even at ambient fluid pressure, although the apparent stress state was below the yielding stress, indicating the need to consider swelling stress in initial water injection scenarios. In summary, our data suggest that water-clay interactions may reactivate pre-existing faults due to (1) the change of the frictional properties and (2) the build-up of swelling stress.

How to cite: Rast, M., Madonna, C., Selvadurai, P. A., Salazar Vásquez, A., Wenning, Q., and Ruh, J. B.: Fault reactivation in clay-rich rocks – effects of water-clay interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16309, https://doi.org/10.5194/egusphere-egu24-16309, 2024.

Many volcanoes host a hydrothermal system,  responsible for a large fraction of volcanic eruption. These eruptions do not expel magma but involve the forceful ejection of pre-existing rocks, volcanic gases, and steam, posing a significant threat to human safety. Recent catastrophic incidents underscore the difficulty in foreseeing sudden hydrothermal explosions, exposing our limitations in prediction. The challenge lies in the absence of distinct precursory signals, making it difficult to anticipate these events. These eruptions may be triggered by the introduction of mass and energy originating from magma, or alternatively, by the development of mineralogical seals above vents, devoid of any direct magmatic influence. Understanding and predicting these hydrothermal phenomena remain critical for mitigating their potential human and environmental impacts.

In the ERUPT research project, we study the geoelectrical response of volcano hydrothermal systems (VHS).  Here, we focus on the laboratory scale, where we amalgamate electrical properties, namely SIP (Spectral Induced Polarization) measurements, with X-ray pore-scale (4D µCT) imaging to unravel the intricate electrical signatures of volcanic systems on rock samples collected from Gunnuhver region (Iceland). SIP  is a geophysical method that measures the complex electrical impedance of a material as a function of a wide range of frequencies (Zimmermann et al., 2008). It is particularly useful for characterizing the electrical properties of porous media, and have been widely used to study rock samples from VHS (e.g., Lévy et al., 2019). SIP responses are sensitive to factors like surface area, pore size distribution, fluid content, as well as movement of fluids within the rock. On the other hand, X-ray µCT is an imaging technique that uses X-rays to create detailed, 3D images of the internal structure of a sample, such as internal morphology, porosity, other structural features of rocks at a micrometer scale, and quantify fluid pathways and flow dynamics within the rock. The synergy of combining these two methods can provide a more comprehensive understanding of the geoelectrical properties and internal structure of a rock sample as follow: by analyzing SIP responses at different frequencies and correlating them with the µCT images, we gain insights into how variations in geoelectrical properties relate to the movement of fluids within the rock matrix, as well as the influence of alteration or precipitation of minerals. As a first step, we developed a unique experimental set-up that enables to combine both methods (SIP and µCT) simultaneously. The noval prototype was thoroughly designed following specific technical features (e.g., dimensioning, materials) to ensure an optimal SIP signal acquisition under well controlled conditions of temperature and pressure, together with a high resolution 4D µCT imaging. This integrated approach is valuable for studies in geophysics, hydrogeology, and reservoir characterization, among other various relevant domains.

References

Lévy, L. et al. (2019) ‘Electrical resistivity tomography and time-domain induced polarization field investigations of geothermal areas at Krafla, Iceland: Comparison to borehole and laboratory frequency-domain electrical observations’, Geophysical Journal International, 218(3), pp. 1469–1489. https://doi.org/10.1093/gji/ggz240.

Zimmermann, E. et al. (2008) ‘A high-accuracy impedance spectrometer for measuring sediments with low polarizability’, Measurement Science and Technology, 19(10). https://doi.org/10.1088/0957-0233/19/10/105603.

How to cite: Omar, H., Bultreys, T., Caterina, D., Nguyen, F., Vanhooren, L., Manoorkar, S., and Hermans, T.: Combining Spectral Induced Polarization and X-ray micro-Computed Tomography imaging to reveal pore-scale dynamic processes occurring in volcanic hydrothermal systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18323, https://doi.org/10.5194/egusphere-egu24-18323, 2024.

We investigate the evolution of poro-mechanical, transport properties and strength characteristics of different sandstones during the cyclic underground hydrogen storage (UHS). Therefore, we selected three different types of sandstones: fine-grained St Bees (∅ =19~22%), coarse-grained Castlegate (∅=18~20%), and coarse-grained Zigong (∅=7~11%). These sandstones exhibit significant porosity, grain size, and mineralogical differences. The samples were imaged using micro-CT to characterise their initial microstructure and then subjected to cyclic loading experiments under hydrostatic as well as various deviatoric stress paths. The aim is to simulate the in-situ stress during cyclic UHS at depths of ~1.5-3km. The permeability of the samples was measured at different stress points. After completing the cyclic loading tests, we performed repeat micro-CT characterization as well as scanning electron microscopy (SEM) analysis to record the permanent changes in the microstructure caused by the stress cycles. The experimental results show that at shallower depths (low-stress state), the high porosity Castlegate sandstone (∅=18~20%) and the St Bees sandstone (∅=19~22%) exhibit an increase in elastic modulus during the tests, experiencing strain hardening due to compaction. The permeability of both sandstones decreases with an increase in mean stress, independent of the stress path. The fine-grained St Bees sandstone shows more significant accumulative inelastic strain and higher permeability loss than the coarse-grained Castlegate sandstone at the same stress state. In contrast, the low-porosity Zigong sandstone (∅=7~11%) shows no significant changes in mechanical properties, and its permeability loss is related to the closure of the initial microcracks. At greater depths (high-stress conditions), the mechanical and transport properties of the fine-grained St Bees sandstone exhibit an evident dependence on the stress path. During stress cycling under deviatoric stress conditions, the rock experienced a noticeable weakening indicated by a reduction in elastic modulus. The porosity of the sandstone decreased by 0.8~1.4% due to the combined effects of compaction and dilatancy, with a permeability loss exceeding 50%. The application of deviatoric stress led to lower permeability than hydrostatic tests conducted under the same mean stress. In contrast, the coarser-grained Castlegate and Zigong sandstones show an insignificant stress path dependence in their mechanical and transport properties. Due to compaction, these sandstones experience increased intergranular contact, leading to reduced porosity, increased elastic modulus, and strain hardening. The lower-porosity Zigong sandstone shows a higher sensitivity of permeability to stress than the higher-porosity Castlegate sandstone, which is related to its more complex pore structure. Microstructural analysis reveals that factors such as porosity, particle size, microfractures, and the presence and distribution of compliant components like clay minerals are the primary causes for the variations in the poro-mechanical and transport properties of the three sandstones under cyclic stress. Therefore, in addition to the depth of the reservoir, grain size (and their distribution) and mineralogical characteristics play a significant role in the selection of hydrogen storage candidates.

How to cite: Wen, M., Wang, Q., and Busch, A.: Evolution of poro-mechanical and transport properties of sandstones under different cyclic stress paths: Implications for underground hydrogen storage., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19226, https://doi.org/10.5194/egusphere-egu24-19226, 2024.

Pressurized fluid injection into underground rocks occurs in applications like carbon sequestration, hydraulic fracturing, and wastewater disposal and may lead to human-induced earthquakes and to surface uplift. Yet, the full mechanical response of the underground to those injections is largely unknown. As the underground cannot be observed directly, experimental studies are crucial for understanding its mechanical reaction to fluid injection. Yet the need to maintain high pressure flow while tracking deformation complexes the execution of such experiments in comparison to standard mechanical tests. In this study we use a unique-transparent porous medium, made from chemically sintered Polymethyl Methacrylate (PMMA) beads, to simulate the underground rocks. We inject into the medium fluid at increasing pressure while measuring its internal plane-strain field as it deforms. We find that although the medium is constrained in its periphery, internal strains still occur perpendicular to the flow, compensated by the medium itself. While the medium’s overall strain shows clear reversibility, the internal perpendicular strain variations show very little to no recovery at all. Flow simulations over permeability fields, derived from the measured strain, reveal that internal strain variations have a significant impact on preferential flow within the medium. Together with the measured strain, the simulations results constitute a strong base for modeling the local heterogenous coupling between preferential flow and deformation in the underground due to fluid injections.

How to cite: Bachrach, A. and Edery, Y.: Coupling Preferential Flow and Flow-induced Strain in Heterogeneous Rock-like Medium , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20486, https://doi.org/10.5194/egusphere-egu24-20486, 2024.

Wettability is a key factor controlling the flow and distribution of multiphase fluids in reservoirs. The study of reservoir wettability is of great significance to understand the mechanism of oil and gas migration and improving oil and gas recovery. The oil reservoirs have various wetting characteristics, including water wet, neutrally wet and oil wet states. The conventional methods for reservoir wettability analysis, including the contact angle, Amott index and the USBM index, have been currently established in the petroleum industry. The contact angle method is relatively simple, but it is not suitable for the reservoir samples with complex mineral composition and strong heterogeneity. Amott and USBM method based on oil-water displacement are encountering problems such as long test time and large errors, when applied to low-permeability and tight reservoirs. This study takes the low-permeability and tight sandstone reservoirs in the Ordos Basin as an example. Based on the measurement of oil and water content in the rock using the nuclear magnetic resonance, the Amott-Harvey index was tested. Then the twin powder sample was sequentially extracted for 72 hours with the fluorescence spectrum of the extracted organic matter was tested every 12 hours. The fluorescence spectrum of the crude oil in the same production layer was also tested, and then the relationship between the fluorescence characteristics of the extracted organic matter and the Amott-Harvey index was analyzed. Finally, a new method for rapid and qualitative evaluation of reservoir wettability was established. The conclusions were as follows: (1) Crude oil represents the characteristics of free hydrocarbons in the reservoir, and its fluorescence spectrum has a fluorescence intensity peak around 375 nm. (2) The firstly extracted organic matter contains both free hydrocarbons and adsorbed hydrocarbons, and its fluorescence spectrum has a double peak with a wavelength of 375nm as the main peak and 460nm as a secondary peak. The last extracted organic matter has a lower proportion of free hydrocarbons and a higher proportion of adsorbed hydrocarbons compared with firstly extracted one. The fluorescence spectrum mostly shows double peaks with the same fluorescence intensities at the wavelength of 375nm and 460nm. (3) The ratio of fluorescence intensity of the extracted organic matter at a wavelength of 460nm to 375nm (I_460nm/I_375nm) may reflect the coverage proportion of the rock surface by adsorbed hydrocarbons, and thus can be used as a parameter indicating the wettability of the rock. The I_460nm/I_375nm value of the last extracted organic matter has a strong correlation with the Amott-Harvey index, therefore it can be used to qualitatively evaluate the reservoir wettability.

How to cite: Yan, H., Wang, Z., Liu, K., Yu, J., Zheng, G., Ji, L., Li, Y., and Zhang, Y.: An innovative method to qualitatively measure the wettability of sandstone reservoirs based on the sequential extraction and fluorescence analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20513, https://doi.org/10.5194/egusphere-egu24-20513, 2024.

The formation of granitic domes via exfoliation jointing produces some of the most celebrated and hazardous landforms on Earth. In 1904, G.K. Gilbert outlined three mechanisms to explain exfoliation jointing as: (1) related to the original cooling of the rock body, (2) related to decompression of the rock body as it is exhumed to the surface of the Earth, or (3) related to processes at Earth’s surface - a hypothesis recently supported by observations of thermal cycling in crack initiation and propagation. Despite more than a century of study, our understanding of the mechanisms driving exfoliation jointing remains incomplete. This research seeks to address the question: is the formation of exfoliation joints more sensitive to surface processes (e.g., biotite weathering, thermal cycling), topographic, or regional (i.e. tectonic) stresses? To test this hypothesis, we predicted the orientation of fractures subject to variable geologic conditions with a multi-scale weathering model of damage and fracture propagation implemented in the finite element method. We present predictions resulting from thermal contraction during cooling of the rock body, depressurization during rock exhumation, and regional tectonic compression. We then compare fractures generated under variable topographic stresses, surface weathering processes, and rock geochemistry (i.e., biotite fraction and orientation). By improving our understanding of how significantly pre-existing geologic conditions and rock fabrics influence fracturing, we can work towards disentangling this effect on observed fracture orientations and better interpret paleo-stresses for major tectonic events or potentially paleo-topography. Additionally, enhancing models for weathering mechanics and fracturing in granitic bodies may reveal sensitivities to changes in climate and critical zone evolution, with implications for the forecasting of rockfall hazards in relation to projected temperature and climatic changes.

How to cite: Reynolds, A., Lang, K., and Arson, C.: A comparison of exfoliation joint formation mechanisms: what is the role of surface processes?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1197, https://doi.org/10.5194/egusphere-egu24-1197, 2024.

Deep-seated, high-alpine rock slides frequently occur in highly schistose, fractured, anisotropic rock masses. Many studies have shown that pre-existing geological structures are decisive for a rock slide’s initiation and kinematics, as they provide weakness zones that may be reactivated in the rock slide process. Besides this structural pre-disposition, internal deformation processes by brittle rock mass fracturing play an important role in the evolution of a rock slide. Nonetheless, the effect of multiscale rock mass fracturing due to the rock slide process is yet to be fully understood. Especially, as it is challenging to measure, characterize, and to numerically model these processes. In our contribution, we present three deep-seated rock slides located in the European Alps in heavily foliated, fractured rock masses with failure volumes above 500.000 m³ each. Focusing on these case studies, we investigate the internal deformation processes with a combined approach, comprising field mapping, laboratory testing, remote sensing, and numerical modelling.

During extensive geological field surveys, we mapped the geomorphological rock slide features and characterized the structural framework of each study site, yielding geometrical models of the rock slides. This provided the basis for our 2D distinct element modelling studies using UDEC, backed by lithological and rock mechanical laboratory investigations.

Whilst UDEC allows for modelling large displacement of blocks bounded by pre-existing discontinuities, it lacks the capability to simulate fracture initiation and propagation of new failure paths within intact blocks, thus neglecting brittle rock mass fracturing. We circumvent this constraint by tessellating the intact rock mass into random polygons – referred to as Voronoi elements. Here, we adapted the original Voronoi technique by assigning an asymmetry to the Voronoi elements, characterized by an elongated axis to consider rock mass anisotropy related to schistosity. By applying this approach, we modelled the fractured, anisotropic, metamorphic rock masses as a combination of pre-existing, field-related structures within a matrix of small, asymmetric Voronoi elements.

In order to confirm the model outputs, we used terrain and deformation data derived from various remote sensing techniques – e.g. satellite based synthetic aperture radar, terrestrial laser-scanning (Riegl VZ 4000) and several campaigns of unmanned aerial vehicle photogrammetry.

In our study, we were able to reproduce the failure mechanism and kinematics of all three rock slides in accordance with our remote sensing deformation data. Thereby, the asymmetric Voronoi tessellation proved to be feasible in reproducing the brittle rock mass fracturing processes in remarkable agreement with our observations in the field. Thus, our results show, how the formation and kinematics of deep-seated rock slides are controlled by the reactivation of pre-existing geological structures and brittle rock mass fracturing. In doing so, our integrated field, laboratory, and numerical modelling approach further contributes to a better understanding of rock slide initiation and kinematics in complex geological media.

How to cite: Gerstner, R., Frießenbichler, M., Avian, M., Maschler, A., Fey, C., Valentin, G., Keuschnig, M., Mair, V., Goldschmidt, F., and Zangerl, C.: High-alpine rock slides controlled by pre-existing geological structures and brittle rock mass fracturing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1670, https://doi.org/10.5194/egusphere-egu24-1670, 2024.

Weak interlayers in reservoir slopes can significantly impact the construction and operation of hydropower projects. Considering the existence of multiple weak interlayers facing outward on the slope within the left bank slope of Caiziba pumped storage plant located in Fengjie, Chongqing, there is an urgent need to identify the effects of high-frequency, rapid, and cyclic water level changes on its stability. The study involved creating a geological model and using 3D laser scanning and 3D printing technology to prepare several groups of weak interlayers with varying roughness. Shear tests were conducted under natural and fully wet conditions to investigate the strength deterioration of weak interlayers with different roughness after multiple wetting-drying cycles. The results were verified through numerical simulation. The results indicate that the shear strength increases with the roughness of the weak interlayer under the same number of cycles. Additionally, the residual curve stepwise decreases with an increase in the number of cycles. When the roughness remains constant but the number of cycles is different, an exponential decrease in the shear strength of the interlayer occurs with an increase in the number of cycles. The impact of the number of wetting-drying cycles on the strength gradually weakens. After six cycles, the shear strength decreases by 90% and stabilizes at a value that is positively correlated with the roughness. Multiple wetting-drying cycles significantly impact the strength of the weak interlayer and the slope stability in the reservoir area. This study summarizes the deterioration law, which can serve as an important reference for analyzing the slope stability in similar projects.

How to cite: Zheng, D., Liu, J., and Zhou, H.: Deterioration laws of weak interlayers on slopes during wetting-drying cycles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3004, https://doi.org/10.5194/egusphere-egu24-3004, 2024.

When excavating a tunnel, the stresses are distributed asymmetrically along the tunnel cross-section. Other factors, particularly slope friction force and excavation speed, can also contribute to the deformation and displacement of a tunnel. Despite this, several authors have used the complex potential method to predict the ground deformation surrounding the tunnel. However, their applicability to the ground response caused by the asymmetric stress distribution around the mine wall is analyzed in this context. This project, therefore, proposes an approximate solution on the slope to predict the mine cross-section deformation. The solution is based on the complex potential method to predict analytically and numerically the ground deformation around the tunnel. However, two variables called the “complex potential functions” for the Laurent series expansion are used for the stress redistribution to the tunnel boundary conditions. Data from the Datong mine case are used to justify the proposed analytical solutions. The solution is an essential guide for analyzing deformations in complex geological conditions and structures, such as steeper slopes.

How to cite: Mabe Fogang, P. and Huo, B.: Contribution of programming language to novel mine risk assessment project, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3021, https://doi.org/10.5194/egusphere-egu24-3021, 2024.

Quantifying the changes in elastic properties of rocks during deformation is an important task. Effective Medium Theory (EMT), as formulated by Sayers & Kachanov (1995) relates the crack fabric (or damage) to the elastic properties. EMT has been successfully applied in the forward sense to predict the evolution of elasticity and related acoustic velocities in response to prescribed changes in crack density; and in the inverse sense to recover crack densities from laboratory measurements of acoustic velocities.  However, EMT fails to predict an important observation from laboratory studies of rock deformation: cyclic loading under uniaxial and conventional triaxial loads of rock samples can produce significant increases in Poisson’s ratio. These increases correlate with increasing number of cycles and with increasing crack density. This phenomenon has been known since the work of Walsh (1965), Brace et al. (1966) and Zoback & Byerlee (1975). More recent work by Heap & Faulkner (2008) and Heap et al. (2009; 2010) has extended the findings across a range of different lithologies.

Published EMT equations predict Poisson’s ratios that stay constant or decrease with increasing crack density. Resolving this discrepancy is important because Poisson’s ratio may play a key role in producing stress rotations in the damage zones of faults, thereby making them ‘weak’ and prone to slip even when the normal stress is high e.g. the San Andreas Fault (Faulkner et al., 2006; Healy, 2008). Building on the work of David et al. (2012 & 2020) incorporating the effects of crack closure, sliding on cracks (Kachanov, 1992) and grain boundaries (Sayers, 2018) during loading, and delayed back-sliding during unloading, closed form micromechanical equations have been derived to describe increases of Poisson’s ratio with increasing number of cycles. Critically, increases in Poisson’s ratio are predicted even without including the effects of new cracks. Examples are shown comparing the predicted changes in Poisson’s ratio using the newly derived equations to data from uniaxial and triaxial laboratory tests on cracked rocks.

How to cite: Healy, D.: Increases in Poisson’s ratio due to cyclic deformation in cracked rock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3893, https://doi.org/10.5194/egusphere-egu24-3893, 2024.

Parametric analysis of laboratory Acoustic Emission (AE) during rock deformation laboratory experiments has revealed periodic trends and precursory behaviour of the rupture source, as crack damage nucleates, it grows and coalesces into a fault zone. Due to the heterogeneity of rocks and the different effective pressures, finding a full prediction of rupture mechanisms is still an open goal.

4x10cm cylindrical samples of Alzo granite were triaxially deformed at confining pressures of 5-40 MPa, while AE are recorded by an array of twelve 1MHz Piezo-Electric Transducers. We trained a Time Delay Neural Networks (TDNN) on key seismic attributes derived from AE, such: Event rate, i.e. the negative log time difference between successive events; Amplitude, i.e. the average max amplitude of all waveforms for each single event AE; Source mechanism estimated from first-motion polarity spheres (King et al., JGR, 2021); Seismic scattering, i.e the ratio between high and low frequency peak delays (King et al., GJI, 2022); Vp/Vs ratios from vertical P-wave velocities and horizontal S-wave velocities for individual AE (King et al., GJI, 2023).

These timeseries are then classified by the TDNN as variations in stress and strain (target parameters). TDNN require continuous, regularly sampled data but AE are discrete and irregular. To transform the training data for the TDNN, parameters are smoothed in a weighted moving window of 100 AE events, where weighting is given towards high amplitude events that occur close in space together. Data processing is applied to waveform data from all experimental condition. Despite the inherent complexity in the raw data, clear increasing or decreasing trends are repeated at different experimental conditions.

Hyperparameters for the neural network are optimised using a Genetic Algorithm (GA) by evaluating the misfit between training target (mechanical data) and model output. Each model is trained on the 10 MPa dataset and validated on the 40 MPa dataset. Roles are reversed and the results summed. This approach ensures consistent trends in the training data (waveform parameters) whilst reducing bias towards a particular dataset. We then investigate 120 configurations for the training data following a ‘leave-one-out’ strategy. E.g., a model is trained on 5, 10 and 20 MPa datasets whilst omitting the Event rate parameter. The model is then validated on the 40 MPa dataset.

Model output on validation datasets demonstrate that the TDNN can classify AE-derived parameters as increasing variations in stress and strain. 10 and 40 MPa demonstrate the best fit and are likely linked to the GA optimisation, highlighting biases driven by the training data. Forecasting results for strain and stress reveal notable over- and under-estimations of values. However, both 10 and 40 MPa are generally accurate to within 20% further highlighting the feasibility of using a TDNN for forecasting the development of new fracture under conventional triaxial conditions.

How to cite: Vinciguerra, S., King, T., and Benson, P.: Using AE based Machine Learning Approaches to Forecast Rupture during Rock Deformation Laboratory Experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5440, https://doi.org/10.5194/egusphere-egu24-5440, 2024.

Catastrophic failure is the end result of a progression of damage towards brittle failure on a variety of system scales in the Earth. However, the factors controlling this evolution, and the relationship between deformation and the resulting earthquake hazard, are not well constrained.  In particular, induced seismicity is a growing cause of concern in the engineering required for the net-zero carbon transition, including subsurface storage of carbon and geothermal energy production, and mining for critical metals. Here we address the question of how to optimize operational controls to minimize induced micro-seismicity in a ‘scale-model’ laboratory experiment where we can simultaneously image the underlying damage using acoustic emissions (sound) and x-rays (vision). We confirm that using continuous servo-control to maintain a constant acoustic emission event rate slows down deformation compared to standard constant strain rate loading, and demonstrate that it also suppresses micro-seismic events of all sizes, including extreme events, and reduces the proportion of seismic to total strain. We develop a new model to explain these observations, based on the observed evolution of microstructural damage and the fracture mechanics of subcritical crack growth.  The model is validated with high precision (r~99%) by comparison with the independently-observed stress history and acoustic emission statistics.

Qualitative inspection of comparable grey-scale x-ray volumes between the two experiments (peak stress and post-failure after unloading) showed that at peak stress microcrack damage accumulated initially, in both samples and in the same area of each sample, as localised pore collapse, pore-emanating and Hertzian tensile intra- and trans-granular cracks and pore-emanating shear and tensile inter-granular cracks. These features were mostly similar in length and aperture, although the sample loaded only under constant strain rate showed a few longer and more open cracks. Strain localisation was apparent at the same stage in both samples, but there was some evidence of earlier en-echelon microcrack localisation in the sample loaded under a constant strain rate. Post-failure, microcracks were longer and more open in the sample loaded under a constant strain rate than in the sample loaded under a constant AE event rate. The visible proportion of damaged rock was greater, with a broader shear zone around two to three grains wide (compared with <1-2 grains) and a greater degree of cataclasis throughout. Off-fault microcracking was limited, but there were some trans- and inter-granular microcracks that extended up to four to five grains long in both samples. These were more common in the sample loaded under constant strain rate and tended to be more open. Finally, branching of the fault zone appeared to be more pronounced in the sample loaded under a constant strain rate.

Our results explain the effectiveness of seismic event rate control on seismic hazard mitigation in mining settings, and imply that it may be more effective in managing the risk from induced seismicity in a pre-emptive way than the commonly-applied ‘traffic light’ system, which is based on reacting after the fact to extreme events.

How to cite: Main, I., Mangriotis, M.-D., Cartwright-Taylor, A., Curtis, A., Butler, I., Bell, A., and Fusseis, F.: Progressive rock failure under different loading conditions – sound and vision, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5553, https://doi.org/10.5194/egusphere-egu24-5553, 2024.

Macroscopic equilibrium statistical mechanics is first used to interpret and predict thermally driven microfracture in rock. Application of the theoretical framework to three heating and cooling experiments, performed on granite and reported between 1989 and 2017, provides strong evidence that the temperature-, pressure- and volume-dependent average microfracture population within a given rock volume can be treated as an equilibrium thermodynamic variable.  This observation, in turn, suggests that thermoelastic microfracture, in rock and similar granular solids, can be predicted and interpreted using standard, process- and history-independent equilibrium thermodynamics.  In order to place equilibrium rock fracture and healing in context, we then consider nonreversible, permanent, i.e., nonequilibrium fracture. Here, pictorial, physical, and quantitative analyses of several common, thermally driven rock fracture processes are presented, including: a) terrestrial thermal exfoliation of single grains from diurnally heated rock surfaces, b) non-terrestrial thermal exfoliation of thin, near surface rock layers, as recently observed, e.g., on Bennu, c) terrestrial and non-terrestrial thermally-driven through cracking, and d) initiation of c).  We show how the form of the continuum momentum and energy conservation equations for thermoelastic materials – here, rock- provides a powerful, intuitive framework for quickly visualizing and roughly predicting the fracture/weathering processes in a) through d).

How to cite: Keanini, R. and the US, Israel, UK, Japan, France Rock Fracture Collaboration: Equilibrium (reversible) and nonequilibrium (permanent) fracture in rock: equilibrium statistical mechanics theory and experiments, and physical/intuitive analysis of common nonequilibrium fracture modes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7023, https://doi.org/10.5194/egusphere-egu24-7023, 2024.

Rocks exhibit a brittle failure mode, leading to system-size failure through cataclastic faulting processes involving microfracture coalescence and frictional sliding, resulting in localized deformation. In contrast, the ductile failure mode can be described as a distributed deformation at the macroscopic scale, although there may be significant grain-scale heterogeneities. The transition between these two modes is an important research area because it is assumed to occur at the base of the seismogenic zone where large earthquakes may nucleate. Understanding strain evolution and partitioning between brittle and ductile failure modes may shed light on the preparation process for large earthquakes. To investigate the transition from brittle to ductile deformation, we performed two series of experiments on Carrara marble core samples: conventional triaxial experiments with acoustic emission recording at GFZ Potsdam, and dynamic in situ 4D X-ray imaging experiments on beamline BM18 at the European Synchrotron Radiation Facility.  Carrara marble is used as a rock model because this transition can be achieved at room temperature. We performed the experiments at room temperature and confining pressures between 5 and 100 MPa. For the synchrotron experiments, we segmented the images and implemented digital volume correlation (DVC) analyses between tomogram acquisitions to quantify the evolution of volumetric and shear strain components during the transition from the brittle to ductile regime. The results show that the transition is controlled by the dynamics of microfractures, even in the ductile regime. Below 40 MPa of confining pressure, deformation localizes along faults, particularly at 5 and 10 MPa. At 40 MPa, tomograms reveal the formation of a localized shear zone and macroscopically distributed deformation, resembling a semi-brittle regime. The DVC reveals the spatial extent of the strain directed into faults. A limited number of acoustic emissions recorded at this confining pressure revealed the prevalence of aseismic activity during deformation. Above 40 MPa, deformation shifts to a non-localized pattern at the core sample scale, involving the opening of microfractures, possibly due to the cataclastic flow mechanism accommodating this regime.

How to cite: Prastyani, E., Cordonnier, B., McBeck, J., Wang, L., Rybacki, E., Dresen, G., and Renard, F.: Revealing the transition from brittle to ductile failure mode in Carrara marble through in-situ 4D X-ray imaging and acoustic emissions experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8848, https://doi.org/10.5194/egusphere-egu24-8848, 2024.

Mature fault zones are formed by abrasive wear products, such as gouge, which results from the frictional sliding occurring in successive slip events. Shear localization in fault gouge is strongly dependent on, among others, fault mineralogical composition and grain size distribution, originating a wide variety of microstructural textures that may be related to different types of fault motion from aseismic creep, slow earthquakes to fast slip events. Within a quartz fault zone, one can encounter different stages of maturity, ranging from an incipient and poorly developed fault zone (i.e. discontinuous and thin gouge layer) to a mature fault zone that has experienced a lot of wear from previous sliding events (i.e. well-developed gouge layer). The localization of deformation within a mature gouge layer has been identified as possibly responsible for mechanical weakening and as an indicator of a change in stability within the fault.

However, to upscale the physics of shear deformation, we need to unveil the physical parameters and micro-mechanisms that govern shear localization. To gain insights on the role of dynamic changes in grain size (i.e. fragmentation), in slip behavior and fault rheology, we performed 2D numerical simulations of quartz fault gouges in a direct shear configuration using the Discrete Element Method (code MELODY). We can reproduce angular particles that can fragment during the simulation as the fault gouge accumulates strain. These experiments were performed to understand the micro-mechanical processes happening during fragmentation and shearing at a constant normal stress. Three mixtures of quartz were sheared to reproduce different initial grain size distributions within the fault (average grain sizes 100 μm, 10.5 μm, and a 50% mixture of both). The minimum grain size was set to 10 μm, meaning that all the coarser particles are subdivided into smaller ones (size 10 μm) that can fragment during the experiment.

Thanks to visual and data outputs, we can observe how particles behave during the compaction and shearing of the gouge. We use four main parameters to describe fault gouge evolution: the damage of coarse particles, the force chains, the change of porosity, and the kinetic energy linked to each particle breakage. Moreover, these numerical experiments were designed to reproduce and be directly compared with shear experiments realized on a double direct shear apparatus in the Laboratory (Casas et al., in prep). The fragmentation algorithm in the code can reproduce the shear localization observed within the real quartz microstructures and the progressive formation of Riedel bands. The connection between numerical and laboratory experiments gives important information on the connection between grain size distribution, shear localization, Acoustic Emissions, and the resulting fault slip behavior. In this context, the proportion between small/coarse particles within the fault plays an important role in controlling fault rheology.

How to cite: Casas, N., Mollon, G., and Scuderi, M. M.: The role of grain fragmentation in understanding shear localization via DEM simulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9339, https://doi.org/10.5194/egusphere-egu24-9339, 2024.

Understanding the damage processes in clay-bearing rocks is a decisive factor in geological engineering, and for instance considering nuclear waste deep geological repositories. But, more generally they may also contribute to localized deformation, and thus the rupture of fault gauges in seismic zones. However, owing to their complex mineralogy, multiscale microstructures and anisotropy, the mechanisms of clay-rich rock damage and their chronology are not yet well understood..

Here we focus on the impact of micro-damage on ultrasonic wave propagation velocity, which is confronted with the corresponding full deformation fields calculated by digital image correlation (DIC). 

The aim is to associate the acoustic signature with the active deformation mechanisms identified by DIC. To this end, an integrated experimental approach is proposed to  characterize localization and to identify the related deformation micro-mechanisms  during uniaxial compression of natural clayey rock samples (Tournemire shales) with two simultaneous measurements: 1) the evolution of P-wave velocity within the sample by active acoustics, 2) the development of the 2D mechanical full field by digital image correlation.

Both experimental techniques are well known, but the innovation of our approach is to combine simultaneously both measurements. Deformation localization is a multiscale problem, which obviously occurs at the sample scale, but also at the fines scales of the microstructure. Therefore, we developed two different experimental setups. On the one hand, during uniaxial compression with a standard MTS loading frame the macro-scale localization patterns are characterized by optical observations, which image resolution is well suited to the cm sample scale (sample diameter: 3.6 cm and double in length). On the other hand, in order to characterize the initiation of micro-damage at the microstructure scale of the composite type of rock, the same loading protocol is reproduced (while keeping the acoustic diagnosis) on smaller scale mm-sized specimens (sample diameter : 8 mm, double in length), using a home-designed miniature loading frame fit for an environmental scanning electron microscope (ESEM). The latter analysis is carried out under  controlled relative humidity  of RH = 80%, hence preventing the samples to dry out due to the high vacuum

A similar acoustic signature is identified at both scales of observation, in spite of the variations of experimental conditions imposed by the environmental SEM. We are therefore confident to be able to understand the fracturing process from micro-cracking initiation (microscale) to sample failure (macroscale), and to assess its impact on ultrasonic wave propagation.

How to cite: Lusseyran, M., Dimanov, A., Bonnelye, A., Fortin, J., and Tanguy, A.: Multi-scale experimental deformation and damage initiation of clay-rich rocks  : Coupling ultrasonic wave propagation and  full field deformation measurements by digital image correlation (DIC), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9811, https://doi.org/10.5194/egusphere-egu24-9811, 2024.

Rock physics theory and experimental data suggest that fracture growth in rock proceeds not only as a function of synchronous stress and environmental conditions but also as a function of past fracture growth in response to those conditions. ‘Stress memory’ or ‘fatigue-limit’ fracture mechanics phenomena such as the Kaiser effect epitomize this idea. Many questions exist, however, as to if and how these phenomena impact the growth of fractures under natural environmental conditions. For example, to what extent does the orientation of past experienced stresses manifest in a rock’s response to stresses of the same magnitude?

Here we test for a memory of intergranular thermal stresses in two natural granite boulders of the same lithology for which we have 1 and 3 years of known temperature history, respectively. We hypothesize that cores extracted from the exterior portions of the boulders – that have necessarily experienced more and larger temperature fluctuations – will have more ‘memory’ of peak temperatures than those cores extracted from the boulder centers. In turn, we hypothesize that outer cores will crack less in response to temperature cycling than inner cores. For the first boulder, we measured P-wave velocities and connected porosities before and after 4 different oven heat treatments – heating up to 40, 45, 50 and 65 °C at a rate of at 20 °C/hr and cooling at an ambient rate over several cycles each. For two transects of cores extracted from the natural upward facing surface down, and the natural west-facing surface inward, we found that porosities increased after each subsequent heat treatment, but by larger amounts with distance away from the outer rock surface, as hypothesized. P-wave velocities, however, both increased and decreased with different heating cycles and positions. Therefore, for the second boulder, we extracted a top-down transect of 5 cores and, using a special-made rig, found that the samples exhibit significant P-wave velocity directional anisotropy. We subjected these cores to the same heat treatments as those of the first boulder, but this time orienting the samples identically in the oven with respect to their original positions in the boulder. Preliminary data show similar results as the first boulder, with the outermost core cracking the least (as interpreted from porosity changes) relative to the inner cores. Ongoing work will examine changes in P-wave velocity in different directions relative to measured anisotropy as a function of heat treatment cycles. This work has important implications for understanding if and how, with ongoing global warming, Earth’s rocks will respond to ‘new’ temperatures. 

How to cite: Eppes, M.-C., David, C., Heap, M., Baud, P., Bonami, T., Dahlquist, M., Keanini, R., Lacroix, C., Rasmussen, M., Rinehart, A., El Alaoui, Y., and Windenberger, A.: Temperature “Memory” and Natural Rock Fracture at Earth’s Surface, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9825, https://doi.org/10.5194/egusphere-egu24-9825, 2024.

In the context of the global environmental crisis and the urgent need for energy transition and efficient energy storage solutions, salt caverns have gained attention as promising reservoirs for hydrogen. However, current literature predominantly focuses on deriving macroscopic constitutive relations, lacking crucial insights into the underlying physical mechanisms of deformation and damage active at various microscopic scales. This study addresses this gap by undertaking qualitative and quantitative investigations into the micro-mechanisms of rock salt, employing advanced micro-scale observation techniques. Natural rock salt from diverse mines and re-synthetic salts, produced through the cold compaction of grinded natural halite powder, are used to encompass a wide range of microstructural morphologies. Initial microstructure characterization involves SEM, EBSD, and CT, followed by classic uniaxial compressive tests coupled to optical microscopy monitoring. High-resolution images of the sample surface are continuously captured during testing, allowing for 2D full field measurements by subsequent application of digital image correlation techniques : the analysis of relative displacements of markers randomly distributed on the sample surface enables the retrieval of surface displacement fields and the calculation of the corresponding local strain fields over statistically representative domains. Segmentation of digital images and quantitative identification, specifically focusing on crystal slip plasticity and grain boundary sliding using an in-house computation program, reveal the complex local interactions of different micro-mechanisms. The estimation of the relative contributions of these mechanisms to global deformation all along the loading path, along with an analysis of the impact of salt grain size, provides insights into physically grounded micromechanical constitutive relations. These findings are essential for the safety assessment of industrial applications involving rock salt caverns with respect to short-term mechanical loading conditions relevant to daily hydrogen filling and withdrawal.

How to cite: Li, X., Dimanov, A., Bornert, M., Hallais, S., and Gharbi, H.: Multiscale experimental investigation of crystal plasticity and grain boundary sliding in rock salt using digital image correlation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10099, https://doi.org/10.5194/egusphere-egu24-10099, 2024.

In rocks and other consolidated geomaterials, static or dynamic excitation leads to a fast softening of the material, followed by a slower healing process in which the material recovers all or part of its initial stiffness as a logarithmic function of time. This requires us to exit the framework of time-independent elastic properties, linear or not, and investigate non-classical, non-linear elastic behavior and its time dependency. Softening and healing phenomena can be observed during seismic events in affected infrastructure as well as in the subsurface. Since the transient material changes are not restricted to elastic parameters but also affect hydraulic and electric parameters as well as material strength – documented for instance by long lasting changes in landslide rates – it is of major interest to characterize the softening and recovery phases.

To characterize this behavior in a controlled environment, we perform experiments on Bentheim sandstone in a Materials Testing System triaxial cell with pore pressure and confining pressure control. Our sample is subjected to various static loading cycles in both dry and water-saturated conditions, while an active acoustic measurement setup allows us to monitor minute P-wave velocity changes, which can then be directly tied to dynamic elastic modulus changes.

Our transducer array allows us to observe the dynamic softening as well as the recovery processes in the sample during repeated loading phases of various time lengths. Observations indicate high spatial, frequency and lapse-time sensitivity of the observed velocity changes, indicating a rich landscape of concurrent effects and physical phenomena affecting our sample during these simple experiments.

To investigate the spatial and directional dependency of the velocity changes, we restrict the analysis to direct and reflected ballistic waves. Our observations indicate that, while stress-induced classical effects are clearly anisotropic as expected, the non-classical effects do not exhibit significant anisotropy. This allows us to rule out a number of physical phenomena as the cause for the non-classical effects. Most importantly, we conclude that the microscopic structures responsible for the reversible softening and healing processes are different from the cracks that induce the anisotropic acousto-elastic effect.

How to cite: Asnar, M., Sens-Schönfelder, C., Bonnelye, A., Dresen, G., and Bohnhoff, M.: Non-linear softening and relaxation in rocks and geomaterials: a laboratory perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10753, https://doi.org/10.5194/egusphere-egu24-10753, 2024.

Frost damage in porous materials is a weathering mechanism that can cause dangerous rockfalls or damage to built cultural heritage. The volume expansion of 9% when water freezes can be one of the cause of frost damage. This does not, however, explain why partially saturated porous stones also show damage despite the fact that ice should have room to grow. By performing experiments both at the scale of a single pore and in a real stone, we investigate the mechanism of frost damage at low water saturations at the pore scale and how it relates to macroscopic damage. We observe that the meniscus at an air-water interface confines the water in the pores. Because of this confinement, ice that forms will exert a pressure on the pore walls rather than growing into the pore. The amplitude of stress is found to be larger in small pores and when the meniscus has a larger contact angle with the walls. The contact angle is also observed to increase in the case of multiple freeze-thaw cycles, which increases the likelihood of damage. We find that cracks start first in the ice (being weaker than the confining material), followed by damage in the material itself. Remarkably, when multiple air-water interfaces are induced within limestone samples through a hydrophobic surface treatment, the stones are much more susceptible to frost damage than are uncoated stones, with cracks appearing preferentially at the hydrophilic-hydrophobic interface. This shows that indeed the meniscus confining the water during freezing and consequently the wetting properties are the relevant factors for frost damage in partially saturated porous stones

Reference: R. Le Dizès Castell, R. Sinaasappel, C. Fontaine, S. H. Smith, P. Kolpakov, D. Bonn, and N. Shahidzadeh, “Frost Damage in Unsaturated
Porous Media,” Physical Review Applied, vol. 20, p. 034025, Sept. 2023.

How to cite: Le Dizes Castell, R., Sinaasappel, R., Fontaine, C., Smith, S., Kolpakov, P., Bonn, D., and Shahidzadeh, N.: Frost damage in unsaturated porous media, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11304, https://doi.org/10.5194/egusphere-egu24-11304, 2024.

Recent observations of large earthquakes document the progressive localization of rock damage around future rupture zones that is also coupled with the spatial migration of foreshock sequences (Kato & Ben-Zion, 2020). This implies that the precursory deformation may act as a potential tracer for preparatory process that result in large earthquakes. It has also been observed that self-organization of the localized damage regions can govern the eventual macroscopic brittle failure in geomaterials (Renard et al., 2019). How the presence of fluid controls the self-organized precursory deformation along localized damage zone remains an open question. In this study, we have performed two triaxial compression experiments on dry and water saturated Berea sandstone, using distributed strain sensing (DSS) technology to visualize the strain field on the sample surface (Salazar Vásquez et al., 2022) with high spatial resolution. By tracking components of the strain field, specifically the region on the sample that sustained the largest incremental change in strain, we tested the effect of fluid on the predictability of phase transition between intact and failed state, under the context of critical hypothesis. Strain was progressively localized around the eventual faulting region for both samples, while a slow faulting was observed in the wet sample accompanied by a diffuse deformation pattern and unstable crack nucleation at failure. The results showed that, the failure in the dry sample was preceded by a critical power law acceleration of the largest increment, thus the dynamic faulting occurred in a well-defined singularity. The strain distribution also provided evidence for a predictable evolution of precursors. In contrast, the wet test showed evidence for a first-order transition with an exponential increase in largest increment, leading to an abrupt failure with a transient increase of strain. We interpreted this abrupt transition to be due to the increasing dominance of fluid-driven subcritical crack growth in the faulting. In this process, the local stress at crack tips decreases with crack lengthening, hence impeding the crack interaction and leading to an abrupt development of fault network. Our observation unravels the mechanisms of precursory deformation with fluid-assisted subcritical cracking, which has important implication in forecasting large earthquakes in nature.

References:

Kato, A., & Ben-Zion, Y. (2020). The generation of large earthquakes. Nature Reviews Earth & Environment, 2(1), 26–39. https://doi.org/10.1038/s43017-020-00108-w

Renard, F., McBeck, J., Kandula, N., Cordonnier, B., Meakin, P., & Ben-Zion, Y. (2019). Volumetric and shear processes in crystalline rock approaching faulting. Proceedings of the National Academy of Sciences, 116(33), 16234–16239. https://doi.org/10.1073/pnas.1902994116

Salazar Vásquez, A., Rabaiotti, C., Germanovich, L. N., & Puzrin, A. M. (2022). Distributed Fiber Optics Measurements of Rock Deformation and Failure in Triaxial Tests. Journal of Geophysical Research: Solid Earth, 127(8). https://doi.org/10.1029/2022JB023997

How to cite: Chen, H., Selvadurai, P., Salazar, A., Bianchi, P., Michail, S., Rast, M., Madonna, C., and Wiemer, S.: The influence of fluid pressure on the phase transition of brittle faulting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13010, https://doi.org/10.5194/egusphere-egu24-13010, 2024.

Subcritical crack growth accelerates weathering of rocks and minerals, reduces the strength of rock slope, and affects the stability of subsurface faults. Under special circumstances, the produced cracks can also heal spontaneously (Self healing) regaining a part of the original tensile strength.  These crack behaviors are a manifestation of molecular-scale surface forces acting between the surfaces near the crack tip. As a part of the effort to understand how these forces control the subcritical crack growth and healing of geological materials, we examine the tensile crack behavior of calcite single crystals. A miniature Double-Torsion (DT) test system was developed for testing small plate samples (40 mm x 20 mm x 1.5 mm) cut out of optical-quality calcite single crystals (Iceland Spar crystals). These samples are oriented in such a way that the induced crack is along the (1014) plane (the primary cleavage plane). The main output of the experiment is the crack velocity (vc) vs the magnitude of applied driving force (stress intensity factor K or strain energy release rate G), which is a typical way to summarize the rate-dependent crack behavior. From the experiment, we have learned that (1) calcite exhibits strong healing behavior compared to materials such as glass or (amorphous) quartz in humid air and water, (2) healing is time dependent (the strength of a healed crack increases over time), (3) liquid water (rather than vapor) introduces strong hysteresis in the recracking vs healing behavior.   The obtained laboratory data are used to develop a mechanistic model for predicting macroscale crack behavior in rock, particularly in a water and electrolyte-rich environment.

How to cite: Nakagawa, S., Zhang, Y., Dadras, H., Hao, Z., Voigtländer, A., and Gilbert, B.: Laboratory measurement of subcritical crack growth and healing in calcite using Double-Torsion tests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13669, https://doi.org/10.5194/egusphere-egu24-13669, 2024.

In south-central Idaho, a segment of the Pioneer fault, exposed at Little Fall Creek, has accommodated large magnitude Mesozoic shortening overprinted by Paleogene extension. The resulting 30 m thick fault damage zone records a history of fault reactivation and associated deformation in quartz veins, graphite concentration on slip surfaces, polyphase contractional and extensional microstructures, and micro- to outcrop-scale corrugated, mineralized and polished slip surfaces. The gently west dipping (207°/14°) fault zone separates Ordovician argillite in the hanging wall from Mississippian argillite and quartzite in the footwall and accommodated east-northeast directed shortening. However, polished slip surfaces within the fault zone document top-to-the-west translation with a mean slip vector 15°/272°, consistent with extensional unroofing of the Pioneer Mountains core complex.

Argillite in the fault damage zone varies from proto- to ultra-cataclasite and provides evidence for overprinting of contractional fabrics by extensional fabrics. The fault damage zone is characterized by multiple anastomosing slip-surfaces which indicate a history of slip surface interactions, fault growth, and reactivation. Early deformation features include graphitic foliations and stylolites, SC foliations, and ptygmatic folds consistent with shortening. Quartz veins, mica fish, and slip surfaces coated with graphite, amorphous carbonaceous material, and amorphous quartz phases, overprint the early deformation features and are associated with west-directed extension. Hydrothermal quartz veins that show at least five phases of deformation indicate multiple strain episodes and high strain rates. Raman spectroscopy and scanning electron microscope textural analysis of the graphite in the fault damage zone show a loss of crystallinity toward the primary slip surface. We infer the late-stage meso- to micro-scale features record seismic slip and fluid-rock interactions in a gently dipping fault zone.

How to cite: Petrie, E., Burton, B., Bradbury, K., and Baldassarre, G.: Strain history of the Pioneer fault, Idaho, USA – progressive deformation and associated crystallographic alteration., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13927, https://doi.org/10.5194/egusphere-egu24-13927, 2024.

Rock weathering is ubiquitously observed at or near Earth’s surface as a fundamental component in many landscape evolution process. In arid landscapes, where limited moisture availability restricts the rate and effectiveness of chemical and biological weathering – salt weathering (regarded herein as the physical disintegration of rocks in the presence of salts) is commonly acknowledged as an especially effective mechanism for progressive weathering of rocks. While volumetric expansion and contraction of salts in response to changes in ambient moisture conditions are broadly recognized as the primary drivers of salt weathering, our understanding of the environmental conditions that produce such moisture dynamics in otherwise extremely dry settings, such as hyper-arid deserts, remains largely unknown.

Here, we present preliminary results from field-based acoustic emission (AE) measurements for boulders with salt-laden cracks perched on abandoned shorelines of the hypersaline Dead Sea. Continuous measurements since April 2023 revealed daily fracturing activity displaying a bi-modal distribution with AE activity peaks during the early predawn and afternoon hours when T changes are minimal and RH fluctuations reach maximum or minimum values, respectively. Time-lapse photography revealed a recurring pattern of salts that crystalize along the rock cracks during the afternoon AE peak hours and subsequently disappear towards the predawn AE peak hours. The appearance of salt crystals during lowest RH conditions (warmest afternoon hours) and their disappearance during highest RH conditions (coldest predawn hours) suggests that stresses induced by repeated cycles of salt deliquescence/efflorescence in response to daily fluctuations in atmospheric RH are most likely responsible for the bi-modal distribution of daily fracturing activity. This suite of new field-based measurements of salt-weathering activity in natural hyper-arid settings suggests that atmospheric RH fluctuations and the volumetric changes they induce in hygroscopic salts can be key drivers of progressive rock fracturing in extremely dry and salt-rich environments on Earth and possibly Mars where other moisture sources are limited to effectively non-existent.

How to cite: Mushkin, A., Boroda, R., Malik, U., Lensky, N., Haggai, E., Muravin, B., and Amit, R.: Salt-driven Progressive Fracturing of Alluvial Boulders Along the Hyper-arid Shoreline of the Dead Sea , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14362, https://doi.org/10.5194/egusphere-egu24-14362, 2024.

I will present novel discoveries from theoretical works, laboratory experiments, and observations from planetary space missions that, in the last fifteen years, considerably improved our understanding of progressive failure of rocks on the surface of solar system small bodies. In particular, several studies evidenced the role of thermal fatigue cracking due to the mechanical stresses generated by temperature cycling between day and night in causing progressive cracking and failure of surface rocks.


I will review how the thermal cracking driven by temperature changes and assisted by volatile sublimation actively shape the landscape of cometary nuclei (1, 2), drive cometary activity (3), boulder breakdown (4), evolution of boulder size distribution (5, 6), and surface erosion.

Several theoretical studies (7–10) based on simulations of the stress fields within rocks on asteroids that experience temperature variations of many tens of degrees between day and night (7, 11) showed that subcritical crack propagation produce rock damage. Supporting evidence came from laboratory experiments (7, 12, 13), indicating the importance of the material microscopic structure (14).

Images from space missions revealed asteroid surface features that could be interpreted as the result of the progressive rock cracking and failure due to thermal fatigue, such as the presence of boulders that appear to erode in place (15), exfoliation fractures that can be modelled as the results of crack propagation due to diurnal temperature cycling (16), and preferential alignment of fractures on boulders (17). Preferential meridional crack orientation is known, from study of Earth and Mars (18) to be consistent with cracking induced by diurnal temperature variations.

I will discuss also the time-scales of progressive failure of rocks on the surface of small bodies, which appears to be much faster (17) compared to the typical lifetimes of these objects.

REFERENCES:

1 N. Attree et al., A&A. 610, A76 (2018).

2 A.-T. Auger et al., Icarus. 301, 173–188 (2018).

3 V. Alí-Lagoa, M. Delbo’, G. Libourel, ApJ. 810, L22 (2015).

4 M. R. El-Maarry et al., Geophys. Res. Lett. 42, 5170–5178 (2015).

5 M. Pajola et al., Nat Astron. 1, 0092 (2017).

6 J.-B. Vincent et al., A&A. 587, A14 (2016).

7 M. Delbo et al., Nature. 508, 233–236 (2014).

8 C. El Mir, K. T. Ramesh, M. Delbo, Icarus. 333, 356–370 (2019).

9 J. L. Molaro, S. Byrne, J.-L. Le, Icarus. 294, 247–261 (2017).

10 B. Ravaji, V. Alí-Lagoa, M. Delbo, J. W. Wilkerson, Journal of Geophysical Research: Planets. 124, 3304–3328 (2019).

11 B. Rozitis et al., Sci. Adv. 6, eabc3699 (2020).

12 K. Hazeli, C. El Mir, S. Papanikolaou, M. Delbo, K. T. Ramesh, Icarus. 304, 172–182 (2018).

13 G. Libourel et al., Monthly Notices of the Royal Astronomical Society. 500, 1905–1920 (2020).

14 M. Patzek, O. Rüsch, J. L. Molaro, JGR Planets, in press, doi:10.1029/2023JE007944.

15 A. J. Dombard, O. S. Barnouin, L. M. Prockter, P. C. Thomas, Icarus. 210, 713–721 (2010).

16 J. L. Molaro et al., Nat Commun. 11, 2913 (2020).

17 M. Delbo et al., Nat. Geosci. 15, 453–457 (2022).

18 M.-C. Eppes, A. Willis, J. Molaro, S. Abernathy, B. Zhou, Nat Commun. 6, 6712 (2015).

How to cite: Delbo, M.: Progressive failure of rocks on asteroids and comets: review of recent studies and space mission’s results, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15818, https://doi.org/10.5194/egusphere-egu24-15818, 2024.

Time- and stress-dependency of elastic properties are features particularly observed in a variety of complex solids, ranging from steel, polymers, and cracked structures, to rocks and concrete. Recently, considerable effort has been made to understand the underlying physics of these phenomena commonly regarded as Nonlinear Mesoscopic Elasticity (NME) in laboratory setups.

As a result, various models have been suggested to explain a range of NME phenomena like hysteresis, dynamic softening, and slow dynamics, among others. Due to the high sensitivity of NME to the presence of imperfections or internal damage on solids, there is a growing interest in taking the current models and applying them to construction materials for damage assessment.

Intending to observe and incorporate these models into real-condition structures, we carried out a 1-day multifrequency vibration experiment in a 24-meter-long reinforced concrete test bridge equipped with a pretension system, to investigate the possible presence of internal damage with vibration-based methodologies.

We used the pretension system to subject the specimen to eight compression states in its longitudinal direction (forces of 400kN at the highest, and 280kN at the lowest). At every compression state, we struck the structure in the vertical direction three times on the north and south sides of the bridge with an impulse drop weight. Throughout the whole experiment, we recorded ambient seismic noise at different frequency bands with a 14-six-component sensor array to measure the acceleration in the conventional translational components and the angular velocity (rotation rate), a 14-geophone array of 4.5 Hz of natural frequency, and four pairs of embedded ultrasound transducers were used to estimate relative velocity changes (dv/v) by applying the Coda Wave Interferometry (CWI) stretching technique. internal temperature of the concrete was also recorded to correct our measurements by first-order thermal effects.

At the material scale (ultrasound regime) we observe stress-dependent dv/v at four different locations in the specimen and describe them by using the acoustoelastic effect concept regarded as a classical nonlinear phenomenon. We also analyze the relative velocity drop and the subsequent healing process in the concrete triggered by the action of the drop weight. We used the model of Snieder and Sens-Schönfelder (2017) to numerically describe the relaxation process happening at different time scales in the specimen through a deterministic inversion procedure. The north side of the structure showed to have a higher acoustoelastic effect and higher velocity drops, as well as longer relaxation times, it is important to mention that there is evidence of external cracking in this span of the bridge.

We present preliminary results in the seismic frequency band (structural scale), where we expect to observe the influence of the vertical beams that support the bridge on the spatial distribution of changes in dv/v. Changes in the fundamental frequency of the structure as a function of the stress level are also expected.

With this work, we point towards the development of new nondestructive testing methodologies highly sensitive to small cracks and imperfections using conventional and non-conventional seismic instruments, and linear and nonlinear wave propagation models.

How to cite: Dominguez-Bureos, M., Hadziioannou, C., Niederleithinger, E., and Sens-Schönfelder, C.: Time- and stress-dependent elastic properties in a concrete structure; spotting internal damage footprints, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16652, https://doi.org/10.5194/egusphere-egu24-16652, 2024.

Lineaments are elongated elements in spatial data such as valleys and ridges on topographic maps, or linear lows and highs in aeromagnetic data. Topographic linear depressions (topolineaments) are generally considered as the morphological expressions of easily erodable, elongated rock bodies situated within a mechanically stronger rock mass. In most circumstances topolineaments are even directly interpreted as faults and fractures, which forms the basis for lineament analysis study to understand brittle deformation patterns. However, topolineaments may also be formed by other tectonic and non-tectonic causes, such as alternating layers, foliation traces, dikes etc., or river- and glacial erosion not controlled by any bedrock features. This mix of potential causes begs the question: “How robust is actually lineament analysis for characterizing and quantifying faults and fractures?”. Testing the topolineament vs. fracture-relationship is not straight forward since topolineaments are usually occupied by rivers or creeks and covered with colluvium, which prevents direct observation of rock types and bedrock structures.

Underground excavations allow for continuous logging of bedrock types, rock mass quality and fracture density and orientations, which is done routinely at tunnel face during tunnel construction. Here we make use of such underground data from a large dataset of Norwegian road tunnels to compare the position of topolineaments spatially and statistically with rock fracture density and orientations in the subsurface. The tunnel dataset comprises data from across Norway in areas with widely varying bedrock geology, tectonic evolution, and geomorphology (e.g. etched surface, alpine, lowlands), which allow for an evaluation of the robustness of lineament analysis in various settings. Topolineaments are acquired using a newly developed algorithm (OttoDetect) run on both 10x10m and 50x50m resolution digital elevation models. The algorithm ensures that tunnel data is compared to a homogeneous and reproducible lineament dataset without operator or hillshade illumination biases.

Preliminary results from tunnels in areas with etched geomorphology show that c. 75% of all topolineaments correspond to weakness zones in the bedrock (i.e. very high fracture densities/very low rock mass quality compared to the surroundings). Hit rate increases for longer lineaments, which generally correspond to thicker fault zones. At the same time, only up to c. 60% of all weakness zones mapped at tunnel face can be spatially associated to a topolineaments, which demonstrate that significant brittle deformation is not expressed as topolineaments. Further analysis will be carried out to build a statistically robust dataset on the validity of lineament analysis.

How to cite: Torgersen, E., Arctander, K., Redfield, T. F., Svendby, A. K., Dichiarante, A. M., and Arntsen, M. L.: Are all lineaments the surface expression of faults and fractures? – A novel analysis using tunnel face mapping data from Norwegian road tunnels, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16840, https://doi.org/10.5194/egusphere-egu24-16840, 2024.

Understanding how the rapidly changing climatic forcing is affecting the complex mechanical interactions between rock masses is crucial for assessing the stability of rock slopes and correctly quantifying the associated hazard and risk. Volumetric changes induced by temperature fluctuations cause seasonal cycles of swelling and contraction, potentially resulting in crack formation and propagation (Tourchi, 2023; Jabbarzadeh et al., 2023). Further couplings complicate the picture, and reliable fully-coupled modelling approaches are lacking. This study presents a numerical simulation of quartz diorite rock cliff-atmosphere interactions. The developed numerical model employs a fully coupled thermo-hydro-mechanical (THM) approach, incorporating solid, water, and energy balance equations alongside thermal, hydraulic, and mechanical constitutive equations (Tourchi et al., 2021; Tourchi et al., 2023).

The geometry of the fractured rock cliff was accurately captured in a previous study, where researchers conducted precise field measurements at the 'Požáry test site', a newly established field laboratory for the investigation of fractured rock masses (Racek et al., 2023). This research utilized a cutting-edge monitoring system to tracks temperature fluctuations and displacement changes, offering critical insights into the thermo-hydro-mechanical (THM) processes occurring within the rock mass and their subsequent impact on the stability of the rock cliff. These gauges have also monitored potential plastic strain over microfractures during a two-month observation period. Initial and boundary conditions for analysis, as well as material parameters, are determined from available field and laboratory experimental results. The thermal, hydraulic, and mechanical observations for three different observation points where compare with computed results. Despite limitations in input parameters, the simulation results exhibit a promising agreement with field measurements. The performance and analysis of the in situ test can significantly enhanced the understanding of a complex THM problem and can proved the capability of the numerical formulation to provide adequate modelling capacity.

References

Jabbarzadeh, M., Sadeghi, H., Tourchi, S. and Darzi, A.G., 2023. Thermo-hydraulic analysis of desiccation cracked soil strata considering ground temperature and moisture dynamics under the influence of soil-atmosphere interactions.

Racek, O., Balek, J., Loche, M., Vích, D. and Blahůt, J., 2023. Rock surface strain in situ monitoring affected by temperature changes at the Požáry field lab (Czechia). Sensors, 23(4), 2237.

Tourchi, S., Mánica, M.A., Gens, A., Vaunat, J., Vu, M.N. and Armand, G., 2023. A thermomechanical model for argillaceous hard soils–weak rocks: application to THM simulation of deep excavations in claystone. Géotechnique, pp.1-14.

Tourchi, S., Vaunat, J., Gens, A., Bumbieler, F., Vu, M.N. and Armand, G., 2021. A full-scale in situ heating test in Callovo-Oxfordian claystone: observations, analysis and interpretation. Computers and Geotechnics, 133, p.104045.

How to cite: Tourchi, S., Jabbarzadeh, M., Sadeghi, H., and Alimardani Lavasan, A.: Thermo-hydro-mechanical modelling of rock cliff-atmosphere interaction: the case of the Pozary test site in Czechia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20407, https://doi.org/10.5194/egusphere-egu24-20407, 2024.

Earthquake-like ruptures disrupt the frictional interface between contacting bodies and initiate frictional motion (stick-slip). The interfacial slip (motion) immediately resulting from a rupture during each stick-slip event is usually much smaller than the total slip recorded during the duration of the event. Slip after the onset of friction is generally attributed to the continuous motion of global ‘dynamic friction’. Here, we demonstrate that numerous hitherto invisible secondary ruptures are initiated immediately after each initial rupture by directly measuring the contact area and slip at the frictional interface. Each secondary rupture generates incremental slip that, when not resolved, may appear as steady sliding of the interface. Each slip increment is linked, via fracture mechanics, to corresponding variations of contact area and local strain. Cumulative interfacial slip can only be described if the effects of these secondary ruptures are taken into account. These weaker slip sequences can also be observed in bimaterial interfaces and exhibit strong directional effects. In addition, the seismic moments we estimate based on slip sequences are consistent with the Gutenberg-Richter (G-R) law. These results have important implications for our fundamental understanding of frictional motion and the important role of aftershocks within natural faults in generating earthquake-mediated slip/afterslip.

How to cite: Shi, S., Wang, M., Poles, Y., and Fineberg, J.: Frictional slip sequences in homogeneous and bimaterial interfaces, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2384, https://doi.org/10.5194/egusphere-egu24-2384, 2024.

In recent years, an intriguing feature of back-propagating rupture (BPR) has been reported during some earthquakes (Ide et al., 2011; Houston et al., 2011; Hicks et al., 2020; Okuwaki et al., 2021; Vallée et al., 2023). The occurrence of BPR challenges the classical interpretation of rupture propagation as a “forward” problem, while remaining less understood by the earthquake science community. Here, using fracture mechanics, we first argue that BPR is nothing but an intrinsic component of rupture propagation; however, its observability is usually masked by the superposition effect of interfering waves behind the primary, forward-propagating rupture front. We then suggest that perturbation to an otherwise smooth rupture process can break the superposition effect and hence can make BPR observable. To test our idea, we report results of mode-II rupture propagation from both numerical simulations and laboratory observations. By introducing a variety of perturbations (lateral variation in bulk or interfacial properties along the fault, singular stress drop, coalescence of two rupture fronts, etc.) during rupture propagation, we show that prominent phases of BPR indeed can be successfully excited. We further classify BPR into two modes: higher-order rupture or interface wave, depending on whether the already-ruptured fault is quickly healed and whether additional stress drop can be produced. Lastly, we propose several application potentials for BPR, such as constraining the velocity structure of fault zones, probing the mechanical state of faults, and studying the stability of perturbed slip along a homogeneous or bimaterial interface. Our study refines the understanding of the nature and complexity of rupture process, and can help improve the assessment of earthquake hazards.

How to cite: Ding, X., Xu, S., Fukuyama, E., and Yamashita, F.: Back-Propagating Rupture: Nature, Excitation, and Applications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3912, https://doi.org/10.5194/egusphere-egu24-3912, 2024.

Modeling the seismic cycle requires multiple assumptions and parameters. Providing a quantitative assessment of the model behavior is pivotal for determining the degree of similarity between different scales and modeling strategies and for exploring dependencies with respect to selected parameters. Here we compare stick-slip ruptures nucleating spontaneously in scaled seismotectonic models (i.e., laboratory experiments capturing the first-order physics of the seismic cycle of subduction megathrusts) with slow earthquakes in nature. We rely on two non-dimensional parameters, namely the Ruina number (Ru) and system dimension (D) to quantify model behavior. Ru is proportional to the ratio of the asperity size to the critical nucleation size. Within the rate- and state friction framework, for velocity weakening asperities Ru controls the behavior of the system, which can be either periodic or not, and it can exhibit both slow and fast ruptures. D measures how complicated the system evolution is. D reveals how many variables are required to describe the seismic cycle because it tells us the minimum dimension needed to embed the observed dynamics. 

By coupling the Simulated Annealing algorithm and quasi-dynamic numerical simulations, we retrieve rate and state friction parameters characterizing single asperity models with different lateral extent of the velocity weakening patch. Similarly to slow earthquakes, we found optimal rate and state parameters indicative of low (< 4) Ru. We also found a direct proportionality between Ru and the lateral extent of the asperity. 

Next, we implement tools from non-linear time-series analysis and Extreme Value Theory to compute D from models of different sizes, materials, deformation rates and frictional configurations (single or twin asperities along strike). Our analysis supports the existence of a low dimensional attractor (D<5) describing the dynamics of scaled seismotectonic models. In particular, our models display D=3.0-4.2, which is remarkably similar to D=3.2 of slow earthquakes identified along the Cascadia subduction zone. Under the explored conditions, D appears more affected by the material behavior of the analog upper plate (i.e., gelatin vs. foam rubber) than by the lateral frictional segmentation of the megathrust.

Despite the different spatio-temporal scales, our results support a scenario where scaled seismotectonic models and slow earthquakes share similar dynamics.

How to cite: Corbi, F., Mastella, G., Tinti, E., Gualandi, A., Sandri, L., Rosenau, M., Pardo, S., and Funiciello, F.: The similarity between ruptures in scaled laboratory seismotectonic models and slow earthquakes , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5187, https://doi.org/10.5194/egusphere-egu24-5187, 2024.

Numerical modeling of Discrete Fracture Networks (DFNs) is commonly used to assess the behavior and properties of hydraulic diffusion and seismicity in the Earth’s crust within a network of fractures and faults, and to study the hydromechanical evolution of fractured reservoirs stimulated by hydraulic injection and production. The modelling of such fractures is typically carried out under a quasi-static approximation, and occasionally accounting for elasto-dynamics in single-rupture studies that assume a slip-weakening friction law. 

In this work, we develop a 2D DFN model to simulate fluid-induced seismicity that couples hydraulic diffusion and slip governed by rate-and-state friction on multiple interacting faults. The main goal of this numerical model is to establish a connection between laboratory derived friction parameters and field observations, enabling the inference of the long-term evolution of fractured reservoirs and crustal fault systems undergoing multiple earthquakes and (slow) slip events induced by fluid pressure perturbations.

In the model, the elastic interactions are computed with a boundary element method, accelerated by the hierarchical matrix method. We assessed the convergence of the method at fracture junctions and verified it does not create unphysical singularities. The use of rate-and-state friction makes it possible to model several seismic events over the injection duration.

The simulations will be later used to fit measurements of permeability and friction collected in laboratory experiments, in-situ observations of fault slip and opening from fluid injection experiments at decametric scale, and finally, induced seismicity at reservoir scale.

How to cite: Romanet, P., Scuderi, M., Chaillat, S., Ampuero, J.-P., and Cappa, F.: Towards a 2D model of Discrete Fracture Network with permeability and friction evolution for modeling fluid-induced seismicity , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5767, https://doi.org/10.5194/egusphere-egu24-5767, 2024.

Elevated pore fluid pressures are frequently implicated in governing fault zone seismicity. While substantial evidence from geodetic and geological studies supports this notion, there is a notable scarcity of experimental observations of how fluid pressure influences fault stability during shear. Understanding the precise interplay between porosity, fault slip rate, and frictional stability is pivotal for assessing the significance of processes like dilational strengthening or thermal pressurization in the context of seismic hazards. Here, we prepare fault gouges from the Utah FORGE enhanced geothermal field injection well 16A at depths corresponding to seismic events (between 2050 – 2070m). Experiments were conducted inside a pressure vessel and loaded under a true-triaxial stress state, replicating in-situ stress conditions observed at the Utah FORGE site. The applied fault normal stress and during the experiments were held constant at 44MPa. Pore fluid pressure was varied between successive experiments (13, 20, and 27 MPa) to span a range of effective stresses to examine impacts on fault dilation/compaction and the successive frictional stability. Different fluid pressure boundary conditions: constant volume or pressure were applied to explore how changes in shearing rate influence gouge stability thought the fault drainage state. Our data indicate that the Utah FORGE samples are velocity-neutral and transition to velocity-weakening behavior at elevated pore pressure and shear strains >7. We find dilatancy coefficients e = ∆f/∆ln(v), where f is porosity and v is fault slip velocity, consistent with quartz-feldspathic-rich rocks ranging from 5–12^10^-4, indicating a conditionally unstable regime. Furthermore, our results demonstrate that the boundary conditions for pore fluids influence frictional stability viachanges in effective normal stress. For example, when pore volume has zero flux, an expansion in the void volume during slip results in a decrease in pore pressure, transitioning the system towards frictional stability. Our results indicate that the connectivity of pore conduits may be more important than the imposed pore pressure conditions when considering the impact on fault stability. We suggest that the interplay between fault slip and fluid mobility within a fault is a delicate balance for predicting and managing seismic hazards.

How to cite: Affinito, R., Elsworth, D., and Marone, C.: Fault drainage state and frictional stability in response to shearing rate steps in natural gouge, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6284, https://doi.org/10.5194/egusphere-egu24-6284, 2024.

Seismic faults are often represented using two different and self-excluding conceptual models. In the first representation, seismic faults are seen as the interface between two surfaces of bare rock, with a roughness extending at all scales. These surfaces interact mechanically through a certain number of “asperities” which constitute the “real contact area”. When adopting this view, attention is paid on the statistics of the asperities population in the fault plane. Faults are thus considered as 2D objects, since their thickness is disregarded.

In the second representation, seismic faults are seen as mathematical planes separated by a certain thickness of granular gouge created by abrasive wear of the surfaces during previous slips. This view is analogous to the tribological “third body” theory, and is supported by field observations and experimental evidences of gouge creation in rotary shear and triaxial experiments. It is convenient to adopt this perspective when weakening phenomena within the gouge are to be spatially resolved in the direction orthogonal to the fault plane. Variations along this plane are then ignored, as well as fault roughness, and faults are mostly seen as 1D objects.

Unification of these two representations requires a better understanding of the interactions between geometrical asperities and a layer of gouge, and in particular of the phenomena that lead to the creation of the latter through the wear of the former. In this communication, we present a numerical model which aims at reproducing lab tests of millimetric single-asperity friction and wear. The model is essentially granular in order to represent the progressive degradation of the asperity along sliding, the separation of powdery matter, its successive ejection and reinjection by the contact (thanks to a periodicity in boundary conditions), and the build-up of a gouge layer. It also includes a coupling with continuum mechanics in order to maintain a meaningful stress field in the asperity beyond the region of degradable rock.

Numerical results show that: (i) the rate of wear of the asperity and the counterface are directly linked to the normal load applied to the contact; (ii) an established layer of gouge develops in the interface and controls the friction coefficient; (iii) a constant level of surface roughness is established after a sufficient sliding distance, both for the asperity and the counterface; (iv) an accurate control of the asperity boundary conditions is necessary in order to obtain repeatable friction and wear. These results are a first step towards a better understanding of the wear kinetics as a function of asperity geometry, load, and roughness, before the introduction of thermal aspects (including melting) in a future version of the model.

How to cite: Mollon, G., Clerc, A., Ferrieux, A., Lafarge, L., and Saulot, A.: A granular numerical model for the friction and wear of a lab-scale fault asperity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7508, https://doi.org/10.5194/egusphere-egu24-7508, 2024.

The role of fluid flow in triggering earthquakes in subduction zones is a critical yet complex aspect in seismology. Despite extensive study through geological, geophysical observations, and laboratory experiments, fully understanding and modelling these processes within a coupled solid-fluid interaction framework remain challenging. This study employs a coupled seismo-hydro-mechanical code (i2elvisp) to simulate fluid-driven earthquake sequences in a simplified subduction megathrust environment. We incorporate non-uniform grid resolution, enhancing the resolution of seismic events within the subduction channel. The code integrates solid rock deformation with fluid dynamics, solving mass and momentum conservation equations for both phases, alongside gravity and temperature-dependent viscosity effects. Brittle/plastic deformation is modelled through a rate-dependent strength formulation, with slip instabilities governed by compaction-induced pore fluid pressurisation. Our approach demonstrates the significant impact of fluid pressurisation on deformation localization, achieving slip rates up to metres per second in a fully compressible poro-visco-elasto-plastic medium. By refining the vertical model resolution in the subduction channel to less than or equal to 200 metres, we ensure convergence in terms of event recurrence interval and slip velocity. The models successfully replicate various slip modes observed in nature, ranging from regular earthquakes (including partial and full ruptures) to transient slow slip phenomena and aseismic creep. This research focuses on the parameters influencing the dominant slip mode, their distributions, and interactions along a modelled subduction interface. Our findings indicate that the dominant slip mode and the earthquake sequences are significantly influenced by porosity, permeability, and temperature-dependent viscosity. We explore two distinct viscosity gradients in the subduction channel to represent subduction zones with differing thermal profiles. In 'hot' subduction models, the brittle-ductile transition commences at shallower depths than in 'cold' subduction cases, influencing the nucleation depth of seismic events. These viscosity variations markedly impact model evolution; regular earthquakes exhibit higher velocities and slip rates in 'hot' scenarios, which are also more conducive to hosting aseismic creep or slow slip events. In conclusion, our study elucidates the pivotal role of fluid pressure evolution in seismicity within subduction zones and provides deeper insights into earthquake source processes. Through comprehensive modelling and analysis, we enhance understanding of the complex dynamics governing fluid-induced seismic activity and contribute to the broader field of earthquake source processes. 

How to cite: Hegyi, B., Gerya, T., Dal Zilio, L., and Behr, W.: Fluid driven seismic cycle modelling in subduction zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7803, https://doi.org/10.5194/egusphere-egu24-7803, 2024.

To investigate the frictional behavior of basalt under hydrothermal conditions, we apply sliding experiments using basalt gouge under the temperature of 100-600ºC, effective normal stress of 150MPa, and fluid pressure of 30MPa and 100MPa, respectively. Experiment results under 30MPa pore pressure show that basalt exhibits velocity-strengthening behavior at 100-200ºC and changes to velocity-weakening behavior at 400-600ºC; meanwhile, at 400ºC, velocity dependence of basalt evolves with slip from initial velocity weakening to velocity-strengthening. Results under 100MPa fluid pressure show a similar transition of velocity dependence at 300ºC; however, at higher temperatures of 400-600ºC, velocity strengthening behavior occurs, accompanied by strong slip weakening behavior at the slowest loading rate (0.04μm/s). During the velocity step, the experiment exhibits a rate-dependent creep without transient evolution with slip. Microstructure observation reveals significant differences between samples sheared under 30MPa and 100MPa fluid pressure. At higher fluid pressure and temperatures of 400-600ºC, the porosity of the basalt gouge layer is significantly reduced, and deformation is characterized by pervasive shear with no apparent localization. Such results suggest that the healing process/plastic deformation is activated at higher fluid pressure, leading to slip stability transition and slip-weakening of frictional strength.

How to cite: Zhang, L., He, C., and Barbot, S.: Transition from Unstable Slip to Rate-Dependent Creep Controlled by High Fluid Pressure, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8381, https://doi.org/10.5194/egusphere-egu24-8381, 2024.

        Abundant cherts (nodules and bands) were discontinuously hosted by dolostones of the Mesoproterozoic group Strata (∼1.5 Ga) in the Shanxi graben system, North China, where earthquakes are common. Measurements of the shear strength and stability of granular quartz reveal that quartz is a typical tectosilicate which exhibits high frictional strength and velocity-weakening properties. Conversely, dolomite is usually frictionally weak but velocity strengthening. The two minerals also behave differently during coseismic slip. Due to the high temperature generated by frictional heating, the thermal decomposition of dolomite usually results in calcite, periclase nanoparticles and carbon dioxide. However, quartz melts by friction at high temperatures. In order to investigate the role of quartz in dolomite fault rock during the process of coseismic slip, high velocity shear experiments were conducted on the quartz-bearing dolomite fault gouge taken from Yuguang Basin South Margin Fault (YBSMF), northeast of the Shanxi graben system. Also, we carried out high velocity experiments with the synthetic quartz-dolomite gouge with different mass ratio. For a slip velocity ≥ 0.1 m/s and normal stresses from 1.0 to 1.5 MPa, the friction values of the gouge decrease exponentially from a peak value of more than 0.5 to a steady-state value from 0.1 to 0.4. This high-velocity weakening feature was observed in the synthetic quartz-dolomite gouge as well as in the YBSMF gouge. With the increase of quartz content, the slip weakening distance (D_w) increases from 4.27 to 13.24 m, and the steady-state friction coefficient increases from 0.2 to 0.4 at 1.0 MPa normal stress and 1.0 m/s slip velocity. The textures of the gouge are characterized by grain comminution, R shear planes and localized deformation zone in the friction weakening stage. The slip surfaces are characterized by mirror-like smooth surface and nanoparticle aggregates. The theoretical calculation results show that the temperature inside the gauge layer did not exceed 300 °C during the experiments. However, the microstructures present that the dolomite experienced thermal decomposition, indicating that the temperature at the asperity exceeds 550 ℃. We suggest that thermal decomposition together with flash heating may lead to the slip-weakening behavior of quartz-bearing dolomite gauge, and the addition of quartz will increase of the strength of the dolomite gouge.

How to cite: Huang, J., Zhang, B., Zou, J., He, H., Dang, J., and Zhang, J.: High-velocity frictional behavior and microstructure evolution of quartz-bearing dolomite fault gouge, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8887, https://doi.org/10.5194/egusphere-egu24-8887, 2024.

Fault’s frictional strength and particularly its ability to heal during the interseismic period (fault frictional healing Δμ) is critical to understand the seismic cycle, yet the understanding of temperature and phase-dependent healing characteristics of natural geothermal conditions remains limited. Here we examined the frictional healing of both simulated fresh and chlorite-altered basaltic gouge from Krafla geothermal field (Iceland) under realistic geothermal conditions of water temperature T_f = 100-400 ˚C and pressure P_f = 10-30 MPa (water in liquid, vapor and supercritical state) by performing Slide-Hold-Slide (SHS) experiments. All experiments were performed under a constant effective normal stress of 10 MPa and initiated with a 5-mm run-in slip at a loading point slip rate V of 10 mm/s before the SHS sequence. For each SHS sequence, shearing was held from 3 s to 10,000 s, separated by a slip interval of 1mm. Our mechanical results indicate that frictional healing, the difference between peak friction reached upon re-shear and the steady-state friction before the hold, increases with increasing logarithm of hold time in all experiments, as suggested by previous studies. Meanwhile, frictional healing rate (β = Δμ/log(1+ t_hold/t_cutoff)), commonly regarded as the quantification of the rate of healing, increases with increasing temperature for both fresh and altered basalt. For fresh basalt, β increases from 0.007 at T_f = 100 ˚C to 0.060 at T_f = 300 ˚C (liquid) before dropping to 0.036 at T_f = 400 ˚C (vapor) and eventually increases to 0.096 at T_f = 400 ˚C (supercritical). For altered basalt, β  increases continuously from 0.003-0.007 at T_f = 100 ˚C to 0.013-0.022 at T_f = 300 ˚C and reaches its maximum value of β = 0.024-0.035 at T_f = 400 ˚C (vapor) and β = 0.030 at T_f = 400 ˚C (supercritical). Besides this temperature-dependent relationship, the dramatic decrease of β in fresh basalt to values similar to those of altered basalt when water changed from liquid to vapor state also suggests that the physical state of water can control the healing rate. Subsequent microanalytical analyses (XRPD, XRF, SEM-EDS) performed on the deformed gouges from altered basalts suggest an increase in hydrothermal alteration with increasing temperature, as shown by a depletion in K₂O at T_f ≥ 300 ˚C. SEM-BSE images of fine platy matrices in shear bands formed at T_f = 400 ˚C point towards a dissolution of quartz, pyroxene and plagioclase. Therefore, we suggest that the healing rate of both fresh and altered basalt not only scales with the ambient temperature but is also affected by the physical state of water, particularly in the case of fresh basalt, potentially related to more intense fluid-rock interactions with increasing temperature.

Keywords: frictional healing, frictional healing rate, hydrothermal fluids, basaltic gouge, Krafla geothermal field

How to cite: Wu, W.-H., Feng, W., Gomila, R., Tesei, T., Violay, M., Mortensen, A. K., and Di Toro, G.: Temperature and Physical State of Water Controls Frictional Healing of Basaltic Gouges from Krafla (Iceland), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9165, https://doi.org/10.5194/egusphere-egu24-9165, 2024.

Mineralogy, fabric, and frictional properties are fundamental aspects of natural and experimental faults that concur in controlling the fault strength and the fault slip behavior. Mineralogy controls the fabric evolution influencing the micro-mechanisms at play during fault deformation and needs an in-depth investigation to better understand and foresee the frictional response of experimental faults. Classically, this investigation has been conducted by relating the fault frictional behavior to the post-experimental microstructures. However, this “classical” approach provides a direct but static view of the fault deformation where the evolution of fabric with deformation can be only speculated.

To investigate in “real-time” the deformation micro-mechanisms at play during the experiments, the recording and analyses of Acoustic Emissions (AEs) produced by the deforming fault gouge can provide new insights.

In this study, we present a systematic study of microstructural, mineralogical, frictional, and AEs analysis coming from a suite of frictional experiments in a double direct shear configuration (biaxial apparatus, BRAVA2). We conducted experiments on gouges made of bi-disperse and layered mixtures of quartz and phyllosilicate. These experiments were performed at a constant normal stress of 52MPa and under 100% humidity. The friction evolves with the phyllosilicate content from µ ~ 0.6 for 100% quartz to µ ~ 0.4 for 100% phyllosilicates. At the end of the experiments samples were carefully collected and prepared for microstructural analysis. The fabric of the experimental samples show an evolution from localized to distributed and foliated fabric with increasing amount of phyllosilicate content.

We then integrate specific features of AEs, such as amplitude and AE rate, to unveil the micro-mechanisms at play during the experimental fault deformation. Our results show that the overall AE behavior is controlled by mineralogy. Deformation of quartz gouge produces the largest number of AEs whereas phyllosilicates are almost not producing AEs. Furthermore, the AE behavior of bi-disperse mixtures of quartz and phyllosilicates is strongly controlled by the amount of phyllosilicates. In fact, increasing the amount of phyllosilicate, the number, the rate, and the amplitude of AEs decrease. This behavior could be explained by the lubricant role of phyllosilicates which hinder the interaction between quartz grains favoring foliation sliding as main deformation mechanism and thus reducing the frictional strength. These results suggest that for bi-disperse mixtures the AEs reflect the frictional behavior of the mixture. Layered quartz-phyllosilicates mixtures show instead a non-trivial acoustic emission behavior which cannot be directly related to the measured frictional strength of the layered mixture: friction is controlled by the frictionally weaker mineral phase, whereas the AEs are probably dependent by the interplay between the stronger and weaker phase of the layered mixture.

Our results show that fault fabric together with mineralogy strongly control the micro-mechanisms at play during deformation and therefore the frictional response. Our findings support the use of the AE analysis as a new tool for the investigation of the micro-mechanisms at play during deformation, improving our interpretation of the mechanical behavior of fault gouges.

How to cite: Scuderi, M., casas, N., Volpe, G., and Collettini, C.: Unraveling the micro-mechanics of shear deformation through acoustic attributes of quartz-muscovite mixtures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9541, https://doi.org/10.5194/egusphere-egu24-9541, 2024.

In the last decades, rock mechanics laboratory experiments have allowed framing earthquake physics as a frictional problem. When the accumulated stress on a fault exceeds the frictional forces holding it in place, a rapid acceleration occurs. This movement can be stable or unstable, involving phases of adhesion (stick) and rapid sliding (slip). In these terms, an earthquake results from the release of mechanical energy during one of these slip phases.

Modern friction theories propose that the frictional forces holding the fault in place are controlled by small asperities defining the real contact area (RCA). Therefore, understanding the mechanics of contacts on the fault and their evolution under stress and velocity changes can shed light on the microphysical processes underlying earthquakes.

In the laboratory, it is now possible to investigate the dynamics of experimental faults, predicting their instability behavior based on Rate and State Friction theory and its experimentally obtainable parameters (a-b, Dc). However, these parameters lack an explicit relationship with contact mechanics, necessitating additional measurements complementing the system's state information. One of the most widely used techniques for studying RCA during laboratory experiments involves investigating changes in acoustic transmissivity (velocity, amplitude) of generated and recorded ultrasonic waveforms (UW) passing through the sample during the deformation. At a given wavelength, analytical expressions for these quantities depend on the elastic properties and densities of the fault portion crossed by the wave. Simultaneous knowledge of stress conditions and elastic properties allows the formulation of constitutive laws for the evolution of contacts between fault asperities.

In double direct shear experiments (DDS) within biaxial apparatuses, the sample dimensions (gouge) impose stringent limits on the spatial and temporal resolution of the signal. These limits highlight the current sensor technology's deviation from the ideal behavior.

Here, we present a methodology and a waveform recording and synchronization protocol

implemented on the biaxial apparatus BRAVA2 in the Rock Mechanics and Earthquake Physics laboratory at Sapienza University of Rome. We focus on the types of sensors used and their specifications to provide accurate measurements of the deformation processes occurring within the gouge layers.

Several studies have conducted DDS experiments using UW, but they rarely take into account the characterization of the impulse signal, various reflections in the sample assembly, and conversion modes of the generated waveforms. These are all essential components to identify the interaction of the experimental system with the propagation of the ultrasonic waves, to exploit the received signal in its entirety.

We believe that a careful signal characterization is necessary to fully understand the physical processes during deformation within the sample and, consequently, to attempt upscaling to natural earthquakes.

How to cite: De Solda, M., Mauro, M., Pignalberi, F., and Scuderi, M.: Strengthen and limitations of ultrasonic wave testing: examples from Double Direct Shear experiments on gouge, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9941, https://doi.org/10.5194/egusphere-egu24-9941, 2024.

The interactions between fluids and fault structures play a pivotal role in understanding fault slip behavior. Over the years, various numerical methods have been developed to simulate these interactions. Volume-based methods, like the finite difference method (FDM), excel in their capacity to handle the intricacies of real-world fault structures, including material heterogeneity. On the other hand, the spectral boundary integral method (SBIM) is renowned for its computational efficiency. Recently, a hybrid approach has garnered significant attention, offering the benefits of both volume-based and SBI methods. This hybrid method allows for the consideration of fault structures' heterogeneity while maintaining computational efficiency. In this study, we introduce a novel hybrid method that bridges the SBIM and the FDM to model fluid migration in fault structures. Through rigorous model verification, we establish that our hybrid method can achieve a remarkable speedup of up to one thousand times compared to the FDM. Furthermore, we conducted two parametric studies to address open questions in fluid migration modeling within fault structures. First, we investigate the mobility contrast ratio between the host rock and the damage zone to determine the limits under which we can assume a zero-leak-off interface. Second, we explore the role of fault zone width in maintaining the validity of this zero-leak-off assumption. Building upon these foundational investigations, we demonstrate the possibility of extending the numerical framework to describe fault-fluid interactions considering poroelastic coupling.

How to cite: Wang, Y.-H. and Rafn Heimisson, E.: An efficient hybrid SBI-FD method for modeling fluid migration and fault-fluid interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10564, https://doi.org/10.5194/egusphere-egu24-10564, 2024.

The slip behavior of crustal faults is known to be controlled by the mineralogic composition of the fault gouge. The exact properties determining the frictional behavior of geologic materials, including diverse remains an important question. Here, we use a geochemical approach considering the role of water-rock interactions. As a mechanism, we suspect that the mineral surface charge allows attractive and repulsive forces (Van Der Waals type), and that those forces may influence the static mechanical behavior of clays (cohesion, static friction).  On the other hand, we suspect that the water bound to the mineral surfaces may play a role during shearing.  To address these ideas, we measured the cation exchange capacity (CEC) of 10 different rock and mineral types, including non-clays and a range of phyllosilicate minerals, using CEC as a proxy for the mineral surface charge and the ability to bind water to the mineral surfaces.  For these materials, we conducted laboratory shearing experiments measuring the pre-shear cohesion, peak friction coefficient, residual friction coefficient, post-shear cohesion, and velocity-dependent friction parameters under 10 MPa effective normal stress.  
Our results show that low CEC materials (< 3 mEq/100g) tend to exhibit high friction, low cohesion, and show velocity-weakening frictional behavior. The phyllosilicate minerals exhibit larger CEC values up to 78 mEq/100g and correspondingly lower friction coefficients, higher cohesion, and velocity-strengthening frictional behavior. Zeolite exhibits a relatively high CEC value typical of phyllosilicates, but its strength and frictional characteristics are that of a non-clay with low CEC. This suggests that grain shape and contact asperity size may be more important for non-phyllosilicates. For phyllosilicates, we suggest that the systematic patterns in strength and frictional behavior as a function of CEC could be explained by water bound to the mineral surfaces, creating bridges of hydrogen or van der Waals bonds when the particles are in contact. Such bonding explains the large cohesion values for high-CEC materials under zero effective stress, whereas surface-bound water trapped between the particles under load explains low friction.  Beyond the results of this study, CEC appears to be a controlling factor for other properties such as permeability and even the amount of bound DNA in sediments.

How to cite: Ikari, M. and Conin, M.: Cation Exchange Capacity Quantifies the Link Between Mineral Surface Chemistry and Frictional-Mechanical Behavior of Simulated Fault Gouges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10969, https://doi.org/10.5194/egusphere-egu24-10969, 2024.

During natural and induced seismic activities, pore fluid pressure within fault zones and their surrounding rock may respond differently to stress variations, introducing additional complexities to seismic hazard assessment. While theoretical investigations have recognized the influence of such poroelastic heterogeneity on fault instability, incorporating phenomena like slip-induced dilation or compaction, the chosen poroelastic properties in these studies lack robust constraints from experimental measurements. Addressing this gap, our study focuses on quantifying the heterogeneity of poroelastic properties in the presence of a fresh fault, aiming to elucidate the coupling between poroelasticy and fault dilatancy during fault slip.

In our experimental investigation, we examined the evolving dynamics of pore pressure both on- and off-fault in initially intact Westerly granite samples. Applying confining stress of 100 MPa and a pore pressure of 60 MPa at two sample ends to replicate crustal settings, we induced a sliding fault plane through loading to failure under a constant strain rate. In the faulted samples, we measured the pore pressure response under sudden step loading in the direction of the maximum compression σ₁. Each loading step of around 5 MPa was imposed incrementally increasing the differential stress from 5 MPa to approximately 80 MPa (frictional resistance) after achieving pore pressure equilibrium. Detailed measurements, including displacement, bulk deformation, differential stress, local pore pressure and acoustic emissions were recorded throughout these tests. A spring-slider model coupled with 1-D fluid diffusion was used to try to simulate experimental observations.

Our results indicate that both the shear zone and the bulk exhibit a diminishing Δp/Δσ₁ with increasing differential stress. Measurements within the fault zone consistently yield positive values, surpassing those off the fault, with the discrepancy more pronounced at lower stress levels. In regions farther away from the shear zone, the off-fault response Δp/Δσ₁ presents a smaller value compared to locations proximal to the fault zone and may even exhibit slight negativity. During fault slip, on-fault measurements exhibit an instantaneous increase upon step loading followed by a gradual decrease, as a result of the interplay between poroelasticity and fault dilatancy. These observations were effectively reproduced by the numerical model integrating the poroelastic measurements and rate-and-state fault friction with slip-dependent dilatancy. The implications of this investigation extend to an enriched understanding of the heterogeneity in poroelastic responses between fault zones and host rocks, serving as valuable benchmarks for informing future numerical simulations, particularly in the context of naturally formed fresh faults. 

How to cite: Liu, D. and Brantut, N.: Poroelastic heterogeneity in the presence of a fresh fault: experimental insights and numerical modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10998, https://doi.org/10.5194/egusphere-egu24-10998, 2024.

A growing amount of evidence indicate that aseismic transients driven by overpressure play an important role in the triggering of induced seismicity. Understanding the physical control on aseismic slip development is thus important for seismic hazard assessment. We conducted an investigation into the propagation dynamics of a fluid-driven slip front along a laboratory frictional interface composed of granite. The experiments were carried out under a confining pressure of 90 MPa, with an initial uniform fluid pressure of 10 MPa. Fault reactivation was initiated by injecting fluids through a borehole directly connected to the fault.

Our findings reveal that the peak fluid pressure at the borehole leading to reactivation exhibits an increase proportionate to the injection rate. Employing three fluid pressure sensors and eight strain gauges strategically positioned around the experimental faults, we performed an inversion analysis to image the spatial and temporal evolution of (i) hydraulic diffusivity and (ii) kinematic fault slip during each injection experiment. Our inversion methods integrated both deterministic and Bayesian procedures, facilitating the tracking of the fluid pressure front along the fault interface and the subsequent propagation of the slip front over time.

The migration pattern shares many similarities with natural slow slip events suspected to play a role in the development of natural and induced earthquake swarms or aftershock sequences.  We demonstrate that increasing the fluid injection rate induces a transition from a quasistatic propagation of the slip front correlated with the increase in fluid pressure to a dynamic scenario where the slip front outgrows the fluid pressure front, accelerating during its propagation. Furthermore, we establish that temporarily shutting off fluid pressure during injection induces the propagation of a pore-pressure back-front, which halts the propagation of the slip front, aligning with theoretical expectations.

How to cite: Passelegue, F., Dublanchet, P., Brantut, N., and Chauris, H.: Influence of Injection rate and slip-induced dilatancy on the propagation of fluid-driven slip front, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11200, https://doi.org/10.5194/egusphere-egu24-11200, 2024.

Faults in nature exhibit complex surface characteristics with patches of the fault (asperities) that may slip dynamically while other sections are more prone to creep (Beeler et al., 2011). Asperities forming in nature may be due to the geometric interactions between surfaces within a fault that contribute to complex stress states that are not well understood. Fault roughness is believed to play an important role in the control of the contact conditions established by asperities, directly affecting its potential to slip unstably. How the asperities are formed and how their seismogenic properties evolve due to wear is an important question with implications to slip budget and earthquake potential.

In this study, we performed a triaxial experiment at sequentially increasing confining pressures (P_c = 60, 80, 100 MPa) on a saw-cut sample of Carrara marble. We analysed the quasi-static frictional response that benefited from novel arrays of distributed strain sensors (DSS) obtained using fiber optics. This sensor offered unique insight into the axial strain with a spatial resolution of 2 mm. The frictional behaviour during the first confining pressure step exhibited a dynamic instability in the form of a stick-slip event (SS) that produced a measurable stress drop. In the subsequent confining pressure stages, where an increase in confining pressure translated to increased normal stress, the fault behaved in a stable manner and no dynamic instabilities were produced. This observation is inconsistent with frictional stability theory (e.g. Rubin and Ampuero, 2005) and required pre- and post-mortem campaigns into the surface characteristics and their evolution to explain this abnormal behaviour. Therefore, we employed experimental techniques (pressure sensitive film (PSF), optical and stylus profilometry) along with finite element (FE) model in ABAQUS to characterize the pressure and roughness.

The DSS array showed extensional axial strain closer to the edges of the fault, while only compression was expected in this triaxial loading test. The pre-experimental profilometry revealed an asperity located at the centre of the fault with a curvature ratio of h/L=0.1% inherited from the hand-lapping preparation, which dominated the initial contact conditions prior to the SS and explained the DSS observations. The DSS results were confirmed using a FE model which justified the effect of the fault geometry (h/L) on the strain response. After the SS, wear and smoothening of the central asperity was seen in roughness measurements. The profilometric measurements showed that gouge was deposed adjacent to the high normal stress asperity center (PSF) and were characterized by increased RMS roughness. These small amounts of gouge on the fault surface were sufficient to suppress the seismic response of the asperity. These findings show that the seismic potential of a carbonate (softer) asperity, may be highly influenced by the debris produced during wear. Its impact on earthquake nucleation could provide insight into large-scale earthquake preparation processes on carbonate faults in nature.

References:

• Beeler, M., Lockner, D. L. and Hickman, S. H. (2001), Bull. Seis. Soc. Am., 91 (6): 1797–1804
• Ampuero, J.-P. and Rubin, A. M. (2008), J. Geophys. Res., 113, B01302

How to cite: Michail, S., Selvadurai, P. A., Rast, M., Salazar Vásquez, A. F., Bianchi, P., Madonna, C., and Wiemer, S.: Laboratory Insight into the Evolution of the Seismic Potential of an Asperity due to Wear, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11407, https://doi.org/10.5194/egusphere-egu24-11407, 2024.

Our understanding of earthquake rupture processes is generally limited by the resolution of available observations. In all but exceptional cases, earthquake observations are made at comparatively large distances from the rupture itself, which puts a limit on what spatial scales can be resolved. At the same time, it is clear that small scale processes may play a crucial, if not dominant, role for various seismogenic processes, including rupture nucleation, co-seismic weakening and stress re-distribution.

The Fault Activation and Earthquake Rupture ('FEAR') project aims at collecting and interpreting a multitude of earthquake-relevant observations from directly on and around the process zone of an induced earthquake. To this end, we attempt to activate a natural granitic fault zone in the BedrettoLab, at a depth of ~1km, after instrumenting the fault zone with a multi-domain and multi-scale monitoring system. The goal is to observe and study earthquake rupture phenomena in a natural setting, from unusually close distance.

In this talk, we outline the project status, the science goals, and the plans for the main experiments, which are scheduled for the years 2024 - 2026. Notable milestones we report on include

• the identification and detailed characterisation of the target fault zone
• the beginning of niche and tunnel excavations
• laboratory experiments that characterise the frictional and mechanical behaviour of both gauge material and host rock of the target fault zone
• development of numerical models for 2D and 3D dynamic rupture propagation
• development of tailored monitoring methods for seismicity, strain, temperature, pressure, bio-geo-chemistry and other relevant observables
• development of remote experiment control methods
• test stimulations in a nearby rock volume of similar geology, with an already existing monitoring system, where we tested the influence of pre-conditioning injection protocols
• similar test stimulations in the same volume where we aim at triggering a larger event (target M_w~0)
• active seismic experiments in an underground salt mine, to calibrate the very- to ultra-high frequency (1k Hz - 500k Hz) acoustic emission sensors

Together, these and other efforts constitute the necessary ingredients we need for interpreting the near-source observations that we will collect during the fault activation experiments.

How to cite: Meier, M.-A., Giardini, D., Wiemer, S., Cocco, M., Amann, F., Spagnuolo, E., Selvadurai, P., Tinti, E., Dal Zilio, L., Zappone, A., Pozzi, G., Jalali, M., and Gischig, V. and the FEAR science team: Fault activation from up close, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11536, https://doi.org/10.5194/egusphere-egu24-11536, 2024.

The short seismic record with respect to the return time of large subduction earthquakes and the spatial fragmentation of available geophysical data represent unfavourable conditions for robust hazard assessment. Over the last decade, data from scaled seismotectonic models have become useful in filling the observational gaps of seismic and geodetic networks. Such models allow reproducing hundreds of analogue seismic cycles in a few minutes of experimental time and with the advantage of known and controllable boundary conditions. 

Here we present experimental results from Foamquake – an established 3D seismotectonic model, which simulates megathrust subduction. Recent technical advances in experimental monitoring have allowed us to include into our research a high-frequency camera to record model surface deformation at 50 Hz and a network of 5 accelerometers (located on the model surface) that measure the three components of acceleration at 1 kHz. To analyse the camera data, we used particle image velocimetry (PIV) to derive surface displacements, such as in a dense, homogeneously distributed geodetic network spanning updip to scaled depths that are often offshore and, therefore, typically under-monitored in natural subduction zones.

We performed 33 experiments exploring 10 different geometrical configurations of asperities along the analog megathrust. In particular, we varied the number of asperities, their size, location, and extra normal load. We observed that the rupture pattern of analogue earthquakes predictably changes as the extra normal load varies and the distribution of asperity configurations becomes more complex. Depending on the number and size of the asperities and the size of the barrier between them, we noticed different ratios between full and partial ruptures with different recurrence time (Rt) intervals. In some experiments we detected cascades of ruptures. We used the coefficient of variation (CoV) of recurrence time to quantify analog earthquakes periodicity. Most of our models display quasi-periodic analog earthquakes recurrence with CoV<0.5, but multi-asperity experiments with variable-size and extra normal load lean toward random behaviour as testified by CoV~0.8.

Future investigations include the following steps – exploring this great volume of data using machine learning, looking for spatial and temporal relationships between accelerometer and PIV displacements, and tracking in detail the aseismic processes that may precede and follow earthquake rupture.

How to cite: Latypova, E., Corbi, F., Mastella, G., Bedford, J., and Funiciello, F.: Exploring earthquake recurrence and nucleation processes with Foamquake and a variety of asperity configurations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12986, https://doi.org/10.5194/egusphere-egu24-12986, 2024.

The 2D, plane strain, Finite Element Method-based linear elastic model that I present aims to assess the differential stress response to variations in the geometric configuration of a system of multiple collinear elliptic cracks intercepting a body of rock undergoing elastic deformation. The assumption underlying this simulation is that a collection of thin voids in a continuum medium can replicate the features observed in a system consisting of rough fault profiles in partial contact subjected to shear. The linear elastic model is designed to reproduce the stress and displacement fields around a rough fault, with a specific focus on stress concentration around its contact asperities. The model also allows to record the principal stress field on the domain for a wide range of scales and geometric properties of the system of collinear cracks embedded in the deforming rock. Analyzing the dependence of differential stress on parameters describing the geometry of rough fractures allows for considerations on the primary factors influencing brittle failure. Additionally, the examination of principal stresses around the tips of the cracks helps evaluate the potential orientation of new fracture patterns that may emerge when the yield strength of the deforming material is locally exceeded. The magnitude and orientation of the principal stresses are also crucial for the understanding of fracture coalescence and frictional reactivation of shear cracks in an elastic rock, which in turn is one of the main factors that govern the seismic cycle of natural faults. Furthermore, a comparison of the results of the present model with recent wing crack models of brittle creep suggest that our code may also be useful to obtain estimates of the critical distance between cracks for their interaction to coalesce into larger fractures. The process is assumed to indefinitely continue at greater scales, which offers the chance to propose a model for fault formation and propagation.

How to cite: Manna, L., Toscani, G., Maino, M., Casini, L., and Dabrowski, M.: A model for the formation and propagation of faults from the coalescence of smaller-scale systems of cracks: Finite Element Method-based numerical approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13117, https://doi.org/10.5194/egusphere-egu24-13117, 2024.

The propagation of earthquake fault ruptures in the crust involve the generation of unloading stress pulses sufficiently large to induce dynamic failure of water-saturated rocks under tensional stresses and hydrofracturing. Similar processes are also activated during underground rock mass excavation activities in mines and tunnels. The current knowledge about rock fracturing via dynamic unloading is mainly limited to empirical records and numerical simulations, while there is a general paucity of experimental studies, due to difficulties in reproducing large instantaneous decompressions on rock samples using standard triaxial rigs. Until now rapid decompression and fracturing of large rock samples in dry conditions was reported only by using an unconventional gas-confined vessel.

Here, we report rock-fracture results for newly conceived rock decompression experiments, completed through the innovative use of a “cold-seal pressure vessel” (CSPV) apparatus which is routinely employed in experimental petrology. We applied instantaneous large decompressions on water-saturated rock samples equilibrated at high confinement (up to 200 MPa) and temperatures (up to 540°C). The tested rock samples were fine-grained Westerly granite, coarse-grained tonalite and micritic limestone. During the decompressions the rock samples hydrofractured due to the confinement dropping faster than the pore pressure within the rock. Porosity measurements, SEM imaging and X-ray µCT acquired before and after the tests suggest that the magnitude of dynamic fracturing not only positively correlates with the pressure drops but it mostly increases when the decompression is associated to a phase change of the pore water (e.g. supercritical fluid to subcritical gas) . Water vaporization or degassing imply an instantaneous volume expansion (up to 70 times) which critically enhances dynamic fracture propagation along rock grain boundaries. The induced fractures span from mm-long transgranular cracks to microcracks with submicrometric aperture. Therefore, synchrotron light high-resolution microtomography (final pixel resolution of 0.3 µm) was employed to fully resolve and quantify the 3D fracture networks of these deformed rock samples. Such unique dataset allowed us to determine at different scales the fracture intensity, aperture and connectivity of the dynamically induced fracture networks and to assess the key contribution of pore-water physical state changes on the initial stages of dynamic fracturing in rocks at crustal conditions. Such results will contribute to close a current knowledge gap in rock mechanics.

How to cite: Fondriest, M., Arzilli, F., Cordonnier, B., Carroll, M., and Doan, M.-L.: The effect of pressure drop and fluid expansion during rock fracturing by dynamic unloading, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13433, https://doi.org/10.5194/egusphere-egu24-13433, 2024.

Establishing a constitutive law for fault friction is a crucial objective of earthquake science. However, the complex frictional behavior of natural and synthetic gouges in laboratory experiments eludes explanations. Here, we present a constitutive framework that elucidates the slip-rate, state, temperature, and normal stress dependence of fault friction under the relevant sliding velocities and temperatures of the brittle lithosphere during seismic cycles. The competition between healing mechanisms explains the low-temperature stability transition from steady-state velocity-strengthening to velocity-weakening as a function of slip-rate and temperature. In addition, capturing the transition from cataclastic flow to semi-brittle creep accounts for the stabilization of fault slip at elevated temperatures. The brittle behavior is controlled by the real area of contact, which is a nonlinear function of normal stress, leading to an instantaneous decrease of the effective friction coefficient upon positive normal stress steps. The rate of healing also depends on normal stress, associated with an evolutionary response. If these two effects do not compensate exactly, steady-state friction follows a nonlinear dependence on normal stress. We calibrate the model using extensive laboratory data covering various relevant tectonic settings. The constitutive model consistently explains the evolving frictional response of fault gouge from room temperature to 600º for sliding velocities ranging from nanometers to millimeters per second, and normal stress from atmospheric pressure to gigapascals. The frictional response of faults can be uniquely determined by the in situ lithology and the prevailing hydrothermal conditions.

How to cite: Barbot, S.: Constitutive behavior of rocks during the seismic cycle in non-isothermal, non-isobaric conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14003, https://doi.org/10.5194/egusphere-egu24-14003, 2024.

Understanding earthquake rupture propagation across fault stepovers is pivotal for assessing the seismic hazard, offering vital insights into dynamic rupture processes within intricate fault geometries. However, the role of poroelastic effects within strike-slip fault systems featuring stepovers remains unexplored in dynamic models simulating Sequences of Earthquakes and Aseismic Slip (SEAS). Many existing models neglect poroelastic effects, and among those that consider them, a typical standard value of 0.8 is adopted for Skempton's coefficient B. Furthermore, a single dynamic rupture simulation is unable to address the frequency at which ruptures propagate through the stepover. Instead, these simulations only provide a binary status, indicating whether the ruptures jump or arrest. Thus, the investigation into how poroelasticity influences the likelihood of an earthquake jumping through a stepover emerges as a significant area of study. In response, we introduce a quasi-dynamic boundary element model that simulates 2D plane-strain earthquake sequences. This model incorporates undrained pore pressure responses affecting the fault's clamping and unclamping mechanisms and is governed by rate-and-state friction, with state evolution defined by the aging law. We first illustrate that dynamic rupture occurring in either left-lateral or right-lateral fault stepovers leads to a dynamic decrease (unclamping) or increase (clamping) in the effective normal stress. Dynamic variations of the effective normal stress depend on Skempton's coefficient. Consequently, higher Skempton's coefficients can promote rupture jumping across fault segments even for larger stepover distances. We then conduct a thorough parameter space study, evaluating the effects of Skempton's coefficient variations and stepover width on fault interactions within a fluid-filled porous environment. The likelihood of rupture jumping involves a trade-off between Skempton's coefficient and stepover width. We validate the numerical model by comparing it to an analytical solution that involves a plane strain shear dislocation on a leaky plane within a linear poroelastic, fluid-saturated solid. This validation demonstrates that a simple analytical solution, primarily dependent on fault dislocation and Skempton's coefficient, has the potential to effectively predict the pore pressure change. The critical jumping width for 50% chance of rupture jumping predicted by our model explains the threshold dimension of the fault step, above which ruptures do not propagate. This study highlights the significance of incorporating poroelastic effects on- and off-fault in understanding the dynamic variations of the effective normal stress, which could significantly alter the overall length of fault rupture.

How to cite: Huang, L., Dal Zilio, L., and Rafn Heimisson, E.: The role of poroelasticity in rupture dynamics across fault stepovers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14504, https://doi.org/10.5194/egusphere-egu24-14504, 2024.

The frictional properties of faults are primarily controlled by their mineral composition, as well as ambient and deformation conditions, such as temperature, pore fluid, normal stress, and slip displacement. While many studies have been conducted to decipher how temperature and pore fluid may affect the frictional behavior of faults, less attention has been paid to the slip displacement effects, especially under hydrothermal conditions. By employing a rotary shear apparatus equipped with an externally-heated hydrothermal pressure vessel, we conducted large-displacement (up to 521 mm) friction experiments on chlorite under temperature (T) of 25 to 400℃ and pore water pressure (P_p) of 30MPa. The imposed effective normal stresses were 200 MPa and the slip rates ranged from 0.4 to 10 μm/s. The experiments unveiled significant slip strengthening in chlorite within the temperature range of 25 to 400 °C. Moreover, with increasing temperatures, there was an overall increasing trend in both the rate of slip strengthening and the ultimate frictional strength. For example, under T = 25 °C, the friction coefficients at displacements of 5, 90, and 521 mm were 0.33, 0.49, and 0.59, respectively, in contrast to 0.46, 0.79, and 0.88, respectively, at the same three displacements under T =400 °C. Under all the temperature and displacement conditions, chlorite exhibited velocity strengthening behavior without discernible temperature dependence, although the velocity-dependence parameter (a-b) increased with slip displacement. Microstructural analysis revealed that, the entire layer of the chlorite gouge experienced pervasive and intense shear deformation after slip of 521 mm, with extremely remarkable grain-size reduction. The thermogravimetrical and FTIR data of the deformed chlorite samples, together with the microstructural data, suggest that the dehydroxylation and the distortion of crystal structure of chlorite might occur during the friction experiments conducted at T ≥ 200 °C. Such changes may explain the more pronounced slip strengthening of chlorite with increasing temperatures towards 400 °C. This explanation can be further demonstrated by a comparative experiment conducted under varying temperatures (400°C for the first 100 mm of slip, followed by 25°C for the rest of 100 mm slip), wherein the friction coefficient at T = 25°C during the latter stage of slip remains as high as that at T = 400°C. These findings highlight the importance of slip displacement in controlling the frictional strength and its variations of chlorite-bearing faults at depths, and have profound implications for understanding the fault slip behaviors and earthquake mechanisms in subduction zones.

How to cite: Qin, W., Yao, L., Shao, T., Feng, W., Chen, J., and Ma, S.: Frictional behavior of chlorite in large-displacement experiments under hydrothermal conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14566, https://doi.org/10.5194/egusphere-egu24-14566, 2024.

Many aspects of earthquake physics are still not completely understood given its intrinsically complex nature. Among the others, the nucleation process; when and where an earthquake will occur, as well as its magnitude. Seismology is a commonly used method for studying earthquakes, but it faces challenges in accessing precise information about the physical processes taking place on the fault plane.

Here, we show how laboratory seismology can directly shed light on fault plane dynamics. Our approach involves reproducing in the laboratory on a large biaxial apparatus with a fault length of 2.5 m generated by two analog (PMMA) samples brought into contact. The experimental setup allows to impose both a heterogeneous loading distribution through the use of independent pistons loading the fault in the normal direction and specific boundary conditions (i.e. by modifying stopper and puncher dimensions). The stress state is measured through strain gauges at high frequency (40 KHz) along 15 locations along the fault. The experiments provide insights into two crucial aspects of laboratory earthquakes: (i) the nucleation location of ruptures and (ii) the complexity of the seismic cycle.

Our findings reveal that the initial on-fault stress distribution plays a significant role in both aspects. We observe that ruptures consistently nucleate in locations where the stress ratio τ/σ_n is the highest. Notably, such values change among experiments, challenging the widespread notion that a friction coefficient solely governs the onset of instability. Furthermore, we demonstrate how the heterogeneity of the initial prestress distribution along the fault controls the complexity of the seismic cycle. In certain cases, the seismic cycle manifests as system-size events with complete ruptures occurring regularly in time, devoid of precursors. Conversely, other initial stress distributions generate more complex cycles, characterized by multiple precursors before a main rupture, predominantly occurring in zones of elevated τ/σ_n (referred to as 'friction asperity'). The complexity of the seismic cycle can be described in terms of the number of precursory events, inter-event time, and the size of finite ruptures.

This study, carried out in a long laboratory fault, highlights the complexities that emerge when heterogeneous, hence more realistic, stress conditions are applied, providing valuable insights into the physics of natural earthquakes.

How to cite: Paglialunga, F., Passelegue, F., and Violay, M.: Initial stress distribution dictates nucleation location and complexity of the seismic cycle of long laboratory faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15205, https://doi.org/10.5194/egusphere-egu24-15205, 2024.

Earthquake nucleation is traditionally described using cascading or slow pre-slip models. In the latter, nucleation occurs as the sudden transition from quasi-static slip growth to dynamic rupture propagation. This typically occurs when a region of the fault of critical size L_c, often called nucleation length, is sliding. This transition is relatively well-understood in the context of homogeneous faults. Yet, faults exhibit multiple scales of heterogeneities that may emerge from local changes in lithologies or from its self-affine roughness. How these multiscale heterogeneities impact the overall fault stability is still an open question.

Combining the nucleation theory of [Uenishi and Rice, JGR, 2003] and concepts borrowed from statistical physics, we propose a theoretical framework to predict the influence of brittle/ductile asperities on the nucleation length L_c for simple linear slip-dependent friction laws. Model predictions are benchmarked on two-dimensional dynamic simulations of rupture nucleation along planar heterogeneous faults. Our results show that the interplay between frictional properties and the asperity size gives birth to three (in)stability regimes: (i) a local regime, where fault stability is controlled by the local frictional properties, (ii) an extremal regime, where it is governed by the most brittle asperities, and (iii) a homogenized regime, in which the fault behaves at the macroscale as if it was homogeneous and the influence of small-scale asperities can be averaged.  

Using this model, we explore the overall stability of rough faults, featuring multiscale distributions of frictional properties. We also investigate the stability of velocity-neutral faults that features brittle asperities. Overall, our model provides a theoretical basis to discriminate which heterogeneity scales should be explicitly described in a comprehensive modelling of earthquake nucleation, and which scales can be averaged.

How to cite: Lebihain, M., Roch, T., Violay, M., and Molinari, J.-F.: Impact of multiscale heterogeneities on the nucleation of earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15273, https://doi.org/10.5194/egusphere-egu24-15273, 2024.

During interseismic periods a fault at depth can experience non-constant effective normal stress due to fluctuations in the pore-fluid pressure. Pore-pressure oscillations may influence the healing capability of the fault and ultimately affect its reactivation. Thus, studying the behaviour of faults during interseismic periods is a critical factor in understanding the seismicity. Triaxial tests were conducted using saw-cut (45^o) samples of Pennant Sandstone to investigate the influence of pore-pressure oscillations during slide-hold-slide (SHS) tests (t_h = 900 – 7300s) on its frictional behaviour and fault reactivation. The cylindrical samples were hydrostatically compacted at 30 MPa and pore-pressurized with argon gas at 5, 10 and 18 MPa resulting in effective normal stress (σ’_n) 25, 20 and 12 MPa, respectively. Then the saples were deformed at a constant shear displacement rate ≈ 4.5 μm/s. To overcome the displacement hardening tendency of the sample geometry, we servo-controlled the confining pressure so that the resolved normal stress on the sliding surface is kept constant. Experimental observations revealed a significant influence of pore-pressure oscillation on the frictional behaviour resulting in an increase in both frictional healing and creep relaxation. Moreover, this effect was enhanced as the effective normal stress was increased further. To understand better the underling mechanism(s) that influences these time-dependent processes we coupled the frictional results with permeability measured using the oscillating pore pressure method during the SHS tests. Finally, we tested how the pore-pressure oscillation affected the fault reactivation by conducting creep experiments at constant shear stress while the fault was brought to reactivation via progressive increase in fluid pressure. Our results demonstrated how non-constant effective normal stress history during interseismic periods deeply affects the fault behaviour, with important implications for natural and human-induced seismicity.

How to cite: Bigaroni, N., Mecklenburgh, J., and Rutter, E.: Frictional Response of Clay-rich Sandstone to Pore-Pressure Oscillation Throughout Interseismic Periods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18100, https://doi.org/10.5194/egusphere-egu24-18100, 2024.

Observations suggest that large earthquakes often propagate as self-healing slip pulses but the mechanical reason of this ubiquity remains debated. Pulse-like ruptures differ from the classical crack-like dynamics by the fact that the slipping portion of the fault is limited to the immediate vicinity of the propagating tip. In this work, we first propose a minimal model describing the dynamics of large earthquakes. In its simplest form, the model contains only two free parameters: a dimensionless stress parameter characterizing the initial state of stress along the fault and a ratio of elastic moduli. The model illuminates how self-healing slip pulses can be produced by the paucity of elastic strain energy that arises once the rupture dynamics interplays with the finite geometry of fault zones—even in the absence of additional mechanisms such as rate-dependent friction.

Next, we discuss the example of faults surrounded by a damage zone whose reduction in elastic wave velocity restricts the flow of strain energy to the rupture tip and promotes pulse-like rupture. Using the proposed model, we demonstrate how the contrast in wave velocities and the initial stress level in the fault zone mediate the propagation mode of the earthquake.

How to cite: Barras, F., Aharonov, E., and Renard, F.: The pulse-like dynamics of large earthquakes illuminated by a minimal elastodynamic model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19397, https://doi.org/10.5194/egusphere-egu24-19397, 2024.

Carbon capture and storage technology is a necessary means to achieve the temperature control goal of 1.5 degrees Celsius under the background of peak carbon dioxide emissions and carbon neutrality. The storage of carbon dioxide in oil and gas reservoirs has the advantages of high safety, large storage capacity, and less additional cost. The reservoir-caprock configuration can provide favorable space for the storage of carbon dioxide geological bodies. To make clear the distribution range of geological bodies suitable for carbon dioxide sequestration, taking the middle-south section of the eastern sag of Liaohe as an example, based on the model of the ratio of mud to ground and caprock effectiveness division, the control factors of caprock sealing were analyzed by entropy weight method combined with TOPSIS method, and the effective thickness of reservoir was determined by clarifying the relationship between reservoir lithology, physical properties, oil content and electricity. The results show that the lower limit of the effective caprock mud-to-ground ratio in the sand-mud interbedding sequence is 70.6%, and the sealing ability of caprock is mainly affected by the thickness of the fault and the thickness of the caprock single layer; The two sets of caprocks in the Shahejie Formation and Dongying Formation are relatively stable, with good fault-caprock configuration sealing, and the fault juxtaposition thickness in the Shahejie Formation is characterized by "thick in the north and thin in the south"; The effective reservoirs of the Dongying Formation are distributed in the whole region, the effective reservoirs of Es1 are distributed in the north of Rongxingtun, and the distribution range is smaller than that of the Dongying Formation, while the effective reservoirs of Es3 are mainly distributed in Huangyure area at the northern end of the study area, and the distribution range is further reduced. According to the reservoir-caprock configuration, carbon dioxide storage types can be divided into three types: shallow storage type, deep storage type, and multi-layer storage type. The lower caprock is well sealed and the lower effective thick reservoir controls the deep enrichment of carbon dioxide; The lower caprock is poorly sealed, and the effective thick reservoir in the middle or upper part controls the multi-layer enrichment of carbon dioxide; The lower caprock is poorly sealed, the upper caprock is well sealed, and the upper effective thick reservoir controls the shallow enrichment of carbon dioxide. The relationship between the effective thickness of the reservoir and the sealing ability of the caprock determines the vertical distribution series of carbon dioxide.

How to cite: li, H. and jiang, Y.: Study on Reservoir-caprock Configuration for Carbon Dioxide Sequestration in oil and gas reservoirs , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-86, https://doi.org/10.5194/egusphere-egu24-86, 2024.

Global warming poses a major challenge that humanity will face during the 21^st century, requiring a significant reduction in anthropogenic CO₂ emissions to mitigate the escalating global temperature. Several governments worldwide, including Brazil, have committed to achieving net-zero CO₂ emissions by 2050, which will be impossible without Carbon Capture, Utilisation and Storage (CCUS) deployment. Ranked 12^th globally in CO₂ emissions (and 1^st in South America), Brazil is in the early stages of studying CCUS. At present, CCUS efforts in the country primarily revolve around enhanced oil recovery, with limited exploration of CO₂ injection in alternative geological settings. A total of 31 sedimentary basins span Brazilian territory, encompassing an area of approximately 6.4 million km², 75% of which is situated onshore. The potential for CO₂ storage in saline aquifers is gaining attention globally, proving a successful and effective approach in various sites. In this work we combine the available surface (geological maps, roads, and gas pipelines) and subsurface data (seismic lines and borehole data) to assess the logistics and feasibility of utilizing saline formations in onshore intracratonic basins as CO₂ sinks, aiming to enable Brazil to reach net-zero CO₂ emissions by 2050. Previous studies indicate that the Parnaíba, São Francisco, Amazonas, and Paraná basins present saline formations with favorable characteristics for CO₂ injection, such as adequate depths, porosity, and permeability. Building upon prior research, we introduce the onshore portion of Espírito Santo Basin to the list of potential sinks, where the target saline aquifer is the pre-salt Mucuri Formation. Results show that greenhouse gases emissions from industrial processes are notably higher in the southeast region of Brazil. Within this region, two formations exhibit considerable potential for carbon sequestration in saline aquifers: (i) the Mucuri Formation, located in the onshore Espírito Santo Basin, reaching 350 m of thickness and shallowest depths of about 950 m, and (ii) the Rio Bonito Formation, in the proximities of the São Paulo state, with over 100 m of thickness and shallowest depths of about 650 m. For large-scale projects, CO₂ transport in the region can be accomplished using the available infrastructure and the available gas pipelines, while smaller-scale research projects can utilize trucks, rail, and ships. Brazil's untapped potential for CCUS presents a unique funding opportunity from the private sector, marking a crucial step toward sustainable and impactful climate action.

How to cite: B. Amarante, F., Kuchle, J., and B. Haag, M.: Towards net-zero: assessing the carbon storage potential of onshore saline aquifers in Brazil, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-362, https://doi.org/10.5194/egusphere-egu24-362, 2024.

Despite constituting two-thirds of the sedimentary rock volume, shales are a few of the least understood rocks. The varied depositional processes and environments give rise to complexity and anisotropy in them. Understanding unconventional resources like shales becomes crucial given their abundance in the petroleum systems and reservoirs and their potential suitability for sub-surface carbon and radioactive waste storage. Therefore, paramount significance lies in understanding the petrophysical and rock physical characteristics of shales to develop feasibility models for the sustainable use of these rock types.
 
This investigation focuses on the Barren Measures and Raniganj Formation shales in the Damodar Valley of Eastern India, which are primarily rich in clays, carbon, and iron and are of fluvio-lacustrine origin. These relatively shallow formations can be good sites for storage and sequestration as they are overlain by shaley and clayey formations acting as traps. The anisotropy in shales is even more challenging as its imponderables range from a micro to a macro scale. This changes even further with factors like organic-hosted porosity and maturity. The inherent anisotropy in shales necessitates a multiscale examination. These multiscale discontinuities, coupled with parameters like organic matter and maturity, impact the elastic properties of the rocks, as evidenced by the ultrasonic evaluations.
 
In this study, acoustic characterization of samples was conducted using a benchtop ultrasonic wave propagation setup. The samples were clustered based on their colour and observed megascopic properties. Some sandstones were also included in the study to contrast sandstones with respect to shales. The wave velocities were determined for samples subjected to progressive heating up to 200°C (gas window), and the consecutive changes in the elastic parameters and resultant wave velocities of the rock were studied. Inputs from other methods utilizing different physics, such as FE-SEM, XRD were integrated to refine our interpretation. Notable changes were seen in wave velocities, especially in clusters with elevated organic content, while the density and Vp cross plots gave a good correlation with an R² value of around 0.7.
This study advances our understanding of the impact of temperature on the elastic properties of shales, an aspect less explored than factors like stress and pressure. Thoroughly characterizing these parameters through acoustic methods provides critical insights into shale's storage capacity, carbon sequestration potential, and additional hydrocarbon recovery, specifically with respect to the Damodar Valley shales, aiding India to offset the projected peak of 4 GT CO₂ emissions to achieve the carbon neutral goal promised at COP 26 and fulfilling UN Sustainable Development Goals.

How to cite: Jamwal, V. D. and Sharma, R.: Ultrasonic Evaluation of Shales vis-à-vis Temperature: A Case Study from Permian Damodar Valley Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-800, https://doi.org/10.5194/egusphere-egu24-800, 2024.

Abstract: Fractured carbonate reservoirs are of great importance in the oil industry due to their significant role in global oil reserves and complex nature, where the majority of these reservoirs are naturally fractured, making them complex and challenging for oil recovery. The detection and characterization of fractures are essential for understanding the reservoir's petrophysical properties and hydrocarbon recovery potential as they play a critical role in reservoir performance. In this paper we have used 3D seismic from the Razzak field in the Western Desert, Egypt, with a specific focus on the Alamein dolomite reservoir. The reservoir holds significance due to its prolific oil-bearing nature, and featuring widespread lateral distribution in the northern Western Desert. Additionally, its contribution to an active Mesozoic petroleum system emphasizes its importance. Using Petrel software, the Alamein top and visible faults were identified, leading to the creation of a structural map illustrating the WSW-ENE axes of the Alamein's structural culminations in the southern part of the horst block. Owing to an extensional force during the Jurassic period with a NE-SW orientation, resulting from rifting, was evident, marked by the formation of normal faults associated with the opening of the Neotethys in the NE-SW direction. In the interpretation of 3D seismic data for Alamein dolomite reservoir, only one major listric normal fault was identified. However, the presence of minor faults or fractures, not easily discernible with conventional seismic techniques, is plausible. To address this, volume attributes were applied to detect subtle changes in seismic properties: (i) the curvature operation calculated the dip and azimuth angles, aiding in identifying structural complexities like faults and fractures, (ii) the maximum curvature value highlighted areas of steeply dipping or folded structures, (iii) Edge detection emphasized sharp boundaries, yet no hidden fractures or minor faults were revealed. The variance attribute yielded limited information, but Ant tracking on the variance cube effectively identified hidden minor faults and fractures. Incorporating the Ant track attribute into FRACPAQ software provided an objective methodology for quantifying fracture patterns, revealing NW-SE-oriented fracture segments in contrast to the WSW-ENE orientation of the major fault. Consequently, seismic attributes will unveil concealed fractures, and the application of FRACPAQ will prove effective in furnishing data on fracture orientation and length statistics.

Key words: FracpaQ; seismic attributes; fractured carbonate; Razzak field

How to cite: Elattar, H.A., Collier, R., and Glover, P. W. J.: Using FracPaQ and seismic attributes to assess seismic scale fractures in carbonate reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1674, https://doi.org/10.5194/egusphere-egu24-1674, 2024.

How to cite: Elattar, H., Collier, R., and Glover, P. W. J.: Using FracpaQ and seismic attributes to assess seismic scale fractures in carbonate reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1674, https://doi.org/10.5194/egusphere-egu24-1674, 2024.

Revealing the thermal structure of subsurface is crucial for various projects including geothermal energy exploitation, CCS and hydrocarbon exploration. For instance, temperature is one of the key physical underground parameters governing the type of Geothermal systems, whether injected CO₂ remains in supercritical fluid stage and the depth of Golden Zone at where the hydrocarbon accumulations occur. Thus, understanding the temperature and geothermal gradient change in 1D-2D-3D sense indicates sweet spots and helps geoscientists to build more robust models to reduce the risks.

Based on this concept, this study aims to demonstrate the outcomes of a game-changer method which is the conversion of interval velocities into temperatures, thermal conductivities and heat flows by the help of recently proposed empirical relationships. As a case study, Northern Arabian Plate, SE Turkey is selected due to the neglection of thermal conditions in the area. Therefore, oil & gas industry-wide accepted methodologies have been applied to better understand thermal behaviour of the subsurface and how it has been controlled by regional tectonic edifices including large-scale thrust and strike slip faults.

In terms of methodology, as the first step, dynamic bottom hole temperatures of the wells have been converted into static ones by the help of “Temperature Analyser” web application. The converted temperature measurements have been used to generate regional temperature and geothermal gradient maps for every 500 meters. On the other hand, for 3D temperature models, seismic velocities have been converted into temperature cubes after calibration with the converted BHT measurements. Generated temperature cubes have been reflected on seismic sections to display lateral and vertical variations in temperature behaviour. It also allows the detection of meaningful temperature anomalies corresponding to possible fluid content.

The results reveal that abrupt temperature increase on maps directly coincides with the locations of oil producing fields. The same behaviour was noted globally both for hydrocarbon and geothermal fields. The change in temperature trend is also dominated by regional tectonics of the focus area. Large thrust fault systems act as boundaries for thermal anomaly regions while sinistral Mosul Fault Zone displaces and separates high temperature zones in a NW-SE sense. This movement can be easily associated with the Northern slip of the Arabian Plate since the continental collision occurred in the Miocene.

Based on these observations, the workflows and results of this study can be used for detailed investigation of subsurface geology, thermal conditions, and their effect on potential reservoirs for geothermal and CO₂ storage. Workflows used to generate thermal models might allow the development of more efficient sustainable energy projects not only for the Northern sector of the Arabian plate but also for the other regions of the World.

How to cite: Uyanik, A.: Conversion of Interval Velocities into Thermal Models: A Game Changer Method for Subsurface Energy Exploitation Projects , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1739, https://doi.org/10.5194/egusphere-egu24-1739, 2024.

This study is based on unique field data on a 3-year pilot test during which air containing 8 mol% O₂(g) was injected as a cushion gas into a natural gas reservoir, a carbonate-cemented sandstone aquifer located in the Paris Basin (France) [1]. The oxygen was fully depleted several months after injection completion, meanwhile CO₂(g) was detected around 2–6 mol%; the pH decreased from 8 to 6, while reducing conditions shifted to mildly oxidizing ones with increasing concentration of sulfates in equilibrium with gypsum. After the test completion, the long-term evolution of the aquifer was assessed by a 15-year survey. The pH gradually returned to its near initial state and sulfates were reduced by 2 to 3 times. Data on the release of trace metals (Ba, Cu, Pb, Zn) during and after the test were also available.

Multiphase reactive transport models were developed on these field data using the HYTEC reactive transport code in 2D-reservoir configurations [1]. At the short-term scale, modeling focused on the gas-water-rock reactive sequence during the air injection: 1/ depletion of the injected O₂(g) due to pyrite oxidation, 2/ leading to acidity production and dissolved sulfates, 3/ acidity buffering by calcite dissolution, 4/ followed by gypsum precipitation and CO₂(g) exsolution. At the long-term scale, the modeling tackled with the progressive return to the baseline chemistry of the deep aquifer that was 1/ mostly driven by transport processes and 2/ to a lesser extent, slow water/rock chemical interactions.

These field-based models developed at short and long-term could be used as a workflow for other gas storage facilities, e.g. biomethane, compressed air, and CO₂.

[1] Sin, I., De Windt, L., Banc, C., Goblet, P., Dequidt, D. (2023). Assessment of the oxygen reactivity in a gas storage facility by multiphase reactive transport modeling of field data for air injection into a sandstone reservoir in the Paris Basin, France. Science of The Total Environment 869, 161657.

How to cite: De Windt, L., Sin, I., Banc, C., Petit, A., and Dequidt, D.: Short and long-term multiphase reactive transport processes during a pilot test of air injection into a sandstone gas storage facility, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2177, https://doi.org/10.5194/egusphere-egu24-2177, 2024.

Kerogen is typically categorized in three types: type I is associated with lacustrine, type II associated with marine, and type III associated with terrestrial sources, respectively. The Kerogen type is a crucial factor affecting oil-generative properties as it significantly influences the initiation and potential for hydrocarbon generation in source rocks. Geoscientists traditionally use Rock-Eval pyrolysis to determine kerogen types, maturity, and pyrolysis reaction temperature (Tmax), and calculate hydrocarbon potential, essential factors in assessing oil reserves and understanding the oil window. Such method, however, has insufficient resolution and is time-consuming. In this study, we employ a temperature-dependent infrared (IR) spectroscopy method to precisely determine kerogen type, maturity, and Tmax. Specifically, our IR spectroscopy is combined with a numerical analysis model developed for the analysis of various organic matter samples. Through measurements of the IR spectra of samples at different temperatures (Heating-FTIR), we determine the maximum sedimentary burial temperature and the pyrolysis Tmax of kerogen. By applying the conversion formula by Shibaoka & Bennett (1977), (R₀)^a=R^a+bt_I*exp(cT_m), we derive a virtual vitrinite reflectance, which is strongly correlated with our IR spectroscopy results, with insights into the maturation. This Heating-FTIR technique is a valuable tool for petroleum geology, facilitating the assessment of oil potential and maturity. Future refinement of the numerical model and improvement of the instrumentation are required to apply this technique to broader fields, such as sedimentary temperature for ancient geothermal gradient with better understanding of the sedimentary history.

How to cite: Chen, Y.-Y. and Chang, Y.-J.: Evaluation of Oil Source Rocks Using Temperature-dependent Infrared Spectroscopy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2449, https://doi.org/10.5194/egusphere-egu24-2449, 2024.

Recent years have seen the growth of new techniques that combine conventional stratigraphic and observational approaches to characterizing the type, scope, extent, timing and effects of diagenetic processes with petrophysical measurements of their rock microstructure. These Quantitative Diagenetic (QD) techniques can be used to predict post- and pre-dolomitisation porosities and permeabilities as well as trace the pathway of the diagenetically evolving rock through different stages of diagenesis that may turn a low-quality carbonate reservoir into a high-quality reservoir, or vice versa. While these new QD techniques are becoming useful for the characterization of hydrocarbon reservoirs, they are also extremely useful in the characterization of carbonate reservoirs for prospective CCUS use. This paper will briefly explain some of the main approaches to QD including dolomitisation prediction, petrodiagenetic pathways, reservoir quality fields, and Fracture Effect Index (FEI), before examining how they can be used to ensure that the prospective CCUS target reservoir is sufficiently well characterized that effective reservoir modelling can take place, and that the volume, flow and trapping of CO₂ in the reservoir can be effectively monitored. Dolomitisation is known to be affected by the presence of CO₂, with CO₂ dissolving in aqueous pore fluids to form carbonic acid that directly affects porosity through dissolution and indirectly by affecting the dynamics of the dolomitisation process itself. There are two current QD methods for predicting the change in porosity upon dolomitisation. One is affected by both the direct and indirect effects, while the other is only sensitive to the indirect effects. Both the direct and the indirect effects can be plotted on a petrodiagenetic pathway. The presence of fractures is also a key aspect of how injected CO₂ will flow in a CCUS reservoir. The QD parameter FEI describes the change in permeability of a rock concomitant upon a unit change in fracture porosity (i.e., what increase in flow results from a given increase on fracture porosity). This varies depending upon the degree to which fractures are connected and can be extremely useful in predicting the flow of CO₂ within a fractured legacy carbonate CCUS prospect. In summary, QD approaches have the potential to provide those who need to characterise and model carbonate CCUS prospects with new and useful tools.

How to cite: Mohammed-Sajed, O., Rashid, F., Glover, P., Collier, R., and Lorinczi, P.:  Using Quantitative Diagenesis to characterise and understand carbonate CCUS prospects, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3199, https://doi.org/10.5194/egusphere-egu24-3199, 2024.

          It is pivotal to predict the overall composition of subsurface oil and gas reservoirs to assess their fluidity, phase behavior, and recovery potential. Recognizing the significance of n-alkanes as key constituents of mature oil and gas, this study conducted a thermal simulation experiment of gold tube hydrocarbon generation on the source rock of Gulong Sag. The experiment included comprehensive analysis and measurement of the n-alkanes components in a representative sample. Subsequently, an empirical regression evaluation formula was established to evaluate the n-alkanes composition at various maturity stages. Furthermore, a chemical dynamics model for the formation of individual n-alkanes single molecule components was developed and calibrated based on the principles of chemical kinetics. Combined with the stratigraphic burial history and thermal evolution history of the target area, the distribution and evolution characteristics of n-alkanes components in different evolutionary stages of geological conditions can be quantitatively evaluated and predicted. Moreover, the phase behavior of n-alkanes components can be determined based on the evolution characteristics of these components. Experimental results indicate that the methane yield continues to increase with temperature under both heating rates. Additionally, the yield of n-C to n-C initially reaches its maximum with the temperature increase, and subsequently decreases. Furthermore, the hydrocarbon generation characteristics of n-alkanes follow a Gaussian distribution trend. The kinetic results demonstrate that the activation energy of n-alkanes falls within the range of 190-280 kJ/mol, while the distribution of pre-exponential factors is uneven. By considering the geological conditions, it has been determined that the light component in the Gulong Sag is currently experiencing a favorable generation period, whereas the heavy component has reached its peak formation stage, with some undergoing cracking. The oil and gas produced under these geological conditions exist as single-phase unsaturated fluids within volatile reservoirs. The evaluation value of the experimental regression formula, along with the predicted value from the dynamic model, aligns well with the experimental data, providing a solid foundation for the geological application of the model. Therefore, this research serves as a stepping stone towards furthering our understanding of hydrocarbon composition prediction, as well as evaluating phase behavior, mobility, and recovery of underground oil and gas in conjunction with geological conditions. 

How to cite: Bo, S., Xue, H., and Lu, S.: Simulation experiment evaluation and chemical kinetics prediction of the composition of n-alkanes components, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3696, https://doi.org/10.5194/egusphere-egu24-3696, 2024.

Successful exploration of Permian shale gas in the Hongxing area has broadened shale gas exploration in the Sichuan Basin; however, the target layer is newly discovered, and reservoir research, which is key to shale gas exploration and development, is limited, thus restricting the screening and evaluation of the section containing the shale gas target layer. In this study, using organic carbon, whole-rock mineral analysis, scanning electron microscopy, low-temperature nitrogen adsorption, and other experimental methods, and by systematically identifying different lithofacies, we clarified the organic‒inorganic composition and microscopic pore structure characteristics of Permian shales in different phases of the Hongxing area and revealed the main factors controlling high-quality reservoirs and favorable lithofacies types for exploration. The results show that the shale in the study area mainly features six types of lithofacies: high-carbon siliceous shale (RS), high-carbon mixed shale (RM), high-carbon calcareous shale (RC), high-carbon muddy shale (RCM), low-carbon muddy shale (LCM), and low-carbon calcareous shale (LC). Organic pores are mainly present in RS, RM, RC, and RCM, while inorganic pores are dominant in LC and LCM. The pores are dominantly micropores, some mesopores are present, and very few macropores are present. Among them, the degree of micropore development is mainly affected by organic matter (abundance, maturity, and type), that of mesopores is mainly affected by clay minerals, and that of macropores is mainly affected by siliceous and clay minerals. There are obvious differences in the pore structure of different lithologies. The RS has the highest pore volume and specific surface area, with average values of 13.8×10^-3 cm³/g and 21.57 m²/g, respectively, and its pore morphology is ink-bottle type, with pore diameters mainly <10 nm. The storage space of RM, RC, RCM, and LCM is moderate, with low-carbon argillaceous shale (LM) having the lowest.

How to cite: Li, B., Zhou, N., and Lu, S.: Characteristics and factors controlling Permian shale gas reservoirs in the Hongxing area, Sichuan Basin, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3700, https://doi.org/10.5194/egusphere-egu24-3700, 2024.

The West Netherlands Basin, which has a long history in exploration as a former prosperous hydrocarbon province, is currently a geothermal hotspot. Being exploited since the 1950’s, most of its oil and gas fields are now in their final phase of production. In the past decade, interest shifted to sustainable energy sources. The geothermal industry in the area is developing quickly, helped by the legacy of the hydrocarbon industry: a wealth of publicly available seismic and well data. Currently, the area has 14 realized, and at least 3 projects in the development phase, with the Late Jurassic Nieuwerkerk Formation being the main target.

Conversely to petroleum systems, in which anticlines are the preferential target for hydrocarbon exploration, synclines are the most suitable sites for geothermal exploration. They offer higher temperatures with respect to the limbs and anticlines, and possible remaining hydrocarbons are not expected to be located inside the central portions of the synclines.

The West Netherlands Basin is a former rift basin that developed during the Mesozoic in the framework of the North Sea rift, and subsequently inverted during the Late Cretaceous. The Nieuwerkerk Formation was deposited during the last major rifting phase. Thus, the thickest packages of its fluvial-deltaic deposits are fault-controlled and commonly located in the synclines. The heterogeneity of fluvial reservoirs causes lateral and vertical quality variations in porosity, permeability and net-to-gross ratios. With the hydrocarbon industry focussing on the stratigraphic highs, there is only limited well data available for the central portions of the synclines.

With reprocessed 3D seismic data, our study uses an image processing approach, coupling traditional amplitude mapping with seismic attributes. This will help to reconstruct the evolution of the fluvial architecture of the Nieuwerkerk Formation over time. By tying the seismic with well data, a better prediction of the quality of the sandy bodies per location can be made. These results can be implemented in de-risking geothermal well planning across fluvial reservoirs in inverted rift basins.

How to cite: Weert, A., Vinci, F., Iacopini, D., van der Vegt, P., Tavani, S., and Ogata, K.: From hydrocarbons to geothermal energy: a case study from the Dutch subsurface, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3864, https://doi.org/10.5194/egusphere-egu24-3864, 2024.

How to cite: Chandra, D., Salgado, B. P., and Barnhoorn, A.: Wave velocities as a proxy to forecast deformation during cyclic loading-unloading in porous reservoir rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4657, https://doi.org/10.5194/egusphere-egu24-4657, 2024.

How to cite: Talukdar, M., Behera, C., Heinemann, N., Miocic, J., and Blum, P.: Cushion gas requirements for hydrogen storage in global underground gas storage facilities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5508, https://doi.org/10.5194/egusphere-egu24-5508, 2024.

The changes in land use/cover are essential aspects of studying the impact of human activities on the Earth's surface and global transformations. In this study, utilizing the ESRI Global Land Cover data (ESRI land cover 2020) and the China Land Cover Data (CLCD), along with historical imagery from Google Earth, a comparative analysis scheme for land use classification results was designed. The CLCD dataset was updated, leading to the creation of a land use dataset for the Pan-Pearl River Basin spanning from 1985 to 2020. This dataset was then employed for the analysis of land use changes in the Pan-Pearl River Basin over the past 35 years.The results indicate:(1) Among the seven land use types, the most significant changes in area occurred in the following order: build -up land, cropland, forest land, grassland, shrubland, waterbody, and barren. Notably, there was a substantial increase in the areas of build-up land and forest land, while cropland, grassland, and shrubland experienced significant decreases. The waterbody’area showed a slight overall increase trend.(2) The major land use types undergoing changes varied among sub-basins, with the intensity of land use change ranked as follows: Pearl River Delta region(1.9%) > Coastal rivers in southern Guangdong and western Guangxi(0.20%) > Dongjiang River Basin(0.13%) > Hanjiang River Basin(0.12%) > Xijiang River Basin(0.10%) > Beijiang River Basin(0.08%) > Hainan Island region(0.02%).(3) Within the sub-basins of the Pan-Pearl River Basin, the most significant increase was observed in the area of built-up land, exhibiting a continuous expansion trend with a total increase of 12184 km². This increase was primarily due to the conversion of cropland, forest land, and waterbody. The most significant decrease occurred in cropland, with a total reduction of 10435 km², mainly transitioning to built-up land and forest land. The phenomenon of built-up land encroaching on cropland was particularly prominent, especially in the Pearl River Delta region. Forest land also showed a decreasing trend, mainly attributed to cultivation and the encroachment of built-up land. The reduction in grassland area was more pronounced in the Xijiang River Basin, primarily transforming into forest land, cropland, and built-up land. The study reveals that the rapid development of socio-economics and industry, coupled with an increase in residents' consumption levels, serves as the primary driving force behind land use changes in the Pan-Pearl River Basin. Additionally, land use and management policies play a crucial role as driving factors in the region's land use changes. This research aims to provide a scientific basis for formulating policies related to the region's land resources and land management, holding significant importance for preserving ecological balance and fostering sustainable development in the basin.

How to cite: Fan, W. and Yang, X.: Land Use Change Characteristics in the Pan-Pearl River Basin in China from 1985 to 2020, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7216, https://doi.org/10.5194/egusphere-egu24-7216, 2024.

To meet the climate targets outlined in the Paris Agreement, European Green Deal, and the goal of reducing dependence on fossil fuel imports per the REPower EU Action, decarbonizing and reducing energy consumption in the heating and cooling sector is imperative. This sector, a major contributor to household energy use, plays a pivotal role in achieving sustainable energy goals.

Geothermal energy, particularly through geothermal doublets, stands out as an ideal solution for supplying energy for space heating and cooling. However, the inherent risks associated with fluid exchange with the subsurface make it scientifically or politically challenging in certain areas. Addressing this concern, deep borehole heat exchangers function as closed-loop systems, eliminating fluid exchange with the subsurface.

In this study, we explore the feasibility of repurposing existing oil and gas wells in the Northern Netherlands as deep coaxial borehole heat exchangers to provide heat to local communities. Utilizing analytical solutions, we calculate the thermal power output of 365 gas wells suitable for retrofitting. These wells exhibit bottom hole temperatures exceeding 80°C, capable of delivering temperatures above 60°C or thermal powers exceeding 800 kW, depending on flow rate and inflow temperature.

Our analysis includes assessing the proximity of well locations to high-density heat demand neighborhoods within a 6 km radius, facilitating the provision of supply temperatures for future local heat district networks. Notably, heat loss from well to neighborhood generally remains below 2°C, ensuring sufficient heating power supply to nearby residential areas. Several well clusters demonstrate significant heat over-supply, suggesting the potential for transporting excess heat to more distant locations. In cases where heat supply from wells is too low, in particularly in neighbourhoods with very low building efficiency rating (<E), heat pumps can be utilised to supply the needed energy.

Our findings indicate that repurposing existing hydrocarbon wells as coaxial heat exchangers offers a viable option for providing low-carbon heating to numerous residential areas in the Northern Netherlands. However, the geographical distribution reveals that not all high heat demand neighbourhoods have well sites in proximity, underscoring the importance of implementing a diverse heat supply strategy.

How to cite: Miocic, J., Drenth, J., and van Benthem, P.: Reutilising hydrocarbon wells as deep heat exchangers to decarbonise heating in the Northern Netherlands , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7761, https://doi.org/10.5194/egusphere-egu24-7761, 2024.

Digital rock models are becoming an essential tool not only for the modelling of fundamental petrophysical processes, but in specific key applications, such as Carbon Capture and Underground Storage (CCUS), geothermal energy exploration, and radioactive waste storage. By utilizing advanced imaging and simulation techniques, digital rocks provide indispensable insights into the porous structures of geological formations, crucial for optimizing CO₂ storage, enhancing geothermal reservoir characterization, and ensuring the secure containment of radioactive waste. This abstract aims to present new advances using digital rocks to study these pressing environmental and energy challenges.

Estimating the physical properties of rocks, a crucial and time-consuming process in both the characterisation of hydrocarbon, geothermal and CCUS resources, has seen a shift from traditional laboratory experiments to the increasingly prevalent use of digital rock physics. A key requirement of many forms of pore structure image analysis is that they require binary images showing pore-space vs. non-pore space (mineral phases). These are typically obtained by thresholding grey scale SEM or X-ray tomographic images to separate the two phases. In this paper, we have adapted a 2D process-driven MATLAB model to generate synthetic porous media images, laying the foundation for simulating authentic SEM images. The objective of the computational framework outlined in this study is to train a machine-learning model capable of predicting various types of porosity. Drawing inspiration from recent advances in machine learning applied to porous media research, our approach involves the development of deep learning models utilizing Convolutional Neural Networks (CNN). Specifically, we aim to quantitatively characterize the inner structure of the 2D porous media based on their binary images through the implementation of these CNN models. This framework consists of: (i) Generating synthetic porous media images through a process-driven model, (ii) training a neural network that takes a labelled synthetic image as input and gives two types of porosity as output, (iii) whereupon the trained model can be applied to provide types of porosities for new images that are not in the training database. The generated data are divided into training, validation, and testing datasets. The training dataset optimizes CNN parameters for accuracy, the validation dataset aids in hyperparameter selection and prevents overfitting, and the testing dataset evaluates the predictive performance of the trained CNN model.

This research not only advances the understanding of fundamental geological processes but also plays a crucial role in optimizing the utilization of renewable energy sources such as geothermal and contributing to the effective management of carbon capture and storage initiatives.

How to cite: Alwan, W.S., Glover, P. W. J., and Collier, R.: Using machine learning to discriminate between mineral phases and pore morphologies in carbonate systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8307, https://doi.org/10.5194/egusphere-egu24-8307, 2024.

How to cite: Alwan, W., Glover, P., and Collier, R.: Using machine learning to discriminate between mineral phases and pore morphologies in carbonate systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8307, https://doi.org/10.5194/egusphere-egu24-8307, 2024.

Decarbonisation of the European economy represents one of the current challenges to both society and the energy sector. The advancement and further application of carbon capture and sequestration (CCS) technologies are crucial components of the EU’s effort to become climate-neutral by 2050. The success of CCS depends heavily on understanding the present-day stress field to anticipate reservoir and cap rock response to fluid injection. Despite its importance, many proposed carbon storage sites in the North Sea are located in areas with little to no borehole stress data available, presenting a significant challenge.

Within the ACT project SHARP Storage framework, we have addressed this gap by generating a comprehensive earthquake bulletin for the North Sea, revealing spatial clusters of seismic events with the majority of earthquakes with ML < 4. Focal mechanisms of earthquakes are excellent indicators of crustal dynamics, which are essential for assessing the present-day stress field. Therefore, to improve the understanding of the in-situ stress conditions, we created a comprehensive workflow to evaluate focal mechanisms based on data from the North Sea (Kettlety et al., 2023). First, we developed a routine for the seismological bulletin to aggregate the recorded earthquakes from international seismological centres. The following step included retrieval of the waveforms from data centres and quality control routines, which included dead channels check, exclusion of files with significant recording gaps and low signal-to-noise ratio, and corrections of errors in the station XML files. Then, a subset of data traces with sufficient quality was selected for moment tensor computations using a Bayesian bootstrap-based probabilistic inversion scheme (see Heimann et al., 2018). Using existing focal mechanism solutions for the North Sea region, we calibrated our processing routine and then applied it to selected earthquakes (after 1990, M > 3.5) to expand the existing focal mechanisms database.

The newly computed focal mechanism solutions provide valuable insight into the present-day stress field in areas outside the main hydrocarbon provinces and improve the risk assessment of ongoing and future CCS projects. Furthermore, we will release our processing workflow as an open-source package and a new focal mechanisms database of the North Sea to establish a standard processing routine that can be readily utilised for similar seismological studies.

How to cite: Martuganova, E., Naranjo Hernandez, D. F., Kühn, D., and Barnhoorn, A.: Assessing earthquake focal mechanisms in the North Sea for risk mitigation of large-scale CO2 injections, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8973, https://doi.org/10.5194/egusphere-egu24-8973, 2024.

From 2005 to 2020, Canada achieved a 9.3% reduction in green house gas emission (69 Mt CO₂ eq), meanwhile British Columbia witnessed a 5% increase (3.0 Mt CO₂ eq) from 2007 to 2019. Exploiting unconventional oil and gas resources in northeast British Columbia (NEBC) has become the province’s second-largest source of greenhouse gas emissions. In pursuit of a cost-effective and seismic risk-aware approach for carbon emission reduction, this study evaluates the CO₂ geological storage capacity in NEBC with a focus on repurposing existing injection wells for carbon storage.

We particularly emphasize the Montney and Debolt formations. These formations are the main targets of a diverse array of injection wells, including those for hydraulic fracturing, enhanced hydrocarbon recovery, and wastewater disposal. Three trapping mechanisms in the NEBC area are examined: physical and solubility trapping for wastewater disposal wells in the Debolt Formation, and physical and mineral trapping for hydraulic fracturing and enhanced recovery wells in the Montney Formation. Furthermore, we incorporate an assessment of seismic hazards, informed by the latest insights into injection-induced seismicity in NEBC, as a potential indicator of CO₂ leakage risk.

Our findings underscore the favorable conditions of the Debolt Formation with lower seismicity hazard and a substantial CO₂ storage capacity (19.3 Gt; ~284.4 years of CO₂ emissions in BC). Depleted oil and gas reservoirs within the Montney Formation are also deemed suitable for CO₂ storage, estimated at 1671.8 Mt (approximately 24.5 years), particularly in the Upper Montney due to its higher storage capacity and lower seismic risk.

Overall, this research offers an assessment of CO₂ geological storage potential at the formation-scale in NEBC. The emphasis on well suitability and seismic risks effectively bridges the gap between the regional-scale geological assessments and site-scale engineering evaluations. It paves the path for future studies on addressing more practical topics related to the choices of project sites and injection strategies.

How to cite: Yu, H., Wang, B., Kao, H., Visser, R., and Jobin, M.: Storage potential of CO2 by repurposing oil and gas-related injection wells in the Montney Play, northeast British Columbia, Canada, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9206, https://doi.org/10.5194/egusphere-egu24-9206, 2024.

In the development of hydrocarbon fields, it is becoming known that CO₂ injection (which is sometimes done to improve hydrocarbon production) can cause pore blockage and wettability alteration by the promotion of asphaltene deposition. In hydrocarbon reservoirs, the result is poor oil recovery performance during carbon dioxide (CO₂) injection. If CO₂ is being injected into a legacy hydrocarbon reservoir (i.e., one that still contains residual oil) the same process will occur. Once again, the ability of fluid (this time supercritical CO₂) to flow will be impeded, but it is also possible that asphaltene deposition will also reduce the overall pore volumes in which CO₂ could otherwise be stored. In this work, the residual oil distribution and the permeability decline caused by organic and inorganic precipitation after miscible CO₂ flooding and water-alternating-CO₂ (CO₂-WAG) flooding have been studied by carrying out core-flooding experiments at high pressures and temperatures in an artificial three layer system. For simple CO₂ injection during CCUS operations, flooding experimental results indicate that the low-permeability layers retain a large oil production potential even in the late stages of production, which could impede CO₂ emplacement and provide significant heterogeneity, while the permeability decline due to asphaltene precipitation is more significant in high-permeability rocks. In contrast, we found that CO₂-WAG can reduce the influence of heterogeneity on the oil production, but it results in more serious reservoir damage, with permeability decline caused by CO₂–brine–rock interactions becoming significant. In addition, miscible CO₂ flooding has been carried out for rocks with similar permeabilities but different wettabilities and different pore-throat microstructures in order to study the effects of wettability and pore-throat microstructure on formation damage. Reservoir rocks with smaller pore-throat sizes and more heterogeneous pore-throat microstructures were found to be more sensitive to asphaltene precipitation, making these less attractive for CCUS reservoirs. However, rocks with larger, more connected pore-throat microstructures became less water wet due to asphaltene precipitation to pore surfaces, ultimately leading to a lower pore volume in which CO₂ can be stored. Taken together, there may be a case for not simply injecting CO₂ in CCUS operations, but alternating the CO₂ injection with injection of water in order to stabilise CO₂ flow and reduce formation damage by asphaltene precipitation.

How to cite: Wang, Q., Paul, G., and Piroska, L.: Quantifying flow reduction during injection of CO2 into legacy hydrocarbon reservoirs for CCUS, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9509, https://doi.org/10.5194/egusphere-egu24-9509, 2024.

Though global energy needs continue to grow, fossil fuels, and their associated CO₂ emissions, are increasingly being opposed as our main source of energy. Instead, to achieve net zero greenhouse gas emissions goals, we are currently transitioning to more sustainable sources of energy, such as solar and wind power and geothermal energy, coupled with storage of waste, such as CO₂. However, these new technologies come with their own challenges, as they continue to rely on (re-)use of the subsurface landscape. The intermittency of solar and wind power will require storage of renewably generated electricity. Hydrogen fuel has been marked as a potential energy carrier, enabling us to store large quantities of energy for prolonged periods of time, such as required to supply large industries or communities during winter months. To store this hydrogen fuel, the subsurface offers the largest storage space available, such as in (offshore) depleted hydrocarbon fields, but reproduction of the stored fluid is crucial. Geothermal energy production will require the extraction of hot fluids from depth and will often be performed in populated areas, close to the consumers, meaning that phenomena such as surface subsidence and induced seismicity are highly undesirable. The safe storage of CO₂ for thousands of years also entails fluid injection, but containment is of vital importance to keep the CO₂ out of our atmosphere. So though we have a vast history of exploitation of the subsurface through the oil and gas industry, which we can and should build upon, these new sustainable energy developments also pose their own, new challenges. While fluid production changes the physical equilibrium of the system, these new uses will also impact the chemical equilibrium through the injection of new fluids. Furthermore, containment and safety play an even bigger role than before to ensure the longevity of these new subsurface operations. In this contribution, I will outline what the challenges are that we are facing and how geoscientists can contribute to solving these challenges, across all areas from rock physics, geochemistry and hydrology, to sedimentology, structural geology and policy.

How to cite: Hangx, S.: Same same but different: the scientific challenges when re-using the subsurface for sustainable energy developments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12486, https://doi.org/10.5194/egusphere-egu24-12486, 2024.

In the context of Carbon Capture and Storage (CCS), the porosity of potential storage formations is a critical factor. Our study explores this aspect using computed tomography (CT) to assess how different scanning resolutions impact the accuracy of porosity measurements. We employed three CT systems - Geotek RXCT (resolution ~20-150 μm), Bruker 1272 (resolution ~5 μm), and DELab μCT-100 (resolution ~9 μm) - to scan sandstone cores of varying porosities. The aim was to identify an optimal scanning resolution that balances detail with practicality for CCS evaluations.

This research addresses the challenges in high-resolution CT scanning, such as denoising effects that can alter accuracy, and the complexities of thresholding segmentation across various systems. Additionally, we examined the partial volume effect, crucial for interpreting pore sizes and distributions accurately.

Our preliminary results suggest that scanning resolution significantly affects the perceived porosity. Different resolutions uncover diverse aspects of pore structure, highlighting the importance of choosing an appropriate resolution. Advanced image processing techniques, including effective denoising and accurate thresholding, are vital for reducing errors in porosity measurement.

The study provides valuable insights into the use of CT scanning for CCS applications, emphasizing the need for a balanced approach in resolution selection and sophisticated image processing. These findings are instrumental in enhancing the reliability of geological evaluations for potential CCS sites, contributing to the broader efforts in carbon storage and climate change mitigation.

How to cite: Huang, J.-J. S., Liou, Y.-M., Kioka, A., and Yang, T.-R.: Exploring the Relationship between CT Scanning Resolutions and Sandstone Porosity for CCS Applications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12846, https://doi.org/10.5194/egusphere-egu24-12846, 2024.

Hydrogen sulfide (H₂S) is a harmful gas associated with oil and gas, which seriously affects the safety and effective exploitation of oil and gas, so it is important to evaluate the origin and distribution of hydrogen sulfide effctively. H₂S rich oil and gas reservoirs were found in lacustrine carbonate rocks of Es₄, Dawangzhuang area, Chezhen Sag. In order to clarify the formation mechanism and enrichment law of H₂S in this area, the geochemical characteristics of natural gas, formation water, elemental characteristics of reservoir solid bitumen, and sulfur isotope characteristics of kerogen, salt-rock and crude oil are analyzed comprehensively. It is concluded that H₂S in Dawangzhuang area is caused by thermochemical sulfate reduction (TSR). H₂S in Dawangzhuang area is generated from the northern deep depression zone and migrated to the upper submember of Es₄ carbonate reservoir in the study area through the successor fault activities during and after Guantao accumulation period. This understanding will provide reference for the future development of sulfur-rich oil and gas reservoirs in Dawangzhuang area and the avoidance of high-yield H₂S risk.

How to cite: Han, D., Dong, C., and Ma, P.: The formation mechanism and enrichment patterns of H2S in Es4 of Dawangzhuang area, Chezhen Sag, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13736, https://doi.org/10.5194/egusphere-egu24-13736, 2024.

In recent years there has been rapid development of nanoparticles (NPs). Nanoparticles can be used both as a probe into restricted spaces, such as the pores within a reservoir rock, and as tools for altering wettability or deliberately blocking pore throats to enhance fluid movement in less connected pores. Silica nanoparticles can have functional surfaces allowing them to react specifically to oils or water. Nanoparticles can be used to enhance oil production by releasing oil on mineral surfaces and improving fluid flow. However, they also have the potential for improving CO₂ flow in CCUS reservoirs while enhancing the pore volume available for CO₂ storage. In this paper we evaluate the performance of different non-functionalised and functionalised nanoparticles for enhancement of oil production, CO₂ emplacement and gas flow. Different forms of silica NPs have been made, either unfunctionalized, or functionalised with branched amino-based polymer (hydrophilic) or a silane-based agent (hydrophobic). Their stability has been characterised using a range of laboratory methods. The microscopic performance of the nanoparticles has been measured using contact angle measurements. Their ability to enhance oil production and CO₂ emplacement has been tested using imbibition and drainage experiments. 

The contact angles, measured in the presence of brine, no modified silica NPs, branched amino-based polymer (hydrophilic) modified silica NPs and silane-based agent (hydrophobic) modified silica NPs showed contact angle values of approximately 110°, 116°, 124°, and 136°, respectively. These results show that introduction of nanofluids led to a change in substrate wettability from water-wet to strongly water-wet. Notably amongst the tested nanoparticles the Silane-based NPs demonstrated the highest hydrophilic surface. The spontaneous imbibition tests conducted on various sandstone cores revealed that silane-based NPs yielded the highest oil recovery rates among the tested NPs. Specifically, these nanoparticles showed an approximate 12% and 50% enhancement in oil recovery compared to non-modified silica nanoparticle, and branched amino-based polymer (hydrophilic) modified silica NPs. In summary, nanofluids have been shown to substantially improve the wettability alteration of the rock surface from oil-wet to water-wet, which can lead to improve the volume and flow characteristics of legacy CCUS prospects. Our future plan is to investigate the enhancement of carbon dioxide (CO₂) solubility in brine through the utilization of the prepared nanoparticles, with the objective of advancing carbon capture technologies.

How to cite: Tliba, L., Hetnawi, A., Sagala, F., Menzel, R., Glover, P., and Hassanpour, A.: Characterising functionalised nanoparticles for improving fluid flow for CCUS in legacy hydrocarbon reservoirs , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13835, https://doi.org/10.5194/egusphere-egu24-13835, 2024.

TRANSGEO is a regional development project that aims to explore the potential for producing sustainable geothermal energy from abandoned oil and gas wells in central Europe.  Composed of 11 partner organizations and 10 associated partners in 5 countries, TRANSGEO is developing a Transnational Strategy and Action Plan to address this technical and economic opportunity.  Our primary objective is to support rural communities and industries in the energy transition by providing tools and information that highlight sustainable redevelopment priorities and opportunities.

To reach this objective and promote the switch from fossil fuels to green energy, TRANSGEO is developing reuse procedures for five different geothermal technologies and validating them via numerical modelling, to assess their performance in repurposing existing hydrocarbon infrastructure and determine the optimal reuse conditions and configurations.  The five geothermal technologies are Aquifer Thermal Energy Storage, Borehole Thermal Energy Storage, Deep Borehole Heat Exchangers, Enhanced Geothermal Systems, and Hydrothermal Energy production.  The modelling studies focus on reference sites in our study areas, the North German Basin, the South German Molasse Basin, the Vienna Basin, and the Pannonian Basin.  Comparison of varying wellbore and reservoir parameters in the numerical modelling studies will provide input to a new online well assessment tool which will be available publicly to determine well suitability and guide planning for future reuse projects.  The online tool will be informed by a database of abandoned wells in Austria, Croatia, Germany, Hungary, and Slovenia and will include local reference data, such as geology, topography, heat demand, and utilities.  This will facilitate well reuse by matching candidate wells with local energy demand and heating networks.  Additional work on socio-economic and policy analyses will provide financial and liability information for the 5 different geothermal technologies, across the project countries.  Finally, the partnership will propose a legal policy and incentive framework to facilitate and expand reuse of abandoned wells for geothermal energy production and storage across central Europe.

TRANSGEO is co-funded by the European Commission’s Interreg CENTRAL EUROPE programme.

How to cite: Hofmann, H., Friddell, J., Sass, I., Höding, T., Sieron, K., Svetina, M., Hölzel, M., Philipp, R., Márton, G., Borkovits, B., Bődi, K., Višnjić, A., Kurevija, T., and Vogrinčič, B.: TRANSGEO - Transforming abandoned wells for geothermal energy production, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14959, https://doi.org/10.5194/egusphere-egu24-14959, 2024.

Geological characterization of the “Fonts-Bouillants” helium discovery - France

Russier E^1,2, Géraud Y², Hauville B¹, Tarantola A² ,Beccaletto L³ and Diraison M¹

¹ 45-8 ENERGY, France

² GeoRessources, University of Lorraine, France

³ BRGM, F-45060, Orléans, France

ABSTRACT

Helium is essential for the manufacturing of many of our daily commodities such as optical fibres, computers or cell phones (semiconductors and processors), medical use (MRI scanners) or in other more specific applications such as airlifts, leak detection, gas chromatography or diving. Nowadays, Europe imports 100% of its helium needs from overseas and is facing regular shortages, reason why 45-8 Energy embarked five years ago on helium exploration and production in Europe.

Helium is a noble gas mostly coming from the natural radioactive decay of Uranium and Thorium contained in the crust and the basement. Its migration and accumulation are strongly linked to a vector fluid that can be CO₂, N₂, CH₄ or water. Helium and its vector fluids are then trapped and sealed in a rock reservoir.

The Fonts-Bouillants area is located at the southern edge of the Paris Basin at the vicinity of the French Massif Central and Limagne rift. The 45-8 Energy project aims to jointly produce He and CO₂ from a gas which is naturally seeping through the major Saint-Parize fault (SPF).  Geological origin and migration pathway of He are therefore key questions to define the exploration guide, in particular to locate production wells to produce the seeping gas and process it. A multidisciplinary approach involving geology, geophysics, petrophysics and geochemistry has therefore been deployed.

Because geological context was hardly documented in this area, a wide range of geophysical data were acquired or reprocessed and coupled with field geology to build a regional geological model. The initial geological model was considerably updated and a hidden and thick Late Palaeozoic depocenter was especially highlighted below the Mesozoic series. Well data in nearby analogous basins as well as outcrops enabled rock collections to conduct petrophysical and geochemical characterization. The main reservoirs discovered currently are in Triassic and Jurassic sandstones, and fault like Saint-Parize fault acted as barrier and drain. 

Our outcrops petrophysical and geochemical study highlight the importance of Late Palaeozoic basin for the helium system:

• As a potential rock source, with higher U-Th concentrations (3-13.5, 8-24 ppm) than typical crustal U-Th concentrations (1.8 and 7.2 ppm, [1]).
• As a potential migration pathway and reservoir, with sandstones and conglomerates porosity higher than 20% and permeability higher than 100 mD.

Finally, gas sampling was performed in local natural springs, but also during well testing conducted in shallow boreholes which have encountered gas bearing reservoirs in the Mesozoic along the SPF. Helium generation system was modelled with geochemical data from the rocks and the fluids and from the volumetric capacity of the Palaeozoic basin.

Keywords: Helium exploration, Geophysics, Petrophysics, Geochemistry

Themes: Helium exploration

References:

[1] Krauskopf, K. B., & Bird, D. K. (1967). Introduction to geochemistry (Vol. 721). McGraw-Hill New York.

How to cite: Emma, R., Yves, G., Benoît, H., Alexandre, T., Laurent, B., and Marc, D.: Geological characterization of the “Fonts-Bouillants” helium discovery - France, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16266, https://doi.org/10.5194/egusphere-egu24-16266, 2024.

Natural gas storage is currently considered as one key pillar of the EU strategy to ensure security of energy supply. In view of CH₄ storage, Oldenburg [1] demonstrated in a theoretical study that using CO₂ as cushion gas instead of CH₄ has the advantage of increasing natural gas storage capacities of geologic reservoirs by up to 30 %. This is due to CO₂ undergoing a significant density change when its’ critical pressure is exceeded during CH₄ injection. Kühn et al. [2] investigated a comparable scenario with CH₄ gas storage in a closed cycle with CO₂ in one reservoir to temporarily store and reuse wind and solar energy. However, the potential qualitative degradation of the stored CH₄ due to mixing with the CO₂ cushion gas has not yet been sufficiently addressed in terms of the impact of the modeller’s choice of diffusion coefficients. Hence, the present study focuses on a quantitative assessment of the mixing behaviour of CH₄ and CO₂ under consideration of dynamic binary diffusion coefficients in a reference numerical simulation benchmark [1,3]. The TRANSPORTSE numerical simulator [4,5] is used with dedicated measures to mitigate the initially high numerical dispersion, introduced by the benchmark’s relatively coarse grid discretisation. The simulation results show that the mixing region is substantially reduced if dynamic binary diffusion coefficients are applied instead of a global constant for both gas components. Consequently, it is demonstrated that previous numerical assessments of natural gas storage with a carbon dioxide cushion gas overestimate the simulated CH₄-CO₂-mixing area, and thus the calculated mixing losses. Hence, combined gas storage of CH₄ and CO₂ is more efficient than expected so far.

[1] Oldenburg, C. M. (2003) Carbon Dioxide as Cushion Gas for Natural Gas Storage. Energy Fuels 17(1), 240−246. https://doi.org/10.1021/ef020162b

[2] Kühn, M., Nakaten, N. C., Streibel, M., Kempka, T. (2014): CO2 Geological Storage and Utilization for a Carbon Neutral “Power-to-gas-to-power” Cycle to Even Out Fluctuations of Renewable Energy Provision. Energy Procedia, 63, 8044-8049. https://doi.org/10.1016/j.egypro.2014.11.841

[3] Ma, J., Li, Q., Kempka, T., Kühn, M. (2019) Hydromechanical Response and Impact of Gas Mixing Behavior in Subsurface CH4 Storage with CO2-Based Cushion Gas Energy & Fuels 33 (7), 6527-6541. https://doi.org/10.1021/acs.energyfuels.9b00518

[4] Kempka, T. (2020) Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Advances in Geosciences, 54, 67-77. https://doi.org/10.5194/adgeo-54-67-2020

[5] Kempka, T., Steding, S., Kühn, M. (2022) Verification of TRANSPORT Simulation Environment coupling with PHREEQC for reactive transport modelling. Advances in Geosciences, 58, 19-29. https://doi.org/10.5194/adgeo-58-19-2022

How to cite: Kempka, T. and Kühn, M.: Geologic CH4 storage with CO2 cushion gas: using dynamic binary diffusion coefficients instead of a global constant in numerical simulations is more precise and results in lower mixing losses, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17184, https://doi.org/10.5194/egusphere-egu24-17184, 2024.

Since the end of the XIX century, many wells have been drilled worldwide for both Oil & Gas exploration purposes. Most of them are now abandoned and subjected to mining closure because exhausted or sterile. In the new epoch of energy transition scenario, the possibility to adapt and reuse these existing boreholes to exploit geothermal energy seems very promising. In fact, considering that approximately 40% of the total costs for a new geothermal project are devoted to drilling activity, the possibility of repurposing abandoned oil and gas wells offers a wide range of applications and exploitation of underground heat uses. The drilled borehole available data (e.g., underground temperature, lithology) provide helpful information about the sub-surface reservoirs, reducing the mining risk level, and wells allow direct access to the sub-surface heat energy. However, to develop a commercially viable geothermal power/thermal generating system, one must consider several factors, i.e., available prospecting, drilling and reservoir technologies, energy costs in the area, and resource durability.

This research aims to analyze the potential and feasibility of deep closed-loop systems solutions for heat and power energy production in Italy, in areas characterized by both normal and anomalous geothermal gradients and the distribution of available abandoned oil and gas wells. A prominent result is the development of a workflow leading to the feasibility assessment of deep closed-loop systems development, based on the identification of suitable abandoned O&G wells through the geological and thermal underground characterization and wells construction characteristics (diameter, depth, borehole material). Furthermore, a sensitivity analysis of the main parameters affecting most of the retrofitting of abandoned wells for geothermal purposes is performed thanks to thermal FEM modelling.

Finally, identifying the possible end-users in a suitable case study area, this research work provides preliminary insights into the quantity of thermal energy and electric power that this technology could produce.

How to cite: Facci, M. and Galgaro, A.: Numerical sensitivity analysis of energy performance of geothermal deep closed loop heat exchangers derived from the reuse of abandoned oil and gas wells, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17674, https://doi.org/10.5194/egusphere-egu24-17674, 2024.

The dynamics of sedimentary basins is a complex combination of synchronous generally non-linear processes. In these natural systems, fluids migration and associated transfers play a fundamental role, even more so as they represent resources that are or may become essential for human societies. One way of assessing the potential of sedimentary basins is to model their past behaviour numerically.

Basin models have been developed since the 1990s for the needs of the oil industry, with the initial aim of assessing the thermal history, i.e. the maturation and expulsion of hydrocarbons from source rocks with variable kinetics and initial composition. These models are used for hydrocarbon prospect assessment in a wide range of sedimentary basins. They have evolved with the integration of the simulation of compaction mechanisms and fluid migration by Darcean single-phase or multi-phase flows. Still with an operational objective in mind, one of these models has been extended to simulate the transport of thermal energy and chemical elements in fluids, thereby helping to assess the geothermal and large-scale storage potential of a basin. The explicit representation of faults and unconformities, as well as the calculation of seal or reservoir formation fracturing as a function of fluid pressures, enables the plumbing system to be represented on a basin scale. In this network of drains, single- or multiphase fluids carrying compounds can interact with the rocks, according to the principles of reactive transport. Some of these simulations are being experimented using AI techniques. In these digital experiments, elements tracking could be a true added value for basin’s dynamics understanding.

A coupled simulation of this kind, combining conductive and advective thermal physics, mechanics (particularly of porous media), the hydraulics of multiphase fluids in porous media, chemistry of reactive transport and even the impact of bioactivity on basin’s fluids, representing geological processes in the subsurface on a large scale, makes it possible to quantify mass and energy transfers in the past. The result is a physically balanced model of the current spatial distribution of pressure, stress, temperature, mass, solid or fluid elements.

These results can be useful both in economic applications for first-order assessment of the resources of any sedimentary basin and in the scientific field for defining the boundary conditions of more specialised models. Initial experiments demonstrating the use of multiphysics models on a basin scale for CCS applications and geothermal energy assessment will be shown.

How to cite: Gout, C., Cacas-Stentz, M.-C., Traby, A., and Collard, N.: Modelling dynamics of sedimentary basins: using geological history to predict subsurface activities at large-scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18605, https://doi.org/10.5194/egusphere-egu24-18605, 2024.

With the advancement of exploration theory and technology, deep and ultra-deep carbonate rocks have gradually become an important new field for the development of oil and gas resources. High-quality carbonate reservoirs have become the focus of attention for oil and gas exploration and research in deep and ultra-deep fields. The Tarim Basin is the largest intracontinental oil and gas basin in China. The thick carbonate strata developed in the Lower Paleozoic are the main layers for oil and gas exploration, and the Ordovician carbonate strata are the main oil and gas producing layers. The predecessors have studied the tectonic evolution, sedimentary background and rock types of the Ordovician in the Tarim Basin. Combined with the analysis of the sedimentary thickness, lithology distribution and seismic profile structure of the Early Ordovician, it is believed that the Lower Ordovician sedimentary period inherited the Cambrian sedimentary pattern and transformed it into a gentle slope sedimentary background with a 'uplift-sag 'pattern, with obvious differentiation. Under the sedimentary background of the gentle slope of the Penglaiba Formation, the three paleo-uplifts of southwestern, northern and central Tarim are inherited geomorphological highs, and the inner gentle slope tidal flat facies is developed. The thickness of the stratum is obviously thinner, and it is mainly developed to represent the tidal flat environment. The periclinal part around the paleo-uplift is the middle gentle slope, which is characterized by dolomite and limestone interbeds. The proportion of granular rocks is high, which is a favorable development area for granular beaches. In this study, the deep drilling cores of the Lower Ordovician Penglaiba Formation in the central Tarim Basin were taken as the key research object, and the lithofacies, reservoir characteristics and dominant reservoir control factors of dolomite reservoirs were systematically analyzed by using macro-micro, qualitative-quantitative reservoir petrology analysis methods. Through research, it is clear that the rock types of the Lower Ordovician Penglaiba Formation in the central Tarim Basin are mainly crystalline dolomite and ( residual ) granular dolomite, and also contain a small amount of limestone, siliceous rocks and transitional rocks. There are various types of reservoir space, mainly including non-fabric selective dissolution pores, intercrystalline pores and various fractures. Combined with previous studies on the genesis and diagenetic evolution of the Lower Ordovician dolomite in the Tarim Basin, it is considered that the development of high-quality dolomite reservoirs in the Lower Ordovician Penglaiba Formation in the central part of Tarim Basin is controlled by many factors. It is the result of a combination of favorable sedimentary facies belts, short-term sea-level changes, exposure and dissolution, early dolomitization, and late tectonic hydrothermal adjustment and transformation.

How to cite: Li, X. and Xu, Q.: Development characteristics and controlling factors of deep dolomite reservoirs of Lower Ordovician in Tazhong area, Tarim Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20142, https://doi.org/10.5194/egusphere-egu24-20142, 2024.

Seismic data integration plays a pivotal role in enhancing the capabilities of geological modelling

software. Our current research focuses on the improvement of seismic interpretation tools within the

PZero software in the framework of the Geosciences IR project lead by the Italian Geological

Survey GitHub - andrea-bistacchi/PZero. The objective is to seamlessly incorporate seismic data

into the geological modelling workflow, enabling more comprehensive and accurate models of

subsurface structures.

The initial phase of our work involved using various libraries to import and analyze seismic

datasets, either 2D or 3D within the PZero framework. We have successfully achieved the

importation of SEGY files into PZero, marking a significant milestone in our efforts. Integrating

seismic data is a crucial step that sets the foundation for constructing detailed geological models,

allowing us to enhance our understanding of subsurface geological features.

Soon, our research trajectory aims to develop advanced algorithms for stochastic simulation tailored

explicitly for modelling clastic sedimentary alluvial plains. The ongoing work includes developing

advanced stochastic simulation algorithms tailored for modelling clastic sedimentary environments,

relevant to both conventional energy resources and emerging sustainable energy storage solutions.

These advancements in seismic data integration and simulation within PZero will significantly

contribute to the field of reservoir modelling. They provide a robust framework for predicting the

behavior of subsurface energy storage systems, which is pivotal in the transition to a low-carbon

energy future.

How to cite: Hussain, W. and Bistacchi, A.: Advancements in Seismic Data Integration and Stochastic Simulation for Geological Modeling in PZero, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22365, https://doi.org/10.5194/egusphere-egu24-22365, 2024.

Geothermal heat and power generation are two of several competing uses of deep groundwater reservoirs. The stress state of these slowly regenerating systems is increasing as drinking water production from shallow resources suffers from anthropogenic influences: overexploitation, total depletion, and deterioration through anthropogenic contaminants are exacerbated by a climate change-induced failure to recharge. In contrast to shallow groundwater, deep aquifers are shielded from short-term influences, like contamination, by protective overlying strata. The groundwater age is generally much higher, indicating slow regeneration processes because the system is nearly closed in its natural state. Recent data, however, suggests that this assumption is not valid for many geothermal systems with high flow rates. Therefore, a careful assessment of all competing operations using deep groundwater reservoirs is required. It should focus on the interactions between different aquifers and those within the reservoirs, all leaving their marks on the waters’ hydrochemical composition.
The lack of dedicated monitoring wells around geothermal production and injection sites makes it difficult to quantify the development of these reservoirs’ flow patterns. Furthermore, regular complete analysis data are usually only available at long intervals of 12 months. Still, short-term flow path development is a crucial factor in assessing heat extraction efficiency. Impaired extraction due to preferential flow paths forming between production and injection sites will degrade overall operation efficiency.

Here, we challenge state-of-the-art practices for monitoring deep aquifers. Based on decades worth of hydrochemical data on groundwater extraction at both single-well operations and geothermal doublets, we discuss how hydrochemical signatures can help determine the grade of sustainability at which deep geothermal wells are operated. Acknowledging that these assessments depend on an adequate database, and that deep groundwater research is plagued by notorious data scarcity, we tested the application of virtual sensors to these wells was tested. Lab experiments complete the analysis, quantifying the kinetics of the fluid-matrix interactions between the injector and producer.

The outcomes of this dissertation include a statistically reproducible algorithm assessing how sustainably a well is operated, focusing on the inherent dynamics at play in deep groundwater. A series of regression analyses conclude that the databases associated with deep groundwater wells are still insufficient to train virtual sensors; however, they allow conclusions on the required minimum amount of hydrochemical analyses needed to adequately represent inter-seasonal fluctuations in the aquifer. Fluid-matrix interactions result in threshold values for hydrochemical changes, which serve as a trigger to review well operation strategies and to update hydraulic and thermal models. They also indicate that changes are likely following an increasing dynamic because the surface area available for reactions increases geometrically and in roughness. Our calibrated model shows that decreasing the injection temperatures and adding CO₂ as a scaling inhibitor significantly increases the reservoir's reactivity.

Hydrochemical data can provide valuable insight into the flow processes in deep reservoirs, which are inaccessible otherwise. With smart sampling procedures and a tailored set of parameters, acquiring relevant data becomes feasible with relatively small financial investments.

How to cite: Dietmaier, A. and Baumann, T.: Hydrochemical Indicators for Sustainable and Optimized Geothermal Use of Deep Groundwater , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1522, https://doi.org/10.5194/egusphere-egu24-1522, 2024.

We develop a computation framework from scratch that allows us to conduct 3D numerical simulations of groundwater flow and heat transport in hot fractured reservoirs to find optimal placements of injection and production wells that sustainably optimize geothermal energy production.

We model the reservoirs as geologically consistent randomly generated discrete fracture networks (DFN) in which the fractures are 2D manifolds with polygonal boundary embedded in a 3D porous medium. The wells are modeled as line sources and sinks.
The flow and heat transport in the DFN-matrix system are modeled by solving the balance equations for mass, momentum, and energy.
The fully developed computational framework combines the finite element method with semi-implicit time-stepping and algebraic flux correction.
To perform the optimization, we use various gradient-free algorithms.

We present our latest results for several geologically and physically realistic scenarios.

How to cite: Partl, O. and Rioseco, E. M.: Optimization of geothermal energy production from fracture-controlled reservoirs via 3D numerical modeling and simulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4164, https://doi.org/10.5194/egusphere-egu24-4164, 2024.

Monitoring of geothermal reservoir deformation is essential for the normal development of the Enhanced geothermal system (EGS). Coda wave interferometry (CWI) with ambient noise is regarded as an effective and low-cost monitoring technique and draws more and more attentions. But the connection between the obtained CWI measurements and the undergoing physical changes of deep reservoir is still not so clear. In this study, we take Rittershoffen geothermal system (France) as a case study and conduct a series of forward simulations regarding the propagation of scattered wavefield through the deformed model considering acoustic-elastic effect based on Code_ASTER (mechanical loading) and SPECFEM2D (wave propagation). The simulations are based on a two dimensional numerical model with a scale of 12km (width)×20km (height), in which the upper reservoir model contains 8 layers to mimic Rittershoffen geothermal reservoir, the lower sub model with multiple circular inclusions is set to scatter the waves emitted from point source at bottom and produce scattered wavefield; two seismic stations are located at the top of the model. The model is first verified by reproducing the seasonal variation of relative wave velocity changes obtained from ambient noise cross-correlation functions (ANCCF) induced by the underground water table elevation changes. Based on the validated model, we study the effect of in-situ reservoir deformation on CWI measurements by modelling the hydraulic pressure increases on an open hole and the aseismic slip of an embedded fault which is based on the case of hydraulic injection of GRT-1 well, Rittershoffen. The result indicates the induced small reservoir deformation in both situations can be detected by CWI measurements, which helps us to have a better understanding about the connection between the obtained CWI measurements and the undergoing deformation of deep geothermal reservoir.

How to cite: Wang, Y., Azzola, J., Zigone, D., Lengliné, O., Magnenet, V., Vergne, J., and Schmittbuhl, J.: Geothermal Reservoir Deformation Monitoring Based on Coda Wave Interferometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4549, https://doi.org/10.5194/egusphere-egu24-4549, 2024.

In western Austria, especially in Tyrol, the potential for geothermal energy use is unexplored. The project GeoEN Inntal is aiming to determine this potential and the risks for the use of geothermal energy in a complex tectonic setting in the Inn valley. The valley is bordered by the Permo-Mesozoic sedimentary succession of the Northern Calcareous Alps in the north and the south-east, as well as Upper Austroalpine basement units in the south. Since the early Late Cretaceous, during Eoalpine orogeny, the Austroalpine basement and its Mesozoic cover were involved in nappe stacking and folding. The nappe stack was again refolded and faulted in the Palaeocene-Eocene collision of Adria and the European distal margin, as well as in Oligocene-Miocene, when post-collisional processes lead to an eastward extrusion of crustal blocks, out-of-sequence thrusting and the development of major faults. One of these faults is the Inntal shear zone, a sinistral ENE-trending shear zone, controlling the course of the Inn valley (“Inntal” in German). The shear zone has a multi-phase activity and is kinematically linked with the Brenner normal fault south of Innsbruck, the Alpine basal thrust at the Alpine front, and the Sub-Tauern ramp, rooting below the Tauern window. As the deep subsurface of the Inn valley was only explored geophysically, but no exploration boreholes were drilled, little is known about these structures at depth. Here we present the reprocessed Inntal part of the TRANSALP seismic section, which serves as a base for multidisciplinary modelling approaches. As part of the GeoEN Inntal project, we present first results from our 3D modelling of fault geometry, results from mechanical modelling of the Inntal shear zone, as well as first temperature gradient assessment from hydrological modelling.

How to cite: Hinterwirth, S., Ortner, H., Schreilechner, M., Binder, H., Lüschen, E., Jud, M., Hoyer, S., Bottig, M., and Hintersberger, E.: From seismic re-processing to mechanic modelling, a new interpretation of the TRANSALP seismic section as a base for future geothermal energy projects, lower Inn Valley, Tyrol, Austria, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6571, https://doi.org/10.5194/egusphere-egu24-6571, 2024.

Naturally fractured geothermal reservoirs (NFGR) represent a challenging frontier for sustainable energy exploration and production. These reservoirs are characterised by the presence of complex fracture networks controlling hot fluid movement at depth. Unfortunately, these networks cannot be directly observed, and their properties need to be modelled. Classically, these models are based on statistic data obtained from outcrops and borehole data. Outcrops allow characterisation of the geometry of networks at a scale up to 100’s of meters. However, the analogy between surface and subsurface is not trivial and surprisingly, the notion that different fracture sets are genetically related is rarely used. Borehole core data provide the only direct sampling of the subsurface. However, cores are challenging and expensive to obtain. As an alternative, geophysical borehole images are acquired to observe fractures in the subsurface. However, the quality of these images is variable, making their interpretation uncertain. In this study, we aim to minimize the uncertainties related to fracture picking in borehole images to accurately recognise specific part of the network interpreted from the surrounding outcrops. This approach will provide new perspective in the characterisation of NFGRs.

We test our approach on the Geneva Basin, located in westernmost part of Switzerland. There, the Lower Cretaceous carbonate units are expected to host geothermal resources. Recently, an exploration well, GEo-01 was drilled in the Canton of Geneva to evaluate the geothermal potential of the Lower Cretaceous reservoir. The basin is bounded to the north by the Jura and to the south by the Saleve mountain and the Borne massif where the Lower Cretaceous rocks are outcropping. This area extends for about 1500 km².

In these outcrops, we introduce the concept of discontinuity associations where sets of fractures, veins and stylolites which formed under a similar stress regime are grouped together. The characterisation of discontinuity associations allows to map the orientation of the maximal principal paleostress (σ₁) of genetically related discontinuities. This method is a more robust way of reconstructing fracture-forming deformation events than assigning one deformation event per discontinuity set. We consistently identify three distinct associations over the investigated mountain ranges. Those associations are formed before the onset of fold-and-thrust belt and therefore constitute a background fracture network expected to be found in the targeted geothermal reservoir.

To prove this hypothesis, we looked for the same discontinuity associations in borehole images of GEo-01. This well disposes of a unique dataset of five independent interpretations of the same 122 m interval of Lower Cretaceous series. To quantify and reduce interpretation uncertainties, our study involves a comparative statistical analysis of these interpretations. The outcomes of this  analysis facilitate the identification of intervals where interpreters reached consensus and those where discrepancies emerged. We delved into the factors influencing interpretation agreement or divergence, considering fracture attribute variability, image log quality variation, and geology. We define guidelines to interpret fractures in the borehole images of the Lower Cretaceous of the Geneva Basin and ultimately validate the presence of the three discontinuity associations as background fractures in the geothermal reservoir.

How to cite: Bruna, P.-O., Hupkes, J., Doesburg, M., Bertotti, G., Moscariello, A., and Caudroit, J.: Best practices in surface and subsurface natural fracture characterisation to advance carbonate geothermal reservoirs insights: a spotlight on the Geneva Basin, Switzerland. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7499, https://doi.org/10.5194/egusphere-egu24-7499, 2024.

The Upper Rhine Graben (URG) is the central segment of the European Cenozoic Rift System which is known for hosting some of Europe’s major geothermal anomalies. Historically, the region has been explored for its hydrocarbon resources and more recently the area has been targeted for deep geothermal energy. Since then, several heat and/or power plants have been commissioned and are currently in operation. However, despite the gradual development of the sector, the technical potential of the URG remains under-exploited. While the first deep geothermal projects benefited from thermal anomalies known at surface, new projects require costly exploration techniques to ensure a right combination of elevated temperature and sufficient permeability.

Several numerical modelling studies have attempted to reproduce thermal anomalies by integrating a complex three-dimensional geometry of the URG and assuming a topography-induced forced convection largely dominating free convection. As a result, the authors observe a basin-wide graben-perpendicular flow from the graben shoulders towards its center, with an upflow axis approximately below the Rhine River. These conclusions are in contrast to previous geochemical studies which suggest that deep brines discharged from the granitic basement are rather homogeneous on a large scale and have a common origine in deep Triassic sedimentary formations with temperatures close to 225 ± 25 °C. The brines would then migrate through sedimentary layers and permeable fault zones in the basement, from the center of the graben to its western flank, where they would flow up into horst structures such as Soultz or Landau. Moreover, this deep brine circulation in the central part of the graben is thought to be almost completely decoupled both from the circulation of less saline fluids in its upper part, in the Tertiary layers, and from flows along bordering faults, which would be characterized by a rapid recycling of meteoric water via deep circulation loops.

Here, we suggest that thermal anomalies in the French western border of the graben result from deep convective cells developing in the basement along the inclined basement-sediments interface without any help from external pressure forces. Therefore, we used the GeORG public database to build a simplified three-dimensional numerical model of the central part of the URG. Results are obtained using the OpenGeoSys software. Conceptual numerical experiments of thermo-hydraulically coupled simulations were carried out, assuming density-driven convective heat transport with thermal dependence of density and viscosity parameters. The first series of models were constructed without any faults, and we show that an integration of basement morphology, a depth-decreasing basement permeability and a fixed heat flow condition at the base of the model is sufficient to trigger multiple upwellings in the basement within a few million years. Current on-going work is to further calibrate the model to reproduce known existing temperature records and to observe how the integration of permeability heterogeneity or one or more fault zones can reorganize the convective system, thus allowing to trace an effective permeable pattern at larger scale. 

How to cite: Ogandaga Capito, R.-N. and Bruel, D.: Influence of basement morphology on hydrothermal convection in the Upper Rhine Graben , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8838, https://doi.org/10.5194/egusphere-egu24-8838, 2024.

The mechanical stimulation of a fault or fracture in the case of an Enhanced Geothermal Reservoir (EGS) is generally reliant on inducing shear dilation of a targeted discontinuity. However, this same process can lead to the nucleation of a potentially-damaging seismic event. Here, a reservoir stimulation technique known as preconditioning is demonstrated experimentally for the first time. This technique consists of initially increasing the effective normal stress along the fault, in practice corresponding to a period of fluid production. Following this the fault is locally unloaded, corresponding to fluid injection. As the unloading continues, a slipping patch may form, eventually leading to dynamic rupture. However, the previously-induced high effective normal stress further along the fault acts as a fracture energy and reduced-stress-drop barrier, potentially resulting in rupture arrest. Here, a highly-instrumented (strain gauges, accelerometers, acoustic sensors, displacement sensors, load cells) biaxial apparatus is used to demonstrate this procedure, making use of the translucence of polymethylmethacrylate (PMMA) and a high-speed camera to image the development of propagating ruptures. It is demonstrated, as previously predicted from theory, that preconditioning has the ability to halt dynamic ruptures and may therefore be a viable stimulation technique resulting in reduced hazard in EGS stimulation. Specifically, experiments are performed at nominal normal stresses of 60, 90, and 120 bar, with preconditioning (or normal stress increase) corresponding to approximately 8, 16, and 24%; in addition to control cases with no preconditioning. Generally, preconditioning slows rupture propagation at 8% normal stress increase and completely halts it for larger values of preconditioning. It further results in a reduced shear stress drop, increased fracture energy, and reduced slip velocity. These results may one day have further implications for the potential of controlled stress release along natural faults.

How to cite: Fryer, B., Noël, C., Arzu, F., Lebihain, M., and Passelègue, F.: An experimental demonstration of fault preconditioning for reduced seismic hazard, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9960, https://doi.org/10.5194/egusphere-egu24-9960, 2024.

Utilizing existing deep mining systems for geothermal extraction not only facilitates the development of geothermal systems but also helps meeting the cooling requirements for deep mining operations. In this study, a thermo-hydro-mechanical model of geothermal extraction in deep mines is developed to investigate the evolution of mine galleries stability and temperature, and the temperature changes in geothermal production wells. The uncertainty in system responses is predicted through the Bayesian Evidential Learning framework.

Due to our limited understanding of the material properties and the scarcity of measurement data, uncertainties emerge in the forward simulations. Ideally, a comprehensive uncertainty analysis would be conducted to predict all possible outcomes and assess any risks. However, In light of the intractability of performing comprehensive uncertainty analyses in scenarios with vast unknown data, particularly due to the computational overhead of multiple inverse problem-solving, we employ the Bayesian Evidential Learning framework, which provides a feasible and rapid alternative for approximating prediction post-distributions and choosing the most informative data sets. Before implementing BEL, we employed Latin Hypercube Sampling to create 500 sets of realizations for forward simulations, and subsequently utilized global sensitivity analysis to evaluate the data's informational value, aiming to diminish the uncertainty in predictions. In this paper, the BEL framework is utilized to achieve two: firstly, to stochastically predict the responses of the system (stability and temperature) within the BEL framework, using machine learning to discover direct correlations between predictors (sensitive parameters) and targets (system responses). Subsequently, newly collected data can be utilized to predict the approximate posterior distributions of the corresponding gallery stability, temperature, and production well temperature, thus circumventing traditional data inversion steps. This framework can be adjusted to accommodate any predictions related to subsurface conditions; hence, our second goal involves predicting the system's long-term responses within the BEL based on short-term data collection, forecasting posterior distributions from the acquired short-term data, and validating the efficacy of this approach.

Our study indicates that in practical engineering, by (1) obtaining data of material properties and (2) key responses of short-term simulation, it is possible to predict the critical responses of the system in long-term geothermal extraction, thereby maximizing the information content of any measurement data while minimizing budget constraints and computational costs.

How to cite: Zhang, L., Daniilidis, A., Dieudonné, A.-C., and Hermans, T.: Bayesian Evidential Learning Approach to Uncertainty Quantification in THM Model of Geothermal Energy Extraction in Deep Mines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10336, https://doi.org/10.5194/egusphere-egu24-10336, 2024.

Ineris and BRGM published a good practices guide and recommendations for the management of seismicity induced by deep geothermal energy operations. It includes a method to assess the seismic incident hazard, defined as an event whose intensity could cause nuisances for the population, affect the local buildings and infrastructures and which could adversely impact the operating conditions and even the continuation of the project. This method has been developed based on more than 50 case studies consisting of projects representative of different types of geothermal systems where induced seismicity occurred or not. Based on an iterative approach, the method recommends hazard assessment at each key step of a geothermal project to benefit from the additional knowledge it bring. The main key steps identified are the initial assessment before frilling occur, a reevaluation just after drilling and a reevaluation before any potential stimulations. Hazard assessment is based on a decision tree approach, involving specific criteria for each project phase. The seismic incident hazard is rated with a score between 0 and 3. At the lowest level (0), no specific measures to manage induced seismicity are required. For level 1 and 2, monitoring and management methods must be developed. At level 3, the project is considered to be beyond potential induced seismicity management (it’s deviating from the plan) and operations must be suspended pending the outcome of further investigations. This method has been tailored for helping and guiding operators and French administration to consider and manage induced seismicity hazard for every deep geothermal project. 

How to cite: Maury, J., De Santis, F., Peter-Borie, M., Klein, E., Dominique, P., and Contrucci, I.: Seismic hazard related to deep geothermal operations (Part II) : iterative methodology for hazard assessment , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10490, https://doi.org/10.5194/egusphere-egu24-10490, 2024.

Besides the amount of a thermal fluid produced and its chemical composition, temperature is one of the key parameters for the utilization of hydrothermal heat. This applies in particular to geothermal fields with low temperature/low enthalpy, as too low extraction temperatures can mean the failure of a project, while higher temperatures can enable electricity generation or generally better economic efficiency. This concerns the undisturbed (natural) temperature in the reservoir as well as that of the produced fluid at the surface, which depends on the well completion, the undisturbed reservoir temperature, and the depth and contributions of hydraulically active zones. Subsequently, improved forecasts of both the undisturbed temperature and the production temperature with a valid estimate of their uncertainty are required to provide a reliable basis for field development and risk assessment.

In the national projects Geothermal-Alliance Bavaria and KompakT, we studied the temperatures in the North Alpine Foreland Basin in Bavaria, Germany. The carbonate rocks form one of Europe’s most important reservoirs for the use of deep geothermal energy, and projects for district heating and electricity generation have been realized here for more than 30 years.

We developed a good practice workflow for the correction of low-quality bottom hole temperature (BHT) values based on a probabilistic Monte Carlo approach. Using this workflow, we corrected BHTs from over 300 hydrocarbon and geothermal wells and predicted the natural temperature field inside the study area. The resulting temperature model is based on risk scenarios and contains a range of uncertainty, the extent of which depends on the uncertainty of the correction input parameters at the individual locations.

To study the short-term and long-term thermal behavior in the reservoir and the wellbore during production conditions, in 2019, a fiber optic cable was installed below the pump into the reservoir of a geothermal production well. We used distributed temperature sensing (DTS) to observe the hydraulically active zones and to thermally derive their contribution to the available heat amount.

The knowledge gained underlines the importance of flow zone characterization and can be used to improve existing temperature models and estimate what temperatures can really be expected during extraction.

How to cite: Schölderle, F. and Zosseder, K.: The Uncertainty of Temperature Predictions and the Influence of Flow Zones on the Production Temperature in a Low Enthalpy Geothermal Field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10641, https://doi.org/10.5194/egusphere-egu24-10641, 2024.

Geothermal projects relying on superhot rocks (SHR), such as the Japan Beyond-Brittle Project, Iceland Deep Drilling Project, and Newberry Volcano, aim to harness heat from geothermal reservoirs where water reaches a supercritical state (temperature ≥ 400 °C and pressure ≥ 22 MPa). Such projects could multiply the power output of geothermal power plants by a factor of ten, positioning them at the forefront of the energy transition. However, a major challenge hindering the widespread application of SHR is the fact that supercritical water resources are often found in regions of the crust where rocks exhibit ductile behavior, a rheological regime where the formation of large-scale fractures and cracking is hindered. However, these fractures are crucial for enabling water flow, and currently, the evolution of rock permeability and other hydraulic properties in this context remains largely unknown.

This study presents experiments conducted in TARGET, a newly designed gas-based triaxial apparatus located at EPFL, CH. Cylindrical cores of Lanhélin granite (40 x 20 mm) were deformed under an effective confining pressure of 100 MPa, temperatures ranging from 200 to 800 °C, and a strain rate of 10^-6 s^-1. Continuous recording of sample permeability using the pore pressure oscillation method was carried out during deformation. Moreover, post-mortem samples were retrieved and scanned at the ESRF synchrotron facility (Grenoble, FR) and the tomographs were used to reconstruct the 3D crack network. Flow in the sample was then modelled using the Avizo XLab extension and permeability was computed in the x y and z direction.

We report that Lanhélin granite transitions from being in the localized regime with the formation of a sample scale fracture to becoming ductile between 600 and 800°C. In the brittle, localized regime, sample permeability remains relatively constant throughout deformation. In the ductile regime, sample strength is halved, and beyond the initial decrease upon loading, permeability increases monotonically by more than an order of magnitude. These results suggest that sample bulk controls the sample permeability in our experiments. In localizing samples, fractures do not connect the ends of the rock core but concentrate all strain after nucleation, limiting permeability improvement through micro-cracking in the bulk. In the ductile regime, where no localization occurs, the bulk permeability of the rock continuously improves with strain. Flow modeling in post-mortem samples yielded permeability values up to seven orders of magnitude greater than in-situ measurements. This substantial difference is attributed to the effect of confining pressure on the crack network aperture. Despite this absolute difference, our modeling results confirm that flow in nominally ductile samples is controlled by bulk cracking rather than macroscopic fractures. Our study demonstrates that low-porosity rocks in the ductile regime can be more permeable than often anticipated. These results hold significant implications for the engineering of SHR reservoirs, showcasing the potential for permeability enhancement in ductile rocks.

How to cite: Meyer, G., Shahin, G., Cordonnier, B., and Violay, M.: Permeability of experimentally deformed ductile granite derived from in-situ measurements and post-mortem X-ray tomography: perspectives for superhot rock reservoirs., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10980, https://doi.org/10.5194/egusphere-egu24-10980, 2024.

Deep geothermal projects can trigger seismic events depending on geological context and operations. This seismicity is generally of low magnitude but, in some cases, larger events may occur, which could lead to geothermal project abandonment and could present risk to neighboring populations. Thus, development of deep geothermal projects requires the management of induced seismicity to control it and to avoid any surface disturbance. It is from this perspective that, in 2023, Ineris and BRGM published a guide of good practices and recommendations for operators and French administration involved in deep geothermal energy.

The guide provides recommendations for assessing geothermal-induced seismic hazard, depending on the type of geothermal system and its intrinsic and operational characteristics, at each key step of a project (i.e. from the exploration phase until the end of the project). A worldwide review of deep geothermal projects, carried out with the aim of identifying key factors triggering induced seismicity, has enabled the definition of the most relevant criteria to take into account in the hazard assessment. In this review, geothermal projects were chosen to be representative of different types of geothermal systems (e.g. deep sedimentary aquifers, volcanic and plutonic regions, deep dry crystalline basements, etc.) and operating conditions (e.g. well configuration, type of operation, etc.). Moreover, the review includes projects associated with several episodes of induced seismicity, ranging in magnitude from microseismicity (M < 2) to large seismic events (M > 5), as well as projects marked by the absence of induced seismic activity.

From the 53 projects and 77 seismic episodes analyzed in this review, we can state that not all geothermal projects are equally prone to seismic events. The occurrence and the intensity of induced seismicity are the results of interactions between several natural (intrinsic) and anthropogenic (operational) factors, often concomitant and dependent on each other. Seismic response of analyzed projects appears to be largely different depending on the type of geothermal system. Indeed, the type of geothermal system characterizes reservoir porosity, as well as heat transfer and fluid circulation modes in the reservoir. Other key factors include the presence of faults that can be critically loaded and/or connected with the basement, the use of EGS technologies, and situations where injected and produced volumes are highly unbalanced. These results allowed defining key criteria for seismic hazard assessment methodology proposed within the good practice guide.

How to cite: De Santis, F., Maury, J., Klein, E., Peter-Borie, M., Contrucci, I., and Dominique, P.: Seismic hazard related to deep geothermal operations (Part I): identification of key criteria for hazard assessment , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11082, https://doi.org/10.5194/egusphere-egu24-11082, 2024.

Meteoric water may or may not infiltrate deeply into high-relief mountain ranges. Along its subsurface circulation path, the water heats up according to the background geothermal gradient and eventually emerges at lower elevation as thermal springs. Whether such topographically-driven circulation establishes or not depends on the host rock’s permeability and/or the hydraulic head. In terms of permeability, fault zones play an important role as they can provide preferential flow paths for fluids. This is particularly the case of active fault zones along which recurring slip counteracts clogging caused by mineral precipitation often found along non-active structures. Thus, the investigation of 4D fault and fracture geometries and their kinematics is a means to understand the locations and dynamics of geothermal systems in orogenic belts. Here, we present preliminary results from the ongoing GeoTex research project, which aims at better defining the geothermal potential of the Rhône Valley, an area of rugged topography in SW Switzerland. The Rhône Valley represents a geothermally active zone within the Alpine orogen, which is characterised by numerous thermal springs, regional-scale faults and enhanced seismic activity. It is therefore a promising setting to explore further for exploitation.

Based on structural data from fieldwork and quantitative remote sensing, we characterise fault geometries (i.e., spatial orientation, relationship of intersecting fault families as well as kinematics) in the vicinity of known thermal springs. Observable paleo-fluid pathways marked by veins and rock alteration are being considered as analogues for recent thermal water circulation. These circulation paths are linked to major Alpine structures in the underlying basement units, such as large-scale strike-slip faults or the axial planes of uplifting basement domes. Our results suggest spatial correlations between the locations of hydrothermal springs and the 3D structure of the host massifs. Specifically, basement–cover contacts exert geometric and lithologic control at some sites, whereas locally dilatant domains along strike-slip faults as well as intersections of fault families focus outflow at other sites. Through the above approach in combination with seismological data, we have derived conceptual models for fluid flow, which may help to predict the locations of blind active geothermal systems elsewhere in the Rhône Valley.

How to cite: Schmid, T. C., Herwegh, M., Berger, A., Truttmann, S., Diamond, L. W., Wanner, C., Van den Heuvel, D. B., Madritsch, H., and Diehl, T.: Structural characterization of hydrothermal fluid pathways in orogenic belts: Insights from the GeoTex project, Rhône Valley, Switzerland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11792, https://doi.org/10.5194/egusphere-egu24-11792, 2024.

The carbonate Upper Jurassic aquifer (UJA) in the South German Molasse Basin (SGMB) is the most important exploration horizon for geothermal energy supply in Bavaria. The UJA shows a complex hydrogeology caused by a heterogeneous geology with karstic features and deep fault zones.

The great interest in the Upper Jurassic aquifer for geothermal energy supply led to an increasing construction of geothermal power plants in the greater Munich area. Today, there are 18 geothermal power plants in this area used for district heating and electricity generation.

To guarantee a long and sustainable use of the geothermal resource, understanding the dynamics within the reservoir is important. Tracer tests are a key tool for investigating groundwater flow paths, detecting potential thermal breakthroughs, and minimizing negative interactions between geothermal power plants.

In recent years, several tracer tests have been conducted, and the growing number of projects will lead to even more tracer testing in the coming years. Future tracer tests need to be carefully designed, as there is, up to now, only a limited number of traditional tracer substances available for use in a deep geothermal setting under high temperatures and pressures. Therefore, we developed tracer management for the UJA, including guidelines for different tracer tests, the suitability of different tracers for usage in a geothermal setting, and recommendations for individual locations.

How to cite: Winter, T. and Zosseder, K.: Developing tracer management for long and sustainable use of the Upper Jurassic geothermal reservoir in South Germany, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17350, https://doi.org/10.5194/egusphere-egu24-17350, 2024.

Natural or artificial fluid flow in deep fractured reservoirs, such as Enhanced Geothermal Systems (EGS), is primarily controlled by open fractures and faults, and is considered a key element for hydraulic performance. Flow along these fractures is strongly affected by channeling between fracture asperities and by deposits sealing the open fracture space due to mineral precipitation. Fracture asperities and fracture sealing also impact the mechanical behavior of fractures, especially their mechanical stiffness. Here, we study both the permeability and the stiffness of a rough fracture at the field scale during its closure.We base our approach on a well established self-affine geometrical model for fracture roughness. We develop a finite element model based on the MOOSE/GOLEM framework and conduct numerical flow experiments in a 256 × 256 × 256 m^3 granite reservoir hosting a single, partially sealed fracture under variable normal loading conditions. Navier-Stokes flow is solved in the embedded 3-dimensional rough aperture, and Darcy flow is solved in the surrounding poroelastic matrix. We study the evolution of the mechanical stiffness and fluid permeability of the fracture-rock system during fracture closure by considering the asperity yield and the depositing of fracture-filling material in the open space of the rough fracture. The evolution of the fault volume, fracture normal stiffness and permeability are monitored until fluid percolation thresholds are exceeded in two orthogonal directions of the imposed pressure gradient. Finally, we propose a physically based law for the stiffness and permeability evolution as a function of the fault volume. It is demonstrated that during closure, stiffness increases exponentially as the fault volume decreases. A strong anisotropy of the fracture permeability is also evidenced when reaching percolation thresholds.

How to cite: Schmittbuhl, J., Deng, Q., Cacace, M., and Blöcher, G.: Mechanical stiffness and permeability of a reservoir-scale rough fracture during closure, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17666, https://doi.org/10.5194/egusphere-egu24-17666, 2024.

During the Vendenheim deep geothermal project (Strasbourg Eurometropole, France), large induced seismic events led to the arrest of the project. Two important features of the induced seismicity were unexpected : the large distance to the wells of a cluster of seismic events (4-5km) and the occurrence of the largest event Mlv3.9 at the bottom of the wells, six months after shut-in. To better understand the mechanisms of seismicity, we develop within the framework of the DT-GEO project (Horizon Europe) a large-scale model (8kmx8kmx8km) of the area. We aim at performing in-silico experimentation to reproduce the geophysical responses of the geothermal reservoir with different geological geometries, different geomechanical properties and constrained with a variety of crustal stress conditions and variety of the external forcing representing the anthropogenic control. The model is based on the MOOSE/GOLEM framework (finite element approach) and integrate the public regional geological model GEORG that includes major lithologies and large-scale faults of the area. We will present the preliminary of coarse-grained simulations of the natural fluid circulation and fluid injections.

How to cite: Abreu Torres, J., Hutka, G., Cacace, M., Blöcher, G., Magnenet, V., and Schmittbuhl, J.: Large-scale reservoir modeling of the Vendenheim geothermal site (France), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18451, https://doi.org/10.5194/egusphere-egu24-18451, 2024.

Heat and cold storage in the subsurface as well as geothermal energy provision intrinsically involve cyclic pumping operations, often in fields of several boreholes. We investigated the pressure and flow-rate fields resulting from the simultaneous periodic operation of two boreholes. The interference in pressure experienced by further (monitoring) boreholes can be assessed by an analytical solution when assuming radial flow from and to the pumping wells. This solution is derived through superposition, relying on the known solution for periodic pumping in a single well. The pressure gradient field, indicative of flow direction, is distorted from radial form, implying dominant flow and consequently heat advection between the two boreholes. We compared the analytical results to observations from field tests conducted in four boreholes located close to the northwestern banks of an artificial freshwater reservoir, the Kemnader See, at the southern city-limits of Bochum, Germany. In the light of the derived solutions, the field observations allow us to assess the role of inhomogeneity and fracture flow for the flow focusing between the pumping wells. Solving and investigating the hydraulic problem constitutes the necessary first step towards devising schemes for the optimization of cold and heat storage or geothermal energy provision by varying the period and phase of pumping coeval operations in several boreholes.

How to cite: Mumand, F. and Renner, J.: Flow focusing associated with doublet operation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18565, https://doi.org/10.5194/egusphere-egu24-18565, 2024.

The technology envisioned in the DeepU project (Deep U-tube heat exchanger) is expected to revolutionize the geothermal energy sector, increasing the accessibility of deep geothermal resources for low-carbon heating and power generation. The ultimate project goal is to create a deep (>4 km) closed-loop connection in the shape of a U-tube exchanger by developing a fast and effective laser drilling technology. The project comprises the development of a novel drilling technique and its application via geothermal modeling at selected sites. A prototype of a drill-head has been realized, combining the laser system with drill strings, sustaining the coupled action of laser and cryogenic gas. The fine particles of drilled rocks are ejected to the surface in the gas stream via the borehole annulus. This contribution focuses on the project’s activities related to the laser-rock interactions studied in the experimental laser drilling tests based on previous works (Seo et al., 2022; Li et al., 2022a, 2022b). Three types of lithologies were selected for initial laboratory tests: granite, sandstone, and limestone (50 x 35 x 15 cm). Constant rates of penetration (ROP) upwards of 20 m/h have been achieved in all lithologies with borehole diameter reaching 18 cm. The petro-thermo-mechanical phenomena occurring during laser drilling, such as spallation, melting, and evaporation, were recognized and described. The drilling process was investigated by thermocamera imaging providing information about the most effective process induced by heating the rocks, up to 700°C. The laser working parameters and experimental setup were optimized regarding observed phenomena. In the next step, sections of boreholes were cut out and examined. The microscopic observations on the thermal unaffected and affected rocks’ thin sections have been performed with the use of polarized optical microscopy and scanning electron microscopy revealing micro-fracturing patterns of the rock induced on rock samples by the heating processes. The change of physic-mechanical properties of rocks was investigated and acknowledged in geothermal models. This innovative and comprehensive study revealed macro- and micro-scale phenomena occurring during laser drilling, contributing to the successful development of this new drilling method and subsequently its application for exploitation of geothermal energy from depths below 4 km.

This research is funded by the European Union (G.A. 101046937). However, the views and opinions expressed are those of the author(s) only and do not necessarily reflect those of the European Union or EISMEA. Neither the European Union nor the granting authority can be held responsible for them.

References

Li, G., Shi, D., Hu, S., Ma, C., He, D., and Yao, K., 2022a, Research on the mechanism of laser drilling alumina ceramics in shallow water: The International Journal of Advanced Manufacturing Technology, v. 118, p. 3631–3639, doi:10.1007/s00170-021-08190-0.

Li, Q., Zhai, Y., Huang, Z., Chen, K., Zhang, W., and Liang, Y., 2022b, Research on crack cracking mechanism and damage evaluation method of granite under laser action: Optics Communications, v. 506, p. 127556, doi:10.1016/j.optcom.2021.127556.

Seo, Y., Lee, D., and Pyo, S., 2022, The interaction of high-power fiber laser irradiation with intrusive rocks: Scientific Reports, v. 12, p. 680, doi:10.1038/s41598-021-04575-z.

How to cite: Slupski, P., Zampieri, E., Di Sipio, E., Manzella, A., Pasquali, R., Pockele, L., Romanowski, A., Sassi, R., Steinmeier, O., and Galgaro, A.: Deep U-tube heat exchanger breakthrough: combining laser and cryogenics gas for geothermal energy exploitation – a perspective of laser-rock interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18812, https://doi.org/10.5194/egusphere-egu24-18812, 2024.

A geothermal doublet has been installed in a sedimentary reservoir for direct-use heating on the TU Delft campus, targeted to supply around 25 MW of thermal energy at peak conditions. This contribution presents the implementation and initial data collection from the doublet, including an initial evaluation of the logging and coring campaign. Nearly half of Netherlands natural gas consumption is allocated to heating, and the on-campus CO₂ emissions from heating exceed 50%. The doublet has been designed with two primary aims of research and commercial heat supply, with the wells being completed in December 2023. The project will be operated by a commercial entity, and built into a larger thermal energy system including a high temperature underground storage system, with the first energy production planned in 2025. The research questions relate to field-scale geothermal operations, e.g. how reliable is the long-term energy production?, how do materials perform in the long-term? and how can geothermal projects be best monitored? The research programme involves the installation of a wide range of instruments alongside an extensive logging and coring program and monitoring network. The doublet has been cored, with substantial continuous samples from the heterogenous reservoir, alongside a large suite of open hole well logs in the reservoir and through casing logs in overlying geological units. A fiber-optic cable will monitor distributed pressure throughout the producer reservoir section, at approximately 2300m depth, which will be installed during commissioning. A local seismic monitoring network has been installed in the surrounding area with the aim of monitoring very low-magnitude natural or induced seismicity. The project is a key national research infrastructure and is being incorporated into the European EPOS (European Plate Observing System, https://www.epos-eu.org/), such that accessibility and data availability will be as wide as possible. All observations will be included in a digital-twin framework that will allow to make better decisions in future geothermal projects.

How to cite: Abels, H., Barnhoorn, A., Daniilidis, A., Bruhn, D., Drijkoningen, G., Elliott, K., van Esser, B., Laumann, S., Van Paassen, P., Vargas Meleza, L., Vondrak, A., Voskov, D., and Vardon, P.: A newly installed research infrastructure for geothermal energy in a subsurface sedimentary reservoir for direct-use heating: the TU Delft campus geothermal project, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20414, https://doi.org/10.5194/egusphere-egu24-20414, 2024.

The Balmatt geothermal doublet, developed and managed by VITO (Flemisch Institute of Technological Research), targets the fractured Lower Carboniferous Limestone reservoir in the Campine Basin at the depth of 3000 m to 4000 m. The development of the project started in 2015 and the operation began in 2018. The geothermal plant consists of two active wells, one injection well and one production well. The geothermal production had to be suspended after the occurrence of a stronger M_L 2.2 event on the 23^rd of June 2019 which triggered a red alert status on the local traffic light system (TLS). Production was then resumed in April 2021, following an extension of the seismic monitoring network and an update of the TLS. Activities were suspended again in November 2022 after another strong M_L 2.1 event was induced. Thanks to the network extension, current investigations aim at understanding in detail the main structural features (active faults) and hydromechanical processes involved in the generation of such larger events which will contribute to improving seismic forecast possibilities for future monitoring operations. Here we present insights into ongoing data processing to create a high resolution unbiased (complete) seismic catalog providing the basis for future interpretation of the spatio-temporal and energetic behavior of seismicity towards different production settings. Our current work focuses in particular on the development of an automatic detection routine based on continuous data of the deep borehole sensor (installed at the depth of 2052 m) by combining a machine learning based automatic events detection algorithm and template matching method. The events detection in the continuous data is complicated by the periodic malfunctioning of the sensor and the presence of aseismic noise which leads to the large number of false events detection. To address this issue and to minimize the number of false detections, we employ frequency and amplitude analysis of the seismic data. Secondly we analyze source attributes of the detected events which involve source mechanism inversion and source parameter determination as well as clustering analysis and constraining source location for noisy small magnitude events. Further more the comparison between the production data (injection pressure, temperature, volume etc.) with the results from the seismic analysis will provide us with better constrain on the hydromechanical characteristics of the reservoir and the relation between the geothermal operations and seismicity at Balmatt geothermal site. 

How to cite: Gautam, R., Kinscher, J. L., Schmittbuhl, J., Broothaers, M., and Laenen, B.: Generation of High Resolution Seismic Catalog Associated With the Production Phase 2021 - 2022 at the Balmatt Geothermal Site, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20667, https://doi.org/10.5194/egusphere-egu24-20667, 2024.

This study explores the power generation potential of enhanced geothermal systems (EGS) at depths of >15 km, where continental crust typically exhibits ductile behavior at temperatures above 400 °C. We employed a numerical model to evaluate the response of such deep crustal rock to fluid injection-induced pressurization and cooling. Our simulations indicate that circulating 80 kg/s of water through rock initially at 425 °C could yield ~100-120 MWth (approximately 20 MWe) for two decades. Even after a century of fluid circulation, fluid temperatures at the production wells exceed 250 °C and thermal energy output exceeds 40 MWth. However, achieving effective permeability in the stimulated volume is crucial to developing an exploitable resource; our model suggests that bulk permeability values between ~10^-15 and 10^-14 m² in a rock volume of 0.1 km³ are optimal. This range balances the need to avoid excessive injection pressures and the risk of rapid thermal depletion. As the reservoir cools, the transition from ductile to brittle behavior in rock is assumed, reducing fluid pressures but increasing the risk of fluid pathway short-circuiting, a common challenge in EGS operations. Our theoretical investigation underscores the importance of geological (e.g., rock temperature and permeability) and operational (e.g., injection rate) factors in harnessing the energy potential of the ductile crust. However, practical implementation hinges on revolutionary advancements in deep drilling technology and a better understanding of rock behavior under high temperature and pressure conditions.

How to cite: Scott, S., Yapparova, A., Weis, P., and Houde, M.: The power generation potential of enhanced geothermal systems in ductile crust at >15 km depth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20785, https://doi.org/10.5194/egusphere-egu24-20785, 2024.

The injection of geothermal water into subsurface rock formations often induces a cascade of seismic events. However, a comprehensive understanding of the resulting temporal and spatial seismic density evolution remains elusive. In this study, we meticulously analyze both spatial and temporal earthquake probability density distributions. Leveraging data from multiple injection sequences obtained from the EPOS TCS-AH through the EPISODES Platform, our objective is to elucidate the spatiotemporal evolution of seismic activity across distinct phases of water injection. We extend our focus to quantify seismicity during the post-injection phase and assess whether the largest magnitude event in each sequence aligns with the derived distribution. This time, our primary emphasis is on conducting the above-mentioned analysis on the 09/1993 Soultz-sous-Fôret sequence. Our research is supported by Horizon Europe under grant agreement No. 101058129 as part of the DT-Geo Project.

Our findings reveal a distinctive characteristic of seismic spatial density, marked by a sudden decay at extended distances. Remarkably, there is no significant divergence in spatial density decay observed before and after the cessation of injection. Furthermore, we observe that the occurrence of the maximum magnitude event coincides with the peak of the probability spatial density. Shifting to temporal density, we identify a close correlation with the increase in injection volume, displaying a skewed normal distribution. Notably, the maximum magnitude event aligns with the peak of the probability temporal density.

In essence, our research substantively contributes to a quantitative comprehension of the dynamic features governing the temporal and spatial evolution of seismic density during intensified water injection scenarios.

How to cite: Wang, Z., Lengliné, O., and Schmittbuhl, J.: Quantifying the 4D Seismic Density Evolution Caused by Geothermal Injection, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21025, https://doi.org/10.5194/egusphere-egu24-21025, 2024.

The storage of surplus power generated by wind turbines or solar panels under favourable weather conditions is of significant importance for the successful transition of current energy systems. Surplus power can be used for hydrogen production or for charging batteries. However, there is also an option to store surplus power as mechanical energy due to the injection of water into the deep underground.

Basic investigations for the storage of mechanical energy were performed at the geothermal research well “Horstberg” in Germany. Here a large and highly conductive artificial fracture was created in the Buntsandstone formation at approximately 3800 m depth. For the hydraulic stimulation 20.000 m³ of fresh water were injected at a pressure of about 300 bar. In succeeding production tests, the water was produced back at a pressure level of about 200 bar and a significant portion of the energy used for injection would have been retrievable. Further injection and production tests, originally designed for cyclic heat extraction, showed that approximately half of the electric energy necessary for injection could have been recovered while producing. Obviously, a large portion of the hydraulic pump energy is stored in the underground as mechanical energy due to ballooning of the fracture and due to elastic compression of water and rock surrounding the fracture.

The efficiency of energy storage can be improved significantly by implementing a horizontal well design with multiple fractures. This is shown based on model calculations. If water is injected in parallel artificial fractures the static pressure level between the fractures increases, water losses into the far field decrease and the back-production is improved. Furthermore, overpressure reservoirs and low permeable rock are favourable. Thereby the injected water remains in the closed surrounding of the fractures and the complete artesian back production at high pressure is ensured. Overpressure formations seem to be widespread in the deep underground of sedimentary basins as in the North German Basin.

Mechanical energy storage in the deep underground should be combined with geothermal heat extraction. At the test site Horstberg thermal water at a temperature of more than 100°C was produced in cyclic tests. Numeric modelling results suggest that a thermal power of appr. 1 MW can be extracted by cyclic production in the long term via the large fracture in Horstberg.

For the realisation of this storage concept several challenges have to be met. Besides the creation of good underground conditions, the handling of the produced saline water and its reinjection without scaling or corrosion are serious issues. On the other hand the storage of surplus power as mechanical energy in the underground and its retransformation to power can be more efficient than the conversion into hydrogen and less expensive than battery storage. The reuse of abundant deep wells for energy storage could be a cost-efficient starting point for this concept.

How to cite: Tischner, T. and Jung, R.: Storage of mechanical energy and heat extraction via artificial fractures in low permeable rock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21854, https://doi.org/10.5194/egusphere-egu24-21854, 2024.

Context

As part of the Deep-HEAT-Flows project (https://deep-heat-flows.voog.com), we have collected a comprehensive geological and petrophysical dataset of crystalline reservoirs formed within fault zones and at the contact of igneous intrusions across Finland, evaluating their potential as deep geothermal reservoirs. Our investigations involve a range of laboratory-based experiments encompassing measurements of rock density, elastic wave velocity, electric resistivity, porosity, and permeability under various confining pressures, and the thermal properties of 120+ samples collected from diverse crystalline rocks. Additionally, we apply mineral and pore space caracterization techniques including petrography, micro-XRF spectrometry, SEM-EDS, hyperspectral imaging, and CT scans to understand the processes that control crystalline reservoir formation.

Findings

Our findings highlight a common trend among various petrophysiscal parameters: rock density, resistivity, elastic wave velocity, thermal conductivity, and heat capacity typically reduce as the porosity increases, a characteristic observed across many sedimentary and volcanic rocks. Reservoir quality is primarily determined by the morphology of the pore network, encompassing fractures and interconnected moldic, sieve, and interparticle pores. The most promising reservoir properties were observed in rocks intersected by regional shear zones and therefore affected by intense brecciation, cataclasis, and hydrothermal alteration, leading to a notable porosity of ~20% and permeability in the order of 10⁻¹² m² (1 darcy). Moreover, the contact margin of rapakivi intrusions also include fractured and hydrothermally altered rocks that have significantly high porosity and permeability. In detail, rocks dominated by fractures typically have little porosity (<4%) and exhibit extremely high permeability (~10⁻¹² m²) only at low confining pressures, which sharply decreases to ~10⁻¹⁹ m² as the confining pressure surpasses 20–30 MPa (corresponding to depths around 700–1000 m). From our dataset, only fractures linked to mineral dissolution have the potential to sustain permeability above 10⁻¹⁶ m² at 50 MPa confining pressure (simulating depths of ~2 km). Conversely, rocks that underwent cataclasis and hydrothermal alteration exhibit comparatively milder permeability reductions, maintaining high values even when subjected to high confining pressures of 50 MPa. Throughout the entire dataset, a consistent observation emerges: mafic minerals are commonly substituted by chlorite and epidote, suggesting hydrothermal alteration processes occurring at relatively high temperatures (200–300 °C).

Implications for geothermal exploration

Exploring deep geothermal resources in crystalline settings offers a promising solution for direct space heating, industrial applications, and electricity generation. However, the typically low porosity and low permeability of crystalline rocks remain a key obstacle in deep geothermal exploration. The identification of hydrothermally altered rocks as potential deep geothermal reservoirs could mark a substantial shift in geothermal exploration within crystalline regions, broadening target prospects beyond the conventional focus on volcanic and rifting areas. Brecciation, cataclasis, fracturing, and mineral dissolution collectively contribute to the creation of exceptional reservoir properties, which have been widely overlooked in deep and ancient (over a billion years) crystalline settings. Our results hold paramount importance for identifying highly productive permeable zones within crystalline settings and also to the advancement of Enhanced Geothermal Systems that could prioritize the creation of more intricate fracture networks through thermal and chemical stimulation.

How to cite: Heap, M., Bischoff, A., Luoto, T., Reuschlé, T., Vuoriainen, S., Spitz, M., and Leon-Stackow, M.: A comprehensive petrophysical databank of crystalline reservoirs for assessing deep geothermal exploration targets in Finland and abroad, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4423, https://doi.org/10.5194/egusphere-egu24-4423, 2024.

Southern Finland crystalline basement was formed and modified during the 1.9–1.8 Ga Svecofennian orogeny, which constitutes a portion of the Fennoscandian Shield. The bedrock comprises supracrustal and early to late-orogenic igneous rocks of mafic to felsic compositions and is characterized by an overall high metamorphic grade associated with high-T and low-P conditions. The bedrock was subjected to multiple tectonic events of distributed deformation, first during a compressional stage, then followed by an extensional stage and finally  a transpressional stage.

Hence, the Kopparnäs study site in southern Finland has been subjected to several stages of ductile and brittle deformation. The site has been studied by the Geological Survey of Finland for several years, with special research emphasize on a subvertical E–W orientated multi-core fault zone that intersects granites, amphibolites, and migmatites. A drillhole cuts through this fault zone at 100 m depth. This drillhole has beed studied using downhole instrumentation, such as optical and acoustic imaging and diverse geophysical surveys (fullwave sonic, magnetic susceptibility, gamma density, natural gamma radiation, drillhole caliper and various electrical loggings). In addition, a comprehensive study of the drill core enables detailed geological and petrophysical characterization of the fault architecture, including recognition of fractures, alteration zones, and mineralization across the fault and its host rocks. These studies together with fluid flow measurements with a packer system, enable us to define subsurface properties for this fault zone.

The initial results suggest that faulting strongly impacts the petrophysical characteristics of the rock, typically increasing porosity and reducing bulk density. This change is most likely related to the fracturing at the site being often associated with mineral alteration and dissolution. These events altered and deformed the multiple fault cores in distinctively manner, affecting the subsurface fluid flow which can be observed in fluid chemical composition differences.

These studies are part of the FLOP project (FLOw Pathways within faults and associated fracture systems in crystalline bedrock) and the Deep-HEAT geothermal energy project. 

How to cite: Engström, J., Bischoff, A., Kortunov, E., Markovaara-Koivisto, M., Ovaskainen, N., Nordbäck, N., and Paananen, M.: Structural, petrophysical, and geophysical characterization of a fault zone in southern Finland – application for subsurface fluid flow in granitic settings, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6192, https://doi.org/10.5194/egusphere-egu24-6192, 2024.

    This study identifies the key geothermal features in the metamorphic terrane within the Yuli belt (a metamorphic mélange) in Ruisui, eastern Taiwan, including fault-related pathways for hot fluids, cap rocks, and naturally fractured reservoirs, and builds a structural permeability distribution model. A preliminary geothermal reservoir model is created for a depth of 3 km, by integrating geological field analysis, magnetotelluric (MT) surveys, hot spring geochemistry, and data from 3 exploration wells. The low resistivity zones shown in MT results are beneath a unit of amphibole-albite schist, containing numerous ultra-mafic blocks at different sizes. We tentatively interpret the four MT low-resistivity zones as four possible reservoirs, which are associated with high-density jointed and faulted quartz-mica schists, underneath cap rocks of amphibole-albite schists, which reveal poor development of joints. The geochemical analysis of hot spring water indicates a high concentration of sodium ions, potentially originating from the amphibole-albite schist. We also observe surface exposures where water up to 50-60^oC flows up through NW-SE trending sub-vertical faults with the downhole temperature up to 200^oC at a depth of 0.9km.

    To determine whether the NW-SE vertical fault zones are suitable for open structures, which appear to be the major geothermal up-flows, we measure the orientation of faults and 3 sets of joints, most of which are sub-vertical in the field. The principal stress orientations are adopted as σ1 vertical, σ2 N120^oE, and σ3 N30^oE, by combining GPS observation, focal mechanisms of shallow earthquakes, and fault slickenlines measured in exposures. We conduct fault-slip inversion and obtain the stress ratio phi=0.51. Utilizing the Mohr-Coulomb failure criterion coupled with selected parameters, such as in-situ principal stresses, fluid pressures, and rock mechanical properties, our model indicates that steeply-dipping NW-SE trending (N120^o-130E^o) joints and faults are mechanically prone to open as fluid infiltrating or injecting during thermal events.

    We furthermore measured the fracture length and density at the aforementioned hot spring exposure where 50-60^oC hot fluid flows out from an NW-SE trending sub-vertical fault. The measuring result shows that the density of the fractures (or joints) decreases away from the fault core, thus we anticipate the joints tend to form on the microfractures created by the fault, which explains an increase of the permeability toward the fault. As for estimating the structural permeability along the NW-SE fault and open joints, the fracture aperture is calculated using linear elastic fracture mechanics, and then the permeability is estimated by using the cubic law for fluid flow in rock fractures. By doing so we obtain the structural permeability in the NW-SE fault zone, which exponentially decreases away from the fault core, and the permeability value ranges from 10^-10 to 10^-13 m² at a distance of 10 m.

How to cite: Huang, Y.-C., Lee, J.-C., Ho, G.-R., Chiang, C.-W., Lu, Y.-C., Song, S.-R., Yang, C.-H., Chen, C.-H., and Chen, Y.-G. C.: Reconstructing the geothermal reservoir model and estimating the structural permeability variation in the metamorphic terrane in Ruisui, Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7157, https://doi.org/10.5194/egusphere-egu24-7157, 2024.

Natural faults when subjected to stimulation by fluid injection may result in slip acceleration because pore pressure (Pf) increases in the rock volumes inside and surrounding the fault zone leading to reduction of effective normal stress (σ_n’). Slip mode ranges from aseismic creep to seismic ruptures defining a spectrum of fault-slip behavior. Fault stimulation experiments will be conducted in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Switzerland) to understand fault reactivation processes on a target well-identified fault zone, fully instrumented to monitor deformation and seismicity during both fluid injection and fault reactivation. This is envisioned in the ERC-Synergy FEAR (Fault Activation and Earthquake Rupture) project. In BULGG, the target fault zone has both a sub-centimetric fault core containing fault gouge and granite asperities in contact and other fractures in the surrounding rock volume.  Therefore, it becomes important to define the frictional properties and slip mode of both gouges and bare rock surfaces taking advantage of a laboratory controlled experimental environment.

Fault stimulation by increasing Pf was simulated in laboratory following an injection protocol suitable for the BULGG fluid stimulation. Experiments were performed on the target fault gouge and on bare rock surfaces made of nearby Rotondo Granite. We employed a rotary shear apparatus (SHIVA) allowing the fluid injection under a controlled shear stress. First, we imposed the stresses measured at depth in the underground laboratory, halved due to apparatus limitations: 7.5 MPa σ_n’, 7.5 MPa confining pressure and 2.5 MPa Pf. Second, we imposed a slip rate of 10^-5 m/s for 0.01 m to have a reference texture. Third, we applied a shear stress so that an equivalent slip tendency of 0.35 (equal to the one measured in the target fault) is achieved (ca. 2.7 MPa) keeping it constant. We then increased stepwise the pore fluid pressure by 0.1 MPa every 150 s. After fault slip initiation, the maximum allowed slip velocity was 0.1 m/s. Between each of the experimental stages, permeability and transmissivity were measured with the gradient or Pf oscillations methods.

We show that reactivation occurs at lower Pf in bare rock surfaces (4.7 MPa) with respect to MC fault gouge (6.5 MPa), suggesting that the effective coefficient of friction, the ratio of shear stress and σ_n’, is larger in gouge (0.58) than in bare rock surfaces (0.49). Moreover, upon the application of last Pf step, reactivation is slower in fault gouge (150 s delay) with respect to bare rock surfaces (10 s delay), consistently with the lower hydraulic transmissivity measured for target fault gouge with respect to bare rock surfaces (i.e., 10^-19 vs 10^-17 m³). Our experiments also show that creep and dilatancy precede reactivation in fault gouge, whereas reactivation is sudden and not preceded by dilatancy in bare rock surfaces.

We suggest that well-oriented and smooth bare rock surfaces might be easily reactivated similarly to what observed for fault gouge during fluid stimulation. Our data and observations will contribute to shed light on the mechanics of faults and induced earthquakes by fluid stimulation experiments.

How to cite: Aretusini, S., Cornelio, C., Volpe, G., Pozzi, G., Spagnuolo, E., and Cocco, M.: Fault core structure affects fault slip during fluid injection: insights from laboratory friction experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8226, https://doi.org/10.5194/egusphere-egu24-8226, 2024.

Geothermal energy is considered a sustainable energy source for the transition to a carbon-neutral economy. In Central Europe, sufficiently hot source rocks are buried deep underground and comprise tight crystalline basement formations. To extract their thermal energy, hydraulic stimulation is used to create efficient heat exchangers in the context of Enhanced Geothermal Systems (EGS). Successful geothermal reservoir initiation requires a broad understanding of the hydro-mechanical coupling in fractured rock masses. For this reason, a decimeter-scale true-triaxial setup has been developed to conduct injection-driven shear tests under various stress conditions.

To gain a deeper insight into the hydro-mechanical processes involved in hydraulic stimulation, a true triaxial compressive apparatus at the decimeter scale is employed. The experimental setup consists of 30 x 30 x 45 cm cuboidal granite specimens, each containing an oblique saw-cut laboratory fracture with different surface properties. The fracture is crossed by two boreholes equipped with packers to isolate a fracture interval. Fluid injection into the isolated intervals follows the typical HTPF (hydraulic testing of pre-existing fractures) scheme, including stepwise pressure increases and decreases. Stress boundary conditions are introduced by three sets of oil-filled flatjacks, contained within a steel frame which allows a more realistic and accurate replication of the stress conditions experienced by geological formations during hydraulic stimulation experiments. Stresses for individual tests are manipulated from hydrostatic to strike-slip conditions to test for different end member states of slip tendency. Fracture and rock deformations are recorded by 16 linear variable differential transformer (LVDT) sensors mounted externally along the edges of the specimen, volume changes in the flatjacks and a newly developed borehole deformation probe (mini-SIMFIP).

How to cite: Osten, J., Jalali, M., Cadmus, A., Welsing, L., Schaber, T., Cook, P., Guglielmi, Y., Fuentes, R., and Amann, F.: Hydraulic Stimulation Experiments in a Decimeter-scale True Triaxial Compressive Apparatus, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8619, https://doi.org/10.5194/egusphere-egu24-8619, 2024.

We investigated the usefulness of the fractal diffusion equation, also known as generalized radial flow (GRF) equation, to characterize hydraulic properties and flow dimensions of the subsurface. Unlike other methods for deriving hydraulic properties that require selecting the flow dimension, analyses based on the GRF equation in principle constrain both, flow dimension and hydraulic properties. We utilized the GFR equation to analyze periodic pumping tests carried out in boreholes penetrating gneiss rocks in the research mine Reiche Zeche, Freiberg, Germany. These tests involved one injection borehole, where flow rate and injection pressure were recorded, and four monitoring boreholes, where pressure responses were monitored. Phase-shifts and amplitude ratios were derived through interference analysis, involving a comparison of the periodic signals of injection and monitoring pressure, as well as injectivity analysis, consisting of a comparison of the periodic flow rate and injection pressure. The pumping tests were conducted at three distinct intervals within the injection borehole, isolated by a double-packer probe and selected based on the characteristics of the fractures intersecting the borehole.  One interval contained a natural fracture zone characterized by a high fracture density with a high mean aperture. The others were previously hydraulically stimulated. While one of them had a single pre-existing fracture, the other was entirely intact before the stimulation that led to an induced fracture with feather geometry, as typical for a borehole that does not follow a principal stress axis. Several observations suggest that the gneiss volume is hydraulically heterogeneous: a) the hydraulic properties and flow dimensions vary with pumping period; b) estimated diffusivity values and flow dimensions differ for interference and injectivity analyses; c) discernible differences in diffusivity values and flow dimensions along diverse hydraulic paths, as determined by interference analysis. Furthermore, pressure dependence in hydraulic properties and flow dimensions are observed for all intervals. The hydraulic response of the fault-zone interval exhibits a greater sensitivity to variations in mean pumping pressure than the two stimulated intervals.

How to cite: Jimenez Martinez, V. A., Cheng, Y., and Renner, J.: Fractal diffusion analyses of periodic pumping tests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9336, https://doi.org/10.5194/egusphere-egu24-9336, 2024.

Fault zone geological and geometrical complexities are prime parameters playing a fundamental role in controlling the characteristics of both natural and induced seismicity. In the Bedretto tunnel (Switzerland), the Fault Activation and Earthquake Rupture (FEAR) project aims at triggering a Mw = 1 seismic event through fluid injection and stimulation of a natural fault zone situated in a large-scale (> 10⁶ m³) fractured granite reservoir. The limited exposure of the fault zone in the tunnel, however, restricts the possibility to constrain in detail the geometrical and geological characteristics of the experimental target. Therefore, in order to constrain the geological and geometrical characteristics of the target fault zone, we have integrated structural analyses, borehole and core logging, and borehole ground penetrating radar (GPR).

Preliminary field investigations in the tunnel allowed to identify the complex fault structure characteristics, fault rock properties, and slip tendency in the current stress field of the selected fault zone. These results were compared to the structural observations obtained from field surveys and remote sensing, constraining the slip history, and lateral extent of the set of natural fault zones occurring on the surface above the Bedretto tunnel. Indeed, the lateral extent of the selected fault has been confirmed through the logging (optical/acoustic televiewer, fracture intensity, fracture typology) of exploration boreholes and the analyses of the related cores. The comparison between the geological characteristics of fault zones in the cores and the characteristics of the selected fault zone exposed in the tunnel allowed to confirm the occurrence of the same typology of fault zone further away from the exposure in the tunnel. In addition, GPR logging of the exploratory boreholes provided fundamental insights on the lateral continuity of the identified fault zones on the tunnel wall, as well as those identified in the borehole/core logging.

All geological and geometrical information have been integrated into a preliminary 3D geometrical model (in Leapfrog Geo), representing the overall geometry of the selected fault zone. This preliminary geometrical model has been validated against synthetic GPR profiles, computed through GPR forward modelling along the exploration boreholes.

The integrated results define the selected fault zone as a 3-7 m wide zone of higher density (up to 5/m), of variably oriented secondary fractures, and bounded by two main slip surfaces. The slip surfaces are irregularly decorated by phyllosilicate-rich gouge patches, filling the roughness of the fault surface. The lateral extension of each discrete fracture does not exceed 30 m in length, but the overall lateral continuity of the fault zone exceeds several hundreds of meters.

The presented integrated characterization approach allowed us to constrain a geologically-sound, first-order 3D geometrical model of a complex natural fault zone, validated against geophysical forward modelling. These preliminary results have fundamental implications for the expected experimental planning and outcomes, modelling and injection strategies, project logistics, as well as the design and deployment of the monitoring network around the stimulated fault zone.

How to cite: Ceccato, A., Achtziger-Zupančič, P., Pozzi, G., Shakas, A., Zappone, A. S., Escallon, D., Hetrich, M., Jalali, M., Ma, X., Meier, M.-A., Osten, J., Amann, F., Cocco, M., Giardini, D., and Wiemer, S.: Characterization and 3D geometrical modelling of a complex fault zone for earthquake rupture experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9736, https://doi.org/10.5194/egusphere-egu24-9736, 2024.

Interest in engineered geothermal systems (EGS) has grown in the last decade due to their recognition as a low-emission, renewable energy source. EGS reservoirs with sufficiently high temperatures are located at depths of several kilometers, where the permeability of the crystalline basement rocks is insufficient for geothermal energy extraction. Permeability enhancement is accomplished through hydraulic stimulation, either by hydraulic shearing of natural fractures or shear zones, or through hydraulic fracturing of intact rock. The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland serves as an in situ test-bed where hectometer-scale hydraulic stimulation experiments are conducted to better understand the seismo-hydromechanical response of fractured crystalline rock masses (Ma et al. 2022).

The geothermal testbed of the BedrettoLab is located in a 100 m long enlarged section of the Bedretto tunnel in the Swiss Central Alps, with an overburden of more than 1000 m of granite. Several characterization, monitoring, and two stimulation boreholes were drilled. One of the stimulation boreholes (referred to as ST1) is 400 m long, 45°-dipping, and was equipped with a multi-packer system that partitions the borehole into 15 intervals.

In this work, we present the structural and seismo-hydromechanical characterization of eight stimulation intervals closely observed using a dense monitoring network (see Plenkers et al. 2023 for the detailed network layout). We injected relatively small fluid volumes (0.35–14 m³) following a standardized injection protocol to compare the response of the targeted geological structures in each interval. Depending on the transmissivity of the interval, the stimulation was conducted pressure- or flow rate-controlled with several steps at constant pressure/flow rate. Despite the similarly oriented structures in each interval, the observed seismo-hydromechanical behavior is complex and heterogeneous. The detected seismicity follows multiple steeply-dipping and NE-SW striking planes (Obermann et al. 2024), which coincides with the direction of known pre-existing fault structures obtained from the geological characterization. In most intervals, a clear bilinear behavior on the pressure vs. flow rate plot marks a strong increase in injectivity above a certain reactivation pressure. Analysis of these reactivation pressures in comparison with the stress field, fracture and seismic cloud orientations implies that the stimulation mechanism is hydraulic shearing of the fractures rather than elastic opening (also known as hydraulic jacking).

References:

Ma, X., Hertrich, M., Amann, F., Bröker, K., Gholizadeh Doonechaly, N., et al. (2022). Multi-disciplinary characterizations of the BedrettoLab -- a new underground geoscience research facility. Solid Earth, 13(2), 301–322. https://doi.org/10.5194/se-13-301-2022

Obermann, A., et al. (2024). Picoseismic response of hectometer-scale fracture systems to stimulation with cm-scale resolution under the Swiss Alps, in the Bedretto Underground laboratory. In preparation for JGR: Solid Earth.

Plenkers, K., Reinicke, A., Obermann, A., Gholizadeh Doonechaly, N., Krietsch, H., et al. (2023). Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments: Advances in the Bedretto Reservoir Project. Sensors, 23(6), 3315. https://doi.org/10.3390/s23063315

How to cite: Bröker, K., Ma, X., Gholizadeh Doonechaly, N., Rinaldi, A. P., Obermann, A., Rosskopf, M., Hertrich, M., and Giardini, D. and the BedrettoLab Team: Systematic multi-stage hydraulic stimulation experiments in a hectometer-scale fractured rock volume at the Bedretto Underground Laboratory, Switzerland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10618, https://doi.org/10.5194/egusphere-egu24-10618, 2024.

Understanding fault slip nucleation within the reservoir interval and its propagation beyond the reservoir is essential. Analytical and numerical studies have shown that, depending on the type of operation (injection/depletion), fault slip can nucleate at external or inner corners along the displaced fault system, driven by positive peak shear stresses. In the case of depletion, slip patches gradually start at the inner corners and grow towards the inner part of the reservoir, merging with further depletion. Conversely, injection or increased pore pressure leads to slip patches at external corners, potentially propagating beyond the reservoir into the overburden and underburden. We conducted triaxial experiments on small-scale (mm scale) cylindrical samples containing an entirely displaced vertical fault to investigate fault reactivation and slip nucleation in such settings. Two types of stress paths, monotonic and cyclic, were applied to examine the effects of stress patterns on slip nucleation. For this purpose, we utilized strain gauges to measure differential compaction along the displaced fault directly on the small-scale samples. Direct measurements with a strain gauge network adjacent to the displaced fault system during the monotonic test revealed that differential compaction intensifies from the top of the sample towards the internal corner at the center of the fault where different layers are juxtaposed vertically, indicating a variation in the stress field surrounding the fault plane. Furthermore, results from the cyclic test showed that the differential compaction increases with an increasing number of cycles. Our direct measurements near the displaced fault plane confirm/match the anomalies and peaks in stress observed in previous numerical and analytical studies.

How to cite: Naderloo, M., Jansen, J. D., and Barnhoorn, A.: Fault Nucleation and Reactivation in Displaced Fault Systems: An Experimental Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11634, https://doi.org/10.5194/egusphere-egu24-11634, 2024.

In tight crystalline basement rocks such as granite, faults are known to be substantially more hydraulically conductive than the rock matrix. However, most of our knowledge of rock permeability in the laboratory and in the field relies on indirect inference, static measurements, or before/after datasets, and the spatio-temporal evolution of the permeability field during faulting remains unknown. Specifically, we would like to determine at which stage of the faulting process does permeability change most, and the degree of permeability heterogeneity along shear faults.

We conducted a series of triaxial deformation experiments in initially intact Westerly granite, where faulting was stabilised by monitoring the acoustic emission rate. At many stages from pre- to post-failure states, we paused deformation and imposed macroscopic fluid flow to characterise the overall permeability of the material. In addition, we measured the pore pressure distribution in the sample, and estimated apparent permeability at different locations along the fault, from the intact ligaments to damaged regions. We monitored the propagation of the macroscopic shear fault by locating acoustic emissions.

We find that average permeability increases dramatically (by around 3 orders of magnitude) near the peak stress, where the fault (as seen by acoustic emission locations) is not yet through-going. Post-peak evolution shows a more gradual increase in overall permeability, with local heterogeneities remaining along the fault, primarily controlled by small-scale fault geometry and the existence of undamaged regions as imaged by acoustic emission locations.

We conclude that permeability change and fluid flow focussing occurs at very early stages of faulting, and do not require substantial slip. Our results highlight the key role of fault geometry in the fine-scale permeability structure of basement rocks.

How to cite: Brantut, N., Aben, F., and Farsi, A.: Spatio-temporal evolution of permeability during quasi-static fault growth in granite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11723, https://doi.org/10.5194/egusphere-egu24-11723, 2024.

Human activities in the subsurface such as geothermal energy production, CO₂- and hydrogen storage, and gas extraction can affect the regional stress field and lead to induced seismicity. Gas production from the Groningen gas field in the northeast of the Netherlands has led to more than 300 shallow earthquakes with local magnitudes M_L > 1.5 and up to a maximum magnitude of M_L 3.6, resulting in substantial damage to buildings. Recent earthquake localization studies show that seismicity dominantly occurs on complex normal fault systems, at the depth of the Permian (Rotliegend) reservoir. These faults were formed during multiple tectonic phases from the Late Paleozoic to Early Cenozoic and may comprise breccias, cataclasites, fault gouges and clay smears. The fault strength and slip behaviour are controlled by its composition and microstructural state (porosity, grain size and shape, and presence of foliation within the fault core). Fluid-rock interactions and diagenetic processes during and after fault activity may have altered these characteristics and, hence, the strength and slip behaviour of the fault. Knowledge on the state and composition is thus required to reliably predict the maximum stress drop and seismic energy release upon fault reactivation. However, such knowledge is still lacking at present day.

With this study, we aim at characterizing the microstructures of fault gouges in the Groningen faults. We assess the mineralogy, porosity, and grain size distribution of natural samples from faulted core samples derived from the Groningen gas field. Well-log data is presented to show the representativeness of these samples in the larger context of the gas field. The observations on natural microstructures are then used to define simplified geometrical representations or scenarios that can be used as input for microphysical models. Microstructural characterization involves optical microscopy for quantitative petrography of both bulk rock and selected regions of interest (ROI) within the fault zone. Scanning Electron Microscopy (SEM) with Backscattered Electron (BSE), Cathodoluminescence (CL), and Energy-Dispersive X-ray Spectroscopy (EDX) is employed to analyse porosity, grain size, shape, and mineralogy of faulted regions.

Preliminary results show that the compositions of fault rocks differ from the host rock and that along-fault variability in mineralogy, cementation, and grain size are important to consider. We distinguish between four main types of fault gouges in the Groningen Rotliegend, based on their microstructural characteristics: (1) gouges consisting of quartz and feldspar grains embedded in a very fine clay matrix, (2) very fine-grained quartz-rich gouges, (3) quartz-cemented gouges, and (4) anhydrite-cemented gouges. We expect that induced fault movement in the first two gouges occurs by reactivation of the earlier produced fault gouges. Since quartz and anhydrite cementation is concentrated in the faults, reactivation of the latter two presumably occurs by cataclastic processes and gouge formation from the adjacent bulk rock rather than the cemented gouge. This suggests that a well constrained fault diagenetic history is required to infer which components of the fault material governs its frictional behaviour and hence the related seismic hazards. 

How to cite: Willingshofer, E., Arts, J., Rodrigues, D., Nader, F., Drury, M., Matenco, L., and Niemeijer, A.: Microstructural Characterization of Fault Rocks from the Groningen Gas Field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11990, https://doi.org/10.5194/egusphere-egu24-11990, 2024.

Geothermal energy plays a growing role in the transition to renewable and carbon free energy sources. A challenge for many geothermal fields is how to enhance water-rock heat exchange either by creation of new fractures, or by blocking short-circuiting large conduits. Here we report a novel approach for blocking large conduits (faults and large fractures) using heat sensitive epoxy resin foam designed to be transported as discrete resin droplets to specific regions that are then activated (foamed and cure) in-situ at targeted temperatures. In contrast with alternative methods for reducing geothermal rock permeability such as silicate gels or heat responsive polymer microbeads targeting small aperture fractures < 0.1 mm, the epoxy foam can reduce the permeability of fractures with apertures up to several millimeters. Results from laboratory 2-D glass fracture model provide insights by visualizing the transport phase and subsequent temperature-sensitive foaming and curing transformations with associated flow pathway blocking. Modeling results for transport and foaming in simple fracture networks considering rheological properties and foaming (volume expansion) behavior will be presented. On going activities of rheological resin characterization; tuning of the foaming-curing to different temperature ranges; and consideration of resin dispersion using small droplets for enhanced transport will be discussed.  

How to cite: Or, D., Parashar, R., Yang, Y., Bishwokarma, M., and Karra, S.: Heat Sensitive Epoxy Foam for Permeability Alteration in Fractured Geothermal Fields , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13098, https://doi.org/10.5194/egusphere-egu24-13098, 2024.

The SIMFIP (Step-rate Injection Method for Fracture In-situ Properties) probe is a downhole displacement tool designed to measure injection-driven fracture displacement in an isolated borehole interval in three dimensions. The resulting displacement and interval pressure data can be used to estimate fracture characteristics such as fracture stiffnesses, strength and hydraulic properties from borehole measurements. SIMFIP results have also been successfully implemented in a stress inversion routine that allows the calculation of the full stress tensor from a single measurement.

As part of the SPINE (Stress Profiling IN Enhanced geothermal systems) project, a laboratory-scale deformation tool, the mini-SIMFIP probe has been developed. The probe, with a diameter of 20 mm and a length of 65 mm, can be installed in laboratory to study the 3D deformation of intact rock and fractures in different type of rocks. Preliminary tests were conducted in a decimeter-scale true triaxial test apparatus containing a cuboid granite sample with an oblique saw-cut laboratory fracture. Two boreholes crossing the fracture can be isolated and equipped with the mini-SIMFIP probe. Fluid injection into the isolated interval opens the fracture or induces hydraulic shearing under anisotropic stress conditions. The resulting dataset can be used to quantify measurement uncertainties associated with the field SIMFIP protocol, to benchmark stress inversion protocols against known stress boundary conditions, and gain better insight into hydro-mechanically coupled processes.

How to cite: Schaber, T., Osten, J., Jalali, M., Cadmus, A., Welsing, L., Cook, P., Guglielmi, Y., Fuentes, R., and Amann, F.: Exploring the Hydro-Mechanical Behavior of Fractures Utilizing the mini-SIMFIP Probe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15749, https://doi.org/10.5194/egusphere-egu24-15749, 2024.

Bentheim sandstone is regarded as a conventional georeservoir rock even at great depth, due to its mineral composition, homogeneity, micro- and macrostructure. Therefore it has been extensively tested for a variety of applications to understand its physical and mechanical properties under changing environmental conditions.

A recent study has shown how the simultaneous change of pressure, temperature and pore pressure, therefore recreating environmental conditions at selected depths, affects the evolution of permeability at depths, both when the rock is buried and when the rock is exhumed. The interaction between those variables has a complex effect on the permeability of Bentheim Sandstone, which could not have been identified by assessing individually the role of a variable. These results show that the permeability of such rock could be overestimated with classical studies and highlight the importance of investigating rock mechanical and hydraulic properties at georeservoir conditions. These experiments have been performed on intact samples.

However, rocks at depth contain fractures and faults, which may alter the interconnectivity of the pore space, hence the permeability of the rock itself. The deformation and failure of Bentheim Sandstone at high strain resulted in permeability loss due to the formation of comminuted material and grain crushing which lowered the pore space interconnectivity. No fractured sample has been tested under simultaneously changing environmental conditions.

To fill this gap, we replicate the experimental procedure used to test intact samples of Bentheim sandstone, both under simultaneously changing conditions and under a sequential variation of different variables, after the sample has been brought to failure. Our goal is to understand the importance of fractures on the permeability evolution at different simulated depths.

How to cite: Fazio, M. and Sauter, M.:  Permeability evolution of a fractured, porous and permeable sandstone at simulated georeservoir conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18390, https://doi.org/10.5194/egusphere-egu24-18390, 2024.

Natural faults and fractures form a critical component of fluid flow in low permeable reservoirs such as tight carbonates for a wide variety of applications including geothermal energy extraction. Fractured systems often control permeability in these reservoirs at the first order where properties of these networks are defined by fracture orientation, intensity, aperture, and connectivity. Accurately quantifying these network properties is vital in generating representations of the fracture networks at reservoir depth.

In regions with limited subsurface data (borehole and seismic), field data and outcrop analogues become an important source for characterising the fracture networks for modelling reservoirs at depth. Outcrops can be used to define several properties of the networks and information on the variation in the fracture distribution across defined areas.

The Franconian Basin is a major tectonic structure in Northern Bavaria containing Mesozoic sediments up to 3500m thick. It is a relatively under-researched region where limited subsurface data is available in comparison to the south in the Molasse Basin where geothermal exploration and production is well established with extensive subsurface datasets widely distributed. Increased geothermal gradients have been identified in Northern Bavaria, including surrounding the major urban areas presenting an opportunity to improve the understanding of the geothermal potential of the region. Several of the identified reservoir units in this region are primarily composed of low permeable carbonates where faults and fractures control primary reservoir flow. These units are also present as outcrop analogues in the Franconian Alb which can be utilised for surface fracture characterisation.

We present results analysing the variations in the fault and fracture systems from across the region captured from 1D measurements and 2D and 3D imaging of quarry and cave outcrops. Using these results, stochastic fracture models of the parts of the region can be generated, providing realisations of the fracture networks which can contribute to assessing the permeability and geothermal potential of the reservoirs in Northern Bavaria.

How to cite: Smith, R., Prabhakaran, R., Jakob, F., and Koehn, D.: Variations in fracture distribution across Northern Bavaria – Towards large-scale geothermal fracture models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18458, https://doi.org/10.5194/egusphere-egu24-18458, 2024.

Understanding the flow of fluids in the subsurface is of vital importance to geo-energy exploitation and disposal of waste fluids. In most situations, individual cracks can be more effective for fluid transport than fluids flowing through the porous matrix of the rock. The enhancement of a single crack in a low permeable rock can be over 1000 times. However, for both porous matrix flow and crack flow, the bulk permeability and crack transmissivity are all affected by the stress state in the lithosphere.

This study aims to investigate the influence of the Terzaghi effective normal and shear stress on the transmissivity of cleavage cracks under upper crustal conditions. Penrhyn slate was selected as samples for the experimental study because of its low porosity (<1%), permeability and slaty cleavage. The matrix permeability of Penrhyn slate is very low, ranging from 10^-20 to 10^-22 m² when the effective pressure is in the range of 10 to 53 MPa measured by the oscillating pore pressure method (OPPM). The crack transmissivity ranged from 10^-18 to 10^-23 m³ when the effective pressure changed from 10 to 280 MPa. The experimental results show that the evolution of crack transmissivity of a single fracture under several cycles of pressurization and depressurization is similar to the trend found in permeability. The first application of the normal stress on fracture surfaces always produces a nonrecoverable loss in crack transmissivity. In the subsequent pressurization and depressurization, partially recoverable variations in transmissivity were observed, suggesting a linear elastic behaviour of crack closure. The highest peak effective pressure attained in the stress history affects the extent of subsequent recoverable crack transmissivity. When the fracture surface is subjected to a new higher peak stress, the crack transmissivity will no longer recover to its former low level but to a lower level, indicating a permanent transmissivity loss. Thus, the transmissivity has a memory of the previous maximum stress the fractured rock was subjected to.

The influence of shear stress on crack transmissivity was studied in Solnhofen limestone and Carrara marble samples with saw-cut ground smooth fractures and compared with the rough cleaved fractures of Penrhyn slate. The influence of shear stress was studied in two situations: (a) the stable (no-slip) condition at shear stress less than needed to promote slip on fracture (b) at shear stresses high enough to yield slip on the fractures. In situation (a), the cyclic increase and decrease of shear stress led to a continuous decrease in crack transmissivity. The magnitudes of this decrease in crack transmissivity decrease with more cycling. The transmissivity tended to decrease to a lowest value eventually but this lowest value can be regenerated by slip on the fracture. In situation (b), a single slip can decrease crack transmissivity. The decrease in crack transmissivity can be attributed to the formation and smearing of frictional wear products or gouges. Under the progressive compaction, there exists a lowest level of crack transmissivity which is independent of the normal stress.

How to cite: Yang, L., Mecklenburgh, J., and Rutter, E.: The evolution of crack transmissivity under normal and shear stress before and after slip, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19263, https://doi.org/10.5194/egusphere-egu24-19263, 2024.

Rocksalt formations are critical candidates for storing natural gas, hydrogen, compressed air energy, and radioactive waste. While pure, undisturbed rock salt deposits exhibit low porosity and impermeability when buried deeply, excavation leads to near-field microcracking and dilatancy in the salt, increasing porosity and permeability. Over time, the connectivity of brine- or water-vapor-filled microcrack networks in deformation-damaged salt is expected to decrease, partly due to dissolution-precipitation healing. In this study, we employ 4D (time-resolved 3D) microtomography to investigate the long-term evolution of dilated grain boundary and microcrack networks developed in deformation-damaged natural salt through brine-assisted processes. Our findings reveal substantial microstructural modification and healing occurring over periods ranging from days to a few months. Cracks and dilated grain boundaries undergo crystallographic faceting, necking, and migration, effectively "recrystallizing" the material and resulting in increased tortuosity and decreased connectivity of the crack network. Understanding the complex interplay between microcracking, healing, and permeability changes in deformation-damaged rock salt is of utmost importance for optimizing storage and disposal applications in geomechanics and physical chemistry. Our research contributes valuable insights to this field and informs the sustainable development and management of rock salt formations for diverse energy storage and waste management needs.

How to cite: Ji, Y., Spiers, C., Hangx, S., de Bresser, H., and Drury, M.: Evolution of Microcracks in Damaged Natural Salt: Insights from 4D imaging, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19710, https://doi.org/10.5194/egusphere-egu24-19710, 2024.

The brittle-ductile transition is an important mechanical shift within the Earth's lithosphere and, as a crucial interface between ascending hot magma and colder host rock, plays a fundamental role in fluid migration within hydrothermal systems. Traditionally assumed to occur at temperatures between 350 °C and 400 °C, recent studies challenge this assumption, revealing a temperature transition range dependent on minerals. Experimental and numerical investigations highlight significant variability, ranging from 260 °C in wet quartz to 700 °C for dry orthopyroxene within homogeneous mineral compositions.

This study delves into the complexities of this rheological barrier in the El Teniente Mafic Complex in Chile (currently a copper mine and formerly a hydrothermal system at the BDT), unraveling its impact on the migration of magmatic fluids. Building on previous research suggesting a self-sustaining mechanism that facilitates fluid movement over the brittle-ductile transition through overpressure-permeability waves and the formation of a dense, multi-episode vein network, our focus is on understanding deformation mechanisms and the localization of deformation and permeability at the BDT. Heat transfer models propose a dual paradigm of conduction and convection, adding to the complexity.

To address these challenges, we conducted a comprehensive series of physical and mechanical measurements on 34 cylindrical samples from the El Teniente Mafic Complex. This included analyses of density, porosity, elastic wave characteristics, and electrical conductivity under variable water conductivities. Elastic wave measurements were performed using 2.25 MHz transducers on both dry and saturated samples. Permeabilities were determined by the pulse decay technique for compact rocks, complemented by triaxial tests and local strain measurement using strain gauges, along with acoustic emission measurements on dry samples subjected to confinements similar to those near the mine.

Our results highlight consistently low porosity (below 1%) in the samples, with electrical conductivity, permeability, and strength controlled by veins. Particularly, at lower salinities, the metallic particle content and the orientation of the vein with respect to the loading axis significantly influence electrical conductivity and phase. At higher water conductivities, behavior is governed by connected porosity. Furthermore, favorably oriented veins emerge as crucial controllers of both permeability and mechanical resistance.

These observations align with a convection heat flow model in a porphyry system, providing significant insights into the complex interaction between rock and vein properties. The study uniquely focuses on fossil high enthalpy systems, shedding light on their complex behavior. Additionally, the article discusses constraints on model variables, fostering a comprehensive understanding of the brittle-ductile transition in magmatic-hydrothermal systems.

How to cite: Robbiano, F., Orellana, L. F., and Violay, M.: Physical and mechanical characterization of veined rocks: Implications to a porphyry model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20381, https://doi.org/10.5194/egusphere-egu24-20381, 2024.

Accurately predicting fluid-fluid interface displacement in fractured reservoirs is paramount for optimizing subsurface operations, particularly in the context of enhanced oil recovery and geological carbon sequestration (GCS). However, a comprehensive understanding of two-phase flow behavior in fractures, including the impact of fracture closure, fluid viscosity ratio, and capillary number, is yet to be achieved. To address this challenge, we have developed an analog experimental setup to investigate the intricate relationship between fracture surface roughness and fluid-fluid interface displacement. Our experimental setup features a transparent fracture flow cell with self-affine rough-walled surfaces that are matched to each other above a chosen length scale (denoted below as the correlation length) and a precisely controlled mean aperture. Realistic synthetic fracture geometries were generated numerically. They are characterized by their Hurst exponent, fracture closure, and correlation length. High-speed imaging captures the dynamic spatial distribution of fluid phases within the fracture plane during drainage processes in a given fracture geometry. The mean aperture can be varied between experiments for a given geometry of the fracture walls. We investigate a comprehensive range of capillary numbers, spanning both viscous and capillary-dominated regimes, vary viscosity ratios, and characterize the resulting displacement regimes. Our results reveal a profound impact of fracture closure and correlation length on trapping efficacy, particularly in the capillary-dominated regime. These findings can be interpreted in terms of residual trapping of CO₂ during GCS in fractured reservoirs.

How to cite: Rezaei, A., Gomez, F., Borgman, O., Neuweiler, I., and Meheust, Y.: Impact of Capillary Number, Fluid Viscosity Ratio, and Fracture Closure on Two-Phase Flow Regimes in Geological Fractures: An Experimental Study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20658, https://doi.org/10.5194/egusphere-egu24-20658, 2024.

When analysing fractured rock outcrops, the fracture network's topology and length statistics are of fundamental importance. Past literature focused on adopting a non-parametric approach for the unbiased estimation of fracture length data mean, and with some additional steps, the variance of a population. However, technology improved, and necessities shifted. Now it is possible to quickly obtain dense length datasets with thousands of measurements and the emergence of stochastic DFNs increased the demand for parametric solutions to correctly fit several types of distributions. These conditions highlighted an absence of works on these topics. Of particular interest is the right censoring bias effect of the interpretational boundary on the fracture length statistics. We tackle this problem by applying survival analysis techniques, a branch of statistics that includes methods for modelling time to event data and correctly estimating the model’s parameters with data affected by censoring. Synthetic testing has been carried out, showing a reliable estimate of the distribution parameters with up to 80% of the total measurements being censored. Moreover, it is shown that the correction is independent from the orientation of the fracture set or boundary geometry. We propose FracAbility, a new open-source Python package capable to both analyse the topology of fracture networks and, by using the latest SciPy version, correctly fit different parametrical distributions on length data with right censored measurements. The library and the proposed approach have been applied to real world data, successfully correcting length distributions affected by censoring.

How to cite: Benedetti, G., Casiraghi, S., Bistacchi, A., and Bertacchi, D.: FracAbility: A python toolbox for survival analysis in fractured rock systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22156, https://doi.org/10.5194/egusphere-egu24-22156, 2024.

The safe and effective deployment of geothermal energy and storage of carbon dioxide requires an assessment of potential induced seismicity and fracturing through the seal above the reservoir. To aid such assessment we have built the SRIMA tool (Seal and Reservoir Integrity through Mechanical Analysis) and we have made it available online. The tool can be used in the Standard extended Seismic Hazard Analysis, which is part of the seismic hazard and risk assessment for geothermal projects in the Netherlands. SRIMA is a fast semi-analytical tool that provides a scenario-based analysis of pressure and temperature changes around an injection well, the resulting stress changes on nearby faults, reactivated fault area, the maximum credible earthquake magnitude, the resulting PGV distribution and an estimate of damage. SRIMA also computes the potential for development of tensile fracture in the seal and base. SRIMA has been designed to give first-order estimates of these results. The speed of the calculations facilitate them to be performed in a stochastic framework, which allows the assessment of failure probabilities.

SRIMA is based on semi-analytical expressions for the fast calculation of temperatures, pressures, and induced poro-elastic and thermo-elastic stresses due to the injection of cold fluid. The expressions for flow and induced stresses have been developed for a homogeneous, isotropic layer cake model under radial symmetry. In the injection layer the flow is assumed to be fully developed and temperature transfer is in an advective way. In the bounding seal and base layers, the pressure and temperature dynamics are assumed diffusive. The derived expressions capture the first-order characteristics of the pressure, temperature and stress changes. Validation of the expressions has been achieved through comparison with finite-difference and finite-element codes for temperature, pressure, and stress changes around an injection well. A fault without offset cutting through the seal, reservoir, and base can be specified within the model space. Poro-elastic and thermo-elastic stress changes are transformed to fault stresses and fault criticality. The fault area over which stresses are critical (i.e. fault reactivation occurs) is used to estimate the magnitude of the largest credible earthquake for each model scenario, assuming that the entire reactivated fault area participates in a single event, slip cannot propagate beyond the reactivated area, and all assumed slip over the fault area is seismic slip. An ensemble of magnitudes is converted to exceedance curves of peak ground velocities (PGV), using nationwide developed Ground Motion Prediction Equations. The resulting PGV distribution as a function of epicentral distance serves as input for calculating the probability of exceeding Damage State 1 using empirical fragility functions for unreinforced masonry buildings. This contribution will summarize details and assumptions behind each step.

How to cite: Fokker, P., Buijze, L., Pluymaekers, M., Maaijwee, C., Mijnlieff, H., Mos, J., Vogelaar, B., de Vries, S., and Vrijlandt, M.: SRIMA: A fast tool to assess seismicity and seal integrity related to fluid injection., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2591, https://doi.org/10.5194/egusphere-egu24-2591, 2024.

The gas-bearing Yinggehai Basin in the northern South China Sea is characterized by high pressure and temperature conditions. The Miocene strata in the eastern slope exhibit intense overpressure with a pressure coefficient exceeding 2.2. Understanding the characteristics of overpressure and clarifying its causes are very important for natural gas exploration and development in reservoirs with intense overpressure. This study identifies and characterizes the pressure structure in the eastern slope of Yinggehai Basin by utilizing formation pressure data from formation tests (MDT) and drill pipe tests (DST) in combination with conventional logging data (acoustic wave, density, and resistivity). The pressure distribution exhibits a distinct three-stage ladder structure. Overpressure initiates in the Lower Pliocene Yinggehai Formation, with a pressure coefficient of 1.2. The formation pressure increases sharply in the underlying Upper Miocene strata (1^st member of the Huangliu Formation), with a pressure coefficient ranging from 1.4 to 1.8. The pressure coefficient increases to between 2.1 and 2.3 in the 2^nd member of the Huangliu Formation.

We utilized the logging curve combination method, Bowers effective stress method, and Bowers acoustic-density crossplot method to differentiate between two types of overpressure origins: loading and unloading type, and further evaluated their contribution rate to overpressure. Loading overpressure primarily accounts for the overpressure observed in the Yinggehai Formation, contributing to pore pressure with a range from 51% to 93%. In contrast, the overpressure in the Huangliu Formation is predominantly of the unloading type, contributing to pore pressure from 35% to 46%. Additionally, we analyzed the genetic mechanism of overpressure, utilizing lithology, subsidence rate, organic matter maturity, and seismic attribute data. We find that the loading overpressure in the Yinggehai Formation is attributed to unbalanced compaction of mudstone due to rapid sedimentation (sedimentation rate > 500m/ Ma). The unloading intense overpressure in the Miocene strata is most likely caused by the vertical transmission of overpressured fluid along faults and fractures. Our findings provide implications for complex pressure structure, overpressure evaluation, and genetic mechanisms in sedimentary basins.

How to cite: Yang, B., Meng, Q., and Hao, F.: Intense fluid overpressure in the eastern slope of the Yinggehai Basin, South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2963, https://doi.org/10.5194/egusphere-egu24-2963, 2024.

In the aggregate quarries in Çiftalan and Ağaçlı (Eyüpsultan, Turkey) regions, grovak-shale type rocks belonging to the Thracian Formation are used as concrete and asphalt aggregates. After crushing, these rocks are classified into 0-5 mm, 5-12 mm and 12-24 mm sizes and marketed as concrete and asphalt aggregates. Aggregates with sizes larger than 5 mm can be used directly in concrete and asphalt applications without any washing process, while aggregates with sizes smaller than 5 mm are called fine aggregates and are marketed after the washing process since the methylene blue value is too high for concrete and aggregate. After the washing process, grains smaller than approximately 100 microns in size (especially clay minerals) are stockpiled in waste sites. Within the scope of this study, it is aimed to produce sinter aggregates with strength that can be used in concrete from these waste materials. It was determined that the waste materials with a grain diameter of less than 100 micron consisted of 28.6% quartz, 21.9% albite, 17% muscovite, 24.9% chlorite and 3.6% calcite minerals. In addition to minerals, 3.4% organic matter was detected. In addition, the main oxide compositions of these wastes were determined, and it was found that 54.51% SiO₂, 18.1% Al₂O₃, 19.66% sum of fluxes (CaO, Na₂O, Fe₂O₃, K₂O, MgO) and 5.8% loss on ignition. Within the scope of determining the thermal properties of the wastes, DTA analyzes were carried out and it was determined that two different endothermic reactions took place in the range of 500-600 °C and 700-800 °C, thus dehydroxylation reactions took place in these temperature ranges. In the range of 1000-1100 °C, it was determined that an exothermic reaction, that is, a new phase or several phases were realized. Considering these thermal properties of the wastes, 1100 °C was determined as the sintering temperature. For the sintering process, the samples were first dried at 105 °C and then milled and powdered again. The powdered samples were placed in a metal cell with a diameter of 5 cm and a height of 10 cm and then compressed with a load of 200 kg per cm². The compressed samples were heated to 1100 °C in a high temperature furnace with a temperature increase of 10 °C per minute and kept at 1100 °C for 30 minutes and then allowed to cool in the furnace. Since the height of these sintered cylindrical specimens was less than 10 cm, the Brazilian test method was used to determine their strength. As a result of the experiment, it was determined that the Brazilian test strengths of 3 different specimens reached 8.8 MPa, 9.6 MPa and 15.7 MPa. Considering the strength values obtained, it was determined that ideal products for concrete and asphalt aggregate can be produced from the dust wastes released in these aggregate quarries. 

How to cite: Avci, E. and Tugrul, A.: Utilization of clay-containing aggregate sludge waste as structural concrete and asphalt aggregate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4757, https://doi.org/10.5194/egusphere-egu24-4757, 2024.

The accurate determination of hydraulic conductivity in unsaturated bentonite is important for the modeling of subsurface thermo-hydro-mechanical and chemical processes. This study introduces a new hybrid approach, employing a constrained CatBoost algorithm coupled with a genetic algorithm for hyperparameter tuning. We benchmarked the effectiveness of the constrained CatBoost model against various data-driven regression models, including lasso, elastic net, polynomial regression, k-nearest neighbors, decision tree, bagging tree, random forest, and standard CatBoost. Our findings demonstrate that the constrained CatBoost model excels in providing accurate estimations of the hydraulic conductivity of compacted bentonite during the wetting phase. The model adequately captures the U-shaped correlation between hydraulic conductivity and suction and reflects the influence of temperature changes on hydraulic conductivity. Furthermore, the bootstrapping analysis, conducted across 800 iterations, confirms the stability and robustness of the constrained CatBoost model. This work provides a reliable tool for predicting hydraulic conductivity in diverse environmental and engineering contexts.

How to cite: Taherdangkoo, R., Nagel, T., and Butscher, C.: A  constraint-enhanced machine learning model for predicting hydraulic conductivity of unsaturated bentonite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5213, https://doi.org/10.5194/egusphere-egu24-5213, 2024.

Induced seismicity has attracted increasing research interest in recent times. This phenomenon is generally associated with fluid injection or extraction wells, in energy industry activities such as hydrocarbon extraction/injection, CO2 sequestration, geothermal energy or underground storage of green hydrogen. However, there are other human activities that can induce or trigger seismic events, such as oscillations in the surface water level due to the construction of hydraulic infrastructures.

Surface water level oscillations, whether natural or anthropogenic, can alter the pore pressure regime in the subsurface. Although these alterations usually have a moderate magnitude, they can destabilize faults that were already close to overcoming their slip resistance and eventually trigger an earthquake. On the other hand, the fault slip itself alters the pressure regime due to the undrained response of the porous medium, caused by the sudden deformation of the fault surrounding area. This undrained pressure, which can take up to weeks to dissipate, can alter the stress state of other nearby fractures and trigger new earthquakes or aftershocks.

In this work we present numerical simulations of the underground area around the Itoiz dam (Spain). We simulate the subsurface as a saturated poroelastic medium, with a fully coupled scheme between fluid flow and solid deformation in the porous medium. Through a seismological analysis we start from series of recent earthquakes to locate geological faults. With our numerical model we study how the undrained effect produced by this series of events can destabilize other nearby faults. We also add the effects of the oscillations in the reservoir level following the historical series of the last years.

Our numerical simulations indicate that in the case of the Itoiz dam, the most recent seismic swarm could have been triggered by the stress transfer from the previous events, which, together with the filling of the reservoir, may have destabilized faults that were critically stressed for failure. Our simulations can contribute to explore how the combined effect of the undrained pressure by the fault slip and the oscillations of the reservoir can trigger faults that, due to the natural state of stress, are already close to slip. Also to clarify if the most relevant mechanism is the oscillation of the reservoir level or the undrained pressure trigger, which according to bibliographic analysis can be of the same magnitude. This could apply to other cases of seismicity induced by hydraulic infrastructures or in general by oscillations in the surface water level.

Acknowledgements

This research Project has been funded by the Comunidad de Madrid through the call Research Grants for Young Investigators from Universidad Politécnica de Madrid under grant APOYO-JOVENES-21-6YB2DD-127-N6ZTY3, RSIEIH project, research program V PRICIT.

How to cite: Andrés, S., Santillán, D., Santoyo, M. Á., and Cueto-Felgueroso, L.: Stress transfer and poroelastic mechanisms to elucidate seismicity triggered by reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6340, https://doi.org/10.5194/egusphere-egu24-6340, 2024.

Cobalt is often released into the natural environment through industrial waste from alloying industries and acid mine drainage. Additionally, it exists as a radionuclide (⁶⁰Co) contributing to high-level radioactive waste. Smectite is a mineral that can be useful for adsorption and isolation of this element. In this investigation, Cheto-type montmorillonite (Cheto-MM), a source clay mineral of The Clay Mineral Society's (CMS) with well-established characteristics, was used as the primary material. The study aimed to assess how cobalt adsorption is affected by the adsorption site in the presence of interlayer water and after subsequent dehydration through heating. Adsorption kinetics and adsorption isotherm models were employed to explore the cobalt adsorption mechanism on Cheto-MM.

Results demonstrated notable variations in adsorption characteristics post-dehydration and subsequent shrinkage. Approximately 38% of cobalt was found to adsorb at the edge of Cheto-MM, while about 62% was adsorbed at the interlayer site, indicating the significant influence of the interlayer on cobalt adsorption in Cheto-MM. Adsorption kinetic models showed that the cobalt adsorption kinetics on Cheto-MM can be explained by a pseudo-second-order model. Moreover, isotherm experimental result was best represented by the Langmuir isotherm adsorption model. This study provides fundamental insights into cobalt adsorption characteristics on montmorillonite, emphasizing distinct adsorption sites. Such findings are instrumental in predicting smectite's adsorption behavior in high-level radioactive waste disposal sites in the future.

How to cite: Kim, Y. and Jang, Y.: Cobalt adsorption on montmorillonite: investigating the influence of dehydration on adsorption properties, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8326, https://doi.org/10.5194/egusphere-egu24-8326, 2024.

Large scale subsurface gas storage in porous reservoirs can play an important role in the energy transition. Geological storage of carbon dioxide will mitigate CO2 emissions while underground energy storage, for example in the form of hydrogen gas, can be used to balance out the renewable energy production and demand. To investigate the feasibility of large scale subsurface gas storage in porous reservoirs, simulation models are needed that accurately capture the multi-phase flow behaviour in porous rock. Important input parameters for reservoir simulators are relative permeability and capillary pressure which highly depend on the wettability of the system. In this presentation, we will show results of different experimental techniques to characterize and visualize gas transport in porous rock including a novel experimental device to characterize wettability under the impact of different driving forces.

How to cite: Boon, M.: Experimental characterization of multi-phase flow in porous rock relevant for subsurface gas storage, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8330, https://doi.org/10.5194/egusphere-egu24-8330, 2024.

Scaling refers to the accumulation of solid precipitates on the surfaces of pipes, heat exchangers, and other equipment across various industrial processes, notably within geothermal systems. This process can lead to decreased efficiency and increased maintenance needs, highlighting the importance of accurate prediction and control methods.

Hydrogeochemical models tend to overestimate the extent of scalings, especially if the scalings are caused by a disruption of the lime-carbonic acid equilibrium due to degassing in the geothermal fluid. This is because the models are not capable to describe diffusion-limited crystal growth, partial volume effects, and local saturation states adequately. In this study we introduce a novel approach by coupling a multiphase CFD-model (OpenFOAM) for the description of the gas phases in the produced geothermal fluids with PhreeqC to simulate the hydrochemical effects of the stripping of CO2 by the gas phase. This results in a model with high spatial and temporal resolution which allows to quantitatively differentiate between processes in the fluid and the processes taking place at the solid interfaces (pipe walls or matrix).

The code is validated with published experiments in bubble columns. The coupling results in a highly flexible model which can account for different hydrochemical conditions, different matrix, and varying gas composition using a well established thermodynamic database. Transfer to other hydrochemical conditions is therefore facilitated.

How to cite: Omidi, M. and Baumann, T.: Coupling OpenFOAM and PhreeqC to quantify local disruptions of hydrochemical equilibria due to bubble formation and stripping of CO2, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8331, https://doi.org/10.5194/egusphere-egu24-8331, 2024.

Understanding perturbations caused by underground fluid injection and extraction is essential for steady long-lasting operations of subsurface energy systems (e.g. geothermal systems). The systems are often too complex to obtain precise analytical solutions and computationally expensive to have fine numerical results. Simplified settings give the benefit of understanding the role of driving geological parameters as well as examining the limits of each approach. In this study, we focus on the influence of permeability on pore pressure and present analytical and numerical solutions of spatial and temporal evolutions of pore pressure in an elastic homogeneous porous media with isotropic and anisotropic permeabilities. We use COMSOL Multiphysics to build a 3D finite element model with injection/production wells and investigate where the numerical solutions of the spatially limited volume coincide and diverge from corresponding analytical solutions of pore pressure in infinite media.

How to cite: Sharia, T., Rietbrock, A., Müller, B., and Niederhuber, T.: Pore pressure evolution in media with isotropic and anisotropic permeability - analytical and numerical solutions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10763, https://doi.org/10.5194/egusphere-egu24-10763, 2024.

In Central Europe, a substantial emitter of CO2 in the energy sector corresponds to the thermal energy required for heating and cooling. Seasonal underground heat storage presents a viable option for storing excess heat generated during the summer months for usage in winter, reducing the need for conventional sources of energy. Today, high-temperature aquifer thermal energy storage (HT-ATES) systems are attracting large interest as they represent a sustainable means of meeting heat demand.

In HT-ATES systems, hot water is injected into a reservoir during summer, while exchanged cold water is injected over the winter season. These fluctuations in temperature and pressure have an impact on the geomechanical and thermo-hydraulic properties of both the reservoir and the surrounding layers. Monitoring the changes in the reservoir properties is a critical aspect of running a heat storage system safely and efficiently. We try to determine whether active seismic imaging could be a suitable method to characterize the temporal and spatial evolution of the reservoir.

With view on designing future geophysical assessment and monitoring systems, we first perform thermo-hydro-mechanical (THM) modelling to estimate the variations in the poroelastic properties due to the geothermal processes. Our modeling is based on the characteristics of the DeepStor demonstrator, currently under development in the north of Karlsruhe (Germany), at the Karlsruhe Institute of Technology (KIT). The three layers model includes different mechanical properties with one borehole. The simulation of cyclic hot water injection and production over time allows to quantify its effect on the underground material properties. In addition to assessing the expected operational parameters of the DeepStor demonstrator, we test additional injection schemes with varying underground properties to simulate the different ranges of porosity changes and look at their effects on the elastic properties.

Linking the THM model parameters to seismic sensitive variables such as velocities and impedances, through empirical equations, allow us to determine the conditions under which active seismic surveys could effectively detect these changes. This approach provides a valuable tool for evaluating the potential of active seismic imaging for monitoring HT-ATES systems.

How to cite: Fraile, C., Kohl, T., and Gaucher, E.: Estimation of seismic velocity changes in a HT-ATES system using THM modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11143, https://doi.org/10.5194/egusphere-egu24-11143, 2024.

Fluid extraction from geological formations for purposes of subsurface utilization leads to a pore pressure drop in reservoirs and potentially to compaction and seismicity. The geomechanical behaviour, and thus production-related compaction, of siliciclastic reservoirs is governed by the composition of reservoir sandstones, which includes porosity, grain size distribution, and detrital and authigenic mineralogy.

One siliciclastic reservoir which is undergoing compaction related to fluid extraction is the Rotliegend of the Groningen gas field. In this research, we investigate potential approaches to upscale the sandstone composition from the micro-scale (thin sections, plugs) to the well and reservoir scale eventually.

Previous research has highlighted that among the petrographic properties, the presence of authigenic clays tends to affect the geomechanical behaviour the most. Intragranular clays can affect the overall stiffness of the individual grains while the presence of the pore-filling clays  and clay rims may result in grain slip or pressure solution in loaded rock samples. In particular, we are defining the fractions of the clay-coating minerals (illite, kaolinite, etc.) and their effect on the inelastic deformation of reservoir sandstone as well as paying close attention to the presence of chlorite as its distribution corresponds to the areas associated with increased subsidence and seismicity.

We employed Short-Wave Infrared (SWIR) spectroscopy, a non-destructive and time-efficient technique, to obtain the mineralogical composition of core slabs from the Groningen Gas field. The SWIR data, based on a resolution of 200 µm pixels, allows for a detailed analysis of compositional variation within the Upper Rotliegend Group. Verification of the SWIR results against a comprehensive petrographic dataset, including X-ray diffraction, thin section descriptions, and modal point count analysis highlights that SWIR data primarily captures qualitative variations in mineralogy rather than providing precise numerical values. Combination of sedimentary facies, SWIR spectroscopy data and conventional petrographic studies allows to generate descriptive mineralogical trends for each of the studied wells.

Ultimately, the results of our research will serve as a foundation for selecting the samples, designing geomechanical experiments to test the proposed hypotheses, and as the means to select the upscaling and modelling approaches to develop a detailed model of the Dutch subsurface that matches well the existing heterogeneous structures. The derived 3D reservoir composition model of the Groningen gas field will be combined with the results of the deformation experiments to link reservoir composition to geomechanical behaviour. This will enable an updated, more realistic, 3D geomechanical model of the Groningen gas field that can be utilised by other researchers to better predict future compaction and subsidence.

How to cite: Bublik, D., Mulder, S., and Miocic, J.: From pore to well - identifying clay mineral distribution in sandstones using SWIR spectroscopy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12464, https://doi.org/10.5194/egusphere-egu24-12464, 2024.

The use of bentonite clay in industrial applications is widespread: it is used as an engineered barrier for long-term management of radioactive wastes, CO₂ storage, landfill liners, and contaminant containment. These applications have diverse environmental conditions ranging from various temperatures, pHs, saline contents, and ionic concentrations. Since bentonite is a low permeability clay, anion transport is diffusion dominated but geochemical reactions can also play a significant role and transport will be affected by environmental conditions. In this study, anion transport (bisulfide) under various conditions was examined using experimental and numerical techniques to understand the various geochemical and surface mediated reactions that are occurring in the bentonite. The case study presented is for the use of bentonite in long term storage of nuclear waste but can be extended to other applications.

First, diffusion experiments were performed to examine the transport and reactive nature of bisulfide (HS^-) through bentonite compacted at dry density of 1090-1330 kg m^-3. Experimental data of bisulfide transport were fitted using the inverse solution technique of Hydrus-1D model and different fitting parameters (e.g., diffusion, sorption, and reaction sink). Simulation results suggest that the HS^- sorption/reaction affecting itsdiffusive transport through bentonite can be modeled using a simple nonlinear adsorption process.

Second, batch experiments were performed to understand the maximum allowable sorption that could take place under key geochemical conditions, including temperature, pH, and ionic strength. The results of batch sorption experiments performed suggest that HS^- sorption increases with increasing temperature but decreases with increasing pH and ionic strength.

Lastly, since transport and reactive processes are interconnected, the results of these experiments were incorporated into a 1D transport COMSOL model to understand which geochemical process governs bisulfide transport through bentonite. Various processes were examined including linear and non-linear sorption, reactive transport, and anion exclusion. The model was validated using the experiments and showed that HS^- was retained in the bentonite due to reactive processes and anion exclusion effects.

How to cite: Krol, M., Chowdhury, F., Papry, S., Asad, M. A., Mondal, P., Rashwan, T., and Molnar, I.: Anion Transport Through Bentonite Under Various Geochemical Conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14166, https://doi.org/10.5194/egusphere-egu24-14166, 2024.

In geothermal reservoirs, pore fluid chemistry can affect mechanical and hydraulic properties of rocks inducing mineral dissolution, precipitation, weakening and alteration. Moreover, with increasing pressure and temperature deformation mode can change from localized to ductile (diffused), leading to a major decrease in permeability, thus affecting the exploitability of the reservoir. The effect of fluid chemistry on the transition between localized and ductile deformation of rocks is still marginally understood.

To investigate the effect of water presence and fluid chemistry on localized and ductile deformation, two sets of triaxial experiments (at 20 and 100 MPa effective confinement pressure, in the localized and ductile field respectively) were performed on a porous silicate sandstone (Adamswiller sandstone), dry, with deionized water and with a 6 M NaCl solution (Na⁺ rich solution), with a 0.1 M HCl solution (pH 1 solution) and a 0.1 M NaOH solution (pH 13 solution). To complement the mechanical properties, complex spectral electrical conductivity was measured during deformation to monitor pore fluid ion content; pore fluid was collected at the beginning and at the end of the experiments and analyzed with ICP-MS; post-mortem microstructural analyses were performed.

In the localized domain, both water presence and pore fluid chemistry had a marginal effect on the strength of the rock, leading to a 5/10% strength reduction over dry rock strength indipendently of the pore fluid composition. In the ductile domain, de-ionized water weakens the rock by 25%, a Na⁺-rich or pH 1 fluid leads to a 35% weakening and a pH 13 fluid weakens the rock by 40% over dry rock strength. Spectral electrical conductivity does not change during localized deformation, while it increases by 2 to 5 times during ductile deformation when the rock is saturated with deionized water; conductivity does not change with the Na⁺-rich fluid regardless of the deformation mode and conductivity decreases by half an order of magnitude with both pH 1 and pH 13 fluids both with localized and ductile deformation.

How to cite: Lazari, F., Meyer, G., and Violay, M.: Effects of pore fluid chemistry  on the localized to ductile transition of sandstone., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15531, https://doi.org/10.5194/egusphere-egu24-15531, 2024.

The escalating global temperature necessitates immediate action to mitigate greenhouse gas emissions. Geological Carbon Storage (GCS) technology has emerged as a promising solution, specifically by injecting carbon dioxide (CO2) into geological formations, particularly deep saline aquifers. However, finding ideal geological reservoir conditions, including caprock stability and storage capacity, is a rare occurrence.

This study comprehensively assesses the potential for CO2 storage in the Triemli saline aquifer in Zurich, Switzerland. The goal is not only to demonstrate the feasibility of CO2 storage in Switzerland but also to emphasize the viability of decentralized storage with multiple small injection point, for emitters like medium -sized cities, in regions with geologically challenging subsurface conditions. Through numerical simulations, we explore CO$_2$ injection, migration, and long-term reservoir stability to bridge the gap between theoretical estimates and practical feasibility.  Our findings underscore the potential of deep saline aquifers for CO2 storage in Switzerland, particularly in the Swiss Molasse Basin and the adjacent Folded Jura, identified as crucial regions for effective CO2 storage. Employing advanced methods and strategic injection techniques, such as multiple vertical or horizontal injection points along a single well, could optimize this storage capacity to approximately 3 million tons of CO2 over the same period.

How to cite: Gunatilake, T., Pio Rinaldi, A., Zappone, A., Zhang, Y., Zbinden, D., Mazzotti, M., and Wiemer, S.: Decentralized CO2 storage in unfavorable conditions: An example from Switzerland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16640, https://doi.org/10.5194/egusphere-egu24-16640, 2024.

Deep underground geological disposal is widely accepted as one of the most appropriate ways for the long-term safety and management of radioactive waste. The heat generated by the nuclear waste decay brings elevated temperature increase, which may affect the thermo-hydro-mechanical (THM) behaviour of the host rock. A correct evaluation of the thermal impacts on the host rock behaviour is important for the design of the underground geological disposal.

With the temperature increases, thermal pressurization is observed both in the small (laboratory) and large (in-situ) scale tests. Physically, the overpressure induced by the discrepancy of the thermal expansion coefficient between the solid and fluid phases may potentially induce fracture re-opening and propagation. The host rock located in the middle of two adjacent cells may suffer shear or tensile failure, which is dependent on the intensity of the thermal power and the distance between the neighbouring cells. Some research work also shows that soil characteristics like cohesion, elastic modulus and water viscosity are influenced by the rise in temperature [1]. To investigate the thermally induced change on the mechanical property of host rock, triaxial compression tests were conducted at the University of Lorraine at different temperatures (20, 40, 60, 80, 100 and 150 °C), confining pressures (0, 4 and 12 MPa) and samples orientations (parallel and perpendicular to the bedding plane). The results showed the transitory overpressure induced by the thermal dilation during the initial heating, and the degradation of the mechanical strength of the host rock with the increase in temperature [2].

Based on the experimental observations, the triaxial compression tests are represented in a two-dimensional axisymmetric coupled THM model. The modelling is composed of the two steps: isotropic loading (increase confining stress and temperature), and shear process (increase axial loading). The numerical FEM code is LAGAMINE from the University of Liège. The Callovo-Oxfordian (COx) claystone, relying on its low permeability and good plasticity, has been selected as the host rock for the underground geological disposal in Meuse/Haute-Marne in France. The objective of this study is to introduce thermal-mechanical modelling involved with thermal plasticity. The cohesion of the host rock is defined as a function of the temperature to describe the thermally induced change of mechanical behaviour of the host rock. This model will then be validated against experimental observations in the laboratory and further applied to the large-scale heating test.

ACKNOWLEDGEMENT

This study was performed in the framework of the European Joint Programme on Radioactive Waste Management (EURAD). EURAD has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 847593.

REFERENCES

[1] Laloui, L. and Cekerevac, C., 2003. Thermo-plasticity of clays: an isotropic yield mechanism. Computers and Geotechnics, 30(8), pp.649-660. Doi : https://doi.org/10.1016/j.compgeo.2003.09.001.

[2] Gbewade, C.A.F., Grgic, D., Giraud, A. and Schoumacker, L., 2023. Experimental study of the effect of temperature on the mechanical properties of the Callovo-Oxfordian claystone. Rock Mechanics and Rock Engineering, pp.1-22. Doi : https://doi.org/10.1007/s00603-023-03630-7.

How to cite: Song, H. and Collin, F.: A thermal-mechanical constitutive modelling for Callovo-Oxfordian Claystone in the context of nuclear waste disposal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16781, https://doi.org/10.5194/egusphere-egu24-16781, 2024.

In the field of radioactive waste confinement, the question of gas transfers in clay formations is a crucial issue. A certain amount of gas, such as Hydrogen may be generated during the exploitation phase in the nearfield by the deterioration of the metal components of the system. As the host medium is characterised by a very low permeability, various gas transport processes could occur as a function of gas accumulation and pressurization [1], including the development of preferential gas pathways through the sound rock mass, which could lead to undesirable changes in the favourable containment properties of the host rock.

There is a growing body of experimental evidences [2, 3] that natural heterogeneities and pre-sxisting fractures in clay-rich materials represent preferred weaknesses for the process of opening discrete gas-filled pathways. Capturing the related transport mechanisms therefore requires to go from macroscopic to microscopic scale. Hence, a multi-scale modelling approach that models the micro-scale effects explicitely on their specific length scale and couples their homogenized effects to the macro-scale is proposed in the present work. Based on a periodicity assumption of the microstructure, a relevant Representative Element Volume (REV) is defined based on experimental data, which makes it possible to idealise the flow behaviour of the material microstructure with different families of discontinuities, and an assembly of tubes substituting the porous matrix blocks. This complete hydraulic constitutive model is solved at the scale of the microstructural constituents, and is directly affected by the mechanical effects tackled at the macroscopic scale, which makes the whole model hydro-mechanically coupled.

This model has been subsequently applied to simulate a gas injection test parallel and perpendicular to the bedding of initially saturated samples of Boom Clay [3]. This analysis provides a rather good agreement with the experimental results in terms of pressure response, outflow volume and average axial strain. In addition, it allows to simulated the creation of a preferential flow pathway along the sample axis (Figure 1b, top), which serves as basis to numerically reproduce the development of random pathways through the sample in plane strain state (Figure 1b, bottom), and aims to improve the mechanistic understanding of the gas transport processes at play in clayey barriers.

[1] P. Marschall, S. Horseman, and T. Gimmi. Characterisation of Gas Transport Properties of the Opalinus Clay, a Potential Host Rock Formation for Radioactive Waste Disposal. Oil & Gas Science and Technology Rev. IFP, 60(1):121-139, 2005. 

[2] Harrington, J.F., Milodowski, A.E., Graham, C.C., Rushton, J.C., & Cuss, R.J. (2012). Evidence for gas-induced pathways in clay using a nanoparticle injection technique. Mineralogical Magazine, 76(8):3327–3336.

[3] Gonzalez-Blanco, L., Romero, E., Jommi, C., Li, X. & Sillen, X. (2016). Gas migration in a Cenozoic clay: Experimental results and numerical modelling. Geomechanics for Energy and the Environment, 6:81–100.

How to cite: Corman, G. and Collin, F.: A multi-scale model to study gas transport processes in clay materials, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16823, https://doi.org/10.5194/egusphere-egu24-16823, 2024.

The successful deployment of carbon dioxide (CO₂) geological sequestration in porous media is reliant on the sealing efficiency of the overlying, clay-rich caprock to act as a physical barrier. Clay-rich caprock formations are considered favorable materials to act as a seal due to them characteristically consisting of small pores providing high capillary entry pressures, hence preventing the intrusion of a non-wetting fluid.

The juxtaposition and availability of deep seated (buried) caprock-reservoir systems to carbon capture and storage clusters may not be available. Therefore, the assessment of shallow seated, weakly consolidated caprock-reservoir systems (e.g., Sleipner) will be required. Our experimental campaign tests analogous caprock geomaterials which have relatively high compressibility, representative of shallow seated (buried or less indurated) clay-rich caprocks.

Past experimental campaigns demonstrate that CO₂ breakthrough is dominated by the creation of very localized channels across the sealing barrier which occur at pressures far lower than the one predicted by the Laplace’s equation [1]. However, limited data characterizing these pathways exists. Furthermore, the physical indicators of susceptibility which underly the micro-mechanisms of failure (e.g., fracturing), are still only postulated for clay-rich geomaterials.

where Pc* is the capillary breakthrough pressure [kPa], ψ, reflects pore shape [-], T_s, interfacial tension between water and gas (e.g., CO₂), and θ, represents wettability [°].

An innovative experimental set-up which allowed for the onset of surface crack formation to be captured during gas injection (representing the non-wetting fluid in CO₂ geological sequestration) into intact clay-rich geomaterials is presented. This allowed for the investigation of physical indicators of susceptibility to gas breakthrough via localized pathways.

Results on different fracture patterns when non-wetting gas (i.e., air) is injected into consolidated clay show the formation of large fractures that nucleate from within the sample. Upon air pressurization, before fracture formation, the sample undergoes volumetric deformation (i.e., consolidation), as the resulting action of the vertical stress applied at the air-water interface (menisci). Once a fracture forms deformation stops and breakthrough occurred at lower pressures than traditionally recorded. The mechanisms of air intrusion are expected to be of a similar nature as CO₂ intrusion. Post-mortem assessment of the internal nature of these localized pathways was then visualized using xCT imaging.

As a continuum mechanics framework will not predict fracture formation under our test conditions, it appears that the experimental evidence support the underlying hypothesis that disjoining pressure governs the mechanisms that ultimately control fracture formation and thus, eventually CO₂ breakthrough. The disjoining pressure is governed by the electrostatic double-layer interactions, van der Waal’s dispersion forces, structural forces, and solvation forces.

If the pore size distribution is such that high gas pressures are required to overcome capillarity, the gas pressure will force single clay particles apart, displacing water form adjacent interparticle spaces. This represents a localized failure mechanism at the clay interface, resulting in fracture nucleation. It is expected that clay displaying large swelling pressures will subsequently display high gas entry pressure, termed “pathway dilation”. Therefore, pressurized gas will enter a dilated pathway at lower pressures than anticipated.

How to cite: Allsop, C., Pedrotti, M., and Tarantino, A.: Insight into the micromechanisms of gas breakthrough in water-saturated clay-rich geomaterials – Implications for CO2 sequestration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17117, https://doi.org/10.5194/egusphere-egu24-17117, 2024.

The utilisation of geothermal heat as a form of clean energy is experiencing global growth. Many sites designated for geothermal energy extraction are in regions with elevated heat gradients, such as Iceland and Japan. However, the repercussions of the migration of cold fluids in rock formations at high temperatures, and the subsequent fracturing of the host rock, which is relevant in the context of deep geothermal energy systems, remains insufficiently understood.  In this study, we explore the three-dimensional evolution of fractures in 50×9.8×1 mm³  homogeneous slabs of various brittle rocks, subjected to a thermal shock of ΔT=580°C, through numerical simulations over 10 seconds. Initially, we validate our numerical approach using a benchmark of Al₂O₃  slab, and subsequently, we examine fracture development in granite, basalt, and shales. Our numerical methodology employs a three-dimensional finite-element-based simulator to model thermo-mechanical deformation. The in-house code is a fully coupled THM code which considers damage to predict fracture initiation. Fracture growth is predicted per fracture tip using stress intensity factors. The code uses adaptive meshing and NURBS for fracture surfaces to facilitate mesh-independent fracture growth. In our simulations, we apply triangles and tetrahedra elements to discretise surface and volume elements, respectively. Our results show the development of dozens of fractures in bi- or tri-modality, which penetrate up to 85% of the depth of the slab, for the various simulations. These results demonstrate both qualitative and quantitative agreement between the simulated slab and the benchmark by reproducing the same intertwined short-long fracture patterns and modal distribution of fracture lengths. Furthermore, they illustrate how the fracturing rates (ranging between 1-100 mm/sec), fracture length distribution (unimodal, bimodal or trimodal), and penetration depth of fractures in front of the shock front vary among the different brittle rock types.

How to cite: Suchoy, L., Paluszny, A., and Zimmerman, R. W.: Fracture growth using fully coupled thermo-mechanical model in brittle rocks during thermal shock and resulting network patterns, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17626, https://doi.org/10.5194/egusphere-egu24-17626, 2024.

Concepts of High-Temperature Aquifer Thermal Energy Storage (HT-ATES) (> 50 °C) are investigated for system application in the Upper Jurassic reservoir (Malm aquifer) of the German Molasse Basin (North Alpine Foreland Basin). The karstified and fractured carbonate rocks exhibit favorable conditions for conventional geothermal exploitation of the hydrothermal resource. Here, to further assess the sustainability of HT-ATES development in the Upper Jurassic reservoir, a physics-based numerical analysis is performed. With an estimated heating capacity of ca. 21 MW over half a year, our approach aims at determining numerically the efficiency of heat storage under the in-situ Upper Jurassic reservoir conditions and locally feasible operation parameters.

The numerical models build upon datasets from three operating geothermal sites at depths of ca. 2000-3000 m TVD located in a subset of the reservoir which is dominated by karst-controlled fluid fluxes. Commonly considered as a single homogeneous unit, the 500 m thick reservoir is subdivided into three discrete layers based on field tests and borehole logs from the three considered sites. This introduced vertical heterogeneity with associated layer-specific enhanced permeabilities allows to examine potentially arising favorable heat transfer, and in combination with the facilitated high operation flow rates to evaluate thermal recoveries in the multilayered reservoir.

Computation results reveal that the reservoir layering induces preferential fluid and heat migration primarily into the high-permeability zone, while thermal front propagation into the lower permeable rock matrix is restricted. The simulations further display the gradual temperature increase in the warm wellbore and its surrounding host rock, and the consequent progressive improvement in the heat recovery efficiencies. Despite the elevated permeability that may trigger advective heat losses, heat recovery factor values range from ca. 0.7 over the first year of operation to over 0.85 after 10 years of operation. An additional scenario is examined with fluid injection solely in the high permeable zone, in order to quantify potential improvement in the recovery efficiency by omitting fluid injection in the lower-permeability layers where heat propagation is diminished. This is due to the geometrical shape of the thermally perturbed rock volume as heat losses occur at the interface between thermal front and adjacent reservoir rock. Consequently, conclusions on the performance of the two different system designs under this layered reservoir setting are derived. All simulations account for density and viscosity variation through the IAPWS (International Association for the Properties of Water and Steam) thermodynamic property formulations. Results show that density-induced buoyant fluxes which would considerably decrease thermal efficiencies are inhibited, and thus the prevailing heat transport mechanism is forced convection.

How to cite: Tzoufka, K., Blöcher, G., Cacace, M., Pfrang, D., and Zosseder, K.: Physics-based numerical evaluation of High-Temperature Aquifer Thermal Energy Storage (HT-ATES) in the Upper Jurassic reservoir of the German Molasse Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18636, https://doi.org/10.5194/egusphere-egu24-18636, 2024.

In order to gain insights into the hydraulic and thermal long-term behavior of deep geothermal boreholes during operation, a fiber-optic monitoring (FOM) system was implemented at the Stadtwerke Munich's (SWM) geothermal plant 'Schäftlarnstrasse' in the national project Geothermal-Alliance Bavaria (GAB). The permanently installed fiber optic cables enable continuous measuring of the temperature in boreholes with high spatial and temporal resolution via Distributed Temperature Sensing (DTS) and of acoustics/vibration via Distributed Acoustic Sensing (DAS). In 2019, one production and one injection well were equipped with FOM in different setups and have been collecting data since then. In the injector, the cable was cemented behind the anchor pipe in the upper section of the borehole, whereas the producer was equipped with a fiber-optic cable until the total depth of the reservoir. Additionally, a fiber-optic gauge provides pressure and temperature at the top of the reservoir in this well.

Among other things, precise inflow profiling in the reservoir section of the producer could be carried out using the temperature data, which helps deepen the understanding of the hydraulic behavior of the reservoir. Changes over time in the temperature of the produced thermal water can be traced back to changes in the hydraulically active zones or inflow temperatures in the reservoir. Other factors that influence the wellhead temperature, such as borehole heat losses to the surrounding rock, can be quantified using the DTS data.

At the beginning of 2024, a third fiber-optic cable will be installed in an injection well at the Schäftlarnstrasse geothermal site. A similar setup as in the production well will allow for continuous measuring in the entire borehole until total depth (TD) and pressure and temperature from p/T gauges at the top of the reservoir and at TD. Therefore, both production and injection conditions in the reservoir and wellbore will be continuously monitored at all depths using DTS and DAS for the first time.

We present the first interpretations of the data collected from the newly installed FOM in the injection well as well as insights into the long-term monitoring of the hydraulic and thermal dynamics in the production well to underline the importance of permanent downhole monitoring to ensure efficient and sustainable use of the geothermal reservoir.

How to cite: Andy, A., Schölderle, F., Pfrang, D., and Zosseder, K.: Operational Monitoring of Thermal Dynamics in Deep Geothermal Production and Injection Wells with Fiber Optics from the Surface to the Reservoir, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19122, https://doi.org/10.5194/egusphere-egu24-19122, 2024.

Many natural processes and energy-related project in the subsurface involve the interactions between the porous rock and the fluids inside the pores. In particular, the fluid can interact with the solid matter by various dissolution-precipitation reactions that can modify the microstructure geometry and thus change the mechanical behavior of the rock and its failure potential or directly induce a creep of the rock. These chemo-mechanical couplings can have important implications for storage applications and induced seismicity. In this contribution, we will show discrete element simulations performed at the microstructural scale of porous matter. First, we will show that the dissolution of the cement in sedimentary rocks strongly influence the lateral earth pressure coefficient. The value of this coefficient tends to an attractor by increasing the degree of dissolution, which can lead to stress redistribution at the reservoir scale and promote faulting or induced seismicity. Secondly, we will show a numerical framework coupling discrete element with a phase field model allowing to capture grains shape changes due to local precipitation or dissolution. This model is applied to study the phenomenon of intergranular pressure-solution and allows to reproduce the creep behavior of the material in compaction. It enables also to study the competition between grain rearrangement and pressure solution in fault gouges to induce a rate dependency of the fault mechanical behavior.

How to cite: Rattez, H., Sac-Morane, A., and Veveakis, M.: Weathering and creep in rocks modelled at the microscale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19876, https://doi.org/10.5194/egusphere-egu24-19876, 2024.

Depleted oil and gas reservoirs are attractive sites for CO₂ sequestration. However, the injection of CO₂ into depleted reservoirs carries the potential for significant Joule-Thomson cooling, when dense, supercritical CO₂ is injected into a low-pressure reservoir. The resulting low temperatures around the well-bore risk causing thermal fracturing and/or freezing of pore waters or precipitation of gas hydrates which would reduce injectivity and jeopardise near-well stability. Injection into reservoirs at subcritical pressure also leads to a phase transition from liquid to vapour CO₂. This is accompanied by cooling, due to the latent heat of vaporisation, and dramatic changes in fluid properties including density, compressibility and viscosity.

We present models of non-isothermal flow of CO₂ in the near well-bore region, which demonstrate the controls on cooling and constrain the different pressure-temperature regimes that can emerge. We show that during radial injection, with fixed injection rate, transient Joule-Thomson cooling can be described by similarity solutions at early times. The positions of the CO₂ and thermal fronts are described by self-similar scaling relations. The scaling analysis here identifies the parametric dependence of Joule-Thomson cooling. We present a sensitivity analysis which demonstrates that the primary controls on the degree of cooling are the injection rate and Joule-Thomson coefficient. The analysis presented provides a computationally efficient approach to assessing the degree of Joule-Thomson cooling expected during injection start-up, providing a complement to full numerical simulations.

How to cite: Tweed, L., Neufeld, J., and Bickle, M.: Joule-Thomson cooling and phase transitions during CO2 injection in depleted reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20645, https://doi.org/10.5194/egusphere-egu24-20645, 2024.

Surplus heat as stored in an ATES (Aquifer Thermal Energy Storage) system in summer could partly meet the increasing demand of energy in winter. Better assessment on the wellbore integrity permits sustainable operation of such systems. Therefore, an artesian flow test was conducted in a research well located in Berlin, Germany. In this test, artesian flow of 16.8°C from Jurassic sand at depths from 220 m to 230 m was produced at 14°C and at a flow rate of 3.6m³/h from the annular space between the production casing and the anchor casing. The depth-resolved temperature at the production casing as monitored using the distributed temperature sensing (DTS) technique manifested the depths of the artesian aquifer. A hydro-thermal coupled numerical model for the artesian flow was calibrated by matching the simulated flow rate to the wellhead-measured values. The simulated and the DTS-monitored temperatures suggested that the heating-up in the near-wellbore materials by the artesian flow was hindered by the deployment-related inclusion of water behind the anchor casing, and the cooling in these materials in the shut-in test stage was enhanced by such inclusion. 

How to cite: Blöcher, G., Pei, L., Kranz, S., Cunow, C., Virchow, L., and Saadat, A.: Analysis of Wellbore Integrity using DTS Monitoring and Numerical Modelling in the Practice of ATES, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21553, https://doi.org/10.5194/egusphere-egu24-21553, 2024.

The complex pore throat structure of tight sandstone reservoirs increases the difficulty of reservoir exploration and development, and becomes a key factor affecting oil and gas charging and reservoir oil content control. Quantitative evaluation and characterisation of the pore-throat structure is also important for maintaining and improving oil recovery. In order to determine the microstructure of pore throats, a series of experiments such as casting thin section, scanning electron microscopy, X-ray-rock diffraction experiment and high pressure mercury injection (HPMI) were carried out on the tight oil sandstone samples of the Chang 6 member in the south-eastern Ordos Basin, China. The relationship between each factor and pore throat complexity was investigated using factors such as reservoir physical properties, pore throat characteristic parameters and mineral composition.

The results indicate that the main pore types of the sample are intergranular pores, feldspar dissolved pores, intergranular dissolved pores and micro-cracks. Meanwhile, the pore throat structure of each sample was identified as large pores and small pores by combining the result of HPMI with fractal theory. The small pores of Chang 6 tight reservoirs are the dominant factor affecting the pore throat structure. The development of small pores positively influences the maximum pore radius, median radius, sorting coefficient, maximum mercury saturation and mercury recovery efficiency, indicating that with the development of small pores, the properties of percolation, storage, pore throat connectivity and oil recovery would become better. Fractal theory was used to quantitatively characterise the complex and irregular pore throat structure of the reservoir. The total fractal dimension D ranged from 2.3601~2.7403 with an average of 2.5722. The average fractal dimension D1 was 2.8094 and the average fractal dimension D2 was lower than D1 with an average of 2.5325, indicating the complex structure and strong heterogeneity of the former. D2 has better correlations with the pore throat structure parameters than D1. The heterogeneity and surface roughness of small pores mainly influence the properties of the pore throat structure, especially the percolation and storage space of the reservoir. The development of small pores is a major factor influencing the pore throat structure of the Chang 6 tight oil sandstone. The fractal characteristic of small pores reflects how well the pore throat structure is suited for exploration and development of Chang 6 tight oil.

How to cite: Zhang, H.: Pore-throat structure and fractal characteristics of tight sandstone: A case study in Yanchang Formation, Southeast Ordos Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2263, https://doi.org/10.5194/egusphere-egu24-2263, 2024.

Geological storage of energy and carbon dioxide requires a deep knowledge of the complex thermo-hydro-chemo-mechanical processes that affect the stability and performance of reservoir rocks. In this study, we investigate the thermo-mechanical properties and mesoscale fracture behaviors of four key minerals in granites: quartz, plagioclase, amphibole, and biotite. We use a combination of experimental and analytical techniques to reveal the microscale mechanisms of rock failure under high temperature and tensile loading.

We perform nanoindentation tests under dynamic heating–cooling cycles to measure the reduced modulus and hardness of the minerals. We also conduct mode I fracture tests under tensile loading conditions to evaluate the fracture toughness and tortuosity of the granite. We observe the dynamic crack propagation and fracture morphology of minerals using scanning electron microscopy. We analyze the structural and physio-chemical changes at high temperatures using X-ray diffraction, thermogravimetric analysis, and Fourier’s transform infrared spectroscopy.

We find that the thermo-mechanical properties and fracture behaviors of the minerals are governed by three main factors: the alterations in mineral structure, the aperture of open cracks along cleavage planes, and the degree of heterogeneity due to mineral composition complexity. We identify two different damage modes in granite: catastrophic and non-catastrophic failure modes. We explain the underlying mechanisms of each mode and show that catastrophic failure has precursory signs, while non-catastrophic failure does not.

Our results provide new insights into the microscale mechanisms of rock failure under high temperature and tensile loading, which have implications for the macroscopic understanding of rock behavior in geological storage applications. By elucidating the microscale intricacies, this study enhances our understanding of the multifaceted interactions that influence the stability and performance of rocks, and supports the development of improved geological storage strategies, such as carbon sequestration and enhanced energy storage.

How to cite: Mo, C., Zhao, J., and Zhang, D.: Thermo-mechanical properties and fracture behavior of granite at pore scale: implications for geological storage, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2751, https://doi.org/10.5194/egusphere-egu24-2751, 2024.

Many geothermal plants in the north alpine foreland basin (NAFB) are affected by the precipitation of calcium carbonate and struggle with efficiency losses and sometimes safety problems. The adaption and implementation of predictive maintenance strategies relies on the accuracy of the prediction and make sense if the costs of early maintenance are significantly less compared to the costs of a replacement of failed parts. Therefore, the accurate description of the kinetics of inorganic precipitations have to be extended to include the fluid dynamics and the interaction of the precipitates with different materials used in the geothermal cycle. The experimental concept also applies to fluid-rock interactions which can alter the properties of the reservoir.

The parametrization of transferable hydrogeochemical models performs best with data on a single interface, single crystal level. This data allows to elucidate the underlying process kinetics and improve existing strategies. The combination of Raman spectroscopy and Quartz crystal microbalance (QCM) opened a way to quantify the formation of carbonate precipitates. Here, the QCM can measure the total mass of attached particles while Raman microscopy identifies the crystal polymorph. Running these experiments in microfluidic channels allows to assess the effects of physical stress on the formation and to test inhibition and removal stratgies.

The QCM sensor was placed in a microfluidic channel and tap water (carbonate rich, Ca²⁺ concentration approx. 2.25 mM, pH approx. 7.50) and sodium hydroxide (0.10 M) were injected through the two inlet channels. As the lime carbonic acid equilibrium shifts due to the pH increase, precipitations are formed. The adhesive forces on different materials (SiO2, aluminium, steel) were studied by changing the flow velocities and chemical cleaning processes were mimicked by injecting an acid (HCl).

The precipitation of CaCO₃ on the QCM sensor (sensitivity in the low mg-range) was less than 20% of the theoretical amount. This underlines the importance to include the fluid dynamics into the assessment models. The experimental data under dynamic conditions was modelled with a combination of CFD simulations with PhreeqC: The particle flow velocity and the precipitates formed depend on the depth as well as local equilibrium changes. The preferred location of scaling could be adequately simulated. This quantification of the effects of the shear stress and the material properties on the scaling efficiency is a further step towards predictive maintenance strategies and a solid comprehensive site assessment during the planning stage, which should improve the sustainability and the much-needed attractiveness of this energy sector.

How to cite: Zacherl, L. and Baumann, T.: Pore scale assessment of disrupted hydrochemical equilibria in geothermal systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3433, https://doi.org/10.5194/egusphere-egu24-3433, 2024.

CO₂-fluid-rock interactions during CO₂ injection in tight oil reservoirs, i.e. asphaltene precipitation and geochemical reactions, alter the pore-throat structure and wettability, which will in turn affect the CO₂ displacement efficiency. In this work, pore structures of tight sandstone from the Ordos Basin, China were characterized comprehensively using the X-ray diffraction, cast thin sections, scanning electron microscopy, nuclear magnetic resonance, and high-pressure mercury intrusion. In addition, pore structure and asphaltene precipitation before and after CO₂ displacement were analyzed to clarify the influences of CO₂-fluid-rock interactions on the pore throat structure and wettability of tight sandstone reservoirs, including mineral dissolution, asphaltene precipitation location and aggregation mode. The results indicate that the Chang8 tight sandstone can be divided into three classes based on pore throat combination: narrow channel intergranular pore type (I), wide channel and dissolution pore type, intergranular pore type (II), and mixed type (III). The porosity of the three types gradually decreases and the percentage of small pore throats increases. With the injection of CO₂, secondary pores are formed due to the dissolution of feldspar in Class I reservoirs, and asphaltene precipitates in the macropores, mainly distributed on the surface of mineral particles and clustered in a point-like manner. The impact on the macropore throat is relatively small, and the wettability is reversed towards oil wetting. In the Class II reservoir, feldspar dissolution produces a large number of secondary pores, and the proportion of pore throat of fine and small pores developed by asphaltene precipitation in Clay mineral increases. Asphaltene content in Class III reservoir is precipitated in a large amount in small pores and throats, and accumulates in clusters. Asphaltene precipitation is usually concentrated more closely in the pore throat structure where Chlorite group and Illite are more developed, and core wettability is constantly changing to the direction of strong oil wetness. The asphalt content and wetting reversal index of Class I are the highest, while Class III is the lowest.

How to cite: Zhen, Z., Xin, C., and Yiqian, Q.: Micro mechanism of reservoirs damage caused by CO2 displacement in tight reservoir, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3456, https://doi.org/10.5194/egusphere-egu24-3456, 2024.

CO₂-oil-rock interactions and complex pore structure affect the sweep efficiency (E_S) and wettability, thus having a significant impact on CO₂ enhanced oil recovery in tight oil reservoirs. In this study, we selected 10 rock plugs from the Yanchang Formation, Ordos Basin in China. First, casting thin sections and mercury intrusion capillary pressure were performed to investigate the microscopic pore structure characteristics of the tight rock samples. The results show that pore structure can be divided into three types (RT-I, RT-II, and RT-III) from good to poor qualities. On this basis, CO₂ floodings using the Nuclear Magnetic Resonance technique were performed to investigate the influence of pore structure on the E_S in large (P_L) and small (P_S) pore throat intervals. With the increase of displacement pressure, the oil recovery of RT-I, RT-II and RT-III are about 70.9%, 67.8% and 10.16%, respectively. The E_S of P_L of all samples are similar, while the E_S of P_S decrease subsequently for the three types. Pressure, mineral composition and the complex pore structure are attribute to the differences. On one hand, higher displacement pressure leads to lower interfacial tension and viscosity, resulting in higher oil recovery. On the other hand, CO₂ is more likely to vaporize the light oil components, resulting in the asphaltene precipitation. Quartz with a smooth surface is not easy to precipitate, while most clay minerals are easier to absorb asphaltene and are likely to alter the wettability of pore surfaces. Thus, in comparison to RT-III, the E_S of RT-I with a higher quartz content is higher in P_S. In addition, the worse the relationship between pore structure configurations, the greater the capillary pressure, causing the different E_S between RT-I and RT-II. The findings in this study shed a light on the understanding of complex mechanisms for CO₂ EOR in tight oil reservoirs.

How to cite: Cheng, Y., Qu, Y., and Cheng, X.: Effect of pore structure and CO2-oil-rock interactions on sweep efficiency of CO2 EOR in tight sandstone reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3479, https://doi.org/10.5194/egusphere-egu24-3479, 2024.

Clarifying the size, morphology, connectivity of pores and throat, and pore throat structure is crucial for studying the CO₂ storage efficiency in tight sandstone reservoirs. In this study, a series of experiments were carried out on tight sandstone samples from the Yanchang Formation of the Ordos Basin, China to study pore throat structure, including cast thin section analysis, scanning electron microscopy, pressure mercury intrusion and nuclear magnetic resonance test. The results show that the major reservoir space is typically formed by the combination of intergranular pores and dissolution pores. Pore and throat combination types mainly include large pore-narrow throat, conduit-like pore throat combination, and tree-like network combination. The pore-throat fractal curve has one segment, while the throat has two segments. The fractal dimensions of narrow throat (Dt1) is larger than that the fractal dimensions of wide throats (Dt2), indicating that small throat has higher self-similarity and more homogeneous distribution. The large pore-narrow throat type and conduit-like pore throat combination type contribute the major permeability, the tree-like network combination type has good connectivity of pore throats and contributes the major storage space. The wide throats connecting the intergranular pores constrain the flow of fluids in the tight sandstone. The study of the fluid movability based on bound water modeling shows that the higher proportion of the conduit-like pore throat combination type, the more homogeneous the pore throat structure is, bound water is redistributed on the pore-throat surface to form a thin water film after centrifugation, leading to a higher proportion of movable fluid, and the overall seepage capacity was stronger. A large number of laumontite dissolved pores as well as intercrystalline clay pores exit from the tree-like network combination type, and the strong capillary pressure causes a low proportion of moveable fluid.

How to cite: Lan, S., Cheng, X., and Qu, Y.: Effect of different pore throat combinations on fluids flow of tight reservoirs in the Ordos Basin, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3485, https://doi.org/10.5194/egusphere-egu24-3485, 2024.

The characterization of reservoir rocks depends on the absolute permeability as a crucial parameter. To estimate this property numerically, one can employ a combination of digital rocks and Stokes flow simulation through the Lattice Boltzmann Method (LBM). In previous studies, the LBM has typically been implemented as an iterative process, wherein iterations are repeated until the parameter estimates between consecutive iterations reach a certain threshold level. However, we argue that this termination criterion is unsuitable and may compromise the accuracy of simulation results.

In this study, we investigate the convergence of the LBM through various tests, including the Poiseuille flow between parallel plates and different types of digital rocks (such as dune sand, sandstone, and carbonates). We find that the logarithm of the relative error, when compared to the estimate at infinite time (representing a stable state), demonstrates a linear relationship with the number of iterations. This linear relationship suggests an exponential rate of convergence. On the other hand, if we rely on the difference in errors between consecutive iterations as the termination condition, the simulation may not reach a stable state.

Instead, we propose a more accurate termination criterion for the LBM simulation by analyzing the decay trend of the error difference. This criterion provides a practical and appropriate approach for the characterization of reservoir rocks. Additionally, we offer a theoretical explanation for the convergence rate, which is linked to the spectral radius of the iterative matrix in linear algebra.

How to cite: Liao, Q.: Convergence Behavior of Lattice Boltzmann Method for Pore-scale Modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4648, https://doi.org/10.5194/egusphere-egu24-4648, 2024.

Horizontal well multi-stage and multi-cluster fracturing technology has played an important role in unconventional reservoir development. However, traditional methods rarely investigated fracturing fluid flow characteristics in reservoirs with abundant natural fractures. Furthermore, affected by the stress interference from dense number of fracturing clusters, some perforations do not form fractures. For deep shale reservoirs, failure to consider these contents may result in unsatisfactory fracturing results for deep shale reservoirs.

An innovative approach is introduced in this paper. This innovation can be based on natural fracture development, fracture propagation law and reservoir composition characteristics. The flow regularity of fracturing fluid inside natural fractures was characterized through CT scanning experiments. Numerical simulation is used to analyze fracture propagation in fracturing horizontal well with multi-stage and multi-cluster. Core full component test experiment was conducted to analyze compressibility. In response to the above results, different fracturing process is adopted, the length of fracturing section is adjusted, the number of fracturing clusters is set. Then the fracturing design scheme of each segment is formulated and the fracturing effect and operation lessons of actual wells are analyzed. 

The results show that obvious pressure interference between developed section and undeveloped section of natural fracture, fracturing fluid first enters the extended natural fracture and then enters other fractures. Therefore, the temporary plugging of the extended natural fracture weakens its preferential tendency to enter the fluid and ensures the uniform migration of fracturing fluid to each fracture. Numerical simulation shows that some perforations do not form effective fractures, due to stress interference caused by excessive number of perforations. Hence, a fracturing scheme with fewer perforation times is adopted to reduce stress interference and improve the efficiency of perforating fracture formation. The results also observe that the reservoir contains plenty of brittle minerals such as calcite, which is easy to cause fractures. However, high silicon content easily form, extremely irregular fracture. Consequently, the fracturing scheme of shortening the length of fracturing section is adopted to strengthen the control of fracture expansion. The fracturing evaluation of the effect shows that the initial production of the well was 6.59 ×10⁴ m³ with a flowback rate of 15.5%. Microseismic monitoring data shows that the reconstruction volume has increased from the original 9.70 ×10⁴ m³ to 1.06 10⁵ m³, the fracturing effect is remarkable.

The conclusion of this research supports for deep shale fracturing design and has a vital practical application significance. Compared with conventional fracturing methods, the fracturing performance is significantly improved results from weakening the advanced fluid tendency of natural fractures, decreasing the stress interference between clusters and strengthening the fracture control.

Key words: deep shale; natural fracture; stress interference; brittle mineral content; temporary plugging technology

How to cite: Di, S., Ma, S., Wei, Y., Cheng, S., Miao, L., and Liu, M.: Effective fracturing strategy considering natural fracture, stress interference and rock component—A practical application of Dingshan block in Sichuan Province of China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5027, https://doi.org/10.5194/egusphere-egu24-5027, 2024.

In low-permeability reservoirs, strong molecular interactions exist at the solid-liquid interface, necessitating the overcoming of the threshold pressure gradient in shale oil extraction. A profound comprehension of molecular interactions between oil and reservoir matrix is crucial to develop a productive strategy for enhanced oil recovery. Molecular dynamics simulation has become an important method for analyzing microscopic mechanisms of some static properties and dynamic processes. In this study, a molecular model of shale oil was built based on the reported experimental results and simulation. Subsequently, the molecular model was utilized to build a flow model within three matrix pores: kerogen, quartz, and portlandite. A comprehensive analysis of the interfacial effects and size effects on the threshold pressure gradient was undertaken. Emphasis was placed on elucidating the influence of the adsorption behaviors (stable adsorption, unstable adsorption, non-adsorption) of polar components at the interface on the flow of shale oil. The utilization of the critical shear stress facilitated the accurate prediction of the threshold pressure gradient of shale oil within large pores. Moreover, within the context of the flow model of shale oil in nanoscale pores, we conducted some explorations into oil displacement by CO₂. This work suggests fresh ideas for studying the oil-matrix interactions on the nanoscale and provides theoretical guidance for shale oil exploitation.

How to cite: Cui, F. and Wang, F.: Micromechanical mechanism of oil-rock interaction and the availability analysis of shale oil, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6006, https://doi.org/10.5194/egusphere-egu24-6006, 2024.

Mineral wettability plays a pivotal role in determining residual oil distribution and devising effective displacement strategies for enhanced oil recovery. Through molecular dynamics simulations, we investigated the surface wettability of various typical minerals, revealing the complete spreading of modeled crude oil over most mineral surfaces. Our research introduces an efficient method for calculating the spreading coefficient of modeled crude oil on mineral surfaces, allowing accurate predictions of its spreading state with a notable reduction in calculation time compared to traditional methods. Furthermore, the study explores the impact of various components within crude oil on mineral surface wettability, emphasizing microscopic interactions between these components and minerals. In nanopore channels, the diverse wettability of oil/rock interface results in varied occurrence forms, such as droplets, films, or columns, which are also different in the water flooding process. In addition, with the introduction of various components in crude oil, due to their different interactions with oil/water/rock, we found that these components can be evenly mixed with modeled crude oil or exist at the oil-water interface. Therefore, the introduction of this component changes the properties of oil-water interface, affects the form of oil occurrence and the process of flooding. These insights contribute to a comprehensive understanding of mineral surface wettability and its correlation with crude oil composition, providing valuable guidance for optimizing reservoir management and refining production strategies in the pursuit of enhanced oil recovery.

How to cite: Fan, J. and Wang, F.: Nanoscale Wettability of Oil/Rock Interface and its Impact on Occurrence and Flooding, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6961, https://doi.org/10.5194/egusphere-egu24-6961, 2024.

CO₂ injection can effectively promote the development and utilization of shale oil. The interaction between CO₂, shale oil and pore structure has attracted much attention as a key mechanism for oil resource exploitation. Therefore, we established a pore network model, injected shale oil and CO₂ successively into the model, and studied the occurrence state of miscible fluid at the nanoscale. This study takes the shale of Lucaogou Formation in Jimsar Sag of Junggar Basin as the research object. Nano-CT and scanning electron microscopy are used to observe the pore structure of shale layer by layer, and the pore structure is transformed into a molecular model layer by layer, and finally superimposed into a pore network molecular model. Through the analysis of crude oil group components and chromatography-mass spectrometry, the characteristics of crude oil components are identified and the corresponding molecular models are established. The occurrence state of shale oil in the pore network model is simulated by molecular dynamics. CO₂ is injected into the system to simulate the occurrence state of the miscible fluid. The fluid density in the system is analyzed, the interaction force between CO₂, shale oil and pore structure is calculated, and the capacity models of adsorbed oil, free oil and CO₂ storage are established. The reliability of the model is verified and applied by combining production data and experimental tests. This study plays a crucial role in advancing the understanding of CCUS (carbon capture, utilization, and storage) and the geological theory of shale oil and gas. It also has the potential to overcome the challenges and limitations in shale oil production technology, thus making significant contributions to this field.

How to cite: Lin, X. and Li, Z.: The mutual feedback mechanism between CO2 and multiphase fluids during CO2 injection into shale oil reservoirs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8286, https://doi.org/10.5194/egusphere-egu24-8286, 2024.

The reconstruction of Digital Rock is a crucial challenge in understanding the microstructure of rocks and its impact on pore-scale flow through numerical modeling. This is particularly significant due to the typically large samples required to address inherent uncertainties. Despite notable advancements in traditional process-based techniques, statistical methods, and recent popular deep learning models, there is a limited focus on deep learning approaches specifically tailored for reconstructing rocks with predefined properties, such as porosity. To address this gap, our research employs Artificial Intelligence Generative Component (AIGC) technologies to precisely generate rock structures with specified properties. Our experimental results demonstrate the successful application of our method in reconstructing rock images based on target properties. The generation of randomly reconstructed samples with distinct rock properties holds promise for advancing research in pore-scale multiphase flow and uncertainty quantification in subsequent studies.

How to cite: Cao, Q. and Chen, Y.: Rock Reconstruction with Deep Generative Network, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9227, https://doi.org/10.5194/egusphere-egu24-9227, 2024.

The tight sandstone reservoir features the development of micro- and nano-scale pores and micro-fractures, contributing to a complex pore-throat structure. This complexity results in an indistinct charging sequence of oil and gas in different types of storage spaces during the accumulation period, thereby escalating the challenges associated with the exploration and development of tight oil and gas. Focusing on the Fuyu oil reservoir in the northern Songliao Basin, this study integrates large-field-of-view stitched scanning electron microscopy with mineral surface scanning techniques. Innovatively, a micro-nano scale comprehensive reservoir evaluation method, incorporating both pores and micro-fractures, is proposed. Within the study area, three predominant pore-fracture combination types are identified: intergranular pore-clay mineral shrinkage fractures, intergranular pore-brittle mineral intergranular fractures, and intragranular pore-clay mineral shrinkage fractures. Building upon this, Wood's alloy, exhibiting high-temperature rheological properties, is injected into rock cores under various pressure conditions. It is observed that with increasing injection pressure, the alloy injection process demonstrates a structured order, with a clear preference for charging intergranular pore-fractures over clay mineral-related pores. Furthermore, the alloy injection efficiency curve exhibits a distinctive parabolic shape. Based on the characteristic properties of Wood's alloy and crude oil, the injection pressure is equivalently transformed, reconstructing the micro-scale charging process of tight oil under reservoir conditions. Consequently, a sequential charging model for tight reservoirs is established, encompassing micro-nano-scale intergranular pore-fractures, nano-scale clay mineral intragranular pores, and shrinkage fractures. This model considers parameters such as source-reservoir pressure difference, storage space type, fluid properties, etc. From both qualitative and quantitative perspectives, it clarifies the microscopic accumulation sequence of tight reservoirs. This research focuses on the development of a new multi-scale evaluation method for tight reservoir storage spaces, combining fluid injection with visualization technology. The findings are crucial for the microscopic sweet spot evaluation and efficient development of tight reservoirs.

How to cite: Liu, J. and Jiang, Z.:  Investigation of Pore-Fracture Charging Sequences in Tight Reservoirs Based on Multi-Stage Pressure Injection Experiments with Wood's Alloy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12139, https://doi.org/10.5194/egusphere-egu24-12139, 2024.

In conjunction with scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS), quasi-static nanoindentation has been widely used to investigate the mechanical properties of minerals and organic matters in shale at micro scale. However, due to the limited test efficiency of conventional nanoindentation measurement and the demand of mineralogical identification which is achieved by SEM observation and EDS analysis, the research scheme in previous works can be time-consuming and complicated. This work attempts to develop a new micromechanical research scheme with high test efficiency and automatic mineralogical identification. A newly-developed high-speed nanoindentaion technique is used to characterize the mechanical properties distribution of a shale sample from the Yanchang Formation in the Ordos Basin, China. Then, the mineralogical distribution in the corresponding areas is obtained by using MAPS Mineralogy. Finally, logistic regression is applied to link the mechanical properties distribution and mineralogical distribution, and to realize the automatic mineralogical identification based on nanoindentation results. In addition, to further investigate the influence of characterization experiments on machine learning results, the characterization abilities, including lateral spatial resolution, detection depth, and signal spacing, of the two experimental methods are compared. The detection depth of MAPS Mineralogy is markedly higher than that of nanoindentation, which means that the material volume detected by the two methods is different. The lateral range responding to applied force and incident electrons determines that the signal of data points at the boundary can be a mixture of two or more minerals. The influence of such detection depth difference and boundary effect is also discussed.

How to cite: Jiang, S., Zhao, J., and Zhang, D.: Machine Learning for mechanical classification of organic-rich shale based on high-speed nanoindentation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13458, https://doi.org/10.5194/egusphere-egu24-13458, 2024.

The injection of carbon dioxide (CO₂) into coal seams serves a dual objective, namely augmenting coalbed methane (CH₄) production and providing a geological CO₂ storage solution, thereby offering both economic and environmental advantages. Given the typically water-rich nature of underground porous media, the impact of water on CO₂-enhanced coalbed methane (CO₂-ECBM) becomes a crucial factor that cannot be ignored. In order to assess the influence of varying reservoir initial water saturation and the physical parameters of coal rock on the CO₂-ECBM process, we have developed a comprehensive Thermal-Hydro-Mechanical-Chemical (THMC) model utilizing COMSOL software. The simulation results indicate a substantial enhancement in CH₄ recovery with increased CO₂ injection pressures, demonstrating a positive correlation between higher CO₂ injection pressures, augmented CO₂ storage rates, and enhanced CH₄ production efficiency.

With the increase of displacement time and injection volume, the effective displacement radius increases, but the influence radius increment decreases gradually. The reason is that the competitive adsorption of CO₂ in the matrix has a higher adsorption capacity than CH₄, resulting in a decrease in reservoir permeability. In addition, the higher the water saturation, the lower the influence radius and the displacement effect. The reason is that the existence of reservoir water limits the migration speed of gas in the reservoir to a certain extent. In the mechanical field, the permeability of coal decreases slowly, and the output of methane gas in the production well is affected.

The mechanical properties of coal reservoir are closely related to the evolution of permeability. The mechanical parameters of coal in this paper are mainly Young 's modulus and Poisson 's ratio. The results show that low Young 's modulus and high Poisson 's ratio show higher deformation resistance, and the output effect of CH₄ is better.

How to cite: Qin, C. Y.: Numerical Insights into Water Saturation Effects on CO2 Sequestration in Enhanced Coalbed Methane Recovery, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14077, https://doi.org/10.5194/egusphere-egu24-14077, 2024.

    The characterization of the microscopic pore systems in organic-rich shales is crucial for comprehending the occurrence mechanisms and flow behaviors of shale oil. Although various image processing techniques have advanced the study of shale pore systems recently, challenges such as unclear boundaries of pore structures, interwoven connectivity, high similarity, and complex topological structures remain unresolved. In this study, a comprehensive investigation of the multiscale pore structure in organic-rich shale is presented, through the examination of 20 lacustrine shale samples from the Paleogene Kongdian Formation. These samples were analyzed using a variety of techniques, including N2 adsorption, mercury intrusion capillary pressure (MICP), nano X-ray CT, and Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM). Furthermore, We innovatively propose a machine learning-based multi-objective panoramic segmentation modeling method. This approach allows for precise segmentation and rapid panoramic modeling of various instances across different semantic categories and the gaps between these instances. It enables the creation of a more comprehensive multi-scale porous network model, which will be more conducive to future simulations of multi-physical processes such as fluid dynamics permeation models.
    We combine the dilated convolutions suitable for semantic segmentation with the feature pyramid structure favorable for instance segmentation to achieve precise panoramic segmentation. This approach accurately segments and represents various components in SEM images, including interparticle pores, intraparticle pores, organic pores, microfractures, feldspar, quartz, dolomite, calcite, clay minerals, and organic matter.In the three-dimensional reconstruction of FIB images, we innovatively employ registration based on contextual relationship sequences to accurately expand the reconstruction scope of pore pathways. Simultaneously, the use of an octree data structure index in constructing pore network structures enhances efficiency and speed. 
   The results show that the overall pore sizes range from 5 nm to more than 50 μm, consisting of abundant nanopores and a small quantity of micropores, and the dominant pores are in the range of 5 nm -200 nm. Through three-dimensional characterization of different types of pore networks, the transport behavior of shale oil within nanoslits was simulated, and it is proposed that fluid migration path is mainly controlled by the content of minerals, whether laminae are developed, and organic matter content. This study offers a promising solution for optimizing the automatic processing of microscopic images for pores, the combination of methods can provide pore structure characterization from sub-nanoscale to macroscale, spanning four orders of magnitude, which is crucial for improving the understanding of reservoir mechanisms and the hydrocarbon potential of lacustrine shale.

How to cite: Li, G., Xin, B., and Li, Z.: Applying Machine Learning Methods Based on Panoptic Segmentation, Context Registration, and Octree Indexing for Multiscale Pore Structure and Connectivity of the Organic-rich shales in Bohai Bay Basin, East China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19274, https://doi.org/10.5194/egusphere-egu24-19274, 2024.

The SMILE network, part of the Marie Sklodowska-Curie doctoral network funded by the Horizon Europe (2021-2027) program, undertakes an innovative role in addressing geo-energy project challenges while promoting net-zero greenhouse gas emissions in line with global sustainability goals.

A segment of the SMILE network is dedicated to addressing the issue of ground deformation analysis. This research aims to develop a software tool that capitalizes on Global Navigation Satellite System (GNSS) data, coupled with Synthetic Aperture Radar (SAR) inputs and ground modeling information. This abstract outlines the methodological framework for the anticipated software development.

In the initial phase, we aim to integrate the high temporal resolution GNSS data with the high spatial resolution of SAR data. The goal of this process is to combine the advantages of both data types while minimizing their limitations. SAR provides extensive spatial detail but has limited temporal frequency and directional sensitivity. On the other hand, GNSS data provides comprehensive three-dimensional vectors with high temporal frequency, but spatially limited to the points where GNSS stations are located.

Merging the displacement measurements from GNSS and SAR requires temporal synchronization and the reconciliation of their different displacement vectors: GNSS captures vertical, east, and north components, and SAR measures in the line-of-sight direction. The optimal joint operation for this task is proposed through a Kalman filter. Due to the complexity of building a joint filter, the proposed method seeks to first analyze by considering the projected displacements only in the vertical direction. In this case, the measure in the line-of-sight of the SAR satellite will be projected in the vertical direction.

The next phase will focus on an innovative approach to enhance the covariance matrix within the Kalman filter algorithm. Instead of using a homogeneous and isotropic constant covariance in time, this enhancement strategy will harness observed data as input. Primarily, it will smooth the covariance evolution in time, exploiting past observations; this may improve the stability of the outcomes. The method under development proposes to improve the covariance modeling further by enabling the consideration of anisotropic and non-homogeneous scenarios. Finally, the proposed method aims to integrate the monitoring data into Thermo-Hydro-Mechanical (THM) modeling.

The proposed expansion is expected to bring significant advancements in ground deformation analysis, improving its resolution and precision. The tool will integrate GNSS and SAR datasets into a comprehensive ground deformation analysis suitable for geomatics applications within geo-energy projects.

How to cite: Aponte, O., Gatti, A., Realini, E., Barzaghi, R., and Sansò, F.: An Integrated GNSS-SAR Approach to Improve Ground Deformation Analysis in the Field of Geo-energies: activities planned within the SMILE project, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1896, https://doi.org/10.5194/egusphere-egu24-1896, 2024.

The extraction of geothermal energy faces the hazard of H₂S, a highly toxic and strongly corrosive gas. H₂S exposure can lead to the failure of oilwell cement, decreased extraction efficiency, and even pose serious risks to operational personnels near the wellsite. High temperature is a prominent environmental feature in geothermal resource extraction. However, current research works primarily focus on the corrosion effects of H₂S on cement at moderate to low temperatures. This study utilizes Class G oilwell cement to conduct corrosion experiments of cement by H₂S under high temperature in a H₂S-rich reaction vessel. The impact of H₂S on the structure, chemical composition, and mechanical strength of oilwell cement is analyzed via SEM-EDS, XRD, nanoindentation tests, and unconfined compressive strength tests. The results indicate a reduction in compressive strength for cement samples corroded by H₂S. The surface nano-hardness and elastic modulus of cement samples decrease while the internal values of nano-hardness and elastic modulus significantly increase. Under the corrosion of H₂S, the structure of cement is characterized by a yellow and black surface layer and stratified cracks. The external surface of the cement exhibits a yellow color due to the formation of pyrite (FeS₂), while internally, pyrrhotite (FeS) and gypsum (CaSO₄^.2H₂O) are generated.

How to cite: Zhang, L., Yin, Y., Mei, K., Cheng, X., Wang, Y., and Wang, H.: Degradation pathways and mechanisms of oilwell cement exposed to H2S under high temperatures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2815, https://doi.org/10.5194/egusphere-egu24-2815, 2024.

Within the context of CO₂ injection, understanding the properties of potential caprocks, particularly clay-rich ones such as Opalinus Clay and their potential evolution is crucial for safe and effective carbon capture and storage (CCS) initiatives. This study presents hydrogeochemical models developed to investigate interactions between groundwater, Opalinus Clay caprock and CO₂, focusing on chemical evolution under varying pCO₂ levels across different layers of the clay.

Utilizing a comprehensive dataset derived from CO₂ injection experiments conducted at the Mont Terri Rock Laboratory and published pore water chemistry of Opalinus Clay, hydrogeochemical models of a potential reservoir and caprock system were constructed employing PHREEQC and CHESS geochemical modeling softwares. These models were designed to simulate and comprehend the intricate processes governing groundwater-CO₂-rock interaction within the reservoir-caprock interface and through the stratified layers of Opalinus Clay.

The models aimed to elucidate the chemical evolution of the groundwater as it interacts with the Opalinus Clay under different pCO₂ conditions. By considering variations in pCO₂ levels representative of potential CCS scenarios, the simulations provided insights into the geochemical alterations occurring within the caprock and their implications for its integrity over time.

The findings of these hydrogeochemical models offer valuable insights into the potential consequences of CO₂ injection into reservoirs whose caprocks are formed of Opalinus Clay, informing risk assessment and mitigation strategies for CCS applications. Moreover, these constructed hydrogeochemical models not only serve as a crucial foundation for comprehending the intricate thermal-hydro-mechanical-chemical (THMC) coupling mechanisms within caprocks like Opalinus Clay but also contribute to a deeper understanding of the complex interplay between pore water chemistry, rock properties, and varying pCO₂ levels, essential for ensuring the long-term security and effectiveness of subsurface CO₂ storage.

How to cite: Koç, Ü., Corvisier, J., Bruel, D., and Blanco Martin, L.: Hydrogeochemical Modeling of Opalinus Clay: Insights from CO2 Injection Experiments at Mont Terri Rock Laboratory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3952, https://doi.org/10.5194/egusphere-egu24-3952, 2024.

In the realm of harnessing geothermal energy, groundwater heat pumps exhibit superior thermal efficiency when compared to closed-loop geothermal heat pumps. However, the outlook for progress remains less than promising. A primary obstacle stems from the improper extraction of groundwater, which can disrupt the stress field in subsurface structures and potentially trigger land subsidence. This concern is particularly pronounced in unconsolidated soils found in coastal sedimentary plains, where the additional threat of severe flooding disasters looms. Furthermore, artificial pumping processes may give rise to coupled hydraulic phenomena, exemplified by the reverse water level fluctuations. This transient anomalous changes in hydraulic head occurs in adjacent aquifers during the initial stages of pumping from a confined aquifer, induced by strain propagation. The magnitude of the hydraulic head elevation varies from a few centimeters to several tens of centimeters, posing a challenge to the accurate interpretation of groundwater monitoring data for land subsidence prevention. Geothermal heat pump systems can also induce changes in the temperature and thermal strain of geological layers. Consequently, understanding how this strain-induced hydraulic head responds to temperature fluctuations becomes a research question. In our investigation, the hydraulic and mechanical responses of a three-layer aquifer system to groundwater pumping were assessed through thermoporoelastic numerical simulations. The simulated reverse hydraulic head changes align with field observations documented in the literature. The findings of this numerical investigation indicate that when we recharged the water into the ground, the initial head decrease is likely to occur in proximity to a recharge well within an unpumped clay layer. These deformation-induced head changes eventually dissipate following the hydraulic propagation of unsteady state drawdown from the pumped aquifer into adjacent layers. In the event of reverse water level fluctuation, it is shown that temperature has no obvious influence on the hydraulic variation, both the head variation and happening time. It is also suggested that the propagation of thermally induced strain is slower than that of hydraulically induced strain and the effect appears much later.

How to cite: Yue, L. and Aichi, M.: Numerical Investigation of Reverse Water Level Fluctuations under Non-Isothermal Conditions with a Fully Coupled Thermal-Hydro-Mechanical Model for Geothermal Heat Pump Systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5401, https://doi.org/10.5194/egusphere-egu24-5401, 2024.

Subsurface uncertainty is large because we have limited access to it. Reducing uncertainty is important for geo-energies in order to increase the reliability of the simulation results used to define safe operation conditions. In particular, subsurface uncertainty can be reduced by analyzing ground deformation. A clear example of this is the CO₂ storage project at In Salah, Algeria, where a double lobe ground deformation shape revealed the presence of a vertical fault zone at depth. Here, we propose a workflow to reduce subsurface uncertainty by inferring the subsurface characteristics from ground deformation measurements. As a foundation step of the workflow, we train a supervised neural network-based regression approach for predicting ground deformation caused by reservoir pressurization due to fluid injection. To train the neural network, we use a recently developed analytical solution to assess ground displacement in response to pressurization of a reservoir that is intersected by a permeable or an impermeable fault (https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4503451). The instantaneous solution provided by the analytical solution allows us to generate a large dataset to train the neural network. We have varied eleven variables, including fault and reservoir geometry and mechanical properties. Simultaneously, a simplified parametric space analysis is also performed. Results show that the reservoir thickness, Biot´s coefficient, and the pore pressure buildup impact the displacement the most. This study highlights that an appropriately trained neural network can effectively predict the ground deformation and give insights into the corresponding subsurface characteristics.

How to cite: Guo, T., Wu, H., and Vilarrasa, V.: A Neural Network to Reduce Subsurface Uncertainty Based on Ground Deformation Measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5492, https://doi.org/10.5194/egusphere-egu24-5492, 2024.

PSI Technique remains a powerful remote sensing method in terms of ground deformation monitoring, which makes it useful for monitoring geo-energy projects such as geothermal and CO2 sequestration where movement is always detected. The monitoring is a crucial supporting component to ensure the smooth progress of geo-energy development. However, PSI Technique still faces some problems that affects the accuracy of the detection. This can be caused by data and related to the terrain of the study areas, such as vegetation, buildings, and the direction of the ground deformation. The goal is to counteract this is by combining multiple sensor images, both low resolution and high resolution. These can be obtained from the wide range of satellites available today such as Sentinel-1, TerraSAR, COSMO-SKYMED, and NISAR. This technique is supposed to increase the temporal sampling for a more comprehensive time series, redundancy to improve accuracy and robustness of PSI analysis, wider coverage by exploiting at each site the data that offers best performances. Aside from the expected improvement, some challenges in developing this technique will be presented. Most of the challenges are due to the difference in satellites’ characteristics, such as resolution, pixel spacing, wavelength, and geometry. The technique is planned to be applied on several study areas. The purpose of this is to study the effects of different terrain characteristics on the re-sampling process, tuning processes, and the result of the processing. One of the study areas will serve as the benchmark of the research.

How to cite: Ramlie, M. C., Olea-Encina, P., Crosetto, M., and Monserrat, O.: Improving PSI Capabilities on Ground Deformation Monitoring for The Application of Geo-Energy Projects. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6245, https://doi.org/10.5194/egusphere-egu24-6245, 2024.

2023 was claimed as the beginning of the “Global Boiling Era”. For this reason, geo-energies are key to provide a green and clean future. Geothermal energy, and geologic carbon injection/storage are the main types of geo-energies. Both have in common the underground fluid movement and the consequent ground motion dynamics.

One of the main techniques for analyzing ground motion is Persistent Scattered Interferometry (PSI), which allows us to estimate ground deformation over time from radar satellite data. PSI techniques calculate the temporal displacement in the so-called persistent scatterers by filtering the data based on amplitude value. Generally, the main cause for amplitude variability is the change in the surface properties over time, primarily due to changes in environmental factors (land cover/land use, vegetation dynamics, temporal water presence, or soil moisture). The main concern with these changes is that they may result in phase shifts, which could be misinterpreted as range displacements.

Improving the understanding of the environmental factors could improve the understanding of ground deformation over time. Therefore, environmental factors analysis and PSI can be integrated seamlessly into a workflow since the launch of the Copernicus Programme, combining Sentinel-1 and Sentinel-2 data. The goal of this research is to explore the relationship between environmental variables and amplitude values. This research is framed in the MultidiSciplinary and MultIscale approach for assessing coupLed processes induced by geo-Energies (SMILE) Project. The study site is the Illinois Basin – Decatur Project (IBDP), which is a carbon dioxide injection and storage located in the United States. IBDP is in the north part of Decatur city, the vegetation is mainly woodland and prairie; with a humid subtropical climate, with an annual precipitation over 1000 mm.

The analysis focuses on three areas near IBDP: wetland area, crop area, and woodland area. Performing a spatio-temporal analysis of the vegetation index (NDVI), soil moisture index (NDWI), and temporal water presence (NDWI) obtained from Sentinel-2 data between 2015 to 2023 and comparing these datasets with the amplitude time series from Sentinel-1 imagery for the same period. The results show that the woodland area has a high mean amplitude value with low dispersion; the wetland area has a low mean amplitude value with high dispersion; and the crop area has a medium mean amplitude value with medium dispersion.

The vegetation index for the woodland area has 7 month per year values over 0,4, showing a presence of high photosynthetic activity, which can be related to the low value of dispersion amplitude; while the wetland area only 3 months per year has a value over 0,4, that can be source of the high dispersion amplitude. While the crop area is 6 months over the threshold. The results of the analysis performed showed that the use of environmental indices can help in the interpretation of the dispersion amplitude in the PSI analysis. The next step is to consider more environmental variables such as snow cover, land cover/land use, or temperature in the analysis of amplitude, as well as consider different polarizations, among others.

How to cite: Olea-Encina, P., Ramlie, M. C., Monserrat, O., and Crosetto, M.: Relationship between Environmental Factors and Radar Amplitude: Illinois Basin – Decatur Project case study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6264, https://doi.org/10.5194/egusphere-egu24-6264, 2024.

Long-term seismic monitoring arrays are often deployed to shallow boreholes to reduce the seismic noise. We investigate noise level decay in shallow boreholes. A large number of publicly available data with such deployment is available at the seismic monitoring array near the town of Groningen, which allows also characterization of the seismic noise decay in shallow boreholes in urban environments. We study noise distribution at 4 sites from this array. Each site includes 5 receivers deployed in shallow vertical boreholes with 50 meters intervals between the surface and 200 m depth. We show there is no difference between noise levels during the summer and winter at the borehole instruments. However, we observe diurnal variation at all depth levels. We also show there are higher noise levels throughout weekdays and lower during weekends and state holidays. These changes are not only observed at the surface but also at the deepest receivers. This implies that the dominant source of this noise is anthropogenic and it penetrates to depths of 200 meters even at frequencies exceeding 5 Hz. This observation is contradicting the common assumption that the seismic noise consists of the surface waves.

How to cite: Kalinichenko, O., Eisner, L., Stanek, F., Waheed, U., Hanafy, S., and Jechumtalova, Z.: Decay of seismic noise at shallow boreholes: Observations from Groningen., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7497, https://doi.org/10.5194/egusphere-egu24-7497, 2024.

Injection of dense-phase CO2 in a saline sandstone aquifer involves several processes which ideally work together to ensure effective long-term storage. The main processes are flow of free-phase CO2 (controlled by viscous and gravity forces), residual trapping at the pore scale, structural trapping at the scale of geological heterogeneities and dissolution in the aqueous phase.  Assessment of possible lateral and vertical migration along high-permeability layers or faults and fractures will also require stress-sensitive flow models which consider the phase behaviour of CO2, and the associated coupled thermal-hydraulic-mechanical-chemical processes.
Many insights into these complex processes can be obtained by analysis of the time-lapse seismic data at Sleipner CO2 storage project in Norway, in conjunction with findings derived from the quantitative and qualitative uncertainty analysis of the medium-scale flow experiments at the Mont Terri Rock Laboratory (Switzerland), involving periodic injections over long-term (CO2LPIE). The insights at Sleipner include the effects of internal shale layers and shale breaks in controlling the actual multi-layer CO2 distributions, the likely contribution of different of trapping mechanisms and the effectiveness of the overlying caprock. Another set of insights gained from these data are estimates of the effective use of the pore space at different length scales. The seismic imaging datasets can be used to show that at the scale of whole storage unit the overall storage efficiency is in the range of 2-5%, with the result depending very much on how the storage volume is defined. When the effects of areal and vertical sweep efficiency are considered, the fraction of the pore space occupied by CO2 rises to around 40-50%.
We illustrate these multi-scale processes using seismic data, analytical analysis, example flow models and 3D/4D visualization. Use of Invasion Percolation (IP) flow models is contrasted with multiphase (finite difference) flow simulators. With this approach CO2 migration problems will be addressed, as well as various open-source research codes will be used to develop enhancements for handling fluid mixing between hydrocarbon and CO2 phases in brine-saturated media. Moreover, coupled thermal modelling is found to be important at this site due to significant temperature changes as the CO2 plume expands and rises within the formation. Next to classic PC screen display, we empower the visualization by using the extended reality (XR) platform for geoscience, BaselineZ. This allows for true 3D (holographic) display in virtual reality (VR) as well as remote and interactive collaboration around the Sleipner dataset and thus leads to new insights. 

How to cite: Yun, T. K., Macut, M., Schulze, K., Ringrose, P., and Berg, C. F.: CO2 storage in saline aquifers: multi-scale processes visualized using the Sleipner case, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8100, https://doi.org/10.5194/egusphere-egu24-8100, 2024.

Various geo-energy applications such as geothermal energy, Carbon Capture and Storage (CCS) and underground heat storage are some of the technologies leveraged to realize Paris Agreement goals by cutting greenhouse gases emissions (GHG). However, these applications require in-depth understanding of the effect of coupled Thermal, Hydraulic, Mechanical and Chemical (THMC) processes that take place in the deep subsurface geological layers. This study focuses on the modeling of THM coupled processes in heterogeneous layered reservoirs.

When geological heterogeneities exist in the subsurface, each layer will have its own thermal, geomechanical and geological properties such as thermal conductivity, thermal expansion coefficient, porosity, permeability, Young’s modulus, Poisson’s ratio and so on. These variations in the properties add more complexity to the behavior of coupled THM processes of the system, which in turn requires sophisticated modeling approaches. To reduce complexity, the subsurface layers can be bundled into groups called “geomechanical facies” based on the mentioned material properties from THM characteristics perspective.

The work in this paper is based on key generic features of actual geo-energy applications where simulation modeling has been utilized to demonstrate how coupled THM processes are affected in heterogenous layers compared to homogenous layers. Hypothetical heterogenous layers have been divided into sets of distinct geomechanical facies. OpenGeoSys (OGS) open-source Finite Element based THMC code was utilized to build the run simulation models.

The results demonstrate that variations in the rock thermal, hydraulic and mechanical properties among different neighboring layers have a significant and individually different impact on stress mapping and distributions in addition to strain transfer to the surface. Geomechanical stability of the system parts that are more prone to failure such as fractures and faults were assessed using Factor of Safety (FOS) analysis which are based on stress distribution and rock mechanical properties. Results suggest that geological heterogeneity has more significant impact on hard rocks compared to soft rocks. The latter (i.e., soft rocks) have the ability to maintain sealing capability of caprocks because they are more ductile and have more room for further deformation prior to failure.

The simulation modeling results in this study contributes to the understanding of the key THM processes involved in heterogenous layered systems. Furthermore, this work provides valuable insights towards developing more generic design criteria and predictive models for various geo-energy applications which can be tailored and used in the design of the particular systems.

How to cite: Sidahmed, A. and Mcdermott, C.: How THM Changes in Layered Geological Systems Influence Stability of Fractured Networks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10213, https://doi.org/10.5194/egusphere-egu24-10213, 2024.

Deep geological repositories rely on a natural geological barrier with a low permeability to ensure radioactive waste containment and prevent radionuclide transport into the biosphere. For the Swiss repository concept, the Opalinus Clay formation is considered as the natural geological barrier. After high-level radioactive waste are disposed of in a repository within the host-rock, corrosion of metal containers in anaerobic conditions can result in the release of hydrogen. Due to the low-permeability of the host-rock, elevated gas pressures are expected. If the gas pressure exceeds the minimum principal stress, fracturing of the rock could occur. It is therefore important to assess the possible impacts of this process on the integrity of the repository such as a possible increase of the permeability of the host formation.

Production of hydrogen is anticipated to span more than 100,000 years, and understanding how gas transport occurs in a low-permeable host-rock is therefore an important aspect for the long-term safety of the repository. Gas transport can generally be subdivided in four different mechanisms: (1) advective-diffusive flow, (2) visco-capillary two-phase flow, (3) dilatancy-controlled gas flow and (4) gas transport along macroscopic tensile fractures. Gas flow rate and the microstructure of the host rock largely control the dominating gas transport mechanism, but the exact variables determining the process are poorly quantified.

The GT (Gas Transport) experiment at Mont Terri was designed to study the pressure and deformation effects after injecting gas into the Opalinus Clay. Helium was injected with increasing pressure increments until a gas breakthrough was observed. The borehole into which the helium was injected was surrounded by eight observation boreholes providing deformation and pore pressure observations.The results of the experiment have been used to create a coupled hydro-mechanical model using TOUGH-FLAC, which couples the multiphase flow and heat transport simulator TOUGH3 with the geomechanical simulator FLAC3D. The model uses the helium injection rates as input and computes the resulting pressure and deformation responses. By running models with different transport mechanisms (e.g. two-phase only, both two-phase and dilatancy, allowing fracture formation) and by comparing the pressure and deformation results with the observations, insight is gained into the dominant transport mechanisms in the Opalinus Clay. Preliminary results reveal a slow build-up of strain and a relatively small pressure drop after gas injection begins, suggesting that dilatancy-controlled gas flow occurred.

How to cite: Nuus, M., Rinaldi, A. P., Cuss, R., Sentis, M., Gisiger, J., Graupner, B., Magri, F., Bernier, F., and Jaeggi, D.: Modelling coupled hydro-mechanical processes of gas flow in the Opalinus Clay, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17949, https://doi.org/10.5194/egusphere-egu24-17949, 2024.

Understanding heat transfer mechanisms in subsurface environments is crucial for deep geothermal energy exploitation. Despite multiple studies on heat transfer in porous media, the combination of different heat transfer mechanisms with fracture networks leads to uncertainty in the temperature distribution in geothermal sites.

More specifically, heat convection and conduction are two major mechanisms responsible for heat transfer in porous media. Conduction occurs through rock matrix and is more dominant than convection in intact rocks due to their low porosity. However, fracture networks in rocks increase fluid transfer in certain directions, making convection more important in heat transfer. At a certain point, it becomes difficult to identify which mechanism is more prevalent. Besides this difficulty, achieving a balance between these two key mechanisms and determining the optimal Peclet number is very crucial for the geothermal energy industry to extract adequate volume of water at the desired temperature, which is essential for smooth operation of geothermal systems. In this study, a two dimensional coupled thermo-hydraulic model in COMSOL is developed to simulate heat transfer mechanisms in enhanced geothermal systems on field scale. Characteristic time variables based on hydraulic and thermal diffusivities are defined to monitor heat distribution through the domain caused by each mechanism. Finally, a sensitivity analysis is performed to identify how the temperature is affected by rock (thermal and hydraulic) parameters and fracture patterns. The simulation results indicate that the Peclet number is highly dependent on fracture network.

How to cite: Khezri, K., Jahangir, E., Bruel, D., and Abuaisha, M.: Investigating the Effect of Fracture Properties on Peclet Number of Enhanced Geothermal Systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18589, https://doi.org/10.5194/egusphere-egu24-18589, 2024.