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.

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.

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.

       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.

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.