Content:

EMRP – Earth Magnetism & Rock Physics

EMRP1.2 – Advances in Rock Physics and coupled THM reservoir processes

EGU2020-7237 | Displays | EMRP1.2

Upscaling laboratory measurements: Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources

Michael Heap, Darren Gravley, Ben Kennedy, Albert Gilg, Elisabeth Bertolett, and Shaun Barker

Hydrothermal fluids can alter the chemical and physical properties of the materials through which they pass and can therefore modify the efficiency of fluid circulation. The role of hydrothermal alteration in the development of geothermal and epithermal mineral resources, systems that require the efficient hydrothermal circulation provided by fracture networks, is investigated here from a petrophysical standpoint using samples collected from a well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show that, although quartz and adularia precipitation reduces matrix porosity and permeability, it increases the uniaxial compressive strength, Young’s modulus, and propensity for brittle behaviour. The fractures formed in highly altered rocks containing quartz and adularia are also more planar than those formed in their less altered counterparts. All of these factors combine to enhance the likelihood that a silicified rock-mass will host permeability-enhancing fractures. Indeed, the highly altered silicified rocks of the Ohakuri ignimbrite deposit are much more fractured than less altered outcrops. By contrast, smectite alteration at the margins of the hydrothermal system does not significantly increase strength or Young’s modulus, or significantly decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent permeability of a silicified rock-mass will be higher than that of a less altered rock-mass or a rock-mass characterised by smectite alteration, the latter of which provides a low-permeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses that are both suitable for geothermal energy exploitation (high-permeability reservoir and low-permeability cap) and more likely to host high-grade epithermal mineral veins, such as gold and silver (localised fluid flow).

How to cite: Heap, M., Gravley, D., Kennedy, B., Gilg, A., Bertolett, E., and Barker, S.: Upscaling laboratory measurements: Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7237, https://doi.org/10.5194/egusphere-egu2020-7237, 2020.

EGU2020-6028 | Displays | EMRP1.2

Acoustic signature of fluid substitution in reservoir rocks

Christian David, Joël Sarout, Christophe Barnes, Jérémie Dautriat, and Lucas Pimienta

During the production of hydrocarbon reservoirs, EOR operations, storage of CO2 underground or geothermal fluid exchanges at depth, fluid substitution processes can lead to significant changes in rock properties which can be captured from the variations in seismic waves attributes. In the laboratory, fluid substitution processes can be investigated using ultrasonic monitoring. 

The motivation of our study was to identify the seismic attributes of fluid substitution in reservoir rocks through a direct comparison between the variation in amplitude, velocity, spectral content, energy, and the actual fluid distribution in the rocks. Different arrays of ultrasonic P-wave sensors were used to record at constant time steps the waveforms during fluid substitution experiments. Two different kinds of experiments are presented: (i) water injection experiments in oil-saturated samples under stress in a triaxial setup mimicking EOR operations, (ii) spontaneous water imbibition experiments at room conditions.

In the water injection tests on a poorly consolidated sandstone saturated with oil and loaded at high deviatoric stresses, water weakening triggers mechanical instabilities leading to the rock failure. The onset of such instabilities can be followed with ultrasonic monitoring either in the passive mode (acoustic emissions recording) or in the active mode (P wave velocity survey).

In the water imbibition experiments, a methodology based on the analytical signal and instantaneous phase was designed to decompose each waveform into discrete wavelets associated with direct or reflected waves. The energy carried by the wavelets is very sensitive to the fluid substitution process: the coda wavelets are impacted as soon as imbibition starts and can be used as a precursor for remote fluid substitution. It is also shown that the amplitude of the first P-wave arrival is impacted by the upward moving fluid front before the P-wave velocity is. Several scenarios are discussed to explain the decoupling between P wave amplitude and velocity variations during fluid substitution processes.

How to cite: David, C., Sarout, J., Barnes, C., Dautriat, J., and Pimienta, L.: Acoustic signature of fluid substitution in reservoir rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6028, https://doi.org/10.5194/egusphere-egu2020-6028, 2020.

EGU2020-6615 | Displays | EMRP1.2

Laboratory testing for monitoring of reservoir properties during water injection

Davide Geremia, Christian David, Christophe Barnes, Beatriz Menéndez, Jeremie Dautriat, Lionel Esteban, Joel Sarout, Sara Vandycke, and Fanny Descamps

Since the performances of geological reservoirs are continuously changing during the production history, tools capable of characterizing these changes are becoming day by day of relevant importance. A time-lapse monitoring is an important and widely used way to explore the variations induced by the oil depletion. In an enhanced oil recovery scenario, the seismic survey method has been mostly used to monitor the remaining oil fraction with respect to the injected water, however no particular attention has been addressed to the effects generated by the water-rock interaction, which might induce deformation with no stress variation. Indeed, it is well known that water can induce important mechanical weakening in reservoir rocks.

For that purpose, we performed injection tests on carbonate rocks in a conventional triaxial cell. An essential characteristic of these tests is the very low injection pressure, in order to minimize changes in the effective stresses and focus specifically on the rock - fluid interaction. The test consists in injecting water from the bottom in a critically loaded sample, initially in a dry state, until deformation is induced by the water-air substitution and failure is reached. While testing, the rock sample is instrumented with either 6 (at UCP, GEC lab) or 16 (at CSIRO, Geomechanics and Geophysics lab) P-wave transducers allowing us to perform an active ultrasonic survey with a narrow time intervals.

The described methodology allowed us to monitor how P-wave attributes (amplitude, velocity and frequency), elastic moduli, as well as, permeability and injection rate change while water is flooding the sample increasing the water saturation, and damage is produced by the water – rock interaction. For instance, injecting water into a dry rock sample could produce several patterns of variations in the P-wave velocity, which we ascribed to 1) partial water saturation; 2) water-induced damage with no failure; 3) water-induced failure and, in some cases, 4) total water saturation. More experiments are planned to mimic real EOR operation, like injecting water in an oil-saturated rock sample, with acoustic monitoring as well.

The outcome of this study indicates that combining multiple data sets from different sources is an effective tool for monitoring the exploitation of underground resources. This can certainly enhance our understanding of reservoir properties changing over time and target the attention toward the areas of greatest interest.

How to cite: Geremia, D., David, C., Barnes, C., Menéndez, B., Dautriat, J., Esteban, L., Sarout, J., Vandycke, S., and Descamps, F.: Laboratory testing for monitoring of reservoir properties during water injection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6615, https://doi.org/10.5194/egusphere-egu2020-6615, 2020.

EGU2020-22373 | Displays | EMRP1.2

Anisotropy in soft rocks

Julia Leuthold, Elina Gerolymatou, and Theodoros Triantafyllidis

In this work, the mechanically induced compaction process in highly porous rocks is studied with experimental investigations and constitutive modeling. The focus of the study is on the influence of the inherent anisotropy on the mechanical properties. From a practical point of view, such behavior is of particular interest when considering reservoirs in soft, porous rocks. The reduction in pore pressure, which is linked to the production, leads to the possibility of compaction in the vicinity of the borehole. One effect is the risk of the loss of stability or of increased sand production. Another is the reduction of the permeability locally. The probability of such occurrences and the magnitude of such effects is currently under debate.

Although the formation of compaction bands in porous rocks has already been investigated in several studies, both in the laboratory and in situ, the extent data about the influence of the inherent anisotropy on the mechanical properties of porous rocks is limited. Baud et al. [1] documented an influence of the orientation of the bedding plane on the mechanical behavior of Diemelstadt sandstone and Louis et al.  [2] documented an influence of the bedding plane on the formation of discrete and continuous compaction bands in Rothbach Sandstone.

On the basis of an extensive experimental program of triaxial and isotropic compression, triaxial extension tests as well as investigations with ultrasonic pulse method, the mechanical behavior of a highly porous rock (Maastricht Calcarenite) is analyzed with a special focus on the formation of compaction bands. The test program is performed with samples cored under different inclinations to the bedding plane to study the influence of the inherent anisotropy on the mechanical properties.

Based on the experimental results, the applicability of a constitutive model for the description of the mechanical properties is tested. Furthermore it is examined how the inherent anisotropy may be considered in the constitutive model and different approaches are discussed.

For the numerical simulation a nonlocal model is suggested to simulate the formation of compaction bands. Finally, conclusions are drawn and an outlook on experimental investigations of the influence of compaction banding on the hydraulically properties is given.

 

[1]P. Baud, P. Meredith und E. Townend, „Permeability evolution during triaxial compaction of an anisotropic porous sandstone,“ Journal of Geophysical Research, May 2012.

[2]L. Louis, P. Baud und T.-f. Wong, „Microstructural Inhomogeneity and Mechanical Anisotropy Associated with Bedding in Rothbach Sandstone,“ Pure and Applied Geophysics, July 2009.

 

How to cite: Leuthold, J., Gerolymatou, E., and Triantafyllidis, T.: Anisotropy in soft rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22373, https://doi.org/10.5194/egusphere-egu2020-22373, 2020.

EGU2020-5417 | Displays | EMRP1.2

Time-lapse X-ray imaging of deformation modes in organic-rich Green River Shale heated under confinement

Maya Kobchenko, Anne Pluymakers, Benoit Cordonnier, and François Renard

Shales are layered sedimentary rocks, which can be almost impermeable for fluids and act as seals and cap-rock, or, if a shale layer hosts a fracture network, it can act as a fluid reservoir and/or a conduit.  Organic-rich shales contain organic matter - kerogen, which can transform from solid-state to oil and gas during shale burial and exposure to heat. When the organic matter is decomposing into lighter molecular weight hydrocarbons, the pore-pressure inside the shale rock increases and can drive propagation of hydraulic fractures and strongly modify the permeability of these tight rocks. Density, geometry, extension, and connectivity of the final fracture network depend on the combination of the heating conditions and history of external loading experienced by the shale reservoir. Here, we have performed a series of rock physics experiments where organic-.rich shale samples were heated, under in situ conditions, and the development of microfractures was imaged through time. We used the high-energy X-ray beam produced at the European Synchrotron Radiation Facility to acquire dynamic microtomography images and monitor different modes of the shale deformation in-situ in 3D. We reproduce natural conditions of the shale deformation processes using a combination of vertical load, confining and heating of the shale samples. Shales feature natural mineral and silt lamination and hydraulic fractures easily propagate parallel to these laminae if no overburden stress is applied. However, if the principal external load becomes vertical, perpendicular to the shale lamination, the fracture propagation direction can deviate from the horizontal one. Together horizontal and vertical fractures form a three-dimensional connected fracture network, which provides escaping pathways for generated hydrocarbons. Our experiments demonstrate that tight shale rocks, which are often considered as impermeable, could have hosted transient episodes of micro-fracturing and high permeability during burial history.

How to cite: Kobchenko, M., Pluymakers, A., Cordonnier, B., and Renard, F.: Time-lapse X-ray imaging of deformation modes in organic-rich Green River Shale heated under confinement , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5417, https://doi.org/10.5194/egusphere-egu2020-5417, 2020.

EGU2020-8454 | Displays | EMRP1.2

Seismo-electric Conversion in Sandstones and Shales using 2 Different Experimental Approaches, Modelling and Theory

Paul Glover, Rong Peng, Piroska Lorinczi, and Bangrang Di

The development of seismo-electric (SE) exploration techniques relies significantly upon being able to understand and quantify the strength of frequency-dependent SE conversion. However, there have been very few SE measurements or modelling carried out. In this paper we present two experimental methods for making such measurements, and examine how the strength of SE conversion depends on frequency, porosity, permeability, and why it is unusual in shales. The first is based on an electromagnetic shaker and can be used in the 1 Hz to 2 kHz frequency range. The second is a piezo-electric water-bath apparatus which can be used in the 1kHz to 500 kHz frequency range.

The first apparatus has been tested on samples of Berea sandstone. Both the in-phase and in-quadrature components of the streaming potential coefficient have been measured with an uncertainty of better than ±4%. The experimental measurements show the critical frequency at which the quadrature component is maximal, and the frequency of this component is shown to agree very well with both permeability and grain size. The experimental measurements have been modelled using several different methods.

The second apparatus was used to measure SE coupling as a function of porosity and permeability, interpreting the results using a micro-capillary model and current theory. We found a general agreement between the theoretical curves and the test data, indicating that SE conversion is enhanced by increases in porosity over a range of different frequencies. However, SE conversion has a complex relationship with rock permeability, which changes with frequency, and which is more sensitive to changes in the petrophysical properties of low-permeability samples. This observation suggests that seismic conversion may have advantages in characterizing low permeability reservoirs such as tight gas and tight oil reservoirs as well as shale gas reservoirs.

We have also carried out SE measurements on Sichuan Basin shales (permeability 1.47 – 107 nD), together with some comparative measurements on sandstones (0.2 – 60 mD). Experimental results show that SE conversion in shales is comparable to that exhibited by sandstones, and is approximately independent of frequency in the seismic frequency range (<1 kHz). Anisotropy which arises from bedding in the shales results in anisotropy in the streaming potential coefficient. Numerical modelling has been used to examine the effects of varying zeta potential, porosity, tortuosity, dimensionless number and permeability. It was found that SE conversion is highly sensitive to changes in porosity, tortuosity and zeta potential in shales. Numerical modelling suggests that the cause of the SE conversion in shales is enhanced zeta potentials caused by clay minerals, which are highly frequency dependent. This is supported by a comparison of our experimental data with numerical modelling as a function of clay mineral composition from XRD measurements. Consequently, the sensitivity of SE coupling to the clay minerals suggests that SE exploration may have potential for the characterization of clay minerals in shale gas and shale oil reservoirs.

How to cite: Glover, P., Peng, R., Lorinczi, P., and Di, B.: Seismo-electric Conversion in Sandstones and Shales using 2 Different Experimental Approaches, Modelling and Theory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8454, https://doi.org/10.5194/egusphere-egu2020-8454, 2020.

EGU2020-7213 | Displays | EMRP1.2

Hydraulic characterization of a karstic limestone vadose zone based on multi-methods geophysical measurements and lab testing

Clara Jodry, Céline Mallet, Jacques Deparis, Salma Ammor, Jean-Michel Baltassat, and Mohamed Azaroual

The vadose zone (VZ) is a highly heterogeneous and dynamic system that have a huge impact on fluid flows and heat transfer, from the soil to the saturated zone. In order to characterize flow patterns within the vadose zone, a comprehensive knowledge of spatial hydraulic parameters distribution is necessary. In this matter, geophysical techniques have proven to be efficient, providing various physical parameters and imaging of the underground. Nonetheless, these techniques are mainly used for water-saturated media and an appropriate calibration of the standard petrophysical relationships is necessary for VZ.

This study is carried out in the framework of the implementation, in an agricultural field, of the platform “Observatory of Transfers in the Vadose Zone” (O-ZNS, Centre – Val de Loire, France). The O-ZNS aims to understand and quantify mass and heat transfers in the VZ thanks to an exceptional well (depth – 20 m and diameter – 4m) associated with boreholes dedicated to geophysical measurements and instrumented piezometers. The emphasis is put on developing high-resolution investigations and focused monitoring techniques and sensors for the vadose zone. This observatory offers a unique support to study and establish the relationships to convert physical responses into hydraulic parameters, especially water content, in the VZ of a limestone aquifer.

The geophysical field investigations, conducted prior to the digging of the well, included various scales of observation with 3D Electrical Resistivity Imaging, 2D Magnetic Resonance tomography and crossholes Ground Penetrating Radar tomography. These highlighted three main lithological groups with a few meter-thick soil, a heterogeneous karstified limestone and a massive fractured limestone, all part of the same geological formation. The results put forth the importance of the karstified level heterogeneity on transfers’ behaviour in the VZ, highlighting the presence of clay lens and a disparate water content distribution.

Going further, laboratory investigations have been carried out using field cores in order to characterize the VZ of the Beauce Limestone aquifer. Laboratory analyses enable us to establish Topp’s, Archie’s and CRIM (Complex Refractive Index Model) empirical relations and model. The objective now is to link quantitatively these geophysical field measurements, primarily electrical conductivity and dielectric permittivity, to the medium’s hydraulic parameters (e.g., hydraulic conductivity, porosity, water content). Results from this analysis should bring valuable information on the hydrogeological behaviour of the aquifer system and underline the influence of the observation scales on the estimation of the hydraulic parameter values of the vadose zone.

How to cite: Jodry, C., Mallet, C., Deparis, J., Ammor, S., Baltassat, J.-M., and Azaroual, M.: Hydraulic characterization of a karstic limestone vadose zone based on multi-methods geophysical measurements and lab testing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7213, https://doi.org/10.5194/egusphere-egu2020-7213, 2020.

Keywords: Thermo Hydro-Mechanical (THM), Model Order Reduction (MOR), Parametric solutions, Real time simulations.
 
Radioactive waste is a by-product of nuclear power generation. It is hazardous to all forms of life and the environment. Its radioactive activity naturally decays over time, so waste has to be isolated and confined in appropriate disposal facilities for a sufficient period until it no longer poses a threat. Deep geological repositories constitute one of the most promising options for isolating this type of waste from human and environmental interactions. The analysis and prediction of the behaviour of such systems relies on coupled THM models [1]. The coupled nature of the problem is explained as follows [2]: i) a thermal part including the heat released by the wastes; ii) the mechanical behavior of the canister holding the wastes, the isolation system and the underground host rock; and, iii) the flow of natural water present in any underground porous media.

A coupled THM problem depends on space, time, and on material parameters (for instance, elastic modulus (E), heat conductivity (κ) and hydraulic conductivity (K)) and geometric parameters (for instance, the distance between canisters). We seek for families of solutions depending on these parameters. We would like to provide a real time numerical simulation of the THM problem for any value of the parameters within a range. Real time here, means a solution provided in a few seconds (instead of several hours). Such a solution can be used within an inversion problem, to obtain an best fitting value of the parameters based on some observations, or even in a control situation, where the prediction of the simulation is used to take some decision in the field.
 
Reduced Order Methods (ROM)  are a family of numerical methods able to provide such a solutions. In this work we will present several parametric problems, and show how ROM  [3] can provide real time solution to (simple) THM problems.
 

REFERENCES:

[1] Toprak, E.; Mokni, N.; Olivella, S.; Pintado, X.: Thermo-Hydro-Mechanical Modelling of Buffer. Synthesis Report. August 2013.

[2] Selvadurai, A.P.S; Suvorov, A.P.: Thermo-Poroelasticity and Geomechanics.CAMBRIDGE UNIVERSITY PRESS, 2017.

[3] Diez, P.; Zlotnik, S.; Garcia-Gonzalez, A.; Huerta, A.: Encapsulated PGD algebraic toolbox operating with high-dimensional data. Accepted In Archives of Computational Methods in Engineering, 2019.

How to cite: Moaven, A., Massart, T. J., and Zlotnik, S.: Real time solutions of Thermo-Hydro Mechanical problems with application to the design of Engineered Barriers via Reduced Order Methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10171, https://doi.org/10.5194/egusphere-egu2020-10171, 2020.

EGU2020-22565 | Displays | EMRP1.2

Analysis of THCM coupling in heterogeneous sediments using high-pressure flow-through testing systems

Christian Deusner, Shubhangi Gupta, Andrzej Falenty, Elke Kossel, and Matthias Haeckel

The experimental and numerical investigation of THCM process coupling is important to better understand reservoir geotechnical behavior and sub-surface processes. In particular, when THCM process coupling is dominated by focused fluid migration and localized chemical or microbiological reactions, bulk sediment and, thus, reservoir geotechnical behavior becomes poorly predictable. To improve the understanding of these complicated processes and process coupling on relevant time and spatial scales, it is necessary to combine experimental and numerical simulation approaches, and to develop complementary investigation strategies.    

We use different high-pressure flow-through experimental systems with triaxial testing units in combination with tomographical imaging tools (e.g. X-ray CT and ERT) to simulate and analyze relevant processes in ocean and earth systems. Our geotechnical studies are carried out at high hydrostatic pressures up to 40 MPa and temperatures between -30°C and 80°C. The experimental systems allow testing of large sample specimen (up to a diameter of 150 mm and a height of 400 mm). In particular, we investigate scenarios with heterogeneous phase distributions and dynamic flow conditions, which cannot be interpreted based on the assumption of homogeneous phase distributions in a sensible manner.

Here, we focus on discussing experimental and numerical strategies and problems towards understanding geotechnical behavior of heterogeneous sediments, including issues from gas migration in fine-grained sediments (e.g. silty clays), gas hydrate formation under two-phase flow conditions, and localized failure and shear banding in cemented soils. We present results from recent studies on underground usage including gas production and injection scenarios, which are relevant for the understanding of reservoir behavior, storage scenarios and, overall, marine sediment and slope stability. One of the most important aspects is to improve current strategies for combined and complementary experimental and numerical studies, considering that the overall objective is to understand processes on a reservoir scale.

How to cite: Deusner, C., Gupta, S., Falenty, A., Kossel, E., and Haeckel, M.: Analysis of THCM coupling in heterogeneous sediments using high-pressure flow-through testing systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22565, https://doi.org/10.5194/egusphere-egu2020-22565, 2020.

Maximizing heat exploitation in geothermal systems is crucial for the economic efficiency of many geothermal systems. As the hydraulic flow in most geothermal systems is primarily due to fracture flow, heat transfer processes along the fracture surfaces are essential. However, while flow and mass transport in a single fracture have been studied experimentally and theoretically to a great extent, heat transfer processes have been rarely investigated. Laboratory experiments show the influence of the fracture surface morphology on flow and heat transfer processes, though a physical interpretation has been missing so far. Further, in many geothermal systems but also in many natural hydrothermal systems, the solid and fluid phases are not in local thermal equilibrium. Parameterization of local thermal non-equilibrium models was originally developed for porous media and adoptions to fractures have been cumbersome. In this work, I present a numerical study on heat transfer processes across rough fracture surfaces. Using a three-dimensional steady-state flow model, heat transfer across the fracture surface is studied for both scenarios: assuming and neglecting a thermal equilibrium across phase boundaries. Also, separate fracture morphologies have been studied using natural sandstone probes as well as synthetically generated fractures. The numerical simulations results are compared to laboratory experiments using artificially generated and 3D-printed fracture surfaces of various fracture morphologies for code validation. The full three-dimensional simulations reveal the role of flow channeling effects on the heat transfer taking place along rough surfaces, which is not captured by simulations with reduced spatial dimensions. The simulations results suggest a re-examination of the effective heat transfer coefficient for fractured reservoirs under local thermal non-equilibrium conditions incorporating characteristics of fracture morphology. The simulations results can also be linked to thermal stress generation and possibly explaining the deformations of fracture surfaces observed in the laboratory. However, parameterization of surface roughness is neither distinct nor trivial. Various parameters exist, such as the joint roughness coefficient, Hurst exponent or statistical descriptions, but none has been successfully linked to flow, transport or transfer characteristics. Relating fracture morphology with results of numerical simulations and laboratory findings regarding transfer and transport processes indicate a shortfall of conventional roughness parameterizations to sufficiently describe the observed variation in heat transfer parameters.

How to cite: Heinze, T.: Numerical study of heat transfer across rough fracture surfaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4195, https://doi.org/10.5194/egusphere-egu2020-4195, 2020.

EGU2020-1217 | Displays | EMRP1.2

Temporal change of permeability in macro-fractured granite by accumulation of fine-grained minerals

Yoshitaka Nara, Masaji Kato, Tsutomu Sato, Masanori Kohno, and Toshinori Sato

It is essential to understand the long-term migration of radionuclides when considering rock engineering projects such as the geological disposal of radioactive waste. The network of fractures and pores in a rock mass plays a major role in fluid migration as it provides a pathway for fluid flow. The geometry of the network can change due to fracture sealing by some fine-grained materials over long-term periods. Groundwater usually contains fine-grained minerals such as clay minerals. Therefore, it is possible that the accumulation of such fine-grained minerals occurs within a rock fracture under groundwater flow. In this case, the aperture of a fracture may decrease, which brings about the decrease of the permeability. It is therefore essential to conduct permeability measurements using water including fine-grained minerals in order to understand the permeability characteristics of a rock. However, this has not been investigated well. In this study, we use a macro-fractured granite sample to investigate the temporal change of the permeability that occurs under the flow of water that includes two different amounts of clay.

It was shown that the clay accumulated in the macro-fracture and that the permeability of the macro-fractured granite sample decreased over time. It was also recognized that the decrease of the permeability was more significant under the water flow with the higher clay content. As a result of the observation using microscope, it was recognized that the clay minerals accumulated in the macro-fracture in the granite sample, which decreased the aperture of the fracture. We concluded that the accumulation of clay minerals in the fracture decreased the permeability of the rock. Furthermore, it is concluded that the filling and closure of fractures in rock is possible under the flow of groundwater including clay minerals.

 

How to cite: Nara, Y., Kato, M., Sato, T., Kohno, M., and Sato, T.: Temporal change of permeability in macro-fractured granite by accumulation of fine-grained minerals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1217, https://doi.org/10.5194/egusphere-egu2020-1217, 2020.

The carbon storage and energy development activities in deep geologic zones that potentially affect water quality in shallow aquifers are of central importance in the energy-water nexus. The extraction of natural gas involves hydraulically fracturing deep shale formations. The storing of carbon dioxide in deep geologic formations is pursued to mitigate global warming. Both these activities have the potential to contaminate the shallow aquifers used for potable water and return the greenhouse gases to the atmosphere. In the case of carbon storage, both during and post-injection phases, it is possible for the CO2 and formation brine to leak through natural faults, pressure-induced fractures, or failed well casings. Two scientific challenges have to be addressed to safely store the carbon in the deep formation while protecting the shallow aquifers. The first, characterizing the affected geologic formations, and the second is monitoring the leakage. Monitoring involves determining leakage locations and tracking of the gas and brine plume through the geologic formation between the deep confining layer used for storage and the shallow aquifer. Challenges to the characterization derive from the limitations and sparsity of observational data in deep formations.  Effective monitoring poses both scientific and engineering challenges as the leakage locations not known, and the resulting pathways cannot be predicted easily. This paper presents two studies where intermediate-scale testing systems were used to understand the processes that occur during the leakage of stored supercritical CO2.  The focus of the first study was to better understand the process of CO2 gas exsolution after a leak from the deep confining formation. The second study addresses the issue of monitoring brine leakage from the confined formation where supercritical CO2 is stored. The improved understanding of these leakage processes will help to develop assessment and monitoring systems for storage permeance and protecting shallow sources of potable groundwater. It is not feasible to conduct experiments in the field to obtain both the fundamental process understanding and test and validate developed modeling and monitoring tools due to lack of control of boundary and initial conditions and expense in fully characterizing deep formations. Tests in intermediate scale synthetic aquifers where highly controlled experiments can be conducted to obtain accurate data provide an alternative to overcome this challenge. However, designing test systems and performing tests under ambient laboratory conditions different types of challenges. This paper will present some of the challenges and how they were overcome. The results on new process insights, how the data was used to assess the natural capacity of the aquifer attenuation of leaking gas, and validating inversion methods for site characterization and leakage detection will be presented. Even though this study focused on CO2 leakage, the results will be of value in problems of natural has leakage during hydraulic fracturing in alternate energy development.

 

How to cite: Illangasekare, T. and Askar, A.: Understanding the leakage of greenhouse gasses from seep geologic formations during geologic carbon storage and hydraulic fracturing: intermediate scale testing challenges , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19592, https://doi.org/10.5194/egusphere-egu2020-19592, 2020.

EGU2020-805 | Displays | EMRP1.2

A Pore-Scale Investigation of Fluid Displacement and Residual Trapping Under Intermediate-Wet Conditions

Rumbidzai Nhunduru, Amir Jahanbakhsh, Omid Shahrokhi, Krystian Wlodarczyk, Duncan Hand, William MacPherson, Susana Garcia, and Mercedes Maroto-Valer

Residual trapping in which ganglia of fluid are isolated and immobilised in porous media by capillary forces is innate to several subsurface engineering applications including carbon geo-sequestration. Residual trapping is highly significant in carbon dioxide (CO2) sequestration, as entrapment of supercritical CO2 in rock pore spaces, limits upward migration of the buoyant CO2 plume and enhances long-term CO2 storage security. It is estimated that residual trapping contributes up to 40% of overall trapping CO2 in the first century following injection (1). The amount of residual trapping depends largely on the wettability of the porous rock.

Brine filled saline aquifers have been identified as having the largest potential for CO2 storage with an estimated cumulative storage capacity of 104 Giga-tons of CO2 (2). Likewise, the focus of many studies has been devoted to investigating residual trapping in water-wet, brine filled sandstone reservoirs, and little attention has been given to intermediate-wet and oil-wet carbonate reservoirs. However, until CO2 storage technology reaches maturity, initial CO2 sequestration projects will most likely be conducted in depleted and oil producing carbonate reservoirs due to economic benefits associated with CO2 enhanced oil recovery and the existence of installed infrastructure which can be reassigned for CO2 injection purposes (3). 

Accordingly, in this work, the intrinsically water-wetting surfaces of laser fabricated glass micromodels (4); which are two-dimensional representations of natural porous rock structures, were chemically modified to imitate intermediate-wet reservoir conditions through a silanization procedure. Imbibition experiments were conducted in the micromodels using two proxy, CO2-brine fluid pairs; deionized (DI) water and n-decane as well as DI water and air.

Fluid displacement under intermediate wettability was analysed and compared with water-wet conditions and residual fluid saturations were quantified for different porous structures. The Volume of Fluid method was used to simulate the experiments in OpenFOAM. Results from the micromodel experiments were used to validate the simulations.

This work has demonstrated that fluid displacement during the imbibition process occurs through a series of cooperative pore-filling events under intermediate-wet conditions and the presence of dead-end pores was found to enhance residual trapping of the non-wetting fluid. Coupling experimental and simulation studies provides a unique insight to multiphase flow under intermediate wet conditions.  

 

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (MILEPOST, Grant agreement no: 695070). This paper reflects only the authors’ view and ERC is not responsible for any use that may be made of the information it contains.

 

References

  1. Li X, Akbarabadi M, Karpyn ZT, Piri M, Bazilevskaya E, Experimental Investigation of Carbon Dioxide Trapping Due to Capillary Retention in Saline Aquifers, Geofluids, 2015;15(4):563–76.
  2. Benson; GEA; Iiasa. Chapter 13: Carbon Capture and Storage. Global Energy Asssessment. 2012.
  3. Al-Menhali AS, Menke HP, Blunt MJ, Krevor SC. Pore Scale Observations of Trapped CO2 in Mixed-Wet Carbonate Rock: Applications to Storage in Oil Fields. Environ Sci Technol 2016;50(18):10282–90.
  4. Wlodarczyk KL, Carter RM, Jahanbakhsh A, Lopes AA, Mackenzie MD, Maier RRJ, Hand DP, and Maroto-Valer MM, Rapid Laser Manufacturing of Microfluidic Devices from Glass Substrates. Micromachines. 2018; 9(8)

How to cite: Nhunduru, R., Jahanbakhsh, A., Shahrokhi, O., Wlodarczyk, K., Hand, D., MacPherson, W., Garcia, S., and Maroto-Valer, M.: A Pore-Scale Investigation of Fluid Displacement and Residual Trapping Under Intermediate-Wet Conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-805, https://doi.org/10.5194/egusphere-egu2020-805, 2020.

EGU2020-9408 | Displays | EMRP1.2

Laboratory study of Hydraulic Fracture interaction with artificial interfaces.

Anna Shevtsova, Egor Filev, Maria Bobrova, Sergey Stanchits, and Vladimir Stukachev

Nowadays Hydraulic Fracturing (HF) is one of the most effective stimulation technique for hydrocarbon extraction from unconventional reservoirs, as well as enhanced geothermal applications. Practical applications of HF can have different aims. In one case, we need to stop cracks inside the host rock to avoid some HF breakthroughs into other formations and possible groundwater pollutions. The second situation is when we need to fracture several bedding planes in a reservoir which has a complex structure, especially in case of the presence of multiple natural fractures in unconventional reservoir. It is important to study hydraulic fracturing, its propagation and conditions of interaction with interfaces in laboratory conditions before expensive field application.

The present work demonstrates the results of a laboratory study designed to understand fracture interaction with artificial interfaces. For the first series of experiments, we used some natural materials such as shales, sandstones, dolomites and limestones with different porosity, permeability and mechanical properties. During these experiments we initiated hydraulic fracturing in homogeneous specimens with and without artificial surfaces, modelling natural fractures or bedding planes in unconventional reservoirs. For the second series of experiments, we used a combination of different materials to understand HF propagation in heterogeneous media, to study conditions of HF crossing or arrest at the boundaries between different types of rock. These laboratory experiments were done to create HF simulating natural processes in fractured and heterogeneous rocks or reservoirs.

Series of hydraulic fracturing experiments under uniaxial load conditions were conducted using the multifunctional system MTS 815.04. Before testing, samples were scanned by 3D CT System to characterize the rock fabric, and after testing, CT scanning was repeated to characterize 3D shape of created HF. The dynamics of HF initiation and propagation was monitored by Acoustic Emission (AE) technique, using piezoelectric sensors glued to the surface of the rock to record elastic waves radiated during the process of HF propagation. The experiments were made with different injection rates and fluid viscosities. Changes in radial strain, injection pressure and microseismic data over time were recorded.

As the result, these experiments indicate significant factors (rock heterogeneity, porosity, permeability, fluid viscosity and injection rate), influencing cracks initiation, propagation or arrest on the artificial interface. The fracture propagation and opening are characterized by measured radial deformation, fluid pressure and geometrical orientation in the sample volume. The experiments demonstrated, that fracture easily crossed artificial surface in the homogeneous limestone samples. And cracks initiated in limestone were arrested on the border with shale. In all cases combination of the AE and deformation monitoring allows to indicate fracture initiation, propagation and arrest.

How to cite: Shevtsova, A., Filev, E., Bobrova, M., Stanchits, S., and Stukachev, V.: Laboratory study of Hydraulic Fracture interaction with artificial interfaces., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9408, https://doi.org/10.5194/egusphere-egu2020-9408, 2020.

EGU2020-8736 | Displays | EMRP1.2

Density and magnetic susceptibility relationships in non-magnetic granites; a “wildcard” for modeling potential fields geophysical data.

Emilio L. Pueyo, Mª Teresa Román-Berdiel, Conxi Ayala, Francesca Loi, Ruth Soto, Elisabeth Beamud, Elena Fernandez de Arévalo, Ana Gimeno, Luis Galán, Stefanía Schamuells, Nuria Bach-Oller, Pilar Clariana, Félix M. Rubio, Antonio M. Casas-Sainz, Belén Oliva-Urcia, José Luis García-Lobón, Carmen Rey, and Joan Martí

Geophysical surveying (both gravity and magnetic) is of great help in 3D modeling of granitic bodies at depth. As in any potential-field geophysics study, petrophysical data (density [r], magnetic susceptibility [k] and remanence) are of key importance to reduce the uncertainty during the modeling of rock volumes. Several works have already demonstrated that ∂18O or [SiO2] display a negative correlation to density and to magnetic susceptibility. These relationships are particularly stable (and linear) in the so-called “non-magnetic” granites (susceptibilities falling within the paramagnetic range; between 0 and 500 10-6 S.I.) and usually coincident with calc-alcaline (CA) compositions (very common in Variscan domains). In this work we establish robust correlations between density and magnetic susceptibility at different scales in CA granites from the Pyrenees. Other plutons from Iberia were also considered (Veiga, Monesterio). The main goal is to use the available and densely sampled nets of anisotropy of magnetic susceptibility (AMS) data, performed during the 90’s and early 2000’s, together with new data acquired in the last few years, as an indirect measurement of density in order to carry out the 3D modelling of the gravimetric signal.

 

We sampled some sections covering the main range of variability of magnetic susceptibility in the Mont Louis-Andorra, Maladeta and Marimanha granite bodies (Pyrenees), all three characterized by even and dense nets of AMS sites (more than 550 sites and 2500 AMS measurements). We performed new density and susceptibility measurements along two main cross-sections (Maladeta and Mont Louis-Andorra). In these outcrops, numerous measurements (usually more than 50) were taken in the field with portable susceptometers (SM20 and KT20 devices). Density data were derived from the Arquimedes principle applied on large hand samples cut in regular cubes weighting between 0.3 and 0.6 kg (whenever possible). These samples were subsampled and measured later on with a KLY-3 susceptibility bridge in the laboratory. Additionally, some density data were derived from the geometry and weighting of AMS samples.

 

After the calibration of portable and laboratory susceptometers, density and magnetic susceptibility were plotted together. Regressions were derived for every granite body and they usually followed a linear function similar to: r = 2600 kg/m3 + (0.5 * k [10-6 S.I.]). As previously stated, this relationship is only valid in CA and paramagnetic granites, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). These relationships allow transforming magnetic susceptibility data into density data helping in the 3D modelling of the gravimetric signal when density data from rock samples are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signal.

How to cite: Pueyo, E. L., Román-Berdiel, M. T., Ayala, C., Loi, F., Soto, R., Beamud, E., Fernandez de Arévalo, E., Gimeno, A., Galán, L., Schamuells, S., Bach-Oller, N., Clariana, P., Rubio, F. M., Casas-Sainz, A. M., Oliva-Urcia, B., García-Lobón, J. L., Rey, C., and Martí, J.: Density and magnetic susceptibility relationships in non-magnetic granites; a “wildcard” for modeling potential fields geophysical data., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8736, https://doi.org/10.5194/egusphere-egu2020-8736, 2020.

Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies

 

Michele Pugnetti*, Yi Zhou*, Andrea R. Biedermann*

* Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland (michele.pugnetti@geo.unibe.ch)

 

In AMS (anisotropy of magnetic susceptibility)-based pore fabric studies, the role of ferrofluid impregnation is crucial to ensure significant magnetic measurements. However, no standard methods to test the ferrofluid impregnation of porous media have been proposed so far. The details of fluid behaviour in porous media are important in many fields of natural sciences, but nanoparticle distribution in the fluid is particularly important for magnetic measurements. In this study methods to test the impregnation efficiency of ferrofluid in porous media, and nanoparticle distribution are proposed, using different materials: wood, agarose and TEOS (tetraethylorthosilicate) gel, and synthetic samples of given composition and grain size, as well as natural rocks. Magnetic pore fabric measurements are normally performed on natural porous samples to correlate the direction of maximum magnetic susceptibility with the direction of preferred pore elongation, and preferred flow direction. The advantage of using artificial samples is the possibility to control and adjust some physical parameters, including porosity and pore size, to keep them more uniform or fix them to a given value. This allows investigating the nanoparticle distribution in ideal samples without the influence of additional heterogeneities inherent to natural samples and to determine the lowest porosity value and smallest pore size that is possible to impregnate with ferrofluid. In particular, the agarose and TEOS gel have a uniform porous structure controlled by the gel concentration or chemical agents used in sample preparation. The wood has a wider range of porosity compared to rocks and a known intrinsically anisotropic structure. The synthetic samples have a uniform grain size, mineralogy and structure. First the porosity of the samples was measured, then to impregnate the samples different methods were developed and tested, (1) percolation, (2) standard vacuum impregnation, (3) flow-through impregnation, (4) diffusion process in gel structure. Impregnation efficiency was evaluated both optically and magnetically. Different impregnation methods provide different impregnation efficiency depending also on the investigated material; in particular porosity plays an important role in limiting the impregnation efficiency. Initial experiments indicate that in general, flow-through impregnation is more efficient than vacuum impregnation because it combines the effect of vacuum with the pressure applied to the fluid that is pushed through the sample. The best results on natural samples were obtained using calcarenites with relatively high porosity. These results and the methods proposed here will help advance magnetic pore fabrics studies and impregnation processes in general.   

 

How to cite: Pugnetti, M., Zhou, Y., and Biedermann, A.: Testing the efficiency of ferrofluid impregnation in porous media – recommendations for future magnetic pore fabric studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5434, https://doi.org/10.5194/egusphere-egu2020-5434, 2020.

Unconventional oils are emerging as an alternative hydrocarbon reserve since conventional oil is depleting nowadays. A kind of unconventional oil is bitumen, which is characterized by high density, high viscosity and API gravity less than 10° and these physical properties are temperature sensitive. Therefore, an accurate assessment of variation in petrophysical properties of bitumen as a function of temperature and pressure is interesting in oil exploration industry.

In this work we investigated the role of heavy hydrocarbons (HHC) in changing petrophysical properties of carbonate-bearing rocks of the Majella reservoir performing seismic wave velocity measurements at increasing temperature. The investigated lithology belongs to the Bolognano formation that outcrops naturally in saturated and unsaturated conditions in the northwest sector of Majella Mountain (in Central Italy).

We conducted ultrasonic measurements of compressional and shear wave velocities on HHC-bearing carbonate samples showing different bitumen content and porosity between 10% and 19%. Firstly, we characterized bitumen density by HCl dissolution of the hosting rock, that resulted to be included between 1.14 and 1.26 gr/cm3 at ambient temperature. Then, we calculated HHC content of our samples, spanning from 2% (low HHC-bearing sample) to 16% (high HHC-bearing sample). Our acoustic velocities point out an inverse relationship with temperature. P- and S-wave velocities depict a distinct trend with increasing temperature depending on the amount of HHC content. Indeed, samples with the highest HHC content show a larger gradient of velocity changes in the temperature range of about 60°-50° C, suggesting that bitumen can be in a fluid state. Conversely, below about 50° C the velocity gradient is lower because, at this temperature, bitumen can change its phase in a solid state. Currently, we are analysing the coupling effect of temperature and pressure on HHC-bearing carbonate samples to test the acoustic response of the investigated samples simulating the reservoir conditions.

Our preliminary results highlight a strongly temperature dependence for HHC-bearing carbonate properties and bitumen influences the acoustic response of carbonate rocks. Such petrophysical characterization would provide a better link between seismic parameters and the hydrocarbon properties with important implications for reservoir characterization from seismic data and for production monitoring. 

 

How to cite: Ruggieri, R. and Trippetta, F.: Combination effects of temperature and pressure on the petrophysical properties of bitumen-bearing carbonate rocks: insight for the Majella reservoir (Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14960, https://doi.org/10.5194/egusphere-egu2020-14960, 2020.

Carbonate rocks are common in many parts of the world including the Eastern Mediterranean where they host significant groundwater supplies and are used widely in engineering as building and ornamental stones. Porosity of carbonate rocks plays a critical role in fluid storage and retrieval. The pore structure connectivity, in particular, controls many properties of rocks, and the relationships between the characteristics of individual minerals and the gross behavior of the rock. To study the relationships between porosity, rock properties, pore structure, pore size, and their impact on reservoir characteristics, several carbonate rock samples were collected from four stratigraphic sections exposed near Sidon, south Lebanon. The studied carbonate rocks are related to marine deposits of different ages (e.g., Upper Cretaceous, Eocene and Upper Miocene). In order to understand the pore connectivity, the MICP (mercury injection capillary pressure) is conducted on ten representative samples. Results from the SEM analysis indicate the dominance of very fine and fine pore sizes with various categories ranging in diameter from 0.1 to10 µm. The MICP data revealed that the pore throat radii vary widely from 0.001 to 1.4µm, and that all samples are dominated by micropore throats. The grain size analysis indicated that the studied rocks have significant amounts of silt- and clay-size grains with respect to the coarser sand-size particles; suggesting a high proportion of microporosity. Obtained results such as the poorly-sorted nature of grains, high microporosity, and the high percentage of micropore throats justify the observed low mean hydraulic radius, the high entry pressure, and the very low permeability of the studied samples. These results suggest that the carbonate rocks near Sidon (south of Lebanon) are possibly classified as non-reservoir facies.

How to cite: Salah, M.: Pore Structure and Petrophysical Characterization of Carbonate Rocks from Southern Lebanon , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1920, https://doi.org/10.5194/egusphere-egu2020-1920, 2020.

Understanding the deformation-related thermomechanical state of reservoir rocks under in-situ conditions is essential for modelling the stress distribution and stability of subsurface structures, for example associated with aftershock activity and induced seismicity. Commonly, reservoir modelling approaches make use of the generalized friction criterion according to Byerlee, which distinguishes between depths below and above approximately 6 km. However, numerous studies have shown that thermomechanical rock properties under elevated pressure and temperature conditions differ significantly from those at the surface and among rock types. The significant influence of the geothermal gradient on elastic and inelastic rock properties has already been demonstrated for temperature variations as low as 150 °C. Studies on the effect of in-situ stress and temperature conditions on post-failure behaviour and frictional properties are completely lacking.

In our experimental study we determined the thermomechanical properties of porous Ruhr sandstone samples during conventional triaxial deformation tests to derive stress- and temperature-dependent failure and friction criteria. Effective confining pressures and temperatures applied in the tests cover the range of in-situ conditions equivalent to depths up to three kilometres. Simultaneously, ultrasonic P- and S-wave measurements were performed to determine properties of ultrasound wave propagation (i.e. dynamic elastic properties) as a function of in-situ conditions. Triaxial deformation experiments were conducted at various strain rates to investigate the deformation-rate dependence of the failure and friction criteria and the correlation between dynamic and static elastic properties.

How to cite: Ahrens, B., Duda, M., and Saenger, E. H.: Experimental investigations on the temperature and strain-rate dependence of failure and friction criteria for a porous sandstone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15134, https://doi.org/10.5194/egusphere-egu2020-15134, 2020.

EGU2020-7273 | Displays | EMRP1.2

Fracture reactivation for permeability enhancement in geothermal systems

Alexandra Kushnir, Michael Heap, Patrick Baud, and Thierry Reuschlé

While the deep granitic basement in the Upper Rhine Graben is currently being exploited as a geothermal reservoir at numerous geothermal sites, the Permo-Triassic sandstones that lie directly above the granite are critical to continued regional hydrothermal convection. Here we investigate the propensity for variably sealed fractures to be reactivated during deformation and the role this fracture reactivation plays on permeability enhancement in geothermal reservoirs. We source un-fractured, bedded sandstones and the same bedded sandstones containing a single, variably-sealed fracture from a 400 m-thick unit of Permo-Triassic sandstone sampled from the EPS-1 exploration well near Soultz-sous-Forêts (France) in the Upper Rhine Graben.

31 cylindrical samples (20 mm in diameter and 40 mm long) were cored such that their dominant structural feature (i.e. bedding or natural fracture) was oriented parallel, perpendicular, or at 30° to the sample axis. The initial permeability of the un-fractured samples ranged between 2.5×10-17 and 5.6×10-16 m2 and between 3.6×10-16 and 3.3×10-14 m2 for naturally fractured samples. In un-fractured samples, permeability decreases as a function of increased bedding angle; fracture orientation, however, does not appear to have a discernable influence on permeability. Samples were water-saturated and deformed until failure under pressure conditions appropriate to the Soultz-sous-Forêts geothermal system - Peff of 14.5 MPa - and at a strain rate of 10-6 s-1. All samples developed through-going shear fractures, however, only in samples containing partially sealed fractures did the experimentally produced fractures take advantage of the pre-existing features. In samples containing a fully-sealed fracture, the experimentally induced fracture developed in a previously undeformed part of the sandstone matrix. Further, post-deformation permeability measurements indicate that while sample permeability increased by up to one order of magnitude for a given sample, this increase is generally independent of feature orientation.

Therefore, formations containing sealed fractures may not necessarily be weaker and, as a consequence, may not be more apt to significant permeability increases during stimulation than un-fractured formations. These data can contribute to the development and optimization of stimulation techniques used in the Upper Rhine Graben.

How to cite: Kushnir, A., Heap, M., Baud, P., and Reuschlé, T.: Fracture reactivation for permeability enhancement in geothermal systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7273, https://doi.org/10.5194/egusphere-egu2020-7273, 2020.

EGU2020-15484 | Displays | EMRP1.2

Impact of a partly sealed fault on hydro-mechanical properties of a granite reservoir

Guido Blöcher, Christian Kluge, Mauro Cacace, Harald Milsch, and Jean Schmittbuhl

The fluid flow in Enhanced Geothermal Systems (EGS) is dominated by hydraulically stimulated fractures and faults which are the key elements of their hydraulic performance and sustainability. At the fault scale, the flow performance is influenced by the aperture distribution which is strongly dependent on the fault roughness, the geological fault sealing, the relative shear displacement, and the amount of flow exchange between the matrix and the fault itself. On the mechanical side, stiffness and strength of partly sealed fault might alter or reinforced the mechanical behavior of the fault zone in particular with respect to new stimulations. In order to quantify the impact of chemical soft stimulation in EGS reservoir on the hydro-mechanical properties of a fault-rock system that includes fault-filling material, we conducted numerical flow through experiments of a granite reservoir hosting one single partly sealed fault of size 512x512 m². In order to mimic the chemical alteration of the fault-rock system we sequentially changed the distribution pattern of the fault-filling material by means of a hydro-poro-elastic coupled simulation. Navier-Stokes flow is solved in the 3-dimensional rough aperture and Darcy flow in the related poro-elastic matrix. By means of this model, an evaluation of the local channeling effect through the fault for various degrees of sealing was performed. Based on the obtained results, we derived a macroscopic change of the hydraulic-mechanical behavior of the fault-rock system, e.g. permeability change, fracture stiffness modulus.

How to cite: Blöcher, G., Kluge, C., Cacace, M., Milsch, H., and Schmittbuhl, J.: Impact of a partly sealed fault on hydro-mechanical properties of a granite reservoir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15484, https://doi.org/10.5194/egusphere-egu2020-15484, 2020.

EGU2020-19657 | Displays | EMRP1.2

Permeability heterogeneity during sandstone compaction

Philip Meredith, Nicolas Brantut, and Patrick Baud

Compaction of porous sandstones is generally associated with a reduction in permeability. Depending on porosity and other microstructural characteristics, compaction may be diffuse or localised in bands. Compaction bands have been shown to act as barriers to fluid flow and therefore reduce permeability perpendicular to the band orentiation, and thus also introduce permeability anisotropy. Additionally, the localised nature of compaction bands should also introduce strong permeability heterogeneity. We present new experimental data on sandstone compaction combining acoustic emission monitoring and spatially distributed pore fluid pressure measurements, allowing us to establish how permeability heterogeneity develops during progressive compaction. Three sandstones were tested in the compactant regime: Locharbriggs sandstone, which is microstructurally heterogeneous with beds of higher and lower initial permeability; a low porosity (21%) Bleurville sandstone, which is microstructurally homogeneous and produces localised compaction bands; and a high porosity (24%) Bleurville sandstone, which is also homogeneous but produces compaction in a more diffuse pattern. At regular intervals during compactive deformation, a constant pore pressure difference was imposed at the upper and lower boundaries of the cylindrical samples, and steady-state flow allowed to become established. Following this, local pore pressure measurements were made at four locations, allowing us to derive estimates of the local permeability. In all samples, progressive compaction produced overall reductions in permeability. In addition, localised compaction also produced internal reorganisation of the permeability structure. Localised compaction bands caused local decreases in permeability, while more diffuse compaction produced a more homogeneous overall reduction in permeability.

 

How to cite: Meredith, P., Brantut, N., and Baud, P.: Permeability heterogeneity during sandstone compaction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19657, https://doi.org/10.5194/egusphere-egu2020-19657, 2020.

EGU2020-2148 | Displays | EMRP1.2

New Predictive Model for Relative Permeability of Deformable Gas Hydrate-Bearing Sediments

Gang Lei, Qinzhuo Liao, and Patil Shirish

Global energy demand is expected to grow significantly as the world population and the standard of living increase in the coming decades. As a potential source of energy, gas hydrate, which is a crystalline compound of gas-water mixture formed in stable of high pressure and low temperature, has been intensively investigated in the past few decades. In this work, a new analytical model is derived to study the effect of hydrate saturation on stress-dependent relative permeability behavior of hydrate-bearing sediments. The proposed relative permeability model solves the steady-state Navier-Stokes equations for gas-water two-phase flow in porous media with hydrates. It considers water saturation, hydrate saturation, viscosity ratio and hydrate-growth pattern, and is adequately validated with the experimental results in existing literatures. The model demonstrates that gas-water relative permeability in wall coating hydrates (WC hydrates) is larger than that in pore filling hydrates (PF hydrates). For WC hydrates, water phase relative permeability monotonically decreases as gas saturation increases. However, for PF hydrates, water phase relative permeability firstly increases and then decreases with the increase of gas saturation, which can be explained by the “lubricative” effect of the gas phase that exists between the water phase and hydrates. This work constitutes a comprehensive investigation of stress-dependent relative permeability in deformable hydrate-bearing sediments, which is a key issue for sustainable gas production. It not only provides theoretical foundations for quantifying relative permeability in hydrate-bearing sediments, but also can be used to estimate pore-scale parameters and rock lithology of gas hydrate-bearing sediments using inverse modeling.

How to cite: Lei, G., Liao, Q., and Shirish, P.: New Predictive Model for Relative Permeability of Deformable Gas Hydrate-Bearing Sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2148, https://doi.org/10.5194/egusphere-egu2020-2148, 2020.

EGU2020-16747 | Displays | EMRP1.2

Fluid flow along a rough fracture: impact on hydraulic diffusivity

Qinglin Deng, Jean Schimittbuhl, Guido Blocher, and Mauro Cacace

Fluid flow along fractures or in fractured rock is of great importance in Enhanced Geothermal System, since natural fracture networks generally affect the permeability of the reservoir rocks and therefore the hydraulic performance. The cubic law commonly estimates the permeability of a single fracture, which is only valid for the flow through two smooth parallel plates. In fact, the flow performance is strongly influenced by the aperture fluctuations, which are related to the fracture surface roughness, the fluid-rock interaction process, and the amount of flow exchange between the matrix and the fracture itself, etc.

To quantify the hydraulic performance and get the better knowledge of the more real fracture flow, we conduct numerical simulations of fluid flow in a fracture-rock system hosting one single rough fracture from laboratory to field scales. As an example, a 2D self-affine rough surface is synthetically generated (Candela et al, 2012), with two anisotropic roughness exponents H// = 0.6 along the slip direction, Hperp = 0.8 in the perpendicular direction and a RMS amplitude of 0.1m at the 512m scale. Based on this surface generation, the opening geometry of a rough fracture is obtained as an input structure for finite element mesh generation. On one hand, we apply a lubrication approximation and limit the fracture opening to spatially variable 2D features with lower-dimensional element embedded in a saturated porous. On the other hand, we consider the full 3D features of the fracture opening as the space between two surfaces symmetrical about the mean fracture plane. The simulations are performed in the framework of the Mutiphysics Object Oriented Simulation Environment (MOOSE) combined with a MOOSE-based application GOLEM dedicated to modeling coupled Thermal-Hydraulic-Mechanical (THM) process in fractured geothermal reservoirs.

For the lubrication case, the mass balance equation for a saturated porous medium is described in terms of volumetric averaged mass conservation equations for the fluid phase, with Darcy’s law governing the momentum conservation equation. For the 3D fracture case, the incompressible Navier-Stokes equation is solved for the dynamic pressure and the velocity field inside the fracture only.

We compare the 2D and 3D cases and assess the effects of the nonlinear inertial term (u•∇)u in 3D case especially when the Reynolds number is high. The objective is to evaluate the large-scale hydraulic diffusivity of the fractured domain and its anisotropy owing to the strong contrast between the fluctuating fracture opening, and the homogeneous bulk porosity. The results show that the long-range aperture variations significantly affect the fluid flow, like the channeling effect and the hydraulic diffusivity anisotropy (i.e., along and perpendicular to the fault), which may have strong implications on the spatial distribution of fluid-induced seismic events in faulted reservoir.

How to cite: Deng, Q., Schimittbuhl, J., Blocher, G., and Cacace, M.: Fluid flow along a rough fracture: impact on hydraulic diffusivity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16747, https://doi.org/10.5194/egusphere-egu2020-16747, 2020.

EGU2020-5982 | Displays | EMRP1.2

Interfacial processes at dissimilarly charged mineral surfaces in contact – a surface forces apparatus study

Joanna Dziadkowiec, Hsiu-Wei Cheng, Anja Røyne, and Markus Valtiner

When two mineral surfaces are in close contact, nanometers to microns apart, the proximity of another surface can significantly influence the pathways of chemical reactions happening in the interfacial region. Apart from affecting the kinetics of dissolution and nucleation reactions in spatial confinement, the proximity of charged surfaces can lead to electrochemically induced recrystallization processes. The latter may happen in an asymmetric system, in which two surfaces have a dissimilar surface charge. The charge and mass transferred during electrochemical reactions can induce dissolution or growth of solids and can significantly affect the local topography of surfaces, causing them to smooth out or to roughen. In this work, we present the experimental study of reactive mineral interfaces, immersed in geologically relevant electrolyte solutions, obtained with the electrochemical surface forces apparatus (EC-SFA). EC-SFA setup consists of one mineral surface and one gold surface (working electrode), the surface charge of which is controlled by applying an electrical potential. EC-SFA can, therefore, monitor electrochemically induced surface recrystallization processes. As the SFA technique is based on white light interferometry measurements, the changes in mineral thickness during recrystallization can be determined with an accuracy better than a nanometer over micrometer-large contact regions. Moreover, SFA allows in situ measurement of surface forces acting between mineral surfaces, which can provide additional information about how the surface reactivity influences the cohesion between mineral surfaces by modifying adhesive and repulsive forces acting between them at small separations.

How to cite: Dziadkowiec, J., Cheng, H.-W., Røyne, A., and Valtiner, M.: Interfacial processes at dissimilarly charged mineral surfaces in contact – a surface forces apparatus study , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5982, https://doi.org/10.5194/egusphere-egu2020-5982, 2020.

EGU2020-20857 | Displays | EMRP1.2

Evolution of Modelling Zeta Potential: Impact of Brine Compositions and Concentration

Miftah Hidayat, Jan Vinogradov, Stefan Iglauer, and Mohammad Sarmadivaleh

Electrochemical interactions of calcite with brines in natural subsurface settings have received ample attention in the last decades due to the broad range of their applications. These interactions can be described by an electrical property termed the zeta potential. Many numerical simulation studies using surface complexation modelling (SCM) have been performed to investigate the relationship between the zeta potential and a wide range of salinities and complex brine compositions. Although most of the simulated results, especially in low salinity conditions, successfully match the experimentally measured zeta potential, the simulated zeta potential for high salinity conditions is still poorly understood.

In this study, we present a new approach of SCM to simulate the zeta potential by considering the actual molecular-scale phenomena at the calcite-brine interface. Unlike previous SCM studies, our model considers the hydrated diameter of ions as the distance of approach, which depends on salinity. We also consider the permittivity of the Stern layer as a function of salinity, which is consistent with previous unrelated studies. We calculate the capacitance for each salinity based on the relationship between the hydrated diameter of ions and the permittivity of the Stern layer. Moreover, all calcite-brine surface reactions are described by new equilibrium constants independent of salinity and composition of brines.

Our results show that the simulated zeta potential which is obtained from our SCM at a broad range of salinities is successfully matched with the published experimental data for two different carbonate rock samples as long as the salinity dependence of the hydration diameter and electrical permittivity is accounted for. We find that the potential determining ions (Ca2+, Mg2+, SO42-, HCO3-,CO32-) play a dominating role compared to the indifferent ions (Na+, Cl-) in the calcite-brine surface reactions. The Implications of our findings are significant for wettability evaluation, characterisation of shallow and deep aquifers and CO2 geological sequestration.

How to cite: Hidayat, M., Vinogradov, J., Iglauer, S., and Sarmadivaleh, M.: Evolution of Modelling Zeta Potential: Impact of Brine Compositions and Concentration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20857, https://doi.org/10.5194/egusphere-egu2020-20857, 2020.

In this study, zeta potential has been measured by using the streaming potential method for the intact sandstone in contact with CaCl2 electrolytes. The experimental results show that a positive zeta potential has been observed for the first time for the intact Fontainebleau sandstone under high salinity of CaCl2, and its magnitude increases with increasing ionic strength. It cannot be explained by the Gouy-Chapman theory anticipating a constant potential for high salinities due to the collapse of the electrical double layer. Meanwhile, the brine effluents after the completion of the streaming potential measurements were collected and then pH and brine composition were analysed suggesting that those variations of pH and chemical composition are negligible and cannot explain the polarity change at high salinity. The anomalous positive potential of the intact Fontainebleau sandstone is due to that overcharge of calcium ions sorbed into the mineral surface, which is consistence with previous literature data.

How to cite: Li, S. and Jackson, M.: Critical role of the structure of the mineral-water interface in the zeta potential measured by streaming potential method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22077, https://doi.org/10.5194/egusphere-egu2020-22077, 2020.

Seismoelectric signals result from an electrokinetic coupling phenomena that can be modeled through two approaches: the coupling coefficient or the effective excess charge density. The traditional approach is based on the frequency dependent coupling coefficient that can relate differences in pressure to differences in electrical potential. The second approach is more recent and is related to the description of the excess charge that is effectively dragged by the pore water displacement relatively to the mineral surface. In this contribution, we propose a new model to obtain the frequency dependent effective excess charge density. The electrokinetic coupling is mechanistically up-scaled considering the pore as a straight capillary. This approach, called flux-averaging, takes into account the inertial term of the Navier-Stokes equation to explain both the dynamic permeability and the effective excess charge density dependence with oscillation frequency. The frequency dependent coupling coefficient can then be calculated from this result. The model results are then successfully compared to previous models and published data. This work is a first step to predict seismoelectric electrokinetic coupling in much more complicated porous media in saturated and partially saturated conditions.

How to cite: Jougnot, D. and Solazzi, S.: Modeling the seismoelectric electrokinetic coupling: a new approach to up-scale the frequency-dependent effective excess charge density, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16880, https://doi.org/10.5194/egusphere-egu2020-16880, 2020.

EGU2020-1424 | Displays | EMRP1.2

Seismic attenuation and velocity dispersion due to squirt flow in cracks with rough walls

Simón Lissa, Nicolás D. Barbosa, Eva Caspari, Yury Alkhimenkov, and Beatriz Quintal

We numerically study the effects that roughness in the walls of cracks has on the P-wave modulus dispersion and attenuation due to squirt flow. We emulate the deformation caused by a seismic P-wave by applying an oscillatory relaxation test on numerical rock models having two perpendicular fluid-filled cracks interconnected and embedded in a cubic elastic background. The deformation caused by the P-wave induces a fluid pressure gradient and then, during the consequent fluid pressure diffusion process, the friction between fluid particles dissipate seismic wave energy. In this work, we consider P-wave deformation normal to one of the cracks. We first consider binary aperture distribution for the cracks to analyse where the energy dissipation process takes place. Then, more complex geometries for the roughness of the walls are also considered. In both cases, the cracks have finite length and square-shape and no contact areas between the walls of the cracks were allowed to occur. We show that the arithmetic mean of the apertures controls the P-wave modulus magnitudes at the low- and high-frequency limits. Additionally, two attenuation peaks and modulus dispersion regimes may occur associated with squirt flow. In general, at low-frequencies, the energy dissipation tends to happen inside the minimum aperture of the cracks, and consequently, the minimum aperture determines the frequency at which the low-frequency attenuation peak occurs. For the considered models, we observed that when the percentage of minimum aperture in the cracks is lower than 10$\%$, a second attenuation peak at high frequencies become dominant. The characteristic frequency of this attenuation process is controlled by an effective hydraulic aperture. Finally, we simulate an increase in confining pressure by reducing the crack apertures by a constant value, allowing for contact areas occurrence. In this scenario, the stiffness of the cracks can not longer be explained with the arithmetic mean of the aperture, as the stiffening effect of the distribution of the contact areas plays a much stronger role. In general, from the analysis of the local energy dissipation, different apertures seem to control the energy dissipation process at each frequency, which means that a frequency-dependent hydraulic aperture might be needed to describe the squirt flow process in cracks with rough walls.

How to cite: Lissa, S., Barbosa, N. D., Caspari, E., Alkhimenkov, Y., and Quintal, B.: Seismic attenuation and velocity dispersion due to squirt flow in cracks with rough walls, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1424, https://doi.org/10.5194/egusphere-egu2020-1424, 2020.

EGU2020-2902 | Displays | EMRP1.2

Poroelastic effects of the damaged zone on seismic fracture reflectivity

Edith Sotelo Gamboa, Santiago G. Solazzi, German J. Rubino, Nicolas D. Barbosa, and Klaus Holliger

The presence of fractures has a predominant influence on the hydraulic and mechanical behavior of rocks. These effects are particularly pronounced and relevant for otherwise largely impermeable and stiff formations. There is widespread evidence pointing to the ubiquitous presence of damaged zones surrounding fractures and faults. The enhanced permeability associated with these zones can promote fluid pressure diffusion in the vicinity of fractures when seismic waves travel through the corresponding subsurface volume. This process, together with the inherent mechanical weakness of damaged zones, is expected to affect the seismic reflectivity of fractures and faults. We investigate these effects based on Biot’s theory of poroelasticity. To this end, we consider a 1D layered representation of the fracture and the associated damaged zone in conjunction with embedding elastic and impermeable half-spaces. We compare a fully elastic fracture-background reference model with a model consisting of a poroelastic fracture and damaged zone enclosed within an elastic background. For these two models, we compute the normal incidence seismic P-wave reflectivities at the background-fracture and at background-damaged zone interfaces, respectively. We also include a model that represents the fracture-damaged zone poroelastic system as an equivalent viscoelastic layer. We aim to test the validity of this representation since it would imply that a similar correspondence is possible to establish when more realistic descriptions of the damaged zone are considered. For this additional model, the viscoelastic layer is characterized by its frequency-dependent P-wave modulus, estimated by applying White’s classical upscaling procedure for 1D poroelastic media composed of alternating layers. We test the validity of the elastic-viscolastic model by comparing its reflectivity against the corresponding results from the elastic-poroelastic model. In doing so, we find that the simplified elastic-viscoelastic model faithfully reproduces the reflectivity of its elastic-poroelastic counterpart up to a threshold frequency, at which resonances produced within the viscoelastic layer become dominant. Overall, our results show that, in the seismic frequency range, there is a substantial increase in seismic fracture reflectivity resulting from the combined effects of fluid pressure diffusion and mechanical weakening associated with the surrounding damaged zone. This, in turn, indicates that the seismic reflectivity of a fracture may indeed be dominated by the thickness and physical properties of its surrounding damaged zone rather than by the properties of the fracture sensu stricto.

How to cite: Sotelo Gamboa, E., Solazzi, S. G., Rubino, G. J., Barbosa, N. D., and Holliger, K.: Poroelastic effects of the damaged zone on seismic fracture reflectivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2902, https://doi.org/10.5194/egusphere-egu2020-2902, 2020.

EGU2020-14762 | Displays | EMRP1.2

Effects of fracture connectivity on Rayleigh wave velocity dispersion

Gabriel Quiroga, J. Germán Rubino, Santiago Solazzi, Nicolás Barbosa, and Klaus Holliger

The use of passive seismic techniques to monitor geothermal reservoirs allows to assess the risks associated with their exploitation and stimulation. One key characteristic of geothermal reservoirs is the degree of fracture connectivity and its evolution. The reason for this is that changes in the interconnectivity of the prevailing fractures affect the permeability and, thus, the productivity of the system. An increasing number of studies indicates that the Rayleigh wave velocity can be sensitive to changes in the mechanical and hydraulic properties of geothermal reservoirs. In this work, we explore the effects of fracture connectivity on Rayleigh wave velocity dispersion accounting for wave-induced fluid pressure diffusion effects. To this end, we consider a 1D layered model consisting of a surficial sandstone formation overlying a fractured and water-saturated granitic layer, which, in turn, is underlain by a compact granitic half-space. For the stochastic fracture network prevailing in the upper granitic layer, we consider varying levels of fracture connectivity, ranging from entirely unconnected to fully interconnected. We use an upscaling approach based on Biot’s poroelasticity theory to determine the effective properties associated with these scenarios. This procedure allows to obtain the frequency-dependent seismic body wave velocities accounting for fluid pressure diffusion effects. Finally, using these parameters, we compute the corresponding Rayleigh wave velocity dispersion. Our results show that Rayleigh wave phase and group velocities exhibit a significant sensitivity to the degree of fracture connectivity, which is mainly due to a reduction of the stiffening effect of the fluid residing in connected fractures in response to wave-induced fluid pressure diffusion. This suggests that time-lapse observations of Rayleigh wave velocity changes, which so far are commonly associated with changes in the fracture density, could also be related to changes in the interconnectivity of pre-existing fractures.

How to cite: Quiroga, G., Rubino, J. G., Solazzi, S., Barbosa, N., and Holliger, K.: Effects of fracture connectivity on Rayleigh wave velocity dispersion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14762, https://doi.org/10.5194/egusphere-egu2020-14762, 2020.

The O-ZNS observatory offers a unique geophysical support for characterization, at different scales (from nano- to metric scales) of the heterogeneous Beauce Limestones aquifer. Currently under development at an agricultural site in Villamblain (Centre Val de Loire, France), this observatory is based on an exceptional well (20 m-depth and 4 m-diameter) associated with many external boreholes on an area of around 2 400 m2. It will combine different geophysical techniques and innovative multi-geosciences sensors to image, monitor and understand fluid and heat transfers in the heterogeneous structure of the vadoze zone.

An initial geophysical characterization has been conducted with surface measurements (3D electrical resistivity imaging and 2D Magnetic Resonance Sounding) that gave interesting information on the lithology of O-ZNS site: a silty-clayed soil of a few meter thick, then a highly heterogeneous and karstified limestone and finally, the massive fractured limestone. Cross-hole radar measurements add to these information a description of the initial zone, the soil properties and the water content. Also, data from three boreholes and the collection of core samples as well as logging measurements completed and improved this initial characterization.

All these data have been used to develop a finite element numerical model representing both the study site and the well under Plaxis 2D. Through the realism of geotechnical engineering including deformation, stability and water flow, the idea, is to anticipate the effect of the digging and provide information about the induced damaged zone that will derive. We also look into describing the evolution of this damaged zone depending on the seasoning variation (i.e. from 3 to 5 m) of the groundwater level. All these characterizations will allow us to better focus our field geophysical investigations on monitoring the damaged zone.

The model consists of a description of the different soil layers from the boreholes that includes elastic, microstructural and transport properties, followed by a description of the interface between the soil and the well. The hydraulic conditions will take into account the time-variability of fluxes and the aquifer level. Furthermore, this model is coupled with the construction phasing from a civil engineering point of view. The results will give the evolution of stress and strain induced by the engineering development of O-ZNS well in the host rock as well as an estimate of the material displacement and its elasticity limits. The preliminary modelling generated a result stipulating a damaged zone of 1-2 m around the well at the surface. The magnitude of the damaged zone is reduced with depth. It seems that, at the bottom, the host rock is undamaged.

Undergoing development are focused on refining the model by providing more effective and updated estimations of the soil and structure properties in order to validate or improve the first results together with an estimation of the time evolution of the damaged zone with the water saturation state. Afterward, we will be able to compare and validate these results to pictures and measurements performed during the digging that will start in the spring 2020.

How to cite: Mallet, C., Jodry, C., Laurent, G., and Azaroual, M.: Geophysical estimation of the damage induced by an observatory digging in a limestone heterogeneous vadose zone – Beauce aquifer (France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2411, https://doi.org/10.5194/egusphere-egu2020-2411, 2020.

EGU2020-18106 | Displays | EMRP1.2

Upscaling of elastic properties in carbonates: A modeling approach based on a multiscale geophysical data set

Jerome Fortin, Cedric Bailly, Mathilde Adelinet, and Youri Hamon

Linking ultrasonic measurements made on samples, with sonic logs and seismic subsurface data, is a key challenge for the understanding of carbonate reservoirs. To deal with this problem, we investigate the elastic properties of dry lacustrine carbonates. At one study site, we perform a seismic refraction survey (100 Hz), as well as sonic (54 kHz) and ultrasonic (250 kHz) measurements directly on outcrop and ultrasonic measurements on samples (500 kHz). By comparing the median of each data set, we show that the P wave velocity decreases from laboratory to seismic scale. Nevertheless, the median of the sonic measurements acquired on outcrop surfaces seems to fit with the seismic data, meaning that sonic acquisition may be representative of seismic scale. To explain the variations due to upscaling, we relate the concept of representative elementary volume with the wavelength of each scale of study. Indeed, with upscaling, the wavelength varies from millimetric to pluri-metric. This change of scale allows us to conclude that the behavior of P wave velocity is due to different geological features (matrix porosity, cracks, and fractures) related to the different wavelengths used. Based on effective medium theory, we quantify the pore aspect ratio at sample scale and the crack/fracture density at outcrop and seismic scales using a multiscale representative elementary volume concept. Results show that the matrix porosity that controls the ultrasonic P wave velocities is progressively lost with upscaling, implying that crack and fracture porosity impacts sonic and seismic P wave velocities, a result of paramount importance for seismic interpretation based on deterministic approaches.

Bailly, C., Fortin, J., Adelinet, M., & Hamon, Y. (2019). Upscaling of elastic properties in carbonates: A modeling approach based on a multiscale geophysical data set. Journal of Geophysical Research: Solid Earth, 124. https://doi.org/10.1029/2019JB018391

How to cite: Fortin, J., Bailly, C., Adelinet, M., and Hamon, Y.: Upscaling of elastic properties in carbonates: A modeling approach based on a multiscale geophysical data set, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18106, https://doi.org/10.5194/egusphere-egu2020-18106, 2020.

Elastic waves are commonly studied in geophysics. They are used for example for prospecting, to follow the exploitation of hydrocarbon reservoirs, to study the effect of fluid injection (CO2 storage)… However, the wave frequencies used in the field (sonic – seismic measurements) are not the same as the ones commonly used in the laboratory (ultrasonic measurements), and fluid-saturated rocks are known to be dispersive, i.e the P- and S- wave velocity in fluid-saturated rock change with frequency. The comparison between field and laboratory measurements is therefore not straightforward.

In the ENS facilities, it is possible to subject samples, under pressure (1 to 30 MPa) to forced - oscillations varying from 0.01 Hz to 1 kHz (field frequencies) and 1 MHz (ultrasonic frequencies) using a triaxial cell. Axial and radial strain gauges are installed to record the resulting strains on the sample. Forced-oscillation can be done on 1) confining pressure to get the bulk modulus as function of frequency or on 2) axial stress to get the Young modulus and Poisson ratio as function of frequency.  With this information, it is thus possible to deduce the P- and S- wave velocities with frequency.

The elastic properties were measured on different samples from the Libra oil field, for which logging measurements are available. Thus, the measurements obtained in the laboratory can be compared to the measurements in the field at the same frequency. In addition, the evolution of the velocity with frequency measured in the laboratory allows us to discuss the mechanisms at the origin of the dispersion.  

How to cite: Gallagher, A.: Comparison of the elastic properties of reservoir rocks in the field and the laboratory: link between seismic, sonic and ultrasonic measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1335, https://doi.org/10.5194/egusphere-egu2020-1335, 2020.

EGU2020-2697 | Displays | EMRP1.2

Microstructural control on compaction localisation in granular materials

Lucille Carbillet, Michael J. Heap, Fabian B. Wadsworth, Patrick Baud, and Thierry Reuschlé

Field observations and laboratory experiments have demonstrated strain localisation can develop in porous rocks in response to an applied stress field. Shear fractures and compaction bands are strain localisation features that can form at relatively low confinement during brittle deformation and at higher confinement during shear-enhanced compaction, respectively. Previous experimental studies suggested that the formation and geometry of compaction bands also depends on the microstructural attributes of the rock.

We investigated the influence of microstructure on compaction localisation in porous rocks using sintered glass bead samples, which allowed for a tight control on grain size and shape and sample porosity. During the fabrication process, populations of solid glass microspheres of predetermined size and size distribution are heated above their glass transition temperature. Above this temperature, the glass beads act as viscous liquid droplets. Time-dependent coalescence of droplets that share contact then causes the bead-pack to evolve into a connected system, producing a porous granular material of known microstructural geometries and final porosity.

We previously conducted hydrostatic compaction and triaxial compression tests on synthetic samples of porosity ranging from 10 to 38% with a monodisperse grainsize (diameter ranging from 0.15 to 1.3 mm). Experimental results showed remarkable reproducibility for the same experimental conditions and concurrence with the phenomenology of mechanical behaviour of natural sandstones. After these validation tests, we conducted systematic experiments on monodisperse synthetic samples of 25 and 35% of porosity prepared using glass beads of mean diameter 0.25, 0.525 and 1.15 mm. Triaxial deformation tests were conducted on water-saturated samples, in drained conditions (with a fixed pore pressure of 10 MPa), at room temperature, at a constant strain-rate and at effective pressures corresponding to the regime of formation of compaction bands. Our mechanical data provide indirect evidence for compaction localisation. We have focused our attention on the influence of porosity and grain size on the formation and microstructural attributes (such as thickness, length and tortuosity) of the compaction bands.

How to cite: Carbillet, L., Heap, M. J., Wadsworth, F. B., Baud, P., and Reuschlé, T.: Microstructural control on compaction localisation in granular materials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2697, https://doi.org/10.5194/egusphere-egu2020-2697, 2020.

EGU2020-2992 | Displays | EMRP1.2

Combined numerical and experimental study of microstructure and permeability in porous granular media

Philipp Eichheimer, Marcel Thielmann, Wakana Fujita, Gregor J. Golabek, Michihiko Nakamura, Satoshi Okumura, Takayuki Nakatani, and Maximilian O. Kottwitz

Fluid flow on different scales is of interest for several Earth science disciplines like petrophysics, hydrogeology and volcanology. To parameterize fluid flow in large-scale numerical simulations (e.g. groundwater and volcanic systems), flow properties on the microscale need to be considered. For this purpose experimental and numerical investigations of flow through porous media over a wide range of porosities are necessary. In the present study we sinter glass bead media with various porosities, representing shallow depth crustal sediments. The microstructure, namely effective porosity and effective specific surface, is investigated using image processing. We furthermore determine flow properties like hydraulic tortuosity and permeability using both experimental measurements and numerical simulations. By fitting microstructural and flow properties to porosity, we obtain a modified Kozeny-Carman equation for isotropic low-porosity media, that can be used to simulate permeability in large-scale numerical models. To verify the modified Kozeny-Carman equation we compare it to the numerically computed and experimentally measured permeability values.

How to cite: Eichheimer, P., Thielmann, M., Fujita, W., Golabek, G. J., Nakamura, M., Okumura, S., Nakatani, T., and Kottwitz, M. O.: Combined numerical and experimental study of microstructure and permeability in porous granular media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2992, https://doi.org/10.5194/egusphere-egu2020-2992, 2020.

Scanning electron microscopy (SEM) and helium ion microscopy (HIM) are two of the fundamental tools in the study of the microstructures of shale. A comprehensive comparison of these two techniques in the application of organic pore structure characterization is presented in this work. Owing to the small wavelength of the helium ion, the spot size of the ion beam is not restricted by diffraction aberration, and the convergence angle of helium ion beam can be much smaller than of the electron beam. The microscopic images and reconstruction models indicate that HIM has higher spatial resolution and increased depth of field than SEM. The pores below 10 nm and inner structures of pore networks can be observed via HIM images. The advantages shown in the focused ion beam/helium ion microscopy (FIB/HIM) results are similar to the 2-D HIM images. Smaller pores whose size is beyond the resolution of focused ion beam/scanning electron microscopy (FIB/SEM) can be found, which suggests the connection possibility of the big pores. However, to get reliable pictures, the ion-induced damage on organic matters should be avoided. To lower the beam current and to shorten the dwell time are two effective ways to reduce the beam damage.

How to cite: Zhao, J., Zhang, W., and Zhang, D.: Organic pore structure characterization of shale: a comparison between scanning electron microscopy and helium ion microscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12191, https://doi.org/10.5194/egusphere-egu2020-12191, 2020.

EGU2020-6706 | Displays | EMRP1.2

Digital rock physics and laboratory considerations on a high-porosity volcanic rock

Laura L. Schepp, Benedikt Ahrens, Martin Balcewicz, Mandy Duda, Mathias Nehler, Maria Osorno, David Uribe, Holger Steeb, Benoit Nigon, Ferdinand Stöckhert, Donald A. Swanson, Mirko Siegert, Marcel Gurris, and Erik H. Saenger

Microtomographic imaging techniques and advanced numerical simulations are combined by digital rock physics (DRP) to obtain effective physical material properties. The numerical results are typically used to complement laboratory investigations with the aim to gain a deeper understanding of physical processes related to transport (e.g. permeability and thermal conductivity) and effective elastic properties (e.g. bulk and shear modulus). The present study focuses on DRP and laboratory techniques applied to a rock called reticulite, which is considered as an end-member material with respect to porosity, stiffness and brittleness of the skeleton. Classical laboratory investigations on effective properties, such as ultrasonic transmission measurements and uniaxial deformation experiments, are very difficult to perform on this class of high-porosity and brittle materials.

Reticulite is a pyroclastic rock formed during intense Hawaiian fountaining events. The open honeycombed network has a porosity of more than 80 % and consists of bubbles that are supported by glassy threads. The natural mineral has a strong analogy to fabricated open-cell foams. By comparing experimental with numerical results and theoretical estimates we demonstrate the potential of digital material methodology with respect to the investigation of porosity, effective elastic properties, thermal conductivity and permeability

We show that the digital rock physics workflow, previously applied to conventional rock types, yields reasonable results for a high-porosity rock and can be adopted for fabricated foam-like materials. Numerically determined effective properties of reticulite are in good agreement with the experimentally determined results. Depending on the fields of application, numerical methods as well as theoretical estimates can become reasonable alternatives to laboratory methods for high porous foam-like materials.

How to cite: Schepp, L. L., Ahrens, B., Balcewicz, M., Duda, M., Nehler, M., Osorno, M., Uribe, D., Steeb, H., Nigon, B., Stöckhert, F., Swanson, D. A., Siegert, M., Gurris, M., and Saenger, E. H.: Digital rock physics and laboratory considerations on a high-porosity volcanic rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6706, https://doi.org/10.5194/egusphere-egu2020-6706, 2020.

EGU2020-3888 | Displays | EMRP1.2

Comparison of pore fabric based on high-resolution X-ray computed tomography and magnetic pore fabric in sedimentary rocks

Yi Zhou, Michele Pugnetti, Anneleen Foubert, Christoph Neururer, and Andrea R. Biedermann

Pore fabrics characterize pore geometry and network in rocks. The pore size, connectivity and elongation direction determine the permeation ability and preferred permeation direction. X-ray micro-tomography (XRCT) is a widely used technique to visualize the inner structure of rock samples. Based on XRCT data, digital rock models can be generated and analyzed to visualize and quantify pore shape distribution, pore sizes and the connectivity of pores. To measure the magnetic pore fabric (MPF), samples are impregnated with ferrofluid prior to measuring anisotropy of magnetic susceptibility. This technique could be complementary to existing techniques to capture smaller pores. Empirical relationships exist between pore fabric or permeability anisotropy and MPF, and the aim of this study is to quantitatively test these relationships. In this study, Upper Marine Molasse sandstone (OMM, Belpberg, Switzerland) with 10-20% porosity and relatively homogeneous pore structure, and Plio-Pleistocene calcarenite (Apulia, Italy) with ~50% porosity and complex pore structure, are tested. To understand the pore networks of these rock types, an integrated approach has been applied including standard pycnometer porosity measurements, MPFs, XRCT, and porosity and permeability simulations based on XRCT analyses. The average equivalent diameter of pores based on micro-CT is ~150 μm for the Molasse sandstone, and ~300 μm for calcarenite. XRCT data indicate preferential alignment of the long axes of the pores, and both MPFs and simulated permeabilities are anisotropic in these samples. For calcarenite with large pores, the direction of the maximum magnetic susceptibility coincides with the direction of the maximum grouping of long pore axes. Simulated permeability is affected by other factors in addition to the grouping of long pore axes, including porosity, pore size, connectivity and tortuosity of pores. Therefore, the next step of this study will compare laboratory-measured directional permeabilities with permeability simulations and with MPFs, to investigate their potential for predicting the preferred fluid flow direction in these samples. For the full understanding of MPFs, more types of sedimentary rocks will be analyzed. If MPFs prove a good and quantitative proxy for pore fabric characterization in hydrocarbon and geothermal studies, more measurements can be made in the future, making it possible to investigate regional-scale variations.

How to cite: Zhou, Y., Pugnetti, M., Foubert, A., Neururer, C., and R. Biedermann, A.: Comparison of pore fabric based on high-resolution X-ray computed tomography and magnetic pore fabric in sedimentary rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3888, https://doi.org/10.5194/egusphere-egu2020-3888, 2020.

EGU2020-17144 | Displays | EMRP1.2

Using Gamma-Ray and X-Ray Computed Tomography for Porosity Quantification of Reservoir Analogue Rocks

Abraão Nova, Frederico Ribeiro, Pamalla Oliveira, Daniel Amancio, Cássia Machado, Alexandra Carolina, Marcio Paixão, Antonio Antonino, Enivaldo Barbosa, Antônio Barbosa, Maria Lourenço, Marcos Rodrigues, and Richard Heck

During the last few decades, X-ray micro-computed tomography (µCT) has been largely used to characterize rock properties and to create high-resolution 3D digital image volumes. It has allowed access to important information about porous systems in reservoir rocks. However, the reliable quantification of porosity of rocks which present porous volumes ranging from centimeter to nanometer scale remains a challenge. Assessment of nano scale porous volume is very difficult by image segmentation techniques, due to the intrinsic limits of the x-ray imaging method. Moreover, image processing for analysis of various types of porosity in the same sample, including microporosity could be computationally expensive. We present a method based in the Gamma-Ray computed tomography (axis attenuation) that can substantially improve the limits presented by conventional X-ray microtomography. This study compared the porosity values acquired by typical segmentation methods for microtomography images, and by the values obtained trough the proposed method of gamma-ray computed tomography to calculate the porosity. Results of both approaches were compared to porosity measurements obtained through experimental equipment (helium porosimeter). These analyses were performed in core samples of limestones and sandstones analogous of Brazilian oil reservoirs. The Gamma Ray Attenuation method (axis attenuation) presented a better correlation (R² = 0.9588) to the experimental measurements when compared to the image segmentation methods (R² = 0.9194). The results suggest that Industrial application of gamma ray tomography for precise evaluation of large number of core samples can be highly effective. Furthermore, the gamma ray data can be integrated with data provided by conventional µCT image processing to complement information regarding morphological aspects.

Keywords: Porous System, X-ray microtomography, Gamma Ray tomography,  Reservoir rocks

How to cite: Nova, A., Ribeiro, F., Oliveira, P., Amancio, D., Machado, C., Carolina, A., Paixão, M., Antonino, A., Barbosa, E., Barbosa, A., Lourenço, M., Rodrigues, M., and Heck, R.: Using Gamma-Ray and X-Ray Computed Tomography for Porosity Quantification of Reservoir Analogue Rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17144, https://doi.org/10.5194/egusphere-egu2020-17144, 2020.

EMRP1.4 – Multiscale rock damage in geology, geophysics and geo-engineering systems

EGU2020-3316 | Displays | EMRP1.4

Predicting the proximity to system-scale rupture using fracture networks

Jessica McBeck, John Aiken, Joachim Mathiesen, Yehuda Ben-Zion, and Francois Renard

A fundamental challenge in geophysics is predicting the timing of large earthquakes. A key step in addressing this problem is constraining the factors that indicate the timing of the next large rupture. To isolate the factors that help predict the proximity of the next earthquake, we develop machine learning models to predict the stress distance to macroscopic failure in triaxial compression X-ray tomography experiments on rocks at the stress conditions of the upper crust. In these experiments, we apply increasing axial stress in steps, and acquire a 3D X-ray tomogram at each stress step while the rock is under constant load, revealing the 3D density distribution. Segmenting the density fields provide the locations of rock (voxels dominated by solid), and pores and fractures (voxels dominated by air). We train the machine learning models using the geometry and clustering properties of the fracture networks identified in the tomography scans. We develop extreme gradient boosting (XGBoost) models to predict the stress distance to failure. In experiments on Carrara marble, monzonite, and granite, the models predict the stress distance to failure with r2 values > 0.7. We examine the feature importance to identify the factors that provide the best predictive power of the distance to failure. Measurements of the fracture network clustering and the shape anisotropy of fractures tend to have the highest importance of the features, providing greater predictive information than the fracture volume, fracture length, fracture aperture, and fracture orientation relative to the maximum compression direction.

How to cite: McBeck, J., Aiken, J., Mathiesen, J., Ben-Zion, Y., and Renard, F.: Predicting the proximity to system-scale rupture using fracture networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3316, https://doi.org/10.5194/egusphere-egu2020-3316, 2020.

EGU2020-4092 | Displays | EMRP1.4

Fault (un-)stability and strain partitioning across the brittle-ductile transition

Jérôme Aubry, François Passelègue, Javier Escartín, Damien Deldicque, Julien Gasc, Samson Marty, Marine Page, and Alexandre Schubnel

In the lithosphere, the transition from brittle to ductile deformation corresponds to a regime where brittle fracturing and plastic flow coexist, called the semi-brittle deformation zone. Within these different regimes, a large fault slip spectrum has been observed, from fast to slow earthquakes. Studying the parameters controlling fault (un-)stability and strain partitioning across this transition is fundamental to understand how natural faults behave at varying crustal depths.

To investigate semi-brittle deformation and the conditions promoting it, we report here the results of experiments performed on Carrara marble saw-cut faults in triaxial conditions. We studied the influence of the confining pressure, axial loading rates and initial fault roughness on fault (un-)stability. From mechanical data, we performed strain partitioning calculations to infer elastic, frictional and plastic strain contributions during the deformation process.

We conclude that (laboratory) earthquakes may nucleate within a regime where homogeneous plastic deformation of the bulk and dynamic fault slip may coexist. The contribution of plastic strain is promoted with increasing confining pressure and fault roughness.

How to cite: Aubry, J., Passelègue, F., Escartín, J., Deldicque, D., Gasc, J., Marty, S., Page, M., and Schubnel, A.: Fault (un-)stability and strain partitioning across the brittle-ductile transition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4092, https://doi.org/10.5194/egusphere-egu2020-4092, 2020.

EGU2020-13547 | Displays | EMRP1.4

Damage indicators and failure prediction in Focal Mechanism solutions

Sergio Vinciguerra, Thomas King, Philip Benson, and Luca De Siena

Acoustic Emissions (AE), the laboratory analogue to seismic events, recorded during conventional triaxial deformation tests allow for an unprecedented amount of information on the evolution of fractured media within a controlled environment. This study presents the results of a new and robust derivation of first motion polarity focal mechanism solutions (FMS). 4 x 10 cm cylindrical samples of Alzo Granite (AG) and Darley Dale Sandstone (DDS) underwent systematic triaxial deformation testing (5, 10, 20 and 40 MPa) in order to investigate the relationships between increasing confining pressure, deformation and failure mode and role of pre-existing microstructure. With an average of 11 of 12 waveforms picked using a neural network for each AE, high resolution datasets are obtained that can track the evolution of deformation structure through time. Focal mechanisms are solved using a least squares minimisation of the fit between projected polarity measurements and the deviatoric stress field induced by tensile, shearing and collapse/closing type sources. Results reveal a surprisingly limited dependency on the distribution of shear fracturing in the lead up to dynamic failure. Instead, deformation is driven by the competition between the opening and closure of fractures that is strongly related to the coupling of local stress fields with pre-existing damage.Spatio-temporal trends in mechanism type and AE amplitude allow for clear identification of: a) Fracture Enucleation. This phase is characterised by broadly distributed tensile fracturing that becomes preferentially aligned as confining pressure increases; b) Fracture Growth. The onset is characterized by a discrete increase in low amplitude shearing events and cyclic fracture development that evolves from a dominance of collapse to shearing followed by tensile fracturing which then returns to collapse type. Influences in mechanism dominance due to rock type are highlighted by increased tensile fracturing in AG, which is replaced by shearing in DDS. A reduction in low amplitude tensile events at 10 MPa in both rock types further reveals a switch from axial splitting to planar localisation as confinement increases; c) Crack Coalescence. The cyclic fracture growth prior to dynamic failure and the amount of strain of this phase share a positive log-linear relationship with confining pressure, allowing to identify the potential for real-time failure prediction; d) Dynamic Failure: High amplitude events characterize the propagation of fractures. Taken together results highlight that failure of the studied samples is the result of the complex interaction between distinct regions of dilatant and compactant deformation. Although planar localisation and preferentially aligned flaws play a more significant role at higher confining pressures, it is the initial heterogeneity or patchiness of the regions undergoing damage that control dynamic failure occurrence and the eventual fracture plane features.

How to cite: Vinciguerra, S., King, T., Benson, P., and De Siena, L.: Damage indicators and failure prediction in Focal Mechanism solutions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13547, https://doi.org/10.5194/egusphere-egu2020-13547, 2020.

EGU2020-10236 | Displays | EMRP1.4

An investigation into the controls on fracture tortuosity in rock sequences and the impact on fluid flow in the upper crust

Nathaniel Forbes Inskip, Tomos Phillips, Kevin Bisdom, Georgy Borisochev, Andreas Busch, and Sabine den Hartog

Fractures are ubiquitous in geological sequences, and play an important role in the movement of fluids in the earth’s crust, particularly in fields such as hydrogeology, petroleum geology and volcanology. When predicting or analysing fluid flow, fractures are often simplified as a set of smooth parallel plates. In reality, they exhibit tortuosity on a number of scales: Fine-scale tortuosity, or roughness, is the product of the small-scale (µm – mm) irregularities in the fracture surface, whereas large-scale (> mm) tortuosity occurs as a result of anisotropy and heterogeneity within the host formation that leads to the formation of irregularities in the fracture surfaces. It is important to consider such tortuosity when analysing processes that rely on the movement (or hindrance) of fluids flowing through fractures in the subsurface. Such processes include fluid injection into granitic plutons for the extraction of heat in Engineered Geothermal Systems, or the injection of CO2 into reservoirs overlain by fine-grained mudrocks acting as seals in Carbon Capture and Storage projects.

Although it is generally assumed that tortuosity is controlled by factors such as grain size, mineralogy and fracture mode, a systematic study of how these factors quantitatively affect tortuosity is currently lacking. Furthermore, in anisotropic rocks the fracture orientation with respect to any inherent anisotropy is also likely to affect tortuosity.

In order to address this gap, we have induced fractures in a selection of different rock types (mudrocks, sandstones and carbonates) using the Brazil disk method, and imaged the fracture surfaces using both a digital optical microscope and X-ray Computed Tomography. Using these methods we are able to characterise both the fine-scale (roughness) and large-scale tortuosity. In order to understand the effect of fracture orientation on tortuosity we have also analysed fractures induced at different angles to bedding in samples of a highly anisotropic mudrock taken from South Wales, UK. Results indicate that fine-scale tortuosity is highly dependent on the fracture orientation with regards to the bedding plane, with fractures normal to bedding being rougher than those induced parallel to bedding. Finally, in order to measure the effect of tortuosity on fluid flow, we have carried out a series of core flooding experiments on a subset of fractured samples showing that fracture transmissivity decreases with increasing tortuosity.

How to cite: Forbes Inskip, N., Phillips, T., Bisdom, K., Borisochev, G., Busch, A., and den Hartog, S.: An investigation into the controls on fracture tortuosity in rock sequences and the impact on fluid flow in the upper crust, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10236, https://doi.org/10.5194/egusphere-egu2020-10236, 2020.

EGU2020-14434 | Displays | EMRP1.4

Fluid-driven tensile fracture and fracture toughness in Nash Point Shale at elevated pressure.

Philip Benson, Stephan Gehne, Nathaniel Forbes Inskip, Philip Meredith, and Nick Koor

Fluid-driven fracturing is a key process in enhancing production in both the hydrocarbon and geothermal energy extraction industries. However, whilst a large number of studies have now developed laboratory methods to simulate the process in a range of settings, and across a number of different rock types, data relating the fundamental material parameters (such as fracture toughness) to the overall rock mechanics response as a function of parameters such as confining and pore pressure remain limited. Here we report a new analysis to recover fracture toughness across a range of effective pressures from hydraulic fracturing experiments that use a modified thick-walled cylinder sample mounted in a conventional triaxial deformation apparatus. We use samples that are 90mm in length and 40mm diameter, with a central, axially drilled borehole 12.6 mm in diameter. An array of 16 ports in the engineered, nitrile, sample jacket allows us to record radial strain (4 channels), acoustic emission output (11 channels) and borehole fluid pressure (1 channel) continuously throughout each test. The sample material was Nash Point shale (NPS) from the south coast of Wales, UK, with samples cored both normal and parallel to bedding in order to investigate the effect of anisotropy. Earlier, ambient pressure fracture toughness tests using the Semi-Circular Bend sample geometry had indicated significant anisotropy, values of 0.24 – 0.30 MPa.m1/2 in the Short-Transverse (ST) orientation, and 0.71 - 0.73 MPa.m1/2 in the Divider (DIV) orientation.

Here, we present results from a suite of 9 experiments, 6 with samples cored parallel to bedding (ST fracture orientation) and 3 with samples cored normal to bedding (DIV fracture orientation). We find that the fluid injection pressure required to fracture our annular shell samples is significantly higher for DIV samples than for ST samples, and increases significantly with increasing confining pressure in both orientations; ranging from 10 to 36 MPa for ST samples and 30 to 58 MPa for DIV samples as confining pressure is increased from 2.2 to 20.5 MPa. We note that the fluid injection pressure undergoes a number of oscillations between fracture nucleation and the fracture reaching the sample boundary. Such oscillations are more common in ST samples than in DIV samples, and in experiments at lower confining pressures. We use the magnitude of each pressure oscillation to estimate the associated increment of fracture extension via the proportion of AE energy generated relative to the total energy accumulated when the fracture reaches the sample boundary. This analysis produces fracture toughness values ranging from 0.36 to 2.76 MPa.m1/2 (ST orientation) and 2.98 to 4.05 MPa.m1/2 (DIV orientation) as confining pressure was increased from 2.2 to 20.5 MPa. We further find that the increase in fracture toughness increases essentially linearly with increasing effective pressure, and this trend appears to be independent of orientation and the material anisotropy.

How to cite: Benson, P., Gehne, S., Forbes Inskip, N., Meredith, P., and Koor, N.: Fluid-driven tensile fracture and fracture toughness in Nash Point Shale at elevated pressure., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14434, https://doi.org/10.5194/egusphere-egu2020-14434, 2020.

A benchmark of low permeability measurements has been organized by the Geosciences and Environment Laboratory at University Cergy-Pontoise over the period 2015-2018. The objective of this benchmark was to measure or estimate through modelling the permeability of a single material, selected for its low permeability. A wide range of different approaches were covered, classified into (i) direct measurement methods, including steady-state, transient pulse and oscillatory techniques and (ii) models using microstructural data obtained from imaging or porosimetry techniques. At the beginning, 30 laboratories in 8 different countries volunteered to participate, and at the end results from 24 labs were collected which is remarkable.

The selected rock was the Grimsel granodiorite (Switzerland), so the benchmark was called “KG²B”, which means “K for Grimsel Granodiorite Benchmark”. Two fresh cores with diameter 85 mm and about one meter long each were provided by colleagues from ETH Zürich. The cores were drilled in the Swiss Grimsel test site, an underground research laboratory in hard rock, at a distance between 4 and 6 meters from the tunnel, away from the EDZ. The cores were cut into small pieces (between 2 and 10 cm long) and sent to the participants. The porosity of the Grimsel Granodiorite is less than 1%, and the permeability is in the 10-18 m² range.

The expected outcomes of the benchmark were: (i) to compare the results for each method separately and (ii) between the different methods/models, (iii) to assess the precision of each method, (iv) to study the influence of experimental conditions, especially sample size and the nature of pore fluid, (v) to gather information on the know-how in each laboratory, and finally (vi) to suggest good practice for low permeability measurements.

The benchmark was designed as a blind test, i.e. the results from each lab were not known by the other labs except for the organizers. A dedicated website [1] was constantly updated to allow the participants to follow the progression of the benchmark. It took about three years to manage the benchmark, collect all the data, complete the dataset analysis and publish the results [2,3]. The results collected allowed us to discuss the influence of pore-fluid, measurement method, sample size and pressure sensitivity, as well as the relevance of various models for permeability estimation. The most striking and unexpected result was that regardless of the method used, the mean gas permeability was higher than the mean liquid permeability by a factor approximately 2.

As an introduction to the session, our aim is to show how conducting such a benchmarking exercise can help to answer the questions raised by the session: - How repeatable are permeability measurements, and how dependent are they on the apparatuses and methods? - Which experimental pitfalls exist, what are the underlying assumptions and how might they impact permeability? - Can we define standard experimental procedures to improve permeability measurements in low permeability materials?

 

  • [1] https://labo.u-cergy.fr/~kg2b
  • [2] Geophys. J. Int., 215, 799-824, doi: 10.1093/gji/ggy304, 2018.
  • [3] Geophys. J. Int., 215, 825-843, doi: 10.1093/gji/ggy305, 2018.

How to cite: David, C.: KG²B, a world-wide inter-laboratory benchmark of low permeability measurement and modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6010, https://doi.org/10.5194/egusphere-egu2020-6010, 2020.

EGU2020-20641 | Displays | EMRP1.4

Representativeness of volume investigated by high-resolution X-ray computed tomography in damaged fine-grained rocks

Hugo Saur, Charles Aubourg, Peter Moonen, Pascale Sénéchal, Tiphaine Boiron, and Hannelore Derluyn

We focus on calcareous homogenous shales featuring different degrees of damage along a km-long strain gradient, marked by cleavage development. In a previous study, we used high-resolution X-ray computed tomography (µCT) to document the evolution of the 3D fabric of the fine-grained shales along the strain gradient (Saur et al., JSG, 2020). Our conclusions were based on samples of ~ 2.5 mm3 containing over 10’000 quartz and calcite grains. The objective of the current study is to assess the representativeness of analyses on such small rock samples. To that extent, we first repeat the µCT analysis on multiple samples of the same, limited, volume and assess the variability of the results. These results are then compared to both macroscopic field observations and bulk fabric measurements obtained with AMS (Anisotropy of Magnetic Susceptibility) on larger samples (~ 10 cm3). AMS provides a statistical description of the magnetic susceptibility tensor, and particularly the confidence angle of axis orientation. Generally, this confidence angle is the result of matrix organization and rock magnetism. In this study, AMS is only controlled by the presence of illite particles which reflect the matrix organization. Finally, we perform a subvolume analysis on the µCT images to determine the smallest representative volume characterizing the fine-grained fabric. In light of these analyses we discuss the representativeness of investigated volume of fine grained shales, subjected to different degrees of deformation.

How to cite: Saur, H., Aubourg, C., Moonen, P., Sénéchal, P., Boiron, T., and Derluyn, H.: Representativeness of volume investigated by high-resolution X-ray computed tomography in damaged fine-grained rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20641, https://doi.org/10.5194/egusphere-egu2020-20641, 2020.

EGU2020-2783 | Displays | EMRP1.4

Thermal influences on macroscale rock damage

Brian D. Collins, Greg M. Stock, Martha-Cary Eppes, Antoine Guerin, Michel Jaboyedoff, and Federica Sandrone

Fracture processes in rock have widespread implications in the geohazard, geomorphologic, and civil and mining engineering communities.  Propagation of fractures reduces overall rock mass strength, can lead to large-scale gravitational instabilities, and can cause significant hazard and damage to infrastructure.  The potential for critical fracture in the form of rock falls and rock bursts are often the primary driver for scientific investigations, civil work project planning, and mining investment outlays.  However, slower subcritical fracture from long-term monotonic and/or cyclic stress perturbations often control the eventual more rapid (and more catastrophic) response of rock.  These slower damage mechanisms may result from existing or perturbed tectonic stresses, stress relief from exhumation or excavation, or long-term environmental stressors such as thermal cycling and frost cracking.

Here we investigate the role of thermal cycling in generating subcritical stresses to which virtually all rock cliffs worldwide are exposed.  Our hypothesis – that diurnal and seasonal cycles of temperature can lead to substantial subcritical fracture propagation and eventual critical fracture – has led us to design several field and laboratory experiments to measure both the deformations and the stresses associated with environmental thermal forcing in rock.  Our studies focus on granitic exfoliation environments, common in many mountainous regions of the world, where relatively thin (centimeters to decimeters) exfoliation sheets are able to undergo a full thickness thermal response, and where exfoliation-related rock falls are common and in some places, well-documented.

In cliff environments located in Yosemite National Park (California, USA), our field studies using in-situ measurements (i.e., crackmeters and temperature sensors) have shown that diurnal and seasonal thermal cycles lead to cyclic stresses in the subcritical range, with resultant cumulative and seemingly permanent rock deformation outwards from the main cliff surface.  Additional field studies using thermal IRT (InfraRed Thermography) imaging identify the locations of rock bridges that likely serve as focal points for these thermally-induced stress concentrations.  Although we did not measure the critical fracture conditions that would result in a rock fall, we did, fortuitously, capture the deformation signals leading up to explosive fracture of a nearby granitic 100-m-diameter exfoliation dome during peak temperatures at the site (located ~60 km northwest from Yosemite), thereby proving the efficacy of thermal stresses in driving both long term – and catastrophic – rock damage.  These field studies are substantiated by analytical fracture mechanics solutions which show how rock may eventually fail under these conditions.  These studies therefore serve as proxies for understanding how some rock falls eventually occur under subcritical thermally-induced cyclic stress conditions, but also more generally for how thermal-stress conditions may affect rock damage in a multitude of environments.

How to cite: Collins, B. D., Stock, G. M., Eppes, M.-C., Guerin, A., Jaboyedoff, M., and Sandrone, F.: Thermal influences on macroscale rock damage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2783, https://doi.org/10.5194/egusphere-egu2020-2783, 2020.

Geological and geophysical observations of fault zones reveal that fault cores are surrounded by regions of damaged rocks consist of fractures at a wide range of length scales with decaying intensity with distance from the fault core. The main mechanisms proposed for the development of off-fault damage include slip on faults with geometrical irregularities, migrating process zones, and dynamic damage from the passage of earthquake ruptures. Field observations of relatively deep exhumed fault zones have shown that fault damage zone width scales with the displacement on a fault. In this study, we combine such observations with numerical modeling to test what is the dominant mechanism producing off-fault damage at depth of several kilometres.

The field data [Faulkner et al., 2011] include measurements of micro-fracture damage zone width from small displacement fault zones within the Atacama fault zone in northern Chile that formed at ∼6 km depth within a dioritic protolith. An increase in damage zone width with displacement is clearly seen. We perform simulations of slip on synthetic faults, with roughness properties similar to that of natural faults, and examine how the total slip and roughness characteristics affect the extent and intensity of inelastic deformation to constrain the geometrical and frictional properties that could generate the observed damage. To accurately account for the effects of geometrical irregularities on the fault and allow slip that is large relative to the size the minimum roughness wavelength, we use the mortar finite element method, in which non-matching meshes are allowed across the fault and the contacts are continuously updated. Inelastic deformation of the bulk is modelled with Drucker–Prager viscoplasticity, which is a simple choice for describing cracked medium and is closely related to the Mohr–Coulomb model. Our results indicate that, for the depth and fault lengths in the field data, geometrical irregularities produce the scaling of damage zone width with displacement observed in the field and suggest that this, rather than the other mechanisms, produce most of the damage.

How to cite: Tal, Y. and Faulkner, D.: The effect of geometrical irregularities on damage zone width: Modeling and field observations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5884, https://doi.org/10.5194/egusphere-egu2020-5884, 2020.

EGU2020-9982 | Displays | EMRP1.4

Differences between static and dynamic elastic moduli: Importance of experimental methods

Elisabeth Bemer, Noalwenn Dubos-Sallée, and Patrick N. J. Rasolofosaon

The differences between static and dynamic elastic moduli remain a controversial issue in rock physics. Various empirical correlations can be found in the literature. However, the experimental methods used to derive the static and dynamic elastic moduli differ and may entail substantial part of the discrepancies observed at the laboratory scale. The representativeness and bias of these methods should be fully assessed before applying big data analytics to the numerous datasets available in the literature.

We will illustrate, discuss and analyze the differences inherent to static and dynamic measurements through a series of triaxial and petroacoustic tests performed on an outcrop carbonate. The studied rock formation is Euville limestone, which is a crinoidal grainstone composed of roughly 99% calcite and coming from Meuse department located in Paris Basin. Sister plugs have been cored from the same quarry block and observed under CT-scanner to check their homogeneity levels.

The triaxial device is equipped with an internal stress sensor and provides axial strain measurements both from strain gauges glued to the samples and LVDTs placed inside the confinement chamber. Two measures of the static Young's modulus can thus be derived: the first one from the local strain measurements provided by the strain gauges and the second one from the semi-local strain measurements provided by the LVDTs. The P- and S-wave velocities are measured both through first break picking and the phase spectral ratio method, providing also two different measures of the dynamic Young's modulus.

The triaxial tests have been performed in drained conditions and the measured static elastic moduli correspond to drained elastic moduli. The petroacoustic tests have been performed using the fluid substitution method, which consists in measuring the acoustic velocities for various saturating fluids of different bulk modulus. No weakening or dispersion effects have been observed. Gassmann's equation can then be used to derive the dynamic drained elastic moduli and the solid matrix bulk modulus, which is otherwise either taken from the literature for pure calcite or dolomite samples, or computed using Voigt-Reuss-Hill or Hashin-Shtrikman averaging of the mineral constituents.

For the studied carbonate formation, we obtain similar values for static and dynamic elastic moduli when derived from careful lab experiments. Based on the obtained results, we will finally make recommendations, emphasizing the necessity of using relevant experimental techniques for a consistent characterization of the relation between static and dynamic elastic moduli.

How to cite: Bemer, E., Dubos-Sallée, N., and Rasolofosaon, P. N. J.: Differences between static and dynamic elastic moduli: Importance of experimental methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9982, https://doi.org/10.5194/egusphere-egu2020-9982, 2020.

The mechanical response of natural gypsum rock is relevant in a wide range of engineering applications (e.g. tunnel excavation, stability assessment of underground quarries, oil and gas accumulation). In particular, in underground quarry environments, static loading conditions insisting on the gypsum pillars during and after the exploitation activities (i.e. several decades) require a specific attention to the sub-critical time-dependent deformation of the rock. The short-term stability (referred to the possibility of a failure in consequence to the sudden application of the axial load) does not preclude the possibility of deformation or even failure in the long-term.

In addition, the underground drifts of gypsum quarries are often located below the static level of the groundwater table, requiring a continuous water pumping to allow for the accessibility of the drifts themselves. The end of the quarry activity, coinciding with the interruption of the de-watering operations and the re-assessment of the original level of water table, brings to the new water saturation of the gypsum body. The water fills the connected porosity of the rock, influencing the general stability of the underground voids.

For these reasons, the present work aims to investigate the mechanical response of gypsum rock in time-dependent regime, also considering the influence of water saturation. The study proposes an experimental investigation of the influence of water on the rheology of a natural gypsum facies (i.e. branching selenite gypsum), distinguishing between the mechanical effects of a saturating fluid (in relation to the internal pore pressure), that should also be observed with a non-reactive fluid such as oil, and the water-gypsum chemical interactions. This influence of water is investigated in uniaxial compression, under uniaxial creep conditions and conventional triaxial compression. The new mechanical data are accompanied by microstructural observations of the effects induced in the rock by the mechanical compression, aiming to propose a description of the mechanisms involved in the gypsum deformation process.

How to cite: Caselle, C., Bonetto, S., and Baud, P.: Time-dependent behaviour and water influence on the mechanical response of gypsum rock in underground quarry frameworks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4138, https://doi.org/10.5194/egusphere-egu2020-4138, 2020.

EGU2020-16050 | Displays | EMRP1.4

Multiphysics of transient deformation processes leading to macroscopic instabilities in geomaterials

Antoine Jacquey, Klaus Regenauer-Lieb, Francesco Parisio, and Mauro Cacace

Material instabilities are critical phenomena which can occur in geomaterials at high stress and temperature conditions. They generally result in the degradation of the microstructure organisation, ultimately leading to material failure. These phenomena are relevant to a large variety of geoscientific and geotechnical applications including earthquake physics, fault mechanics, successful targeting of unconventional georesources and mitigation of induced seismicity. Quantifying and predicting the onset of material degradation upon instability remains a major challenge due to our lack of understanding of the physics controlling the behaviour of porous rocks subject to high temperature and pressure conditions.

In the laboratory, rocks gradually transition from a time-independent brittle behaviour to a transient semi-brittle, semi-ductile behaviour upon an increase in pressure and/or temperature. Furthermore, even when subject to constant subcritical stress conditions rocks have been observed to macroscopically fail due to growth of subcritical processes such as stress corrosion. Brittle creep is a phenomenon observed on a variety of rock types (volcanic and sedimentary) and shows a high sensitivity to temperature and stress conditions. In the field, such subcritical transient processes are difficult to detect and can jeopardise the safety of geothermal projects. Transient failure mechanisms in the reservoir have set back geotechnical projects through induced seismicity occurring days or even weeks after stimulation shut in as observed at the Basel geothermal site in Switzerland or at the Pohang geothermal project in South Korea. These observations demonstrate how conventional techniques fail at describing the physics responsible for fault reactivation, which is controlled by dynamic processes resulting from transient multiphysics coupling.

In this contribution, we detail the theory and procedure to develop a constitutive model for rate-dependent damage poro-elasto-plastic material behaviour suitable for porous rocks. To allow for a generic framework for assessing geomaterials instabilities, this model incorporates the potential for microstructure degradation and a path- and rate-dependence. To that purpose, we rely on thermodynamic principles to derive in the frame of the hyperplasticity theory a coupled hydro-mechanical rate-dependent plasticity and damage rheology. We present numerical examples of this new constitutive model at the laboratory scale using experimental data on brittle creep in sandstones and discuss the implications upon upscaling at the reservoir and lithosphere scale.

How to cite: Jacquey, A., Regenauer-Lieb, K., Parisio, F., and Cacace, M.: Multiphysics of transient deformation processes leading to macroscopic instabilities in geomaterials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16050, https://doi.org/10.5194/egusphere-egu2020-16050, 2020.

EGU2020-19084 | Displays | EMRP1.4

To creep or to snap? How induced heat governs the brittleness of matter

Tom Vincent-Dospital, Renaud Toussaint, Stéphane Santucci, Loïc Vanel, Daniel Bonamy, Lamine Hattali, Alain Cochard, Eirik Grude Flekkøy, and Knut Jørgen Måløy

The growth of fractures within mechanically loaded materials often shows two different behaviors. When loaded below a particular threshold in energy release rate, cracks tend indeed to creep at very slow velocities, while the rupture becomes catastrophic beyond this threshold, with propagation velocities approaching that of the material mechanical waves. Understanding according to which of these two behaviors a material is prone to break is of paramount importance, notably in engineering, where the brittle rupture of structures can lead to unpredicted disasters. It is also fundamental in Earth science, as damaging earthquakes  are rather generated by abrupt ruptures in the crustal rocks than by their slow deformations. To explain both behaviors, we focus here on the thermal effects which are auto-induced by the growth of cracks. During their propagation, some of the system’s energy is indeed partly dissipated by Joule heating, which is arising from the friction in a damaged zone around the fracture fronts. The heat hence generated can in return have a significant impact on the physics of the propagation. For instance, the stability of faults is believed to be affected by the thermo-pressurization of their in situ fluids. Independently of this effect, we show, how statistical physics, as understood by an Arrhenius law that includes the dissipation and diffusion of heat around the fracture tip, can explain the full dynamics of cracks, from the slow creep to the fast rupture.

We indeed show that such a model can successfully describe most of the experimentally reported fracture rheology, quantified in terms of velocity / energy release rate relations, in two different types of polymers, acrylic glasses and pressure sensitive adhesives, over eight decades of crack velocities. In these two cases, it is sufficient to assume that these polymers are homogeneous to model their failure. Yet, we in addition illustrate how the thermal disorder, from both the ambient temperature and the propagation induced heat, should interact with the matter typical quenched disorder in fracture energy. Numerical simulations of planar cracks in heterogeneous media indeed show that such quenched disorder helps to trigger hot avalanches in the propagation of cracks, making the overall toughness of a material highly dependent on both its heterogeneities, as it is often reported in the literature, and its thermal properties.

How to cite: Vincent-Dospital, T., Toussaint, R., Santucci, S., Vanel, L., Bonamy, D., Hattali, L., Cochard, A., Flekkøy, E. G., and Måløy, K. J.: To creep or to snap? How induced heat governs the brittleness of matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19084, https://doi.org/10.5194/egusphere-egu2020-19084, 2020.

EGU2020-19783 | Displays | EMRP1.4

Assessment of the mechanism of fracture propagation of soft rock coastal cliffs by using non-local constitutive models

Piernicola Lollino, Nunzio Luciano Fazio, Michele Perrotti, Alessio Genco, Gaetano Elia, and Matteo Oryem Ciantia

The assessment of susceptibility to failure of soft rock coastal cliffs, along with the associated failure mechanism, is not a simple task. Equilibrium conditions depend on the combination of several factors such as structural setting, rock mechanical strength, weathering processes, the hydro-mechanical action of sea waves, the variation of the rock cliff geometry, to mention some of the most important ones. From a geomechanical perspective, the brittle - strain softening behaviour of the rocks plays a key role in the onset and propagation of failure (Ciantia & Castellanza 2015). In particular, the rapid strength reduction occurring after peak under mechanical loading leading to localised deformations within shear fractures is detrimental for rock cliffs. Taking rock brittleness into account in numerical simulations under the framework of continuum mechanics is not straightforward, due to the problems related to a strong dependence of the numerical results from the adopted mesh when strain-softening laws are implemented (Vermeer and Brinkgreve 1994). Nowadays, several regularization techniques are available to control the size of the localised region and prevent the mesh dependence. Within regularization techniques, the nonlocal integral type solution has the advantage of not changing the field equations which facilitates numerical implementation. In this approach, the chosen nonlocal variables are valuated from spatial averages of the field variables in a neighbourhood, and the constitutive model is updated by replacing a local variable with its nonlocal counterpart. Consequently, the constitutive response of a Gauss point is influenced by all the integration points within a neighbourhood, with the size determined through a characteristic length (Bažant and Jirásek 2002). This contribution addresses the problem of the stability of an ideal 2-D plane strain coastal cliff, 20-m high, by means of the use of a non-local constitutive model implemented in a commercial finite element code (Mánica et al. 2018). The numerical results show insights into the evolution of the strain field and the process of slip surface/fracture propagation in the rock cliff as well as highlight the importance of regularising the finite element solution in the presence of brittle materials.

How to cite: Lollino, P., Fazio, N. L., Perrotti, M., Genco, A., Elia, G., and Ciantia, M. O.: Assessment of the mechanism of fracture propagation of soft rock coastal cliffs by using non-local constitutive models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19783, https://doi.org/10.5194/egusphere-egu2020-19783, 2020.

EGU2020-5397 | Displays | EMRP1.4

The onset of dilatancy in rocks

Sandra Schumacher and Werner Gräsle

The onset of dilatancy determines the start of critical fracture growth in rocks under increasing load. For various applications such as the construction of nuclear repositories or dams, a quantitative comprehensive knowledge on the critical conditions leading to dilatancy is required.

Thus, it is important to determine the parameters, which control the dilatant behaviour of rocks, and to analyse their interactions.

We conducted a series of undrained triaxial experiments on two consolidated, fully saturated Opalinus Clay samples from the Mont Terri underground research lab and one sample of Bunter Sandstone from southern Lower Saxony. By testing only a few samples but them extensively, we avoid that the natural material heterogeneity among multiple samples affected our results. Here we show that our approach allows identifying new correlations between different parameters with surprising clarity.

During the experiments, which can take years, the samples are repeatedly exposed to increases in differential stress (σ13) into the dilatant regime but always well below the point of failure. This we achieve by monitoring the pore pressure during the increase in differential stress. The onset of dilatancy becomes visible as clear drop in pore pressure with increasing differential stress.

In addition to the detection of the onset of dilatancy via the pore pressure evolution, pressure diffusion experiments are performed to determine the onset of dilatancy. For this, in the dilatant regime, the differential stress is kept constant and the pore pressure on one side of the sample is de- and increased repeatedly, while the reaction of the pore pressure on the other side of the sample is monitored. With the pore pressure pulse diffusing though our sample specimen, this controlled pore pressure variation induces a transition between dilatant and subdilatant regimes at constant differential stress.

The values for the onset of dilatancy derived by these two methods permit a comprehensive analysis of the dilatant behaviour not only of the Opalinus Clay samples, but also of the Bunter Sandstone sample. Our results show that dilatant behaviour of the tested materials is not governed by only one parameter but by an intricate interplay of several parameters.  Consequently, the development of an equation of state for the dilatant behaviour of different types of rock is achievable. However, due to the multiple parameter dependencies, it will be a time-consuming undertaking.

How to cite: Schumacher, S. and Gräsle, W.: The onset of dilatancy in rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5397, https://doi.org/10.5194/egusphere-egu2020-5397, 2020.

EGU2020-10379 | Displays | EMRP1.4

Influence of the initial damage on fracture toughness and subcritical crack growth in a granite rock

Salvatore D'Urso, Lucas Pimienta, François Passelègue, Federica Sandrone, Sergio Vinciguerra, and Marie Violay

Fracture mechanics is an important tool to assess the slope stability, since this approach offers a methodology to study the fracture stress field in the neighborhood of the joint tips and accurately predict propagation of the joints over time. While the fracture toughness of material has been extensively studied in the past, low interest was given to the influence of initial damage on the subcritical crack growth, despite of its relevance to assess long term slope stability. Here we report new experimental results that address this question.

Starting from the real case of unstable rock mass of “Madonna del Sasso” (Colombero et al., 2015), a series of Cracked Chevron Notched Brazilian Disc (CCNBD) (Fowell, 1995) specimens were failed in a standard Mode I tensile test to investigate the effects of thermal damage on fracture toughness and subcritical crack growth on samples of granite of Alzo.

The degree of initial damage was imposed using slow heat treatment (1°C/min) up to 100, 200, 300 and 400°C, to emulate different levels of rock degradation. The samples were equipped with strain gauges close to the tips of the notch, an extensometer for the Crack Mouth Opening Displacement (CMOD) and twelve Acoustic Emission recorders.

Our results show that fracture toughness decreases with increasing thermal damage, in agreement with previous studies (Nasseri, Schubnel, & Young, 2007). The fracture toughness of undamaged granite is 1.04 MPa m1/2, but 0.65 MPa m1/2 after treatment up to 400°C.

Subcritical crack growth behaviour has been studied for samples treated from 100°C up to 400°C through creep tests on CCNBD specimens. The overall effect of heat treatment is to increase the crack growth rate in granite for a given stress intensity factor. The slopes of stress intensity factor versus crack velocity curves are sensitive to modifications of initial damage’s degree. Trend drops substantially with increase of the temperature of the heat treatment. This shows a substantial increase of the internal damage index n, and a decrease of the time to failure of the CCNBD specimens.

The study highlights the importance of considering both the time-dependent slope behaviour and effects of rocks internal damage, since these conditions could lead to an unexpected premature failure.

How to cite: D'Urso, S., Pimienta, L., Passelègue, F., Sandrone, F., Vinciguerra, S., and Violay, M.: Influence of the initial damage on fracture toughness and subcritical crack growth in a granite rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10379, https://doi.org/10.5194/egusphere-egu2020-10379, 2020.

EGU2020-10115 | Displays | EMRP1.4

Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone

Anne Pluymakers, Richard Bakker, and Auke Barnhoorn

Not all rocks are perfect. Frequently heterogeneities will be present, either in the form of pre-existing fractures, or in the form of sealed fractures. Tensile strength and strength anisotropy of rocks has been investigated for strongly layered rocks, such as shales, sandstones and gneisses, but data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites. We have performed Brazilian Disc tests on limestone samples containing pre-existing fractures and stylolites, investigating Brazilian test Strength (BtS) and fracture orientation. We used Indiana limestone samples, pre-fractured with the Brazilian Disc method, and Treuchtlinger Marmor samples which contained central stylolites. All experiments were filmed. The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ is parallel to the principal loading direction, and 0⁰ to the horizontal axis of the sample. Pre-fracturing Indiana limestone samples results in a cohesion-less planar discontinuity, whereas the stylolites in the Treuchtlinger Marmor samples are discontinuities which have some strength.

The results show that our imperfect samples with a planar discontinuity are always weaker than an intact sample. For the Indiana limestone, with a cohesion-less interface, there is 10 to 75% of weakening, which is angle-dependent. Once the angle is 30 or lower there is no influence from the initial fracture for the orientation of the new fracture. The stress-displacement pattern followed the expectation for Brazilian Disc testing. However, in the samples with a stylolite, strength is isotropic and between 25 and 65% of the strength of an intact sample. For all cases several new cracks appeared, of which the orientation is influenced by the orientation of the stylolite. The fracture pattern and associated stress drops are more complex for high angles. Interestingly, in the samples with stylolites, always more than one fracture was formed, whereas in the samples with a cohesionless interface usually only one new fracture formed, which for natural settings suggests a potential for higher fracture density when hydrofracturing a stylolite-rich interval.

A second difference between these datasets is the amplitude of the pre-existing interface. The effect of amplitude will be qualitatively investigated with a 2D Comsol model, to investigate the location of the first fracture occurring, which can then be compared to the camera data of the experiments.

How to cite: Pluymakers, A., Bakker, R., and Barnhoorn, A.: Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10115, https://doi.org/10.5194/egusphere-egu2020-10115, 2020.

EGU2020-21891 | Displays | EMRP1.4

Monitoring of damage processes in cemented granular materials with acoustic emissions and seismic velocity reduction

Vincent Canel, Xiaoping Jia, Michel Campillo, and Ioan R. Ionescu

Earthquakes or fault core sliding occur naturally in response to long-term deformation produced by plate tectonics. However, the way the damage or fracture process of rocks control the frictional slip is not well understood. It involves indeed materials in very different states: from granular-like materials near the shear band within the highly cracked fault core [1] to almost cohesive state in distant host rocks. To address this issue, we perform controlled laboratory experiments and new numerical simulations of damage in cemented granular materials to study the material evolution from cohesive to granular-like states under external loading. Our synthetic rocks (porous media) are made of cemented glass beads in which the packing density and the cement property (ductile or brittle) as well its content are tunable [2,3]. Two mechanical tests have been conducted: i) under oedometric load in a cylindrical cell with rigid wall; and ii) under triaxial load in a cell with elastic membrane (confined by atmospheric pressure). The fracture processes are monitored by acoustic waves, measuring the longitudinal ultrasound velocity (active detection) [4] and the acoustic emission (passive detection) [5].

More precisely, in the case (i) the fracture process is likely associated with the crack increase, spatially diffused without shear-band formation. For a rock sample cemented by a ductile bond, the damage induced by load appears likely as an anomalous deviation in the master curve of stress-strain whereas the combined acoustic detection provides a very clear evidence with an important sound velocity decrease. Upon cyclic unloading-reloading, we recover a power-law scaling of the sound velocity with the pressure similar to the law in purely granular media but with a finite velocity at vanishing pressure which depends on the residual cohesion of the damaged material. When the drop stress occurs intermittently in fractured samples cemented with brittle materials, we measure not only the sound velocity decrease but also acoustic emissions. In the case (ii) under a triaxial load, we observe the formation of shear-bands, i.e. fractures on the scale of the sample at a load much smaller than those applied in the oedometric loading (i). Again, there is a strong elastic softening (velocity decrease) [4]. Finally, we also compare these experiments with the finite-element modelling of damage and wave propagation in 2D dense cemented disk packings with various cement contents and elasto-visco-plastic properties. This numerical simulation allows to characterize the heterogeneous damage of the material at a microscopic scale.

 

References

[1] C. Marone, Laboratory-derived friction laws and their applications to seismic faulting, Annu. Rev. Earth Planet. Sci. 26 1998, 643-696.

[2] V. Langlois, X. Jia, Acoustic probing of elastic behavior and damage in weakly cemented granular media, Phys. Rev. E 89 2014, 023206.

[3] A. Hemmerle, M. Schröter, L. Goehring, A cohesive granular material with tunable elasticity, Scientific reports 2016.

[4] Y. Khidas, X. Jia, Probing the shear-band formation in granular media with sound waves, Phys. Rev. E 85 2012, 051302.

[5] P.A. Johnson et al., Acoustic emission and microslip precursors to stick-slip failure in sheared granular media, Geophys. Res. Lett. 40 2013, 5627-5631.

How to cite: Canel, V., Jia, X., Campillo, M., and Ionescu, I. R.: Monitoring of damage processes in cemented granular materials with acoustic emissions and seismic velocity reduction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21891, https://doi.org/10.5194/egusphere-egu2020-21891, 2020.

EGU2020-4790 | Displays | EMRP1.4

Experimental Deformation of Sandy Opalinus Clay at Elevated Temperature and Pressure Conditions

Valerian Schuster, Erik Rybacki, Audrey Bonnelye, Anja Schleicher, and Georg Dresen

Studying the mechanical properties of argillaceous rocks is of major interest in geoscience. For example, these rocks are important in engineering applications such as being suitable cap-rocks for the geological storage of carbon dioxide and potential host rocks for the storage of nuclear waste. Furthermore, argillaceous rocks are encountered in different natural settings such as accretionary wedges or fault zones. As a result of their sedimentary and diagenetic history clay rich rocks are often characterised by multiscale textural anisotropy and compositional heterogeneity resulting in anisotropic mechanical and hydraulic properties.
Here, we studied the anisotropic deformation behaviour of Opalinus Clay, collected from the Mont Terri underground laboratory, which is the envisaged host rock formation for nuclear waste disposal in Switzerland. We used the sandy facies of Opalinus Clay, characterized by an irregular wavy lamination of quartz-rich and carbonate-cemented lenses with clay-rich interlayers. Unconsolidated-cylindrical samples cored at 0°, 45° and 90° to the macroscopically visible bedding were deformed in undrained constant strain rate experiments using a Paterson-type deformation apparatus. For each orientation, tests were performed at dry conditions varying either confining pressure (in the range of 50 - 100 MPa), temperature (25 - 200 °C) or strain rate (1*10-3 - 5*10-6 s-1) to study the influence of testing condition and sample orientation on the deformation behaviour. In addition, we deformed a set of back saturated samples at fixed conditions of 50 MPa, 100 °C and 5*10-4 s-1 to investigate the effect of water content on the material strength.
The results show semi-brittle deformation with low yield strength and strain weakening behaviour, in which strain is localized in sub-millimetre to millimetre-wide shear zones at all conditions. Increasing water content reduces, whereas increasing confining pressure increases the peak strength. Samples that were deformed parallel to bedding orientation exhibit the highest strength compared to samples with an orientation of 90° and 45° to bedding. Only for the latter orientations a weak correlation was found between temperature and failure behaviour. The variation of strain rate shows no clear influence for all orientations. Within this test series, there appears to be a potentially greater influence of the porosity on the peak strength for 45° and 90° oriented samples. Clay rich layers seem to have a strong influence on localization and formation of shear zones, in particular for samples oriented at 45° and 90° to bedding. This observation was confirmed by electron microscopy performed on broad ion beam polished surfaces of deformed sample material.
Our experiments reveal that water content, sample orientation with respect to bedding and confining pressure are the most important factors influencing the peak strength of the sandy facies of Opalinus Clay, whereas compositional heterogeneity is responsible for the localization behaviour.

How to cite: Schuster, V., Rybacki, E., Bonnelye, A., Schleicher, A., and Dresen, G.: Experimental Deformation of Sandy Opalinus Clay at Elevated Temperature and Pressure Conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4790, https://doi.org/10.5194/egusphere-egu2020-4790, 2020.

EGU2020-6001 | Displays | EMRP1.4

Poroelastic relaxation in thermally cracked and fluid-saturated glass

Abdulwaheed Ògúnsàmì, Jan Borgomano, Jérôme Fortin, and Ian Jackson

To test theoretical models of modulus dispersion and dissipation in fluid-saturated rocks, we have investigated the broadband mechanical properties of four thermally cracked glass specimens of simple microstructure with complementary forced-oscillation (0.004 -100 Hz) and ultrasonic techniques (~1MHz). Strong pressure dependence of moduli (bulk, Young’s, and shear), axial strain, and ultrasonic wave speeds for dry conditions, attests to essentially complete crack closure at a confining pressure of 15 MPa – indicative of ambient-pressure crack aspect ratios mainly < 2 ´ 10-4.Oscillation of the confining pressure reveals bulk modulus dispersion and a corresponding dissipation peak, near 2 mHz only at the lowest effective pressure (2.5 MPa) – attributed to the transition with increasing frequency from the drained to saturated-isobaric regime. The observations are consistent with Biot-Gassmann theory, with dispersion and dissipation adequately represented by a Zener model.  Above the draining frequency, axial forced-oscillation tests show dispersion of Young’s modulus and Poisson’s ratio, and an associated broad dissipation peak centred near 0.3 Hz, thought to reflect local ‘squirt’ flow and adequately modelled with a continuous distribution of relaxation times over two decades. Observations of Young’s and shear modulus dispersion and dissipation from complementary flexural and torsional oscillation measurements for differential pressure ≤ 10 MPa provide supporting evidence of the transition with increasing frequency from the saturated-isobaric to the saturated-isolated regime – also probed by the ultrasonic technique. These findings validate predictions from theoretical models of dispersion in cracked media and emphasize need for caution in the seismological application of laboratory ultrasonic data for cracked media.

How to cite: Ògúnsàmì, A., Borgomano, J., Fortin, J., and Jackson, I.: Poroelastic relaxation in thermally cracked and fluid-saturated glass, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6001, https://doi.org/10.5194/egusphere-egu2020-6001, 2020.

EGU2020-6743 | Displays | EMRP1.4

Digital rock physics: Segmentation of sub-resolution features

Martin Balcewicz and Erik H. Saenger

Digital rock physics (DRP) became a complementary part in reservoir characterization during the last two decades. Deriving transport, thermal, or effective elastic rock properties from a digital twin requires a three-step workflow: (1) Preparation of a high-resolution X-ray computed tomography image, (2) segmentation of pore and grain phases, respectively, and (3) solving equations due to the demanded properties. Despite the high resolution µ-CT images, the numerical predictions of rock properties have their specific uncertainties compared to laboratory measurements. Missing unresolved features in the µ-CT image might be the key issue. These findings indicate the importance of a full understanding of the rocks microfabrics. Most digital models used in DRP treat the rock as a heterogeneous, isotropic, intact medium which neglect unresolved features. However, we expect features within the microfabrics like micro-cracks, small-scale fluid inclusions, or stressed grains which may influence the elastic rock properties but have not been taken into account in DRP, yet. Former studies have shown resolution-issues in grain-to-grain contacts within sandstones or inaccuracies due to micro-porosity in carbonates, this means the micritic phase. Within the scope of this abstract, we image two different sandstone samples, Bentheim and Ruhrsandstone, as well as one carbonate sample. Here, we compare the mentioned difficulties of X-ray visualization with further analytical methods, this means thin section and focused ion beam measurements. This results into a better understanding of the rocks microstructures and allows us to segment unresolved features in the X-ray computed tomography image. Those features can be described with effective properties at the µ-scale in the DRP workflow to reduce the uncertainty of the predicted rock properties at the meso- and fieldscale.

How to cite: Balcewicz, M. and Saenger, E. H.: Digital rock physics: Segmentation of sub-resolution features, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6743, https://doi.org/10.5194/egusphere-egu2020-6743, 2020.

EGU2020-16134 | Displays | EMRP1.4

Elastic properties of rocks: Why shouldn’t they be constant?

Federica Sandrone, Lucas Pimienta, Laurent Gastaldo, and Marie Violay

How to cite: Sandrone, F., Pimienta, L., Gastaldo, L., and Violay, M.: Elastic properties of rocks: Why shouldn’t they be constant?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16134, https://doi.org/10.5194/egusphere-egu2020-16134, 2020.

EGU2020-10679 | Displays | EMRP1.4

Origin of the temporal evolution of elastic properties during laboratory seismic cycle.

Federica Paglialunga, François X. Passelègue, Mateo Acosta, and Marie Violay

Recent seismological observations highlighted that earthquakes are associated to drops in elastic properties around the fault zone (Brenguier et al., 2008). This drop is often attributed to co-seismic damage produced at the rupture tip, and can mostly be observed at shallow depths. However, it is known that in the upper crust, faults are surrounded by a zone of damage (Caine, Evans, & Forster, 1996). Because of this, the origin of the velocity change associated to earthquakes, as well as its recovery in the months following the rupture remains highly debated.

We conducted stick-slip experiments to explore the evolution of elastic waves velocities during the entire seismic cycle. The tests were run on saw-cut La Peyratte granite samples presenting different initial degrees of damage, obtained through thermal treatment. Three types of samples were studied: not thermally treated, thermally treated at 650 °C and thermally treated at 950 °C. Seismic events were induced in a triaxial configuration apparatus at different confining pressures ranging from 15 MPa to 120 MPa. Active acoustic measurements were carried through the whole duration of the tests and P-wave velocities were measured.

 

The evolution of P-wave velocity follows the evolution of the shear stress acting on the fault, showing velocity drops during dynamic slip events. The evolution of the P-wave velocity drops with increasing confining pressure shows two different trends; the largest drops can be observed for low confining pressure (15 MPa) and decrease for intermediate confining pressures (up to 45 MPa), while for confining pressures of 60 MPa to 120 MPa, drops in velocity slightly increase with confining pressure.

Our results highlight that at low confining pressures (15-45 MPa), the change in elastic velocity is controlled by the sample bulk properites (damage of the medium surrounding the fault), while for higher confining pressures (60-120 MPa), it might be the result of co-seismic damage.

These preliminary results bring a different interpretation to the seismic velocity drops observed in nature, attributed to co-seismic damage. In our experiments co-seismic damage is not observed, except for high confining pressures (laboratory equivalent for large depths), while the change in P-wave velocity seems to be highly related to combined stress conditions and initial damage around the fault for low confining pressures (laboratory equivalent for shallow depths).

How to cite: Paglialunga, F., Passelègue, F. X., Acosta, M., and Violay, M.: Origin of the temporal evolution of elastic properties during laboratory seismic cycle., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10679, https://doi.org/10.5194/egusphere-egu2020-10679, 2020.

Determining elastic wave velocities and intrinsic attenuation of cylindrical rock samples by transmission of ultrasound signals appears to be a simple experimental task, which is performed routinely in a range of geoscientific and engineering applications requiring characterization of rocks in field and laboratory. P- and S-wave velocities are generally determined from first arrivals of signals excited by specifically designed transducers. A couple of methods exist for determining the intrinsic attenuation, most of them relying either on a comparison between the sample under investigation and a standard material or on investigating the same material for various geometries.

Of the three properties of interest, P-wave velocity is certainly the least challenging one to determine, but dispersion phenomena lead to complications with the consistent identification of frequency-dependent first breaks. The determination of S-wave velocities is even more hampered by converted waves interfering with the S-wave arrival. Attenuation estimates are generally subject to higher uncertainties than velocity measurements due to the high sensitivity of amplitudes to experimental procedures. The achievable accuracy of determining S-wave velocity and intrinsic attenuation using standard procedures thus appears to be limited.

We pursue the determination of velocity and attenuation of rock samples based on full waveform modeling and inversion. Assuming the rock sample to be homogeneous - an assumption also underlying standard analyses - we quantify P-wave velocity, S-wave velocity and intrinsic P- and S-wave attenuation from matching a single ultrasound trace with a synthetic one numerically modelled using the spectral finite-element software packages SPECFEM2D and SPECFEM3D. We find that enough information on both velocities is contained in the recognizable reflected and converted phases even when nominal P-wave sensors are used. Attenuation characteristics are also inherently contained in the relative amplitudes of these phases due to their different travel paths. We present recommendations for and results from laboratory measurements on cylindrical samples of aluminum and rocks with different geometries that we also compare with various standard analysis methods. The effort put into processing for our approach is particularly justified when accurate values and/or small variations, for example in response to changing P-T-conditions, are of interest or when the amount of sample material is limited.

How to cite: Boxberg, M. S., Duda, M., Löer, K., Friederich, W., and Renner, J.: Determining P- and S-wave velocities and Q-values from single ultrasound transmission measurements performed on cylindrical rock samples: it’s possible, when…, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9178, https://doi.org/10.5194/egusphere-egu2020-9178, 2020.

The mechanical behavior of soft rocks is dominated by the mechanical properties of the rock itself. Because soft rocks have different physical properties to hard rocks, it is essential to understand the mechanical behavior of soft rocks when tunnels and huge structures are constructed in these. Strain softening is the mechanical behavior of soil and rock materials and is important in understanding soft rock foundation. To investigate the mechanical behavior of siltstone, a sedimentary soft rock, we performed the one-dimensional consolidation tests (hereafter called K0-consolidation test) using a constant strain-rate loading system. We also took high-resolution X-ray CT images of the test specimens before and after the consolidation tests to observe the consolidation deformation. Using Quaternary siltstones distributed in the Boso Peninsula, central Japan as specimens, strain softening in the consolidation process was confirmed in some formations using two test machines at Kyoto University and Nagoya Institute of Technology.

All specimens yielded and the consolidation curves showed over- and normal-consolidation areas. Some specimens’ stress decreased suddenly at increasing strain just before yielding, which can be regarded as a real strain softening because no strain localization could be confirmed within specimens. The stress at the time of the softening differed even for specimens taken from the same formation. Furthermore, the micro-focus X-ray CT images indicated that the specimens had no macro cracks inside. This suggests that strain softening is not due to brittle failure in local areas but due to the softening of the framework structure of the siltstone itself. The samples used in this study are siltstone taken from the Quaternary forearc basin, whose development is related not only to consolidation but also tectonic effects such as horizontal compaction accompanied by plate subduction. Therefore, it is possible that the strain softening confirmed in this study reflects the micro cracks and internal structure that developed during siltstone formation.

How to cite: Kamiya, N., Zhang, F., and Lin, W.: Strain softening of siltstones in consolidation process using a constant strain-rate loading system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12851, https://doi.org/10.5194/egusphere-egu2020-12851, 2020.

EGU2020-902 | Displays | EMRP1.4

Microscale characterisation of damage evolution in curling stones used in international competition

Derek Leung, Florian Fusseis, and Ian Butler

The rocks used to produce curling stones for international competition are only sourced from two localities in the world: Ailsa Craig, Scotland and Trefor, Wales. Curling stones consist of two components: (1) the running band (the ring-shaped bottom surface of the stone which rests on the ice) and (2) the striking band (the convex band on the profile of stones which collides with those of other stones). With a focus on the striking bands, we aim to document the damage evolution of curling stones using synchrotron microtomography (3D characterisation of pristine samples and 4D damage evolution), optical and scanning electron microscopy (quantitative characterisation of pristine samples and microfracture characterisation of damaged striking bands), and petrophysical testing (fracture characteristics and comparative data). These data will be complemented by an on-ice experiment that will determine the mechanics (e.g., stress distribution, contact area, and velocity) of curling stone impacts. Out of four curling stone varieties (from Ailsa Craig and Trefor), we observe the striking bands of three varieties to show macroscopic, incipient to mature, curvilinear fractures. The curvature of these fractures is consistent and does not vary significantly between individual stones and between curling stone varieties. However, the degree of macroscopic fracture development differs between aged striking bands of curling stone types: Blue Trefor (macroscopic fractures not observed), Red Trefor (weakly incipient), Ailsa Craig Common Green (incipient to juvenile), and Ailsa Craig Blue Hone (juvenile to mature). Unfortunately, it is not possible to determine the degree of usage (age) of the selected samples and thus it is not possible to normalize these apparent differences in damage. Given that the striking band limits the lifetime of curling stones, understanding the damage evolution of curling stones can contribute valuable information to the maintenance of curling stones. The rock physics of curling stone impacts is linked to dynamic spalling and more broadly to rock failure, as these processes are ultimately related to the initiation and propagation of fractures.

How to cite: Leung, D., Fusseis, F., and Butler, I.: Microscale characterisation of damage evolution in curling stones used in international competition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-902, https://doi.org/10.5194/egusphere-egu2020-902, 2020.

EGU2020-22562 | Displays | EMRP1.4

Quantitative characterization of fracture networks on Digital Outcrop Models obtained from avionic and terrestrial laser scanner

Gloria Arienti, Matteo Pozzi, Anna Losa, Federico Agliardi, Bruno Monopoli, Andrea Bistacchi, and Davide Bertolo

We present a semi-automatic workflow aimed at extracting quantitative structural data from point clouds obtained with avionic and terrestrial laser scanners (Lidar and TLS). The workflow is characterized by a calibration phase followed by an automatic data-collection phase. The large datasets of “fractures” mapped in this way are analysed with statistical methods allowing to define representative parameters of the fracture network.

In the first phase, the intervention of an expert interpreter with structural geology skills is fundamental to evaluate which features can be interpreted as fractures in the point clouds. In the second phase, an automatic segmentation and classification is performed, based on phase 1 calibration, that allows extracting very large fracture datasets. The main steps in phase 1 are: manual segmentation of facets representing fracture surfaces, orientation analysis and definition of fracture sets (possibly supported by kinematic analysis), definition of orientation parameters to be used for automatic segmentation. Phase 2 analysis proceeds with the automatic segmentation of subset point clouds that include just one fracture set. In these point clouds, facets representing fractures lying on different planes are well separated and disconnected, and this allows applying automatic vectorization techniques that extract individual facets representing single fractures on the outcrop surface. The datasets issued from this processing are analysed with automatic algorithms allowing to define fracture spacing and orientation statistics with a very large support, that would not have been allowed by other methodologies.

How to cite: Arienti, G., Pozzi, M., Losa, A., Agliardi, F., Monopoli, B., Bistacchi, A., and Bertolo, D.: Quantitative characterization of fracture networks on Digital Outcrop Models obtained from avionic and terrestrial laser scanner, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22562, https://doi.org/10.5194/egusphere-egu2020-22562, 2020.

This work proposed an available approach to analyze the property evolution of weak interlayers during immersion softening at micro and macro scales, which combining the advantages of nanoindentation tests and numerical modelling. The weak interlayers has significant impact on the failure process of natural slopes, however, their properties are difficult to be obtained using traditional triaxial compression tests. Because these weak interlayers are consist of clay and rock fragments which leads to the difficult to prepare intact samples. Additionally, the softening properties of these weak interlayers are strongly related to their fillings at micro scale. In this work, the weak interlayers is investigated using nano-scale micromechanical tests and upscaling methodologies, so only small rock fragments are required (see Fig.1). 
In northwestern Hubei China, the mountains often developed several layers of weak interlayers with major lithology as shale which is sedimentary rock with low strength and dense clay particles. We investigated these shale fragments in weak interlayers, which is prone to decrease in strength induced by precipitation erosion. The Gaussian mixture model was used to analyze a large amount of data obtained by statistical grid nanoindentation method. Then the Mori-Tanaka scheme was used to homogenize the elastic properties of the samples and upscale the nanoindentation data to the macroscale. The hardness values which obtain by Berkovich and Cube corner indenter were able to assess the cohesion and friction angle of shale. Finally, these achieved parameters were applied in numerical model, in order to analyze the slope failure caused by the softening of weak interlayers (see Fig. 2).
The results show that: (1) the chlorite and muscovite minerals, which are major proportion of shale, soften or dissolve with the increasing saturation time. The fine mineral particles are gradually stripped from micro structure. As a result, at microscale the compact shale samples sale became loose. The strength of these shale samples are also decrease because water seeped through pores and micro cracks. (2) After water immersion, the friction angle is almost constant, while the elastic modulus and cohesion decrease significantly with increasing saturation time. (3) The shear strength decrease so that the shearing creep occurs along the weak interlayers surface, then bottom sliding surface is cut, which leads to landslide.

How to cite: Xu, J., Feng, Y., and Tang, X.: Investigate the softening properties of weak interlayers in slope failure process using nanoindentation test and simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18394, https://doi.org/10.5194/egusphere-egu2020-18394, 2020.

EGU2020-12430 | Displays | EMRP1.4

Geotechnical characteristics of rocks around the King Sejong Station in Antarctica by Freeze-Thawing test
not presented

Jeongdu Noh and Seong-Seung Kang

The purpose of this study is to evaluate the rock in the extreme regions by conducting a field study and laboratory tests on three rocks of diorite, andesite, grano-diorite around Sejong Station in Antarctica. The King George Island, the research area, is mostly covered by glaciers and partially exposed bedrock along the coast. Around the coastal area and Sejong-bong, andesites, diorites, grano-diorites are distributed and were measured rebound values using Silver Schmidt hammer. This hammer, unlike conventional Schmidt value’s R, calculates Q values using input and output energy. As a result of field study, the average Q value of diorite was estimated 76, which is high compared others, and andesite was estimated 67, which is low compared others, grano-diorite was estimated 72, which is widely scattered. Freeze-Thawing test was performed based on ASTM C-666, KS F 2456. The temperature range of freeze-thawing test is from -20 ℃ to 20 ℃ referred to the published papers, and all rocks are completely saturated without humidity. The temperature holding time was set to 2 hours for temperature inside rock to -20 ℃ when the atmosphere temperature is -20 ℃. The freeze-thawing test was carried out every 20 cycles for porosity, absorption, and slaking durability. The laboratory tests were performed 200 times in total. As a result, up to 100 cycles, the porosity and absorption were not significantly different. Since then, they increased slightly. However, the slaking tended to increase gradually from the 0 cycle. In order to accurately assess the weathering of the three rocks, continuous freeze-thawing tests should be conducted.

 

Acknowledgement : This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MIST (2019R1F1A1048854)

How to cite: Noh, J. and Kang, S.-S.: Geotechnical characteristics of rocks around the King Sejong Station in Antarctica by Freeze-Thawing test, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12430, https://doi.org/10.5194/egusphere-egu2020-12430, 2020.

In situ stress state at shallow depths (<1 km) is important for designing underground systems for various projects such as nuclear waste disposal, carbon dioxide geological sequestration, and geo-resource development. Stress characterization for such projects rely largely on stress measurement data (such as hydraulic fracturing test data). We compile a large number of hydraulic fracturing test data measured in a total of 226 boreholes in South Korea, and attempt to characterize shallow crustal stress over the country. These data are measurements at depths down to 850 m, and classified mostly low-quality based on World Stress Map quality ranking scheme (B-quality: 7%, C: 42%, and D: 51%). We grid the country by 0.25°×0.25°, and find a circular bin size at each grid point using two statistical methods (weighted standard deviation and quasi interquartile range), by which the uniformity of stress orientation can be estimated. As many data are low-quality, we apply this process to two subsets of data (B-C and B-D) to find an optimal stress characterization. Our most optimal characterization results show that bin diameter in most of the country vary between 100 and 200 km, except for southeastern Korea. Bin diameters in southeastern Korea range between 0 and 60 km, which means that stress heterogeneity is especially significant in the region, where lithology varies markedly and several active faults are clustered. The stress orientations in the northeastern part of the country are characterized as intermediate stress uniformity (bin size of ~120 km in diameter) but a systematic horizontal stress rotation (up to ~60°) from that of the deep-seated regional stress. This region is mountainous with altitude as high as 1.4 km. To verify whether the stress rotation is a result of topographic effect, we model stress perturbation using the digital elevation model (DEM) data of the region, which yields stress rotation comparable to measurements. We find that lithology is a particularly important factor that affects stress magnitudes over the country, as the stress magnitudes at the same depth tend to be markedly smaller in sedimentary rocks than in crystalline rocks. Our study, although given data are of fairly low-quality, can provide a basis for shallow stress map of South Korea.

How to cite: Kang, M. and Chang, C.: Effects of topography and lithology variation on in situ stress at shallow depths in South Korea: results from statistical characterization of stress data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8327, https://doi.org/10.5194/egusphere-egu2020-8327, 2020.

Self-organization is not a universal property of matter, it exists under certain internal and external conditions and this is not associated with a special class of substances. The study of the morphology and dynamics of migration of anomalous zones associated with increased stresses is of particular importance in the development of deep deposits, complicated by dynamic phenomena in the form of mountain impacts. An important tool for this study is geophysical exploration. To describe the geological environment in the form of an array of rocks with its natural and technogenic heterogeneity, one should use its more adequate description, which is a discrete model of the medium in the form of a piecewise inhomogeneous block medium with embedded heterogeneities of a lower rank than the block size. This nesting can be traced several times, i.e. changing the scale of the research, we see that heterogeneities of a lower rank now appear in the form of blocks for heterogeneities of the next rank. A simple averaging of the measured geophysical parameters can lead to distorted ideas about the structure of the medium and its evolution. We have analyzed the morphology of the structural features of disintegration zones before a strong dynamic phenomenon. The introduction of the proposed integrated passive and active geophysical monitoring into the mining system, aimed at studying the transient processes of the redistribution of stress-strain and phase states, can help prevent catastrophic dynamic manifestations during the development of deep-seated deposits. Active geophysical monitoring methods should be tuned to a model of a hierarchical heterogeneous environment. Iterative algorithms for 2-D modeling and interpretation for sound diffraction and a linearly polarized transversal elastic wave on the inclusion with a hierarchical elastic structure located in the J-th layer of the N-layer elastic medium are constructed. The case is considered when the inclusion density of each rank coincides with the density of the containing layer, and the elastic parameters of inclusion of each rank differ from the elastic parameters of the containing layer.

How to cite: Hachay, O. and Khachay, O.: Acoustic Monitoring of Anomalous Stressed Zones, Determination of their Positions, Surfaces, Evaluation of Catastrophic Risk., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1322, https://doi.org/10.5194/egusphere-egu2020-1322, 2020.

Due to the large burial depth of the Pliocene Red Layer in Qingyang, Gansu and its special historical causes, its engineering mechanical characteristics are quite different from those of the southern red clay. Lack of systematic data on the internal forces of the lining structure through the stratum tunnel. Therefore, this paper takes the Yinchuan-Xian High-speed Railway Qingyang Tunnel as the research object, through field measurement and finite element simulation to obtain the space-time distribution characteristics of the internal force of the lining structure, the surrounding rock pressure, the deep displacement of the surrounding rock from 5 to 10 m, and the convergent deformation of the support. The reasons for the stress state of the lining-surrounding rock composite structure reflected in the results are analyzed, and the ABAQUS software is used to simulate the tunnel excavation process to compare and verify the lining structure stressing law. Internal force characteristics. The results show that: 1) The physical and mechanical indicators of the Pliocene red layer in the Neogene in Qingyang, Gansu belong to the extremely hard soil-very soft rock critical category. Due to the long consolidation pressure and long consolidation history, it can be obvious on the saturated flooding fault surface. Observation of the characteristics of layered joints proves that this layer of red clay has a tendency of sedimentary diagenesis. 2) The quality of the surrounding rock of the stratum lining structure is good. The horizontal in-situ stress is twice that of the vertical in-situ stress. It can be optimized for the design of III-IV surrounding rock while increasing the side pressure coefficient. 3) The unclosed initial support cannot effectively limit the deformation of the surrounding rock, and the temporary stress can be used to improve the state of stress. The numerical simulation results are consistent with the field measurement laws. 4) This stratum with severe deformation is the cave diameter range of the excavation boundary to the surrounding rock. The deformation area is mainly concentrated in the vault. Delayed excavation of the inverted arch can effectively reduce the stress on the internal lining structure of the inverted arch.

How to cite: Xie, Z. and Wu, X.: Study on Deformation and Force Characteristics of Deep-buried Large Section Expansive Red Clay Tunnel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2248, https://doi.org/10.5194/egusphere-egu2020-2248, 2020.

EGU2020-21768 | Displays | EMRP1.4

Issues with fracturing ice during an ice drilling project in Greenland (EastGRIP)

Ilka Weikusat, David Wallis, Steven Franke, Nicolas Stoll, Julien Westhoff, Steffen Bo Hansen, Trevor James Popp, Frank Wilhelms, and Dorthe Dahl-Jensen

Drilling an ice core through an ice sheet (typically 2000 to 3000 m thick) is a technical challenge that nonetheless generates valuable and unique information on palaeo-climate and ice dynamics. As technically the drilling cannot be done in one run, the core has to be fractured approximately every 3 m to retrieve core sections from the bore hole. This fracture process is initiated by breaking the core with core-catchers which also clamp the engaged core in the drill head while the whole drill is then pulled up with the winch motor.

 

This standard procedure is known to become difficult and requires extremely high pulling forces (Wilhelms et al. 2007), in the very deep part of the drill procedure, close to the bedrock of the ice sheet, especially when the ice material becomes warm (approximately -2°C) due to the geothermal heat released from the bedrock. Recently, during the EastGRIP (East Greenland Ice coring Project) drilling we observed a similar issue with breaking off cored sections only with extremely high pulling forces, but started from approximately 1800 m of depth, where the temperature is still very cold (approximately -20°C). This has not been observed at other ice drilling sites. As dependencies of fracture behaviour on crystal orientation and grain size are known (Schulson & Duval 2009) for ice, we thus examined the microstructure in the ice samples close to and at the core breaks.

 

First preliminary results suggest that these so far unexperienced difficulties are due to the profoundly different c-axes orientation distribution (CPO) in the EastGRIP ice core. In contrast to other deep ice cores which have been drilled on ice domes or ice divides, EastGRIP is located in an ice stream. This location means that the deformation geometry (kinematics) is completely different, resulting in a different CPO (girdle pattern instead of single maximum pattern). Evidence regarding additional grain-size dependence will hopefully help to refine the fracturing procedure, which is possible due to a rather strong grain size layering observed in natural ice formed by snow precipitation.

 

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Wilhelms, F.; Sheldon, S. G.; Hamann, I. & Kipfstuhl, S. Implications for and findings from deep ice core drillings - An example: The ultimate tensile strength of ice at high strain rates. Physics and Chemistry of Ice (The proceedings of the International Conference on the Physics and Chemistry of Ice held at Bremerhaven, Germany on 23-28 July 2006), 2007, 635-639

Schulson, E. M. & Duval, P. Creep and Fracture of Ice. Cambridge University Press, 2009, 401

How to cite: Weikusat, I., Wallis, D., Franke, S., Stoll, N., Westhoff, J., Hansen, S. B., Popp, T. J., Wilhelms, F., and Dahl-Jensen, D.: Issues with fracturing ice during an ice drilling project in Greenland (EastGRIP), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21768, https://doi.org/10.5194/egusphere-egu2020-21768, 2020.

EGU2020-18041 | Displays | EMRP1.4

Coupled processes in clay during tunnel excavation

Antonio Pio Rinaldi, Yves Guglielmi, Alba Zappone, Florian Soom, Michelle Robertson, Paul Cook, Maria Kakurina, Quinn Wenning, Dorothee Rebscher, and Christophe Nussbaum

Tunnel excavations are known to perturb the hosting rock mass at long distances, with changes in the hydrogeological flow affecting, as well as deforming the rock mass, inducing subsidence in a zone above the tunnel. During the extension of the Mont Terri Underground Rock Laboratory, we had the unique opportunity to monitor the final part of the excavation of Gallery18 and the final breaktrough.

The joint effort of two experiments (CS-D lead by ETH Zurich and FS-B lead by LBNL) allowed for a detailed characterization of the poro-elastic response of the rock mass and the Mont Terri Main Fault Zone to the excavation. Geophysical, geomechanical, and hydrogeological monitoring include: (1) pressure monitoring in several borehole intervals; (2) deformation at a chain potentiometer and fiber optics grouted in boreholes (normal to bedding and parallel to fault zone), and platform-tilmeters installed at the tunnel floor, as well as detailed 3D displacement at the SIMFIP probe.

All monitoring systems detected major perturbations starting from 15 days before the breakthrough and continuing for several days after it. We summarize the observations and will combine numerical modelling and observed trend to conceptualized the pattern of poro-elastic deformation. The results of the analysis could help shedding light on the poro-elastic behaviour of clay, providing interesting hints for the modeling community and helping in planning of future nuclear waste repositories in such material.

How to cite: Rinaldi, A. P., Guglielmi, Y., Zappone, A., Soom, F., Robertson, M., Cook, P., Kakurina, M., Wenning, Q., Rebscher, D., and Nussbaum, C.: Coupled processes in clay during tunnel excavation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18041, https://doi.org/10.5194/egusphere-egu2020-18041, 2020.

EGU2020-2348 | Displays | EMRP1.4

3 failure limits to relief

Anne Voigtländer, Rachel C. Glade, and Jens M. Turowski

Reaching the top of a high mountain is a great experience, yet there seem to be several limits. One is the relief of the mountain itself, which constitutes the driving stress consisting of the height, h, and density, ρ of the mass, accelerated by gravity, g and modulated by the slope, α. The material strength required to balance this stress defines the limit to relief. There are three failure modes in which the material strength can be surpassed: shear, compression, and tension. Failure criteria established for shear and compression have been demonstrated to be useful in certain settings, but don’t hold in steep (50-90°), hard and rocky landscapes. For those, we propose a tensile strength limit criterion (TSL). Due to the Poisson effect of normal stress (σn), indirect tensile stresses (σt) arise near free surfaces. The magnitude of these stresses is defined by the Poisson’s ratio (ν) of the lithology and the relief. First-order estimates of different lithologies and their material properties are in good agreement with the height of cliffs and slopes of the same lithology. Similar to the approach by Schmidt and Montgomery (1995) predicting bulk, slope scale material properties from relief, we can invert the tensile strength limit criterion. By this, we can infer material tensile strength and Poisson’s ratio from the maximum slope heights and angle on Earth, and beyond!

In terms of dynamics, the tensile strength limit criterion (TSL) predicts critical yielding at the foot of the slope, causing surface parallel fractures that would lead to further critical yielding and failure slope upward. This pattern of progressive rock failure has been observed in steep rock walls, like El Capitan or Half Dome in Yosemite National Park.

We propose this solely geometrically and stress-controlled criterion not contrary but in addition to existing limit criteria. Implications of the three failure limits to relief are that, (i) over-steepening doesn’t necessarily exist, as there is not only a threshold slope angle but also a threshold height, (ii) there is a transition from one dominant limit and failure mechanism to the other, shifting from shear failure and sliding to toppling and fall, and (iii) internal material property changes, due to chemical and/or mechanical weathering, and subcritical crack growth can evoke a progressive reorganisation of yielding and potential rock failure without external triggering events.

How to cite: Voigtländer, A., Glade, R. C., and Turowski, J. M.: 3 failure limits to relief, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2348, https://doi.org/10.5194/egusphere-egu2020-2348, 2020.

EGU2020-4831 | Displays | EMRP1.4

Petrological constraints on ultra-high pressure metamorphism and frictionite formation in a catastrophic rockslide: The Koefels event (Eastern Alps).

Diethard Sanders, Bastian Joachim-Mrosko, Jürgen Konzett, Julian Lanthaler, Marc Ostermann, and Peter Tropper

The P-T conditions in extremely-rapid gravity-driven rockslides are difficult to constrain from the descended rock mass itself. Here, we report mineralogical observations from the Koefels rockslide and their interpretation. The Koefels event – happened between 9527-9498 cal BP – comprises 3.9 km3 mainly of muscovite + biotite-bearing orthogneiss, and is one of the few large rockslides in silicate-bearing rocks worldwide. Detached by collapse of a valley flank, the rockslide impacted the opposite valley flank: While the lower part of the mass was sharply stopped, the overriding part propagated farther. This led to shear localization along discrete planes and, in consequence, to transient melting by frictional heating. The resulting frictionites comprise thin glassy levels with floating crystal fragments. The bulk composition of the glassy melt corresponds to the composition of the orthogneiss.

            In the frictionites, ultra-high pressure metamorphosed quartz (UPQ) occurs next to unaffected quartz in a glassy matrix. Micro-Raman spectroscopy of unaffected quartz yielded an intense A1 Raman mode at 464 cm-1 ; UPQ shows a shift of this band down to 460cm-1, with some grains showing an internal gradient of up to 3 cm-1 from the core (463cm-1) to the rim (460 cm-1). Some UPQ are rimmed by lechatelierite (SiO2 glass), which never surrounds unaffected quartz grains. Until now lechatelierite formation in frictionites was considered to be a function of temperature only (Heuberger et al. 1984). Because lechatelierite only rims UPQ with outward decreasing band numbers, we interpret lechatelierite formation to be mainly pressure-driven. The completely molten matrix and the lack of glassy rims at the edges of normal quartz indicates minimum temperatures of 900°C. Experimental investigations have shown that the shifted A1 mode of UPQ equilibrates to 464 cm-1 at 1100°C, thus giving an upper limit of the temperature range. The Raman shift of the A1 mode and the presence of lechatelierite strongly suggest that a pressure >23 GPa was attained (cf., McMillan et al. 1991, Fritz et al. 2011, Kowitz et al. 2013).

            The UPQ and lechatelierite rims formed by grain collisions during initial shear localization, when the shear plane was relatively cool. Next, upon rapid frictional heating the glassy frictionite matrix formed and became locally injected into lechatelierite rims. Once formed, the melt prevented high-energy grain collisions. Unaffected quartz (which nevertheless may have seen pressures up to 22 GPa) in the frictionites perhaps escaped UHP overprint due to position in local pressure shadows and/or was sheared out from the adjacent caciritic rock mass into the melt. Our results help to better constrain numerical simulations of P-T-conditions in rockslides. Since our investigation only provides limiting estimates the actual P-T conditions in deep shear levels of rockslides exceeding the volume of the Koefels event might be even higher.

 

References:

Fritz et al. 2011: International Journal of Impact Engineering, 38:440

Heuberger et al. 1984: Mountain Research and Development, 4:345

Kowitz et al. 2013: Earth and Planetary Science Letters, 384:17

McMillan et al. 1992: Physics and Chemistry of Minerals, 19:71

How to cite: Sanders, D., Joachim-Mrosko, B., Konzett, J., Lanthaler, J., Ostermann, M., and Tropper, P.: Petrological constraints on ultra-high pressure metamorphism and frictionite formation in a catastrophic rockslide: The Koefels event (Eastern Alps). , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4831, https://doi.org/10.5194/egusphere-egu2020-4831, 2020.

EGU2020-5125 | Displays | EMRP1.4

P-wave velocity anisotropy in an active methane venting pockmark: The Scanner Pockmark, northern North Sea

Gaye Bayrakci, Timothy A. Minshull, Jonathan M. Bull, Timothy J. Henstock, Giuseppe Provenzano, Hamza Birinci, Calum Macdonald, and Robert Dunn

Scanner pockmark is an active and continuous methane venting seafloor depression of ~ 900 x 450 m wide and 22 m deep. It is located in the northern North Sea, within the Witch Ground basin where the seafloor and shallow sediments are heavily affected by pockmarks and paleo-pockmarks of various sizes. A seismic chimney structure is present below the Scanner pockmark. It is expressed as a near-vertical column of acoustic blanking below a bright zone of gas-bearing sediments. Seismic chimneys are thought to host connected vertical fractures which may be concentric within the chimney and align parallel to maximum compression outside it. The crack geometry modifies the seismic velocities, and hence, the anisotropy measured inside and outside of the chimney is expected to be different.

 

We carried out anisotropic P-wave tomography with a GI-gun wide-angle dataset recorded by the 25 Ocean Bottom Seismometers (OBSs) of the CHIMNEY experiment (2017). Travel times of more than 60,000 refracted phases propagating within a volume of 4 x 4 x 2 km were inverted for P-wave velocity and the direction and degree of P-wave anisotropy. The grid is centred on the Scanner Pockmark and has a y-axis parallel to -34o N. The horizontal node interval is denser in the zone covered by the OBSs and the vertical node interval is denser near the seabed. A 3 iteration inversion leads to a chi2 misfit value of 1 and a root-mean-square misfit of <10 ms. The results show a maximum P-wave anisotropy of 5%, and higher degrees of anisotropy correlates well with higher velocities. The fast P-wave velocity orientation, a proxy for fracture orientations, is 46o N. The top of the chimney possibly links a bright spot mapped at 270 ms in two way travel time using RMS amplitudes of MCS data, to the surface gas emission. The bright spot corresponds to low tomographic P-wave velocity and anisotropy, suggesting that gas is located in a zone with unaligned fractures or porosity. This observation is in good agreement with early multi-channel seismic data interpretations which suggested that the gas is trapped within a sandy clay layer, the Ling Bank Formation, capped by an upper clay layer, the Coal Pit Formation. In the next step, we will invert the travel-times of reflected phases in order to increase the image resolution.  

How to cite: Bayrakci, G., Minshull, T. A., Bull, J. M., Henstock, T. J., Provenzano, G., Birinci, H., Macdonald, C., and Dunn, R.: P-wave velocity anisotropy in an active methane venting pockmark: The Scanner Pockmark, northern North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5125, https://doi.org/10.5194/egusphere-egu2020-5125, 2020.

EGU2020-6669 | Displays | EMRP1.4

Fracture characterisation using frequency-dependent shear-wave splitting analysis of azimuthal anisotropy: application to fluid flow pathways at the Scanner Pockmark area, North Sea

Adam Robinson, Gaye Bayracki, Calum MacDonald, Ben Callow, Giuseppe Provenzano, Timothy Minshull, Mark Chapman, Timothy Henstock, and Jonathan Bull

Scanner pockmark, located in the Witch Ground Graben region of the North Sea, is a ~900 m by 450 m, ~22 m-deep elliptical seafloor depression at which vigorous and persistent methane venting is observed. Previous studies here have indicated the presence of chimney structures which extend to depths of several hundred meters, and which may represent the pathways along which upwards fluid migration occurs. A proposed geometry for the crack networks associated with such chimney structures comprises a background pattern outside the chimney with unconnected vertical fractures preferentially aligned with the regional stress field, and a more connected, possibly concentric fracture system within the chimney. The measurement of seismic anisotropy using shear-wave splitting (SWS) allows the presence, orientation and density of subsurface fracture networks to be determined. If the proposed model for the fracture structure of a chimney feature is correct, we would expect, therefore, to be able to observe variations in the anisotropy measured inside and outside of the chimney.

Here we test this hypothesis, using observations of SWS recorded on ocean bottom seismographs (OBS), with the arrivals generated using two different air gun seismic sources with a frequency range of ~10-200 Hz. We apply a layer-stripping approach based on observations of SWS events and shallow subsurface structures mapped using additional geophysical data to progressively determine and correct for the orientations of anisotropy for individual layers. The resulting patterns are then interpreted in the context of the chimney structure as mapped using other geophysical data. By comparing observations both at the Scanner pockmark and at a nearby reference site, we aim to further contribute to the understanding of the structures and their role in governing fluid migration. Our interpretation will additionally be informed by combining the field observations with analogue laboratory measurements and new and existing rock physics models.

This work has received funding from the NERC (CHIMNEY; NE/N016130/1) and EU Horizon 2020 programme (STEMM-CCS; No.654462).

How to cite: Robinson, A., Bayracki, G., MacDonald, C., Callow, B., Provenzano, G., Minshull, T., Chapman, M., Henstock, T., and Bull, J.: Fracture characterisation using frequency-dependent shear-wave splitting analysis of azimuthal anisotropy: application to fluid flow pathways at the Scanner Pockmark area, North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6669, https://doi.org/10.5194/egusphere-egu2020-6669, 2020.

EGU2020-4707 | Displays | EMRP1.4

How many plates?

Tiantian Chen, Chun‘an Tang, and Yongyi Wang

The spacing of opening-mode fractures in layered materials, such as certain sedimentary rocks and laminated engineering materials, is often proportional to the thickness of fractured layers. Bai, Pollard & Gao (2000) investigated the full stress distribution between such fractures, from which they show that the spacing initially decreases as extensional strain increases in the direction perpendicular to the fractures. But at a certain ratio of spacing to layer thickness, no new fractures form and the additional strain is accommodated by further opening of existing fractures: the spacing then simply scales with layer thickness, which is called fracture saturation. Their conclusion is in marked contrast to existing theories of fracture, such as the stress-transfer theory, which predict that spacing should decrease with increasing strain ad infinitum. Here we show that the principle for 2D equal spaced fracture problem also applies to the 3D polygonal fracture problem. By using 3D mechanical modeling on a spherical shell model under interior expansion, we found that the modeled plate mosaic exactly follows the same principle that the size of formed plates is also proportional to the thickness of the fractured shell. By using a spherical shell model with isotropic, elastic two-layers, we numerically load the shell to fail under a quasistatical, slowly increasing interior pressure in a displacement controlling manner (induced, e.g., by gradual thermal expansion). The fractures only occur in the surface layer. The value at which a particular element breaks is random, but fixed at the start of the fragmentation process (i.e., the disorder is quenched). The probability distribution (PD) of breakdown thresholds is a material property and is known from the start. We account for this local randomness by assigning to each element a failure threshold taken from a Weibull probability distribution (PD), with a parameter defines the degree of material homogeneity, called the homogeneity index. We use a three-dimensional finite element code named RFPA (Rock Failure Process Analysis) to solve the problem. The modeling results show that, under conditions of uniform expansion force from inside the shell, the cracking pattern also follows a global scale law in terms of the thickness of the fractured layer. The numerical modeling demonstrates an important observation that, under conditions of uniform and layer-parallel tension induced by thermal expansion within the spherical shell, surface cracks spontaneously self-organize into quasi-hexagonal tessellations, following the mechanical principle that the hexagonal pattern relieves the greatest strain energy for the least work invested in nucleation and propagation of fractures. If this applies to the problems of Earth tessellations, called Platonics (Anderson, 2002), it implies that the thermal expanded Earth may breakup to form plate-like network as a consequence of thermal-expansion induced rift rather than mantle convective or plutonic causes, and the plate size may be proportional to the thickness of lithosphere. This provides a new explanation on how the plate number should be, and whether there is a pattern in the plate mosaic, issues related to the optimal sizes and shapes of plates in terms of fracture spacing.

How to cite: Chen, T., Tang, C., and Wang, Y.: How many plates?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4707, https://doi.org/10.5194/egusphere-egu2020-4707, 2020.

EMRP2.2 – Observing Earth with Swarm: Results from Six Years in Orbit and Future Perspectives

EGU2020-9616 | Displays | EMRP2.2

Core-mantle boundary flows obtained purely from Swarm secular variation gradient information

Kathy Whaler, Magnus Hammer, Chris Finlay, and Nils Olsen

The Swarm constellation provides information on both along- and across-track magnetic field gradients. Spatial changes of the magnetic vector field elements are described by a magnetic field gradient tensor, whose elements and their uncertainties can be estimated using the Virtual Observatory (VO) concept, whereby data within a cylinder centred on the VO with axis perpendicular to the Earth’s surface are reduced to a central point at satellite altitude. Recent experiments have shown that analysing data collected over a 4 month window provides the best compromise between reducing bias from the way the satellite orbits sample each VO cylinder and preserving information on temporal changes of the field, and that the data provide spatial information sufficient to resolve 300 non-overlapping VOs. We invert annual first differences of the 5 independent gradient tensor elements (providing estimates of secular variation, SV, gradients) at these 300 VOs over the Swarm era for advective velocity at the core-mantle boundary, forcing the flow to have minimal acceleration while providing an adequate fit to the data. We obtain flows similar to those from previous SV inversions but purely from the gradient information. The resolution of the SV gradients is higher than that of the SV itself, resulting in a ~30% increase in the number of effective flow parameters; this is thought to be because the gradients are less affected by long period external signals that are difficult to remove from the data, resulting in an improved signal to noise ratio. Although very little temporal change in the flow is required to reproduce even rapid changes in the magnetic field, we are able to isolate some robust flow changes, in particular regarding changes in the azimuthal flow acceleration, associated with the geomagnetic impulse in the Pacific region in around 2016.

How to cite: Whaler, K., Hammer, M., Finlay, C., and Olsen, N.: Core-mantle boundary flows obtained purely from Swarm secular variation gradient information, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9616, https://doi.org/10.5194/egusphere-egu2020-9616, 2020.

EGU2020-7879 | Displays | EMRP2.2

Mapping 3-D mantle electrical conductivity using Swarm, Cryosat-2 and ground observatory data

Alexey Kuvshinov, Alexander Grayver, Lars Tøffner-Clausen, and Nils Olsen

In this contribution, we report on our recent attempts to detect lateral variations of the electrical conductivity at mid mantle depths (400­ – 1600 km) using 6 years of Swarm, Cryosat-2 and observatory magnetic data. The approach involves a three-dimensional (3-D) inversion of matrix Q-responses. These responses relate spherical harmonic coefficients of external (inducing) and internal (induced) parts of the magnetic potential, derived for geomagnetic variations at periods longer than 1 day and hence mainly describing signals of magnetospheric origin (i.e. external also to satellites, as required). In addition to the inversion results, we discuss potential ways to improve the recovery of 3-D conductivity structures in the mantle.

How to cite: Kuvshinov, A., Grayver, A., Tøffner-Clausen, L., and Olsen, N.: Mapping 3-D mantle electrical conductivity using Swarm, Cryosat-2 and ground observatory data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7879, https://doi.org/10.5194/egusphere-egu2020-7879, 2020.

EGU2020-4651 | Displays | EMRP2.2

Oceanic tidal signals in satellite magnetic data: quo vadis?

Alexander Grayver, Nils Olsen, Chris Finlay, and Alexey Kuvshinov

The continuous high-quality geomagnetic field measurements delivered by the Swarm satellite constellation trio have enabled reliable global mapping of the magnetic signature of ocean tides for several tidal constituents. These signals provide geophysical constraints on the average electrical conductivity profile of the upper mantle below the oceans. In principle, these signals can also sense lateral variations of the electrical conductivity in the oceanic upper mantle, although the amplitude of these effects is small. Additionally, the long-term changes in the climatology of the ocean can be potentially detected by the magnetic satellite signals. Both applications put additional demands on the accuracy and resolution of the extracted signals. This contribution discusses potential ways to meet the required demands and evaluates the feasibility of using the magnetic signature of ocean tides for studying these effects.

How to cite: Grayver, A., Olsen, N., Finlay, C., and Kuvshinov, A.: Oceanic tidal signals in satellite magnetic data: quo vadis?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4651, https://doi.org/10.5194/egusphere-egu2020-4651, 2020.

EGU2020-13612 | Displays | EMRP2.2

The magnetic signatures of oceanic tides in satellite data: A virtual-observatory approach

Jakub Velímský, Magnus D. Hammer, and Christopher C. Finlay

The magnetic signatures of the M2, and more recently also the N2, and O1 oceanic tides have been successfully extracted from satellite observations (Grayver & Olsen, 2019). The traditional method uses the spatial representation of the tidal signals by spherical harmonics. Here we present an alternative approach based on the concept of virtual observatories, motivated by similar development in the analysis of the core field (Mandea & Olsen 2006). All quiet-time, night-side vector magnetic field values observed by the satellite(s) in the proximity of a selected virtual observatory are parameterized by a scalar magnetic potential represented by a cubic harmonic polynomial in a local Cartesian coordinate system. The time-dependence of the polynomial coefficients is constrained by selected tidal frequency, taking into account also the phase and amplitude corrections. The local approach offers several advantages over the use of the global spherical-harmonic base. The disturbances from external field in the polar areas have no impact on the inversion at lower latitudes, and local error estimates can be also provided. In this initial report, we will explore the possibilities of the new technique in terms of resolution, the combination of datasets from multiple satellites and the use of NS and EW field differences from the Swarm A-C pair.

How to cite: Velímský, J., Hammer, M. D., and Finlay, C. C.: The magnetic signatures of oceanic tides in satellite data: A virtual-observatory approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13612, https://doi.org/10.5194/egusphere-egu2020-13612, 2020.

Analysing ionospheric electron density and magnetic field data from several years of the Swarm three-satellite mission we define a dataset of anomalies statistically.  We then use a superposed epoch approach to study the possible relation with a corresponding dataset of earthquakes occurred in the same space-time domain. Two statistical quantities d and n are then established comparing the statistics of the real analyses with simulations to assess the effectiveness of the largest concentrations of anomalies as ionospheric precursors. In detail, d would show how much the real maximum concentration is above the expected typical maximum concentration of a random anomaly distribution; while n value measures how much the largest concentration deviates with respect a typical random deviation: the larger are the d and n values, the more the results of the analysis applied to real data deviate from randomness. The best cases for which the real analyses are well distinct from random simulations are selected when d≥1.5, because the anomaly density is equal to or larger than 50% of random distribution, and n≥4, because the probability to be random is equal to or less than 0.1%.  This is the case of Y magnetic field component with a search in the Dobrovolsky area around each considered earthquake epicentre. The electron density is slightly less effective in the correlation with earthquakes, but still better than a homogeneous random distribution of anomalies.

How to cite: De Santis, A. and the SAFE Team: Statistical analysis of Swarm satellite data for assessing the effectiveness of ionospheric precursors of earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13140, https://doi.org/10.5194/egusphere-egu2020-13140, 2020.

EGU2020-10018 | Displays | EMRP2.2 | Highlight

Reconstructing the propagation of Whistlers observed in ELF during ASM burst sessions from the lightning strikes to their detection and validation of IRI model

Pierdavide Coïsson, Vladimir Truhlik, Janusz Mlynarczyk, Gauthier Hulot, Rémi Madelon, Olivier Bonnot, Pierre Vigneron, Dalia Burešová, Jaroslav Chum, Pawel Rzonca, and Andzej Kulak

New sessions of burst-mode acquisition of the Absolute Scalar Magnetometers (ASM) onboard Swarm satellites have been conducted during 2019 , with the aim of acquiring events covering various geophysical conditions, in terms of geomagnetic latitude, spacecraft Local Time and season, to better understand the conditions under which the ELF component of whistlers is excited and can be detected at satellite altitude and to provide an additional ionospheric monitoring.

Among all candidate events detected using an automatic algorithm specifically designed for that purpose, a selection of remarkable whistler events have been further studied. Firstly, from the estimation of the whistler dispersions, the origin times of the lightning discharge have been estimated and validated with ground data from the World ELF Radiolocation Array (WERA), providing the locations of the lightning strikes and their intensity in the ELF spectral band. These locations have also been validated using data from the World Wide Lightning Location Network (WWLLN) providing measurements.

Subsequently, to reconstruct the propagation path inside the ionosphere of the ELF component of the whistler, a dedicated ray-tracing algorithm has been designed. It uses a background ionosphere model of electron and ions based on the International Reference Ionosphere. For the purposes of producing a ionospheric representation as close as possible to the experimental conditions, the update of the main ionospheric parameters based on worldwide ionosonde data IRTAM has been applied, validating it by using ionosonde data available in the vicinity of specific whistler events. The in-situ electron density measurements of the Electric Field Instrument (EFI) of Swarm satellite have also been used to constrain the model in the topside ionosphere.

We present the recent results obtained during some of these burst sessions, and discuss the possibility offered by this new dataset to validate global ionospheric models and provide a new avenue in ionospheric research, that could be also pursued by the NanoMagSat mission.

How to cite: Coïsson, P., Truhlik, V., Mlynarczyk, J., Hulot, G., Madelon, R., Bonnot, O., Vigneron, P., Burešová, D., Chum, J., Rzonca, P., and Kulak, A.: Reconstructing the propagation of Whistlers observed in ELF during ASM burst sessions from the lightning strikes to their detection and validation of IRI model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10018, https://doi.org/10.5194/egusphere-egu2020-10018, 2020.

EGU2020-11130 | Displays | EMRP2.2

Turbulence and Plasma Inhomogeneity Observed by Swarm Constellation

Paola De Michelis, Giuseppe Consolini, Georgios Balasis, and Jerome Bouffard and the INTENS Team

The ionospheric environment is a complex system where dynamic phenomena, such as turbulence (fluid and magnetohydrodynamics) and plasma instabilities generally occur as a consequence of the coupling processes among solar wind, magnetosphere and ionosphere. It has been suggested that the turbulent character of the ionospheric plasma density also enters into the formation and dynamics of ionospheric inhomogeneities and irregularities, which essentially characterize the active equatorial, mid-latitude and polar regions. The ionospheric turbulence indirectly plays an important role also in the framework of space weather when due to the arrival of solar perturbations the plasma, the energetic particle distributions, the electric and magnetic fields within the magnetosphere and ionosphere are deeply modified thus paving the way for an increase in the ionospheric turbulence. Recent findings within the ESA funded project “Characterization of IoNospheric TurbulENce level by Swarm constellation (INTENS)” permitted us to investigate the role played by the turbulence on scales from hundreds of kilometers to a few kilometers in generating multi-scale plasma structures and inhomogeneities in the ionospheric environment at different latitudes. This presentation reports on the most promising results of the INTENS project regarding the investigation of turbulence and plasma conditions in the topside ionosphere using Swarm data.

How to cite: De Michelis, P., Consolini, G., Balasis, G., and Bouffard, J. and the INTENS Team: Turbulence and Plasma Inhomogeneity Observed by Swarm Constellation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11130, https://doi.org/10.5194/egusphere-egu2020-11130, 2020.

EGU2020-3760 | Displays | EMRP2.2

Integrated Science Operations of CASSIOPE e-POP with the Swarm Constellation for New Studies of Magnetosphere-Ionosphere Coupling

Andrew Yau, Andrew Howarth, H. Gordon James, David Knudsen, Richard Langley, and David Miles

The CASSIOPE Enhanced Polar Outflow Probe (e-POP) was originally envisioned as a low-cost, short-lifetime (18-month) small-satellite mission for investigating polar ion outflows and related magnetosphere-ionosphere coupling phenomena. However, e-POP is currently in its seventh year of continuing operation, as an addition to and as the fourth component of the Swarm constellation of satellites, under the European Space Agency Third Party Mission Programme.

Since 2017, the increased operation duty-cycle of e-POP has enabled the routine extension of its science operations to its full altitude range and to all latitudes, and made possible several new studies of important mid- and low-latitude topside ionospheric phenomena. In addition, the integrated e-POP and Swarm operation takes advantage of the synergy between the orbit characteristics and unique instrument capabilities between e-POP and Swarm, to enable or enhance a host of coordinated studies of magnetosphere-ionosphere coupling: including the Earth’s magnetic field and related current systems, auroral and upper atmospheric dynamics, and ionosphere-thermosphere and ionosphere-plasmasphere coupling processes. We present an overview of these new studies, focusing on their results on the effects of space weather in the ionosphere and upper atmosphere such as anomalous satellite orbit drag and ionospheric scintillation.

How to cite: Yau, A., Howarth, A., James, H. G., Knudsen, D., Langley, R., and Miles, D.: Integrated Science Operations of CASSIOPE e-POP with the Swarm Constellation for New Studies of Magnetosphere-Ionosphere Coupling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3760, https://doi.org/10.5194/egusphere-egu2020-3760, 2020.

The Thermal Ion Imagers on Swarm A-C, and the Suprathermal Electron/Ion Imager on ePOP (now “Swarm-E”) provide a unique view of charged particle distribution functions in the ionosphere at high time resolution (up to 100 images/s). Through high resolution, CCD-based imaging (~3000 pixels/image), ion drift velocity is derived from these images at a resolution of 20 m/s or better, and in general agreement with velocities derived from ground based radars [1] and an empirical convection model [2]. This talk reviews recent scientific applications of this technique, which are wide-ranging and include mechanisms of ion heating and upflow [3,4], M-I coupling via Alfven waves [5,6], electron acceleration and heating by Alfven waves [7,8, 9], intense plasma flows associated with “Steve” [10,11], and electrodynamics of large-scale FAC systems[ 12], among others. In addition, future opportunities made possible by these data will be discussed.

[1] Koustov et al. (2019), JGR, https://doi.org/10.1029/2018JA026245

[2] Lomidze et al. (2019), ESS, https://doi.org/10.1029/2018EA000546

[3] Shen and Knudsen (2020a), On O+ ion heating by BBELF waves at low altitude, JGR, in revision.

[4] van Irsel et al. (2020), Highly correlated ion upflow and electron temperature variations in the high latitude topside ionosphere, submitted to JGR.

[5] Pakhotin et al. (2020), JGR, https://doi.org/10.1029/2019JA027277

[6] Wu et al. (2020a), Swarm survey of Alfvenic fluctuations and their relation to nightside field-aligned current and auroral arcs systems, JGR, in revision.

[7] Liang et al. (2019), JGR, https://doi.org/10.1029/2019JA026679

[8] Wu et al. (2020b), e-POP observations of suprathermal electron bursts in the ionospheric Alfven resonator, GRL, submitted.  

[9] Shen and Knudsen (2020b), Suprathermal electron acceleration perpendicular to the magnetic field in the topside ionosphere, JGR, in press.

[10] Archer et al. (2019), JGR, https://doi.org/10.1029/2019GL082687

[11] Nishimura et al. (2019), JGR, https://doi.org/10.1029/2019GL082460

[12] Olifer et al (2020), Swarm observations of dawn/dusk asymmetries between Pedersen conductance in upward and downward FAC regions, submitted to JGR.

 

How to cite: Knudsen, D.: Recent scientific findings based on high-resolution core plasma imaging of the ionosphere with Swarm and ePOP, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12192, https://doi.org/10.5194/egusphere-egu2020-12192, 2020.

EGU2020-20871 | Displays | EMRP2.2

Using Swarm to study ionosphere-thermosphere coupling

Johnathan Burchill

Properties and dynamics of ionosphere-thermosphere coupling may be investigated using observations from the Swarm electric field instruments (EFI). We illustrate this claim using measurements of vertical ion drift and electron temperature made by the EFIs, within the context of ambipolar diffusion parallel to the geomagnetic field. The associated ambipolar electric field is difficult to measure directly. Rather, under conditions where the ambipolar electric field is assumed to be specified, t