Opalinus Clay Research Articles

Consolidated-undrained triaxial testing of Opalinus Clay: Results and method validation

Specific equipment and procedures developed for geomechanical testing of hydrocarbon caprocks were adopted to conduct truly undrained triaxial tests with Opalinus Clay. The amount of pore pressure development during consolidation, and the resulting effective stress, is managed by equilibrating the samples in vacuum desiccators of different relative humidities (vapor equilibration technique) prior to assembling into the test apparatus. A drained consolidation test was first conducted to determine the appropriate strain rate for consolidated-undrained (CU) triaxial testing. Opalinus Clay samples were then consolidated in the triaxial rig to mean effective stresses in the range from 3 to 52 MPa and eventually sheared. Within the explored stress range elastic and pore pressure coupling parameters were found to be stress dependent. The different stress paths to peak indicate a transition from overconsolidated to rather normally consolidated state, yet failure was in all tests dilatant, i.e. associated with a drop in pore pressure and strain-softening (more so at low effective stress). Accurate pore pressure monitoring enabled the discrimination of different deformation stages during deviatoric loading. In terms of Mohr–Coulomb strength parameters, transition from peak to post-peak strength is manifested by a reduction in the effective cohesion whereas the effective friction angle remains nearly constant. The robustness of the CU testing methodology is demonstrated by (i) diagnostic analyses, (ii) inconsistency of CU tests with two CU tests deliberately loaded faster to explore the effect of strain rate, and (iii) consistency of CU tests with two consolidated-drained tests. Finally, test results of two caprock shales are also shown for comparison. The caprocks are of similar basic properties as the Opalinus Clay and stem from a large data base of tests conducted using the same methodology.

Anisotropic Behaviour of Opalinus Clay Through Consolidated and Drained Triaxial Testing in Saturated Conditions

This paper investigates the anisotropic hydro-mechanical behaviour of Opalinus Clay shale, the host material currently being considered for the construction of a nuclear waste repository in Switzerland. Consolidated and drained triaxial tests on Opalinus Clay from the Mont Terri URL have been conducted in order to derive information on its strength and stiffness properties. Opalinus Clay specimens were tested both parallel to bedding (P-specimens) and perpendicular to bedding (S7 specimens). The considered effective confining stress range (from 2 to 12 MPa) has been selected in order to reproduce possible in-situ stress conditions for the repository. In this work, particular attention has been paid to the experimental procedure in order to ensure consolidated conditions and avoid generation of unwanted excess pore water pressure during drained shearing. The Skempton B parameter has been determined for all the tested specimens in order to ensure saturation. Both single-stage and multistage triaxial testing procedures were adopted in the experimental campaign. The results of the triaxial tests highlight an anisotropic elastic response of Opalinus Clay: S specimens present a more compliant behavior than P-specimens. The values of the Young modulus are found to increase with the increase in mean effective stress. The analysis of the peak and ultimate shear strength results reveals that the material behaves in a similar manner regardless of the considered direction of loading (P and S directions) with respect to the bedding orientation. Peak and ultimate failure envelopes for Opalinus Clay were derived for the investigated stress range.

Permeability Variations Associated With Fault Reactivation in a Claystone Formation Investigated by Field Experiments and Numerical Simulations

AbstractWe studied the relation between rupture and changes in permeability within a fault zone intersecting the Opalinus Clay formation at 300 m depth in the Mont Terri Underground Research Laboratory (Switzerland). A series of water injection experiments were performed in a borehole straddle interval set within the damage zone of the main fault. A three‐component displacement sensor allowed an estimation of the displacement of a minor fault plane reactivated during a succession of step rate pressure tests. The experiment reveals that the fault hydromechanical (HM) behavior is different from one test to the other with varying pressure levels needed to trigger rupture and different slip behavior under similar pressure conditions. Numerical simulations were performed to better understand the reason for such different behavior and to investigate the relation between rupture nucleation, permeability change, pressure diffusion, and rupture propagation. Our main findings are as follows: (i) a rate frictional law and a rate‐and‐state permeability law can reproduce the first test, but it appears that the rate constitutive parameters must be pressure dependent to reproduce the complex HM behavior observed during the successive injection tests; (ii) almost similar ruptures can create or destroy the fluid diffusion pathways; (iii) a too high or too low diffusivity created by the main rupture prevents secondary rupture events from occurring whereas “intermediate” diffusivity favors the nucleation of a secondary rupture associated with the fluid diffusion. However, because rupture may in certain cases destroy permeability, this succession of ruptures may not necessarily create a continuous hydraulic pathway.

Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)

Abstract. We studied gouge from an upper-crustal, low-offset reverse fault in slightly overconsolidated claystone in the Mont Terri rock laboratory (Switzerland). The laboratory is designed to evaluate the suitability of the Opalinus Clay formation (OPA) to host a repository for radioactive waste. The gouge occurs in thin bands and lenses in the fault zone; it is darker in color and less fissile than the surrounding rock. It shows a matrix-based, P-foliated microfabric bordered and truncated by micrometer-thin shear zones consisting of aligned clay grains, as shown with broad-ion-beam scanning electron microscopy (BIB-SEM) and optical microscopy. Selected area electron diffraction based on transmission electron microscopy (TEM) shows evidence for randomly oriented nanometer-sized clay particles in the gouge matrix, surrounding larger elongated phyllosilicates with a strict P foliation. For the first time for the OPA, we report the occurrence of amorphous SiO2 grains within the gouge. Gouge has lower SEM-visible porosity and almost no calcite grains compared to the undeformed OPA. We present two hypotheses to explain the origin of gouge in the Main Fault: (i) authigenic generation consisting of fluid-mediated removal of calcite from the deforming OPA during shearing and (ii) clay smear consisting of mechanical smearing of calcite-poor (yet to be identified) source layers into the fault zone. Based on our data we prefer the first or a combination of both, but more work is needed to resolve this. Microstructures indicate a range of deformation mechanisms including solution–precipitation processes and a gouge that is weaker than the OPA because of the lower fraction of hard grains. For gouge, we infer a more rate-dependent frictional rheology than suggested from laboratory experiments on the undeformed OPA.

Modeling of Gas Migration Through Low-Permeability Clay Rock Using Information on Pressure and Deformation from Fast Air Injection Tests

The characterization of gas migration through low-permeability clay formations has been a focus of R&D programs for radioactive waste disposal, which is also of great importance for shale gas exploration, cap-rock behavior of hydrocarbon reservoirs, and hbox {CO}_{2} sequestration. Laboratory tests have been performed on Opalinus Clay, a Mesozoic claystone that is being investigated in Switzerland as a potential host rock for the storage of nuclear waste. The laboratory program included specific water and air injections tests, as well as oedometer and isotropic compression tests. Undisturbed core samples have been retrieved from a shallow borehole in the Mont Terri Underground Research Laboratory (URL) and from a deep borehole in northern Switzerland. For the shallow cores from Mont Terri URL, largely linear-elastic deformations associated with the gas injection test could be inferred and the change in void ratio was accounted for by the pore compressibility. The corresponding change in permeability was obtained from the results of the water tests, indicating a log-linear relation between permeability and porosity. The derived porosity change and the corresponding change in permeability were implemented in the standard TOUGH2 code, which reproduced the measured gas test results using fitted water retention data derived from laboratory measurements. Similar air injection tests performed on Opalinus Clay cores from the borehole at greater depth showed overall similar behavior, but at lower porosities, lower permeability values, and lower compressibility. These cases indicated nonlinear behavior which was implemented using an effective stress-dependent porosity change and the associated change in permeability. In addition, the anisotropy associated with the bedding planes of the clay formation was considered by assuming different properties for “soft” and “hard” layers to account for storage capacity for the injected gas prior to gas breakthrough. The computed change in the overall porosity could be compared to the measured axial deformation during the gas injection test and was used for calibration of the parameters describing the relationship between the effective stress and porosity, as well as the corresponding change in permeability and capillary pressure.

Frictional Properties of Opalinus Clay: Implications for Nuclear Waste Storage

AbstractThe kaolinite‐bearing Opalinus Clay (OPA) is the host rock proposed in Switzerland for disposal of radioactive waste. However, the presence of tectonic faults intersecting the OPA formation put the long‐term safety performance of the underground repository into question due to the possibility of earthquakes triggered by fault instability. In this paper, we study the frictional properties of the OPA shale. To do that, we have carried out biaxial direct shear experiments under conditions typical of nuclear waste storage. We have performed velocity steps (1–300 μm/s) and slide‐hold‐slide tests (1–3,000 s) on simulated fault gouge at different normal stresses (4–30 MPa). To establish the deformation mechanisms, we have analyzed the microstructures of the sheared samples through scanning electron microscopy. Our results show that peak (μpeak) and steady state friction (μss) range from 0.21 to 0.52 and 0.14 to 0.39, respectively, thus suggesting that OPA fault gouges are weak. The velocity dependence of friction indicates a velocity strengthening regime, with the friction rate parameter (a − b) that decreases with normal stress. Finally, the zero healing values imply a lack of restrengthening during interseismic periods. Taken together, if OPA fault reactivates, our experimental evidence favors an aseismic slip behavior, making the nucleation of earthquakes difficult, and long‐term weakness, resulting in stable fault creeping over geological times. Based on the results, our study confirms the seismic safety of the OPA formation for a nuclear waste repository.

Image‐Based Upscaling of Permeability in Opalinus Clay

AbstractThis paper presents an approach for upscaling permeability in Opalinus Clay from pore scale to a coarser scale, where pores cannot be resolved and where the transport properties are controlled by the geometry of the clay matrix considered as a distinct phase. Microstructures reconstructed from multiscale image data are cellular models with different sizes of basic building blocks, the so‐called voxels. Flow simulations were performed on the basis of the grid that is naturally inherent in the coarser‐scale image data that were acquired by synchrotron X‐ray computed tomography. Thereby, the spatial distribution of permeability at this coarser scale was determined from image data at the pore scale acquired by focused ion beam nanotomography. Then, coarse‐scale microstructures with different clay matrix contents were used as input for flow simulations, which allowed predicting the vertical and horizontal bulk permeability at the mesoscale. In agreement with results of other workers, it turned out that vertical (cross‐plane) permeability Kv decreases with increasing clay content, which can be explained in that the presence of coarse nonclayey grains increases permeability of the porous clay matrix when compared to a matrix consisting only of fine‐grained shales. Mesoscale permeability anisotropy Kh/Kv increases from about 1 to 2 if the clay content increases from 0.2 to 0.9. The predicted permeability behavior is discussed and compared to results from experiments and other predictive models.

Measurement of Cl− : Br−Ratios in the Porewater of Clay-Rich Rocks—A Comparison of the Crush-and-Leach and the Paper-Absorption Methods

Characterization of porewater chemistry in low-permeability, clay-rich rocks provides insights into solute transport mechanisms and the origin and residence time of porewater. Extraction of porewater for chemical quantification is challenging, and several methods have been applied including squeezing, advective displacement, crush and leach, and a relatively new technique that extracts porewater by absorption into a cellulosic paper by capillary action. Here we compare porewater Cl−and Br−mass ratios from samples using the paper-absorption and crush-and-leach techniques. Samples were obtained from Upper Ordovician shales in the Michigan Basin in Ontario, the Opalinus Clay at the Mont Terri Rock Laboratory in Switzerland, and the Upper Ordovician Lorraine Group shale in southern Quebec. The data display consistent and reproducible differences among methods for Cl−and Br−mass ratios, with the paper-absorption method producing systematically lower Cl− : Br−ratios. The observed differences in Cl− : Br−ratios are attributed primarily to anion exclusion effects which are stronger for Br−than for Cl−, resulting in higher Br−concentrations in the largest pores that are preferentially sampled by the paper-absorption technique. In addition, calculations suggest that Cl−is more effective than Br−in forming ion pairs and clusters with neutral or positive charge which can enter the diffuse double layer. This causes a further decrease in the Cl− : Br−ratios for the mobile water. One important message from this work is that different extraction methods should not be expected to converge on a unique porewater Cl− : Br−ratio because each method reflects different proportions of the interlayer, diffuse double layer, and mobile fractions of porewater.

Bayesian inversion of a chloride profile obtained in the hydraulically undisturbed Opalinus Clay: Mass transport and paleo-hydrological implications

The BDB-1 deep inclined borehole was drilled at the Mont Terri rock laboratory (Switzerland) and enabled to acquire relevant data on porewater composition through the Opalinus Clay (OPA) and its bounding formations. Petrophysical measurements were carried out and included water content, water accessible porosity and grain density determination. Conservative anion profiles were obtained by aqueous leaching and out diffusion experiments performed on drillcore samples, and revealed to be consistent with previous studies carried out at the rock laboratory level. Diffusive properties were also investigated using three experimental setups: cubic out diffusion, radial diffusion and through diffusion. These transport parameters were used as a priori values in a Bayesian inversion using a Markov Chain Monte Carlo method to interpret the chloride profile in the Opalinus Clay. Based on a Peclet number analysis using the transport parameters formerly acquired, a purely diffusive scenario enabled specifying the paleohydrogeological evolution of the Mont Terri site from the folding and thrusting of the Jura Mountains to present time and transport parameters.

Microstructure and composition of brittle faults in claystones of the Mont Terri rock laboratory (Switzerland): New data from petrographic studies, geophysical borehole logging and permeability tests

Claystones are considered as a geological barrier. However, the properties of claystone may be modified if these are cut by brittle faults and fractures. Investigations of fault rocks are therefore crucial to evaluate the barrier properties of clay rich formations.The present study is dealing with the characterization of naturally and artificially deformed Opalinus Clay of the Mont Terri rock laboratory in NW Switzerland. Complete core sections, covering the artificially excavation damaged zone (EDZ) and several tectonic fault zones, have been studied using a multidisciplinary approach consisting of geophysical, geotechnical, mineralogical/geochemical and lithological/structural data.The fault zones encountered are characterized by a high density of planar discontinuities, which often show slickenside striations. Under the microscope, the fault zones turned out to be pervasively deformed resulting in open veins and pore space, now filled with calcite and celestine. Fault zone reactivation led to very fine-grained, cohesionless fault gouge and fragmentation of the previously formed calcite veins.Packer tests in boreholes reveal excavation induced, enhanced permeabilities up to 3m borehole depth (1.9m ┴ to gallery floor). Seismic borehole measurements indicate that seismic attributes, which are typical for undisturbed Opalinus Clay, are not reached until 7.5m borehole depth (4.8m ┴ to gallery floor), which is larger than in the usual gallery configuration.We interpret the anomalies in geophysical measurements as well as the elevated permeabilities, measured in the Main Fault (3.6m to 4.2m ┴ to gallery floor), to result from an enlarged EDZ, influenced by the presence of the brittle fault structures. The up to two orders of magnitude higher permeability (compared to the intact claystone) most probably results from excavation induced stress and a local reactivation of fault planes.The results suggest that the presence of a fault zone can alter the extent of the EDZ significantly, and thereby affect the rock integrity, at least in the near field of a repository tunnel.

Coupled THMC models for bentonite in an argillite repository for nuclear waste: Illitization and its effect on swelling stress under high temperature

Subsurface manipulations such as those expected from the disposal of heat-emanating radioactive waste in deep repositories can induce strongly coupled Thermal (T), hydrological (H), mechanical (M) and chemical (C) processes. Adequate coupled THMC models are highly desirable or even indispensable for performance assessment of such repositories, for examples for the analysis of bentonite-based engineered barrier system (EBS) surrounding the emplaced waste. In this study, we present coupled THMC model simulations of a generic nuclear waste repository in an argillite with a bentonite-based buffer. The objective is to evaluate the chemical changes in the EBS bentonite and their effect on mechanical behavior under high temperature, attempting to shed light on whether EBS bentonite can sustain temperatures higher than 100°C without significant impact on barrier's performance.Two scenarios were simulated for comparison: a case in which the temperature near the waste canister peaks at 200°C and a case in which the temperature at the same spot culminate with about 100°C. Simulations for a generic case with Kunigel-VI bentonite as backfill and Opalinus Clay as host rock were conducted for 1000years and reported in the previous study (Zheng et al., 2015). In this paper, simulations for 100,000years have been done for two types of bentonite-based buffer materials: Kunigel-VI and FEBEX bentonite. This enables us to evaluate how different types of bentonite behave in terms of the illitization and its impact on swelling stress and whether we can generalize these results to support decision making. The simulations show the occurrence of illitization in the bentonite buffer and the enhancement of illitization under high temperature. However, FEBEX bentonite undergoes less illitization mainly due to the higher ion concentration in pore water and the lower content of K-feldspar in the bentonite mineral composition. Moreover, the reduction of swelling stress by chemical changes is more pronounced for Kunigel-VI bentonite than for FEBEX bentonite. Overall, the results of our model simulations suggest that an argillite repository with a bentonite-based EBS that is similar to FEBEX bentonite could sustain temperatures much higher than 100°C as far as illitization concerns. Model results also reveal that illitization is stabilized after about 2000years in bentonite near the waste package, but continues in bentonite near the bentonite-argillite interface, which manifests the strong effect of geochemical interaction between EBS bentonite and host rock on long term illitization in bentonite.

Testing High-Voltage Electrical Discharges in Disintegrating Claystone for Isotopic and Mineralogical Studies: An Example Using Opalinus Claystone

AbstractThe radiogenic isotope systematics of clay minerals are complex because of the intimate mixture of minerals from different origins such as detrital and authigenic sources. An important aspect of dating clays is the primary sample preparation and disintegration method. In the present study, a sample of weakly deformed Opalinus claystone from the Mont Terri underground site (Switzerland) was investigated after disintegration by three different methods. The Opalinus Clay was sedimented in the late Toarcian and early Aalenian and reached maximum temperatures of ~85°C during burial in the Cretaceous. The present study reports data from a comprehensive investigation comparing the effects of disintegration by: (1) disc milling; (2) repeated freezing and thawing; and (3) high-voltage discharges. K-Ar age values of the finest clay (<0.1 µm) released by the different disintegration methods are indistinguishable, indicating that the high-voltage liberation method does not influence grains as small as 100 nm. The K-Ar age values of particle-size separates decreased with decreasing particle size. The age values of the 2–6 µm separates correspond to the Carboniferous Period, which reflects the dominance of Paleozoic detritus in that size range. The age values of the smallest separates (<0.1 µm), on average 213 ± 4 Ma, exceed the numerical age of the formation (~177−172 Ma), which show predominance of detrital grains over authigenic grains even in the finest illite. In summary, isotope geochronology data suggest that the high-voltage method can be applied reliably for disintegrating claystones.

Field characterization of elastic properties across a fault zone reactivated by fluid injection

AbstractWe studied the elastic properties of a fault zone intersecting the Opalinus Clay formation at 300 m depth in the Mont Terri Underground Research Laboratory (Switzerland). Four controlled water injection experiments were performed in borehole straddle intervals set at successive locations across the fault zone. A three‐component displacement sensor, which allowed capturing the borehole wall movements during injection, was used to estimate the elastic properties of representative locations across the fault zone, from the host rock to the damage zone to the fault core. Young's moduli were estimated by both an analytical approach and numerical finite difference modeling. Results show a decrease in Young's modulus from the host rock to the damage zone by a factor of 5 and from the damage zone to the fault core by a factor of 2. In the host rock, our results are in reasonable agreement with laboratory data showing a strong elastic anisotropy characterized by the direction of the plane of isotropy parallel to the laminar structure of the shale formation. In the fault zone, strong rotations of the direction of anisotropy can be observed. The plane of isotropy can be oriented either parallel to bedding (when few discontinuities are present), parallel to the direction of the main fracture family intersecting the zone, and possibly oriented parallel or perpendicular to the fractures critically oriented for shear reactivation (when repeated past rupture along this plane has created a zone).

Geochemical Interactions of Plutonium with Opalinus Clay Studied by Spatially Resolved Synchrotron Radiation Techniques.

Plutonium plays an important role within nuclear waste materials because of its long half-life and high radiotoxicity. The aim of this study was to investigate with high spatial resolution the reactivity of the more oxidized forms of Pu(V,VI) within Opalinus Clay (OPA) rock, a heterogeneous, natural argillaceous rock considered as a potential repository host. A combination of synchrotron based X-ray microprobe and bulk techniques was used to study the spatial distribution and molecular speciation of Pu within OPA after diffusion and sorption processes. Microscopic chemical images revealed a pronounced impact of geochemical heterogeneities concerning the reactivity of the natural barrier material. Spatially resolved X-ray absorption spectroscopy documented a reduction of the highly soluble Pu(V,VI) to the less mobile Pu(IV) within the argillaceous rock material, while bulk investigations showed second-shell scattering contributions, indicating an inner-sphere sorption of Pu on OPA components. Microdiffraction imaging identified the clay mineral kaolinite to play a key role in the immobilization of the reduced Pu. The findings provide strong evidence that reduction and immobilization do not occur as linked processes on a single reactive phase but as decoupled, subsequent, and spatially separated reactions involving different phases of the OPA.

Hydromechanical Aspects of CO2 Breakthrough into Clay-rich Caprock

Caprock formations are intended to prevent upwards carbon dioxide (CO2) migration to the surface during CO2 geological storage. Caprock interaction with CO2, as well as its potential consequences, requires to be predicted, and thus, need to be studied experimentally. Laboratory investigations of caprock behavior are complex due to its low permeability, and the scarcity of experimental studies involving high-pressure CO2 injection into caprock representatives puts this difficulty into manifest. In this study, we perform laboratory experiments in an oedometric cell on intact and remolded Opalinus clay (Jurassic shale), evaluating the breakthrough pressure and permeability for liquid and supercritical CO2. Intact and remolded shale specimens present intrinsic permeabilities of 10-21 m2 to 10-20 m2, respectively. Applied axial stress ranges from 27 MPa to 42 MPa and the pressure and temperature conditions are representative of a caprock at a depth of 800 m. We found that the microstructure of the caprock has a great effect on the material properties. The intrinsic permeability of a more tight material (intact Opalinus clay) is around two times lower than that of remolded shale, which has a more open microstructure. Additionally, the intact rock becomes 30 times less permeable to CO2 than the remolded shale, which implies that the CO2 relative permeability is 15 times smaller for intact rock than for remolded shale. On the other hand, CO2 breakthrough pressure for the tighter material is almost three times lower than for the more permeable remolded shale. Breakthrough pressure of the remolded shale ranges from 3.9 MPa to 5.0 MPa for liquid CO2 and from 2.8 MPa to 4.6 MPa for supercritical CO2. For the intact shale, breakthrough pressure is 0.9 MPa for liquid CO2 and 1.6 MPa for supercritical CO2. Thus, the breakthrough pressure cannot be correlated with the intrinsic permeability of the caprock.

Quantification of Viscous Creep Influence on Storage Capacity of Caprock

In the light of growing concerns for the climate change, it is of particular interest for governments to encourage efficient capture and safe storage of large amounts of carbon dioxide in the subsurface. In this perspective and in order to accurately predict the short and long-term response of the reservoir, a precise characterization of the geomechanical properties has to be carried out. In addition to the classical poroelastic properties, time-dependent deformation, such as viscous creep should also be considered. Storage capacity of a caprock may be seriously affected by local creep deformation allowing fast vertical fluid flow through an a priori very impermeable formation. Within this study, we investigate the ease to creep of Opalinus clay (Jurassic shale) under shallow geological storage conditions and predict the propagation of high porosity channels at operational time scales. The effective poroviscoelastic parameters of rock are inferred from the novel laboratory experiments that allow evaluation of time-dependent deformation. The bulk viscosity of the shale is found to be ∼ 1014 - 1015 Pa·s and it decreases with rise of temperature and pore fluid pressure to total mean stress ratio. Furthermore, the propagation speed of high porosity channels (porosity waves) is calculated to be on the order of the centimeters per year.

Modelling of the CO2-Induced Degradation of a Fractured Caprock During Leakage: Potential for a Mechanical Self-Limiting Process

In the present study, we assess the potential for initiating ductile failure in a fractured caprock due to the chemical alteration of its mechanical properties under pressure increase induced by CO2 leakage and fixed in situ boundary conditions. In this view, 2D numerically coupled reactive-transport simulations were set up by using the Opalinus Clay formation as an analogue for a caprock layer. The fractured system was viewed as a compartmentalised system that consists of a main highly permeable pathway, a moderately permeable damage zone and the intact rock. The outputs of the numerical simulations (mineral fraction, porosity changes, gas saturation, pore-fluid pressure) were converted into parameter changes of the yield surface by viewing the rock material of the three compartments (fault, damage zone and intact rock) as a composite system that consists of a clayey solid material, pores and mineral inclusions (such as carbonate and quartz). Three alteration processes were considered: (1) the effect of the mineral fraction and porosity evolution on the yield surface, (2) changes in the resulting poro-elastic properties and (3) the suction effect, i.e. the bounding effect induced by the presence of two phases, water and CO2. Our numerical investigations showed that the decrease in the friction coefficient remained negligible during leakage, while the pre-consolidation stress mainly decreased. Consequently, the damage zone of the fractured system became more collapsible over time, which was driven by low-to-moderate pressure build-up of the fluid penetrating the fault (1 MPa in our case). For the considered case, the initiation of ductile failure is likely under conditions of fixed vertical stress and zero lateral strain. This process could potentially limit the spatial spreading of CO2-induced alteration, although this remains very site specific. We recommend that characterisation efforts be intensified to obtain better insight into the properties of fracture systems in caprock-like formations (with special attention to their initial over consolidation ratio).

Thermal Volume Changes and Creep in the Callovo-Oxfordian Claystone

The Callovo-Oxfordian (COx) claystone is considered as a potential host rock for high-level radioactive waste disposal at great depth in France. Given the exothermic nature of radioactive wastes, a temperature elevation planned to be smaller than 100 °C will affect the host rock around the disposal cells. To gain better understanding of the thermal volumetric response of the COx claystone, a new thermal isotropic compression cell was developed with particular attention devoted to monitoring axial and radial strains. To do so, a high-precision LVDTs system ensuring direct contact between the LVDT stem and the claystone sample through the membrane was developed. A short drainage length (10 mm) was also ensured so as to allow full saturation of the sample under stress conditions close to in situ, and fully drained conditions during compression. High-precision strain monitoring allowed to observe a volumetric creep under stress conditions close to in situ. A drained heating test under constant stress carried out afterwards up to 80 °C exhibited a thermoelastic expansion up to a temperature of 48 °C, followed by thermoplastic contraction at higher temperature. Creep volume changes, that appeared to be enhanced by temperature, were modelled by using a simple Kelvin–Voigt model, so as to estimate the instantaneous response of the COx claystone and to determine its thermal expansion coefficient. The temperature at which the transition between thermal expansion and contraction appeared is close to the maximum burial temperature of the Callovo-Oxfordian claystone, estimated at 50 °C. This is in agreement with what has been already observed on the Opalinus Clay by Monfared et al. (2012) that was interpreted as a thermal hardening phenomenon, showing that the material kept the memory of the highest temperature supported during its geological history.

A multi-stage triaxial testing procedure for low permeable geomaterials applied to Opalinus Clay

In many engineering applications, it is important to determine both effective rock properties and the rock behavior which are representative for the problem's in situ conditions. For this purpose, rock samples are usually extracted from the ground and brought to the laboratory to perform laboratory experiments such as consolidated undrained (CU) triaxial tests. For low permeable geomaterials such as clay shales, core extraction, handling, storage, and specimen preparation can lead to a reduction in the degree of saturation and the effective stress state in the specimen prior to testing remains uncertain. Related changes in structure and the effect of capillary pressure can alter the properties of the specimen and affect the reliability of the test results. A careful testing procedure including back-saturation, consolidation and adequate shearing of the specimen, however, can overcome these issues. Although substantial effort has been devoted during the past decades to the establishment of a testing procedure for low permeable geomaterials, no consistent protocol can be found. With a special focus on CU tests on Opalinus Clay, this study gives a review of the theoretical concepts necessary for planning and validating the results during the individual testing stages (saturation, consolidation, and shearing). The discussed tests protocol is further applied to a series of specimens of Opalinus Clay to illustrate its applicability and highlight the key aspects.

Modelling the Mont Terri HE-D experiment for the Thermal\u2013Hydraulic\u2013Mechanical response of a bedded argillaceous formation to heating

Coupled thermal–hydrological–mechanical (THM) processes in the near field of deep geological repositories can influence several safety features of the engineered and geological barriers. Among those features are: the possibility of damage in the host rock, the time for re-saturation of the bentonite, and the perturbations in the hydraulic regime in both the rock and engineered seals. Within the international cooperative code-validation project DECOVALEX-2015, eight research teams developed models to simulate an in situ heater experiment, called HE-D, in Opalinus Clay at the Mont Terri Underground Research Laboratory in Switzerland. The models were developed from the theory of poroelasticity in order to simulate the coupled THM processes that prevailed during the experiment and thereby to characterize the in situ THM properties of Opalinus Clay. The modelling results for the evolution of temperature, pore water pressure, and deformation at different points are consistent among the research teams and compare favourably with the experimental data in terms of trends and absolute values. The models were able to reproduce the main physical processes of the experiment. In particular, most teams simulated temperature and thermally induced pore water pressure well, including spatial variations caused by inherent anisotropy due to bedding.