Opalinus Clay Research Articles

Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric

The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (pc) of 50–100 MPa, temperatures (T) between 25 and 200 °C and strain rates (dot{varepsilon }) of 1 × 10–3–5 × 10–6 s−1. Using a Paterson-type deformation apparatus, samples oriented at 0°, 45° and 90° to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed pc–T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45° and 90° to bedding at pc = 100 MPa, strain is localized in discrete shear zones. Compressive strength (σmax) increases with increasing pc, resulting in an internal friction coefficient of ≈ 0.31 for samples deformed at 45° and 90° to bedding, and ≈ 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and dot{varepsilon } do not significantly affect deformation behavior of dried samples. However, σmax and Young’s modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength σmax and Young’s modulus E at similar deformation conditions. Strength and Young’s modulus of samples deformed 90° and 45° to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.

Modeling cesium migration through Opalinus clay: a benchmark for single- and multi-species sorption-diffusion models

Sophisticated modeling of the migration of sorbing radionuclides in compacted claystones is needed for supporting the safety analysis of deep geological repositories for radioactive waste, which requires robust modeling tools/codes. Here, a benchmark related to a long term laboratory scale diffusion experiment of cesium, a moderately sorbing radionuclide, through Opalinus clay is presented. The benchmark was performed with the following codes: CORE2DV5, Flotran, COMSOL Multiphysics, OpenGeoSys-GEM, MCOTAC and PHREEQC v.3. The migration setup was solved with two different conceptual models, i) a single-species model by using a look-up table for a cesium sorption isotherm and ii) a multi-species diffusion model including a complex mechanistic cesium sorption model. The calculations were performed for three different cesium boundary concentrations (10−3, 10−5, 10−7 mol / L) to investigate the models/codes capabilities taking into account the nonlinear sorption behavior of cesium. Generally, good agreement for both single- and multi-species benchmark concepts could be achieved, however, some discrepancies have been identified, especially near the boundaries, where code specific spatial (and time) discretization had to be improved to achieve better agreement at the expense of longer computation times. In addition, the benchmark exercise yielded useful information on code performance, setup options, input and output data management, and post processing options. Finally, the comparison of single-species and multi-species model concepts showed that the single-species approach yielded generally earlier breakthrough, because this approach accounts neither for cation exchange of Cs+ with K+ and Na+, nor K+ and Na+ diffusion in the pore water.

Pilot Test of pH Triggered Polymer Sealant for CO2 Storage

Wellbore integrity is a critical subject in oil and gas production and CO2 storage. Successful subsurface deposition of various fluids, such as CO2, depends on the integrity of the storage site. In a storage site, injection wells and pre-existing wells might leak due to over-pressurization, mechanical/chemical degradation, and/or a poor cement job, thus reducing the sealing capacity of the site. Wells that leak due to microannuli or cement fractures on the order of microns are difficult to seal with typical workover techniques. We tested a novel polymer gelant, originally developed for near borehole isolation, in a pilot experiment at Mont Terri, Switzerland to evaluate its performance in the aforementioned scenario. The polymer gel sealant was injected to seal a leaky wellbore drilled in the Opalinus Clay as a pilot test. The success of the pH-triggered polymer gel (sealant) in sealing cement fractures was previously demonstrated in laboratory coreflood experiments [1-3]. pH-sensitive microgels viscosify upon neutralization in contact with alkaline cement to become highly swollen gels with substantial yield stress that can block fluid flow. The leaky wellbore setup was prepared by heating-cooling cycles to induce leakage pathways in the cased and cemented wellbore. The leakage pathways are a combination of fractures in the cement and microannuli at the cement-formation interface. The exact nature of these leakage pathways can be determined by over-coring at the end of the experiment life. We used polyacrylic acid polymer (sealant) to seal these intervals. The process comprises of three stages: (1) injection of a chelating agent as the preflush to ensure a favorable environment for the polymer gel, (2) injection of polymer solution, and (3) shut-in for the polymer gelation. Then, we evaluated the short-/long-term performance of the sealant in withholding the injected fluids (formation brine and CO2 gas). The novel sealant was successfully deployed to seal the small aperture pathways of the borehole at the pilot test. We conducted performance tests using formation brine and CO2 gas to put differential pressure on the polymer gel seal. Pressure and flow rate at the specific interval were monitored during and after injection of brine and CO2. Results of performance tests after polymer injection were compared against those in the absence of the sealant. Several short-term (4 min) constant-pressure tests at different pressure levels were performed using formation brine, and no significant injection flow rate (rates were below 0.3 ml/min) was observed. The result shows more than a ten-fold drop in the injection rate compared to the case without the sealant. The polymer gel showed compressible behavior at the beginning of the short-term performance tests. Our long-term (1-week) test shows even less injectivity (~ 0.15 ml/min) after polymer gelation. The CO2 performance test shows only 3 bar pressure dissipation overnight after injection compared to an abrupt loss of CO2 pressure in the absence of polymer gel. Sealant shows good performance even in the presence of CO2 gas with high diffusivity and acidity. Pilot test of our novel sealant proves its competency to mitigate wellbore leakage through fractured cement or debonded microannuli, where other remedy techniques are seldom effective. The effectiveness of the sealing process was successfully tested in the high-alkaline wellbore environment of formation brine in contact with cement. The results to date are encouraging and will be further analyzed once over-coring of the wellbore containing the cemented annulus occurs. The results are useful to understand the complexities of the cement/wellbore interface and adjust the sealant/process to sustain the dynamic geochemical environment of the wellbore.

Alteration of Bentonite Reacted with Cementitious Materials for 5 and 10 years in the Mont Terri Rock Laboratory (CI Experiment)

The cement–clay interaction (CI) experiment was carried out at the Mont Terri rock laboratory to complement the current knowledge on the influence that cementitious materials have on Opalinus Clay (OPA) and bentonite (MX). Drill cores including the interface of OPA, concrete (LAC = low-alkali binder, and OPC = ordinary Portland cement), and MX, which interacted for 4.9 and 10 years, were successfully retrieved after drilling, and detailed analyses were performed to evaluate potential mineralogical changes. The saturated compacted bentonites in core samples were divided into ten slices, profiling bentonite in the direction towards the interface, to evaluate the extent and spatial variation of the mineralogical alteration of bentonite. Regarding the mineral compositions of bentonite, cristobalite was dissolved within a range of 10 mm from the interface in both LAC-MX and OPC-MX, while calcite precipitated near the interface for OPC-MX. In LAC-MX and OPC-MX, secondary products containing Mg (e.g., M-S-H) also precipitated within 20 mm of the interface. These alterations of bentonite developed during the first 4.9 years, with very limited progress observed for the subsequent 5 years. Detectable changes in the mineralogical nature of montmorillonite (i.e., the formation of illite or beidellite, increase in layer charge) did not occur during the 10 years of interaction.

Deformation and water/gas flow properties of claystone/bentonite mixtures

As a potential engineered barrier material for disposal of radioactive waste in clay formations, claystone aggregate excavated from the Opalinus clay (OPA), its mixture with bentonite MX80 in a mass ratio of 7/3, and pure bentonite were extensively investigated with respect to the hydro-mechanical properties and performances. With these materials, a series of parallel experiments was performed under sequentially applied conditions of hydration with synthetic porewater of the clay formation, consolidation and water flow under increased stresses, and gas injection into the water-saturated and compacted materials under loading. Significant responses of the clay mixtures were observed. Main findings include: (1) the hydration and induced swelling of the mixtures are mainly dominated by bentonite content and dry density; (2) the consolidation decreases the porosity and water permeability exponentially by 2–3 orders of magnitude to low values of 10 −18 -10 −20 m 2 at stresses of 2–5 MPa, depending upon bentonite content; and (3) the gas penetration in the water-saturated and compacted bentonite is characterised by a cyclic pressure rising/dropping process limited in between the upper breakthrough and lower shut-off boundaries, whereas the compacted claystone and claystone/bentonite mixture allow for gas release at low and moderate pressures. The results are helpful for design of the engineered barriers for safe isolation of radioactive waste in repositories.

Fault sealing and caprock integrity for CO<sub>2</sub> storage: an in situ injection experiment

Abstract. The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO2) in the storage formation at depth. One of the critical elements of the safekeeping of CO2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction of the CO2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO2-saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO2 storage at depth. The mobility of the CO2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone.

Numerical analysis of coupled hydro-mechanical and thermo-hydro-mechanical behaviour in buffer materials at a geological repository for nuclear waste: Simulation of EB experiment at Mont Terri URL and FEBEX at Grimsel test site using Barcelona basic model

We investigated the applicability of a thermo-elasto-plastic model using a TOUGH2-MP/FLAC3D simulator to reproduce the coupled hydro-mechanical behaviour at the engineered barrier emplacement experiment in Opalinus Clay and the coupled thermo-hydro-mechanical behaviour at the full-scale engineered barrier experiment in a crystalline host rock, as part of the DECOVALEX-2019 project Task D. In the numerical models, we adopted Darcy's law, Fick's law, and Fourier's law for the advective flux of fluid, vapour diffusion, and conductive heat flux, respectively. We also used the mechanical constitutive law, which explicitly considers the effects of temperature, pore pressure, suction, and strain changes using the elastic model and the Barcelona basic model. In general, the numerical models in the simulator successfully represented the coupled behaviour in bentonite buffer materials at the two long-term in situ experiments in terms of evolution and spatial distribution for temperature, relative humidity, total stress, and displacement. The performance of the models for the dismantling of the in-situ experiments is globally satisfactory compared with the post-mortem results for saturations, water contents, and dry density.

Evaluation of anisotropic poroelastic properties and permeability of the Opalinus Clay using a single transient experiment

The determination of the transversely isotropic poroelastic parameters and the permeability of the Opalinus Clay, a possible host rock for deep radioactive waste disposal in Switzerland, is necessary for better predicting the hydromechanical response around the galleries. Transient experiments with fast undrained loading followed by a drainage phase, established for testing very low permeability claystones like the Opalinus Clay, were carried out on a sample equipped with strain gauges. The data obtained with this procedure showed that the values of a given poroelastic parameter, obtained from different loading paths, are reasonably close to each other, indicating that measurements are reliable and repeatable. An average intrinsic permeability perpendicular to bedding of 1.68 × 10–21 m2 was back-calculated through two consolidation phases, with similar values after loading and unloading. Some parameters, such as drained and undrained bulk moduli, exhibit a strong anisotropy. Conversely, some other parameters, like the Biot coefficients parallel and perpendicular to bedding, are close to each other. The values of these parameters were compared with those from other studies, enlarging the existing database of the hydromechanical properties of Opalinus Clay.

On the concentration-dependent diffusion of sorbed cesium in Opalinus Clay

Cation diffusion coefficients in clayey materials partly appear to be greater than diffusion coefficients of water tracers. The measured values vary between experiments performed at different salinities or different tracer concentrations. This effect is especially pronounced for cations that sorb strongly on the clay surfaces, such as Cs. The observations illustrate the difficulties in applying Fick’s law to cation diffusion in clays and demonstrate the need to find a consistent description of cation diffusion in clays that can be used to predict experiments performed at different conditions. In order to consistently describe Cs diffusion in Opalinus Clay, a multi-site surface diffusion model was implemented in the continuum-scale reactive transport code Flotran. The model combines pore and surface diffusion in one single diffusion coefficient, which accounts for the diffusion of sorbed cations along the clay surfaces.. The contribution from surface diffusion to the diffusion coefficient is directly coupled to the sorption behavior via the derivative of the sorption isotherm. The model parameters include the surface mobilities, which are specific for each cation and sorption site. To derive surface mobilities for Cs, in-diffusion experiments were conducted at eight different stable Cs background concentrations. A set of surface mobilities for Cs on three sorption sites in Opalinus Clay was estimated by fitting the surface diffusion model simultaneously to these experimental data. Moreover, the sensitivity of the model to sorption parameters and surface mobilities was evaluated. The surface diffusion model with the estimated surface mobilities was then successfully tested against independent experimental data for Cs in Opalinus Clay, illustrating the model’s predictive capabilities.

Experimental assessment of the hydro-mechanical behaviour of a shale caprock during CO2 injection

The presented experimental study focuses on the hydro-mechanical characterisation of a shale caprock (Opalinus Clay) in contact with carbon dioxide. The objective of this paper, consists in the evaluation of the material's sealing capacity in terms of entry-pressure, mechanical behaviour and sensitivity of the transport properties to chemo-mechanical effects induced by gaseous and liquid CO2 injection. Two types of Opalinus Clay core samples are tested; shaly and carbonate-rich. The sealing capacity has been evaluated on the shaly OPA according to the stepwise and the residual methods and compared to the results from mercury intrusion porosimetry. The obtained results and the differences associated to the different involved physical processes are discussed and compared with literature data. Injection tests carried out in saturated and unsaturated conditions have revealed that sub-critical CO2 propagation in a water saturated material is not associated with generation of fractures. On the other hand, the generation of capillary forces is affecting the mechanical behaviour beside the sealing capacity. The impact of chemical effects on the permeability of both types of OPA is analysed with long-term CO2 injection tests, where no significant variations of permeability are measured during the exposure time investigated. The challenges related to this type of analysis with laboratory scale experiments are illustrated and new insights on the behaviour of Opalinus Clay when subjected to injection of a non-wetting fluid are highlighted.

Surrogate Model for Multi-Component Diffusion of Uranium through Opalinus Clay on the Host Rock Scale

Multi-component (MC) diffusion simulations enable a process based and more precise approach to calculate transport and sorption compared to the commonly used single-component (SC) models following Fick’s law. The MC approach takes into account the interaction of chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32−. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories.

3D pore microstructures and computer simulation: Effective permeabilities and capillary pressure during drainage in Opalinus Clay

The 3D reconstruction of the pore space in Opalinus Clay is faced with the difficulty that high-resolution imaging methods reach their limits at the nanometer-sized pores in this material. Until now it has not been possible to image the whole pore space with pore sizes that span two orders of magnitude. Therefore, it has not been possible to predict the transport properties of this material with the help computer simulations that require 3D pore structures as input. Following the concept of self-similarity, a digital pore microstructure was constructed from a real but incomplete pore microstructure. The constructed pore structure has the same pore size spectrum as measured in the laboratory. Computer simulations were used to predict capillary pressure curves during drainage, which also agree with laboratory data. It is predicted, that two-phase transport properties such as the evolution of effective permeability as well as capillary pressures during drainage depend both on transport directions, which should be considered for Opalinus Clay when assessing its suitability as host rock for nuclear waste. This directional dependence is controlled on the pore scale by a geometric anisotropy in the pore space.

Shale fault zone structure and stress dependent anisotropic permeability and seismic velocity properties (Opalinus Clay, Switzerland)

Given shale's potential to serve as both a caprock for geological carbon sequestration and as the host for nuclear waste disposal, this study focuses on the structural characterization around a 1.5–3 m thick fault within the Opalinus Clay host rock and petrophysical characterization of both the host rock and fault core. Seven boreholes were drilled at the Mont Terri Rock laboratory in late 2018 to constrain the orientation of the so-called ‘Main Fault’ and several fracture families within the fault core using core and image logs. Contrary to typical fault models, matrix permeability and seismic velocity measurements show similarities in the host rock and within the Main Fault. In the clay-rich samples, measurements performed in laboratory at varying confining pressures indicate p-wave velocities range from 2.60 to 2.95 km/s perpendicular to foliation and 3.38–3.58 km/s parallel to foliation at near in-situ confining pressures, which increase with similar anisotropy at higher confining pressures (200 MPa). Since the permeability of the host rock and fault zone is very low (10−19–10−21 m2) flow is expected to prevail along fractures, where critical stress fault analysis suggests that potential hydraulically conductive fractures are actually conjugate to the Main Fault trend.

Geochemical fingerprinting of key lithologies and depositional processes across the upper boundary of the Opalinus Clay (Aalenian, Middle Jurassic, northern Switzerland)

AbstractThe Opalinus Clay is an argillaceous to silty mudstone formation, notable in Switzerland as the selected host rock for deep geological disposal of radioactive waste. Its upper bounding unit (Passwang Formation and eastern equivalents) is composed of successions of mudstone, sandy bioclastic marl and limestone separated by ooidal ironstone beds. The lithostratigraphic transition is diachronous across northern Switzerland and shows high vertical and lateral lithological variability. To constrain this variability into predictive models, and to identify horizons with properties that could potentially influence radionuclide mobility, the sedimentological and diagenetic processes involved in the genesis of this transition have to be investigated. The present study aims at testing the applicability of X‐ray fluorescence chemostratigraphy to characterise the mixed carbonate–siliciclastic units and understand the complex genesis of the lithostratigraphic transition from the Opalinus Clay towards its upper bounding unit. Sediment drill cores from four locations across northern Switzerland (Mont Terri, Riniken, Weiach and Benken) are analysed using high‐resolution X‐ray fluorescence core scanning. Data are compared to petrographic and additional geochemical data sets (inductively coupled plasma mass spectrometry, scanning electron microscopy with energy dispersive X‐ray analysis, micro‐X‐ray fluorescence mapping) obtained from powdered samples, thin section analyses and drill core slabs. The results demonstrate that the combination of these rapid and non‐destructive measurements along with multivariate data analysis allows the fast and objective classification of lithofacies along complex sedimentary successions. Moreover, it provides quantitative means for differentiating between prominent depositional and post‐depositional processes. The lithostratigraphic transition has been traced by the use of specific elemental proxies as a discontinuity, and its genesis linked to sediment bypassing.

A comparative study on methods for determining the hydraulic properties of a clay shale

SUMMARY A comprehensive characterization of clay shale behavior requires quantifying both geomechanical and hydromechanical characteristics. This paper presents a comparative laboratory study of different methods to determine the water permeability of saturated Opalinus Clay: (i) pore pressure oscillation, (ii) pressure pulse decay and (iii) pore pressure equilibration. Based on a comprehensive data set obtained on one sample under well-defined temperature and isostatic effective stress conditions, we discuss the sensitivity of permeability and storativity on the experimental boundary conditions (oscillation frequency, pore pressure amplitudes and effective stress). The results show that permeability coefficients obtained by all three methods differ less than 15 per cent at a constant effective stress of 24 MPa (kmean = 6.6E-21 to 7.5E-21 m2). The pore pressure transmission technique tends towards lower permeability coefficients, whereas the pulse decay and pressure oscillation techniques result in slightly higher values. The discrepancies are considered minor and experimental times of the techniques are similar in the range of 1–2 d for this sample. We found that permeability coefficients determined by the pore pressure oscillation technique increase with higher frequencies, that is oscillation periods shorter than 2 hr. No dependence is found for the applied pressure amplitudes (5, 10 and 25 per cent of the mean pore pressure). By means of experimental handling and data density, the pore pressure oscillation technique appears to be the most efficient. Data can be recorded continuously over a user-defined period of time and yield information on both, permeability and storativity. Furthermore, effective stress conditions can be held constant during the test and pressure equilibration prior to testing is not necessary. Electron microscopic imaging of ion-beam polished surfaces before and after testing suggests that testing at effective stresses higher than in situ did not lead to pore significant collapse or other irreversible damage in the samples. The study also shows that unloading during the experiment did not result in a permeability increase, which is associated to the persistent closure of microcracks at effective stresses between 24 and 6 MPa.

In-situ X-ray fluorescence to investigate iodide diffusion in opalinus clay: Demonstration of a novel experimental approach

During the last two decades, the Mont Terri rock laboratory has hosted an extensive experimental research campaign focusing on improving our understanding of radionuclide transport within Opalinus Clay. The latest diffusion experiment, the Diffusion and Retention experiment B (DR-B) has been designed based on an entirely different concept compared to all predecessor experiments. With its novel experimental methodology, which uses in-situ X-ray fluorescence (XRF) to monitor the progress of an iodide plume within the Opalinus Clay, this experiment enables large-scale and long-term data acquisition and provides an alternative method for the validation of previously acquired radionuclide transport parameters.After briefly presenting conventional experimental methodologies used for field diffusion experiments and highlighting their limitations, this paper will focus on the pioneer experimental methodology developed for the DR-B experiment and give a preview of the results it has delivered thus far.

Constitutive model development and field simulation of excavation damage in bedded argillaceous rock

Argillaceous rocks are candidate host and/or cap formations for the geological disposal of nuclear wastes in many countries, including Canada, France and Switzerland. The understanding of the long term mechanical behaviour of such rocks is an essential requirement for the assessment of their performance as a barrier against radionuclide migration. Due to the existence of bedding, argillaceous rocks are inherently anisotropic and the development of stress-strain models for their mechanical behaviour needs to take this anisotropy into account. This paper presents two examples of the practical implementation of stress-strain relationships in finite element models to simulate the excavation damage zones (EDZ) in bedded argillaceous rocks. The first example concerns the EDZ around a micro-tunnel in Opalinus Clay, in Switzerland. The second example relates to the EDZ around the century-old tunnel in Tournemire shale, in France. Both examples show the importance of developing robust stress-strain models that can replicate inherent anisotropy of the rock, and of calibrating and validating the models with a comprehensive set of laboratory experiments. The second example shows the additional influence of dessication and fault zone on the extent and shape of the EDZ.

An international model comparison study of controlled fault activation experiments in argillaceous claystone at the Mont Terri Laboratory

We present results from an international model comparison study involving a series of controlled fault activation experiments in Opalinus Clay at the Mont Terri Laboratory, Switzerland. The fault activation experiments were conducted in situ by water injection at variable pressure from boreholes targeting different parts of the Main Fault crossing several tunnels and galleries of the Mont Terri Laboratory. The model simulations focused on (1) an experiment activating a discontinuity (fracture or minor fault) within the damage zone of the main fault, and (2) an experiment activating discontinuities close to the core of the Main Fault. The experimental data consist of coupled hydraulic and mechanical responses monitored at an injection borehole and a monitoring borehole located a few meters away. After overcoming several modeling issues along with necessary model developments, a reasonably good agreement was achieved between the modeling results and the field observations. The fault activation experiments displayed an abrupt flow rate increase associated with a sudden fracture opening and rupture propagation after the injection pressure reached above the estimated normal stress on the fracture. This was followed by an abrupt flow rate decrease, indicating hydraulic closing, once the injection pressure decreases to an estimated 1–2 MPa below the stress normal to the opened fracture. The models were able to capture this abrupt hydromechanical behavior, including an observed dominant opening behavior along with the rupture propagation, while the spatial extent of the shear rupture and the quantity of peak injection flow were the most challenging to predict.

Coupling Between Poromechanical Behavior and Fluid Flow in Tight Rock

Proper characterization of the mechanical and flow properties of participating rock formations is crucial for subsurface geo-energy projects, including hydrocarbon extraction, geologic carbon storage, and enhanced geothermal systems. Application of mechanical and hydraulic pressures changes the porosity of rock and modifies flow paths. For low-permeable or “tight” rock that mainly contains nanoscale pores and serves as the confining layer for underground storage operations, a significant change in permeability may occur due to a small change in porosity. The pore volume changes in nanoporous geomaterials are extremely difficult to measure directly, but can be assessed from the knowledge of the hydro-mechanical response. Experimental methods to measure the stress-dependent permeability and poroelastic parameters of fluid-saturated tight rock are introduced. Eau Claire shale, Opalinus clay (claystone), and Charcoal granite are selected as representative materials for tight rock and their pore structure and material properties are carefully investigated. The porosity–permeability relationship for tight rock is established by adopting a power-law dependence with the exponent value in the range of 15–17, thus being significantly larger than that for a porous reservoir rock. Consequently, even small perturbations of porosity can cause orders of magnitude changes in permeability possessing a risk on the sealing capacity of the tight formations.

Further results on the in situ anaerobic corrosion of carbon steel and copper in compacted bentonite exposed to natural Opalinus Clay porewater containing native microbial populations

AbstractSince 2012, a long‐term in situ corrosion experiment (IC‐A) is being conducted in the Mont Terri Underground Research Laboratory in Switzerland to investigate the corrosion behaviour of candidate canister materials in conditions representative of the Swiss concept for the disposal of high‐level waste and spent nuclear fuel. To date, carbon steel and various types of copper coatings have been retrieved after different exposure periods of up to 3 years, and characterised to establish the composition of the corrosion product, the morphology of the corroded surface, the nature of the interaction between the metal and the surrounding bentonite, and the microbial populations in the bentonite and surrounding porewater. For carbon steel specimens, a complex corrosion product was identified, consisting predominantly of magnetite. Much less alteration on either the metal or the bentonite was observed in the case of copper samples. Low average anaerobic corrosion rates were measured for carbon steel and a very modest amount of alteration was identified on copper. The density and the initial form of the bentonite had a small influence on the rate of corrosion, across all materials. This paper summarises the results of the experimental programme obtained to date and discusses the relationship observed between exposure time and the evolution of the metal–bentonite interface for both carbon steel and copper.