Callovo-Oxfordian Research Articles

Experimental study of poromechanical behavior of Callovo-Oxfordian claystone in undrained triaxial compression and extension tests

The Callovo-Oxfordian (COx) claystone is selected as the host rock in the French project for geological disposal of radioactive waste. In its initial state, the host rock is fully saturated. Due to the low permeability, the pore fluid pressure can locally evolve in a quasi undrained condition due to subsequent mechanical and thermal loading, and then significantly affects deformation and cracking process of geological barrier. Most previous studies were carried out in quasi drained conditions or with constant pore pressure. Poromechanical behavior of COx claystone in undrained condition has been so far rarely investigated. In the present work, we carry out a new series of laboratory tests. Two representative loading paths are considered: triaxial compression with constant confining pressure and triaxial extension with constant mean stress. The variations of strain and pore pressure are measured during the tests. It is found that the poromechanical behavior of the claystone is clearly affected by loading path. The failure strength is higher in triaxial compression than in extension. The evolution of pore fluid pressure is correlated with volumetric strain. Compared with the results obtained in drained tests, it seems that the Terzaghi effective stress can be used for the description of pore fluid pressure effect on failure strength.

Combination of a Failure Index and a Dilatancy-Dependent Permeability Model in Hydro-Mechanically-Coupled Numerical Simulations of Argillaceous Rock Formations of the Callovo-Oxfordian (COx)

Hydraulic-mechanical coupling in claystone is of great importance for repository research, as claystone is considered a possible host rock and hydraulic-mechanical coupled processes can affect the integrity of the barrier. Of particular importance in this context are excavation damage zones and hydraulically-induced microcracks and fractures. In this work, two constitutive equations developed for these hydraulic-mechanical coupled processes are combined in a novel way. The approach exploits the different dominant stress states that prevail in the respective processes. This approach is applied in several numerical simulations for validation and compared with experimental data. The measured data were obtained during gas-injection tests in the scope of the PGZ experiment in the underground laboratory of Bure, France, where the hydraulic-mechanical coupling of the is the Nabla operator claystone was investigated. Five numerical models were calculated, each with a different model configuration, to simulate the excavation damage zone and dilatancy-controlled flow during gas-injection tests. The numerical results show that the developed approach can first satisfactorily predict the extent and behaviour of the excavation damage zone under different stress conditions and the subsequent dilatancy-dependent permeability due to the increased gas pressure.

13C isotopic labeling to decipher the iron corrosion mechanisms in a carbonated anoxic environment

AbstractA two‐step corrosion experiment was performed on a ferritic steel (Armco) in a synthetic solution representing the Callovo–Oxfordian at 120°C. After the development of a carbonated corrosion product layer (CPL) during the first 15 days of the experimental step, corrosion front progression was investigated using 13C marked carbonate species during the second 15 days experimental step. CPL was characterized at each step, in terms of morphology (scanning electron microscopy), composition (energy‐dispersive spectroscopy), and structure (µ‐Raman). 13C corrosion product locations were analyzed by time‐of‐flight secondary ion mass spectrometry. Results evidenced that after a step of generalized corrosion, iron corrosion continues locally at the metal/CPL interface. These results suggest that although a protective siderite layer formed on the iron surface after 15 days, a local dissolution of the carbonate layer at the M/CPL interface occurred. A galvanic effect is developed between the bared surface (anode) and the covered one (cathode). This activates iron oxidation. The precipitation of carbonate corrosion products to the metal/CPL interface is possible by the diffusion of 13CO32− ions from the bulk through the siderite layer.

Determination of the in situ hydraulic parameters of the Callovo–Oxfordian clay rock in a long-term hydraulic interference testing at Andra's Underground Research Laboratory (CMHM)

Abstract. Between 2009 and 2015, Andra has conducted a unique long-term hydraulic interference test in two parallel boreholes 1 m apart from each other and air core drilled in the undisturbed Oxfordian Clay rock at Andra's Underground Research Laboratory (CMHM) in Bure, France. The experimental set-up consisted of six test intervals positioned along the parallel boreholes drilled from a drift located within the Callovo–Oxfordian clay at around 500 m b.g.l. (below ground level). The 20 m boreholes were oriented around 30∘ downwards. The pressure sensors linked to six intervals of 1 m length each allowed for continuous pressure measurement over 6 years and a slanted borehole for the observation of rock displacements (see Fig. 1). Additionally, flow rates were measured during water injection tests that lasted weeks or months. This extensive set of hydraulic data allowed, for the first time, a precise determination of the in situ Callovo–Oxfordian clay rock permeability and anisotropy and its specific storage at the metre to decametre scale. It was achieved based on an automated calibration, using a 3D flow model and explicitly considering all the relevant hydraulic elements acting on the flow system (such as rock mass, borehole disturbed zone and sealing elements) and the drift acting as a steady sink due to the underground ventilation (see Fig. 2). The history matching was performed on the transient pressure measurements. The obtained numerical calibration results were in close agreement with the hydraulic parameter values of the Callovo–Oxfordian clay determined at a smaller scale (single borehole tests and rock core samples in the laboratory), therefore demonstrating the reliability of the very low permeability of the Callovo–Oxfordian clay at the metre to decametre scale. Furthermore, the modelling illustrated the impact of the measurement set-up and its details on the observed hydraulic behaviour. The analysis methodology and the main findings will be presented.

Modeling the convergence of deep drifts in fractured rock based on a continuous equivalent model.: Application to drifts excavated in the Callovo-Oxfordian claystone.

The French National Radioactive Waste Management Agency (ANDRA) is conducting a research program including in-situ experiments at the Meuse/Haute-Marne Underground Research Laboratory (MHM URL), which aims to demonstrate the feasibility of constructing and operating a deep geological radioactive waste disposal in the Callovo-Oxfordian (COx) claystone formation. A drift network of about 2km has been excavated essentially following minor and major horizontal stress directions in the MHM URL since 2000. Continuous monitoring shows that the convergence of the drifts has not stabilized yet, but the convergence rate decreases with time. Moreover, the convergence response strongly depends on the drift orientation. Understanding the time-dependent behavior of the host rock is the key issue for optimizing the design of the disposal. This paper aims to present a constitutive model including threefold anisotropies (elasticity, plasticity and viscosity) to describe the behavior of COx claystone, with a focus on the time-dependent behavior. Transverse isotropic elasticity is considered. Anisotropic Mohr–Coulomb criterion is used to represent the plastic behavior of the host rock. Anisotropic Lemaitre creep law is proposed to model the time-dependent behavior of the COx claystone. Anisotropy is elaborated by introducing an equivalent stress tensor. Model parameters are calibrated based on laboratory tests on sample scale and then adjusted against the field observation in MHM URL. The proposed model is finally applied for modeling the anisotropic convergence observed in the drifts GED and GCS which are respectively excavated in the direction of the minor principal horizontal stress and in the direction of the major principal horizontal stress. The model proved to be able to reproduce the observed convergences for the two directions of the drifts.

Modelling the viscoplastic behaviour of Callovo-Oxfordian claystone with consideration of damage effect

In order to evaluate the performance of deep geological disposal of radioactive waste, an underground research laboratory (URL) was constructed by Andra in the Callovo-Oxfordian (COx) claystone formation at the Meuse/Haute-Marne (MHM). The construction of URL induced the excavation damage of host formations, and the ventilation in the galleries desaturated the host formation close to the gallery wall. Moreover, it is expected that the mechanical behaviour of COx claystone is time-dependent. This study presents a constitutive model developed to describe the viscoplastic behaviour of unsaturated and damaged COx claystone. In this model, the unsaturation effect is considered by adopting the Bishop effective stress and the van Genuchten (VG) water retention model. In terms of the viscoplastic behaviour, the nonstationary flow surface (NSFS) theory for unsaturated soils is used with consideration of the coupled effects of strain rate and suction on the yield stress. A progressive hardening law is adopted. Meanwhile, a non-associated flow rule is used, which is similar to that in Barcelona basic model (BBM). In addition, to describe the damage effect induced by suction change and viscoplastic loading, a damage function is defined based on the crack volume proportion. This damage function contains two variables: unsaturated effective stress and viscoplastic volumetric strain, with the related parameters determined based on the mercury intrusion porosimetry (MIP) tests. For the model validation, different tests on COx claystone under different loading paths are simulated. Comparisons between experimental and simulated results indicated that the present model is able to well describe the viscoplastic behaviour of damaged COx claystone, including swelling/shrinkage, triaxial extension and compression, and triaxial creep.

Potential influence of microorganisms on the corrosion of carbon steel in the French high‐level long‐lived radioactive waste disposal context at 80°C

AbstractIn this study, experiments were carried out to assess the microbially influenced corrosion (MIC) risk in the context of the French high‐level radioactive waste disposal. The exposures were carried out at 80°C in different repository relevant conditions, including the presence of different cement‐grout mixtures as filling material. Biotic conditions with nutrient and nonsterile conditions with indigenous microbes added from Callovo Oxfordian clayey rock and without nutrients were considered. For biotic conditions, specific preparations of microbial inoculum were carried out from samples collected at Andra's Underground Research Laboratory and microorganisms from microbial culture collection centers. Corrosion kinetics were determined using traditional coupons and completed with real‐time corrosion sensors. Microbiological characterizations consisted of cultural approach, quantitative polymerase chain reaction, and next‐generation sequencing. The obtained results show no significant MIC and a reduced risk with the use of more alkaline filling material.

Experimental investigation on the effects of confining pressure and pore pressure on gas/water transport properties of COx argillite

As a preferred host rock for the disposal of high-level long-lived radioactive waste in France, Callovo-Oxfordian (COx) argillite inevitably undergoes gas and water intrusion during long-term sealing. In this study, a series of gas and water permeability tests have been performed on dry, partially saturated, and fully saturated COx argillite to investigate the effects of confining pressure and pore pressure on transport properties of this material. It is first found that gas permeability is very sensitive to the sample damage state for dry samples. Due to the presence of microcracks, the relationship between porosity and gas permeability is not evident, while the Klinkenberg effect is clearly observed in intact samples. Moreover, gas permeability of partially saturated samples is very sensitive to water saturation, especially for highly saturated samples. This indicates that the material has a good gas sealing capacity after the host rock is resaturated during the long-term sealing process. For both low- and highly saturated samples, confining pressure and gas pressure have very different effects on gas permeability of partially saturated argillite. Water permeability of argillite varies between 10−20–10−21 m2 (Kw), suggesting a good water-sealing capacity. The evolution of Kw is more sensitive to the change of pore pressure than to the change of confining pressure, indicating that the effective stress coefficient for water permeability is greater than 1. The intrinsic permeability of this low-permeable clayey rock is discussed at the end of this work.

Modelling the creep behaviour and induced failure of clay rock from microscale viscosity to large-scale time-dependant gallery convergences using a multiscale numerical approach

The large-scale creep behaviour of Callovo-Oxfordian (COx) clay rock is modelled from small-scale viscous mechanisms of the rock using a multiscale numerical approach in the context of radioactive waste repositories. At the mesoscale, the saturated non-homogeneous rock is represented in digital 2D Representative Elementary Areas (REAs) as a cracked composite material consisting of rigid elastic mineral inclusions (quartz, calcite, and pyrite) embedded in a clay matrix. The modelling of these materials is considered within double-scale finite element framework, in which the homogenised responses of the REA to the enforced global kinematics serve as a numerical constitutive relation at the macroscale. To reproduce the rock creeping, two viscous mechanisms have been introduced to consider the creep of the clay matrix: either the viscoplasticity of the clay aggregates or the viscoelasticity of their contacts, or both. Firstly, laboratory biaxial creep tests of clay rock samples are simulated. A three-stage creep stage is reproduced and the creep failure process is discussed. Then, large-scale underground engineering structures are modelled to reproduce its time-dependent behaviour. It is found that the developed multiscale model is able to provide some valuable insights into the large-scale creep behaviour of clay rocks through the morphological and material small-scale characterization of REA.

Induced polarization of clay-rich materials — Part 1: The effect of desiccation

A petrophysical model describing spectral induced polarization (IP) has been developed for clay rocks accounting for the Maxwell-Wagner polarization. It is also used to connect the complex conductivity to the relative permeability of the material. This model is applied to the Callovo-Oxfordian clay rock of the Paris Basin (France) where the Meuse/Haute-Marne Underground Research Laboratory is located. Laboratory experiments are performed using eight clay-rock cores to study the effect of desiccation on their spectral-IP response. The measurements are performed along the foliation plane. Complex conductivity spectra are measured over a frequency range of 1 mHz to 45 kHz. These spectra are fitted with a double Cole-Cole model to extract the evolution of the Cole-Cole parameters with the saturation during the desiccation process. The low-frequency Cole-Cole model corresponds to IP phenomena, whereas the high-frequency Cole-Cole model corresponds to the Maxwell-Wagner contribution. We obtain the value of the first and second Archie’s exponents and we check the relationship between the surface conductivity and the cation exchange capacity of the clay rocks. We are also able to connect the relative permeability curve to the second (saturation) Archie’s exponent. The monitoring of the complex conductivity can be used to predict how the permeability of the clay-rock formation changes with the water content.

Studies on resaturation processes and structural characteristics of the Callovo-Oxfordian claystone using X-ray microtomography and digital volume correlation

Claystone formations in their natural state are favorable for geological disposal of radioactive waste. Nevertheless, disturbances and water loss generated around the excavated areas might change the favorable properties of formations. Similarly, water loss and resaturation during sample preparation can also cause variations in the mechanical properties. To investigate the influences induced by these actions, resaturation processes in a claystone were studied using X-ray microtomography and Digital Volume Correlation (DVC). Structural characteristics of Callovo-Oxfordian (COx) claystone inside the sample during resaturation processes are visualized in the form of images. Strain fields demonstrate the non-uniform deformation behavior of the sample during the resaturation processes, as well as details of local deformations. The results indicate that the effects of resaturation processes on the claystone cannot be ignored for the laboratory testing at the sample scale.

Investigation of strain fields and anisotropy in triaxial tests on Callovo-Oxfordian claystone by X-ray micro-tomography and digital volume correlation

The objective of the present study is to investigate deformation behavior of anisotropic claystone in relation with material heterogeneity. For this purpose, a set of five triaxial compression tests are performed on samples of Callovo-Oxfordian (COx) claystone under in-situ X-ray micro-tomography monitoring. Complete 3D images of the samples full loading history were recorded and analyzed by digital volume correlation method. Full non-uniform strain fields were determined and averaged global strains were calculated. As novelty, we have particularly investigated the effects of bedding planes, initial cracks and confining stress on strain fields. The results indicate that the structural anisotropy of COx claystone has a strong influence on the local strain distribution and cracking process. The confining pressure has also an important impact on the strain fields and failure modes of samples in the triaxial tests. The cracking process of samples is strongly correlated with radial strain distribution.

Self-sealing of fractures in indurated claystones measured by water and gas flow

Self-sealing of fractures in the indurated Callovo-Oxfordian (COX) and Opalinus (OPA) claystones, which are considered as host rocks for disposal of radioactive waste, was investigated on artificially fractured samples. The samples were extracted from four lithological facies relatively rich in clay mineral, carbonate and quartz, respectively. The self-sealing of fractures was measured by fracture closure, water permeability variation, gas penetration, and recovery of gas-induced pathways. Most of the fractured samples exhibited a dramatic reduction in water permeability to low levels that is close to that of intact rock, depending on their mineralogical composition, fracture intensity, confining stress, and load duration. The self-sealing capacity of the clay-rich samples is higher than that of the carbonate-rich and sandy ones. Significant effects of sample size and fracture intensity were identified. The sealed fractures become gas-tight for certain injection pressures. However, the measured gas breakthrough pressures are still lower than the confining stresses. The gas-induced pathways can recover when contacting water. These important findings imply that fractures in such indurated claystones can effectively recover to hinder water transport but allow gas release under relatively low pressures without compromising the rock integrity.

A new experimental device developed to study the creeping behavior of a rock joint under shear stress

In the context of nuclear waste management project Cigeo, the behavior of the fractured Callovo Oxfordian claystone (COx) is studied for its time dependent effect on the long term convergences of the drifts. We present an experimental setup designed to investigate the viscous behavior of a fracture. While few experimental setup already exist with the purpose of studying the time dependent deformation of a rock joint, all of them measure the displacement in the direction of the sheared discontinuities in one point of the sample with devices such as a linear variable displacement transducers (lvdt). The information provided by this sensor comes to measuring the average displacement of the fracture slip. Depending on the configuration of the experimental device, the lvdt sometimes also measures the deformation of the rock medium which cannot always be considered as a rigid body. Which is why we propose in this paper to set a camera up in order to use Digital Image Correlation to get an information of the slip along the fracture.

Experimental study on the physico-mechanical properties of Tamusu mudstone – A potential host rock for high-level radioactive waste in Inner Mongolia of China

Tamusu mudstone, located in Bayin Gobi Basin in Inner Mongolia of China, has been selected as a potential host rock for high-level radioactive waste (HLW) disposal in China. A series of tests has been carried out, including X-ray diffraction (XRD) tests, scanning electron microscopy (SEM) tests, disintegration tests, permeability tests and triaxial compression tests, to estimate the physico-mechanical properties of Tamusu mudstone in this work. The mineral composition of Tamusu mudstone was analyzed and it was considered as a stable rock due to its low disintegration rate, i.e. approximately 0.11% after several wet/dry cycles. Based on the results of permeability test, it was found that Tamusu mudstone has a low permeability, with the magnitude of about 10 –20 m 2 . The low permeability makes the mudstone well prevent nuclide migration and diffusion, and might be influenced by temperature. The triaxial tests show that Tamusu mudstone is a stiff mudstone with high compressive strength, which means that the excavation disturbed zone would be smaller compared to other types of mudstone due to construction and operation of HLW repositories. Finally, the properties of Tamusu mudstone were compared with those of Opalinus clay, Callovo-Oxfordian (COx) argillite, and Boom clay to further discuss the possibility of using Tamusu mudstone as a potential nuclear waste disposal medium.

3D Simulations of gas injection on callovo-oxfordian claystone assuming spatial heterogeneity and anisotropy

A series of gas injection tests on Callovo-Oxfordian (COx) claystone from the Bure underground research laboratory (URL) in France were carried out at the British Geological Survey (BGS). The tests were performed using a triaxial apparatus specifically designed to capture small volumetric strains induced by the injected gas flow and consequent material dilatancy. The long-duration experiments were monitored throughout. Measurements also included pressure, stresses (axial and radial stresses prescribed for each test stage), rate of gas inflow, gas outflow volume as well as pore-pressures observed at various points of the sample. A coupled hydro-gas-mechanical 3D numerical model has been developed to simulate the tests. Initial permeability is assumed heterogeneous throughout the specimen and embedded fractures are incorporated in the formulation. Gas pressure-induced deformations during the test lead to variations of permeability due to changes in matrix porosity and, especially, fracture aperture as well as fracture orientation due to material anisotropy. A programme of sensitivity analyses involving the variation of different aspects and parameters of the model contributes to a better understanding of the phenomena and highlights its complexity. The model is able to reproduce the observed behaviour of the tests.

Effect of anisotropic creep on the convergence of deep drifts in Callovo-Oxfordian claystone

The French National Radioactive Waste Management Agency (ANDRA) is conducting a research program including in-situ experiments at the Meuse/Haute-Marne Underground Research Laboratory (MHM URL), which aims to demonstrate the feasibility of constructing and operating a deep geological radioactive waste disposal in the Callovo-Oxfordian (COx) claystone formation. A drift network of about 2km has been excavated essentially following minor and major horizontal stress directions in the MHM URL since 2000. Continuous monitoring shows that the convergence of the drifts has not stabilized yet, but the convergence rate decreases with time. Moreover, the convergence response strongly depends on the drift orientation. Understanding the time-dependent behavior of the host rock is the key issue for optimizing the design of the disposal. This paper aims to present a constitutive model including threefold anisotropies (elasticity, plasticity and viscosity) to describe the behavior of COx claystone, with a focus on the time-dependent behavior. Transverse isotropic elasticity is considered. Anisotropic Mohr–Coulomb criterion is used to represent the plastic behavior of the host rock. Anisotropic Lemaitre creep law is proposed to model the time-dependent behavior of the COx claystone. Anisotropy is elaborated by introducing an equivalent stress tensor. Model parameters are calibrated based on laboratory tests on sample scale and then adjusted against the field observation in MHM URL. The proposed model is finally applied for modeling the anisotropic convergence observed in the drifts GED and GCS which are respectively excavated in the direction of the minor principal horizontal stress and in the direction of the major principal horizontal stress. The model proved to be able to reproduce the observed convergences for the two directions of the drifts.

Numerical investigation of the couplings between strain localisation processes and gas migrations in clay materials

Deep geological repository is the preferred solution in many countries to manage radioactive wastes, such as in France where the Callovo-Oxfordian (COx) claystone is the candidate host rock. In such clay rock formation, the drilling of storage gallery creates an Excavation Damaged Zone (EDZ) with altered flow properties in the short term, while corrosion processes release large amounts of gas in the long term. Assessing the evolution of gas pressures in the near-field and predicting the effect of the EDZ on gas transport remains a major issue. This paper presents a second gradient two-phase flow hydro-mechanical (H2M) model tackling the multi-physics couplings related to gas transfers and fractures development. The EDZ is reproduced by shear strain localisation bands using a microstructure enriched model with a second gradient approach. The gas migration is captured by a biphasic fluid transfer model. The impact of fracturing on the flow properties is addressed by relating the permeability and the water retention curve to mechanical strains. Using this tool, numerical modelling of a drift in the COx claystone is performed with the aim of emphasising the influence of the HM couplings on gas migrations at nuclear waste disposal scale.

Time-Dependent Behavior of Callovo-Oxfordian Claystone for Nuclear Waste Disposal: Uncertainty Quantification from In-Situ Convergence Measurements

The sustainability of geotechnical infrastructures is closely linked with their long-time behavior. In fact, there is not a straightforward procedure to predict this behavior, and very often, the back analyses of observed data are the best tool to understand their long-time response. In-situ observations of drifts constructed in the Callovo-Oxfordian (COx) claystone, the potential host formation for geological radioactive waste disposal, in France exhibit a progressive convergence. These convergence measurements with quite significant dispersions reveal a considerable uncertainty of time-dependent behavior of this argillaceous rock that can strongly affect the transmit loading to liners, hence the long term stability of the drift. Consequently, the uncertain quantification of the creep behavior of COx claystone presents an important task before analyzing the safety of the waste disposal system. In this work, this challenge was conducted by using the well-known Bayesian inference technique. For this aim, on the one hand, the effectiveness of the classical and hierarchical Bayesian techniques to quantify the epistemic and aleatoric uncertainties of the time-dependent behavior of the host rock were investigated using synthetic data. On the other hand, we dealt with the uncertain quantification of the Lemaitre parameters that characterize the visco-plastic behavior of COx claystone thanks to the real data of in-situ convergence measurements of drifts.

Effect of spatial variability of creep rock on the stability of a deep double-lined drift

This work aims at investigating the effect of aleatoric uncertainty of creep rock properties on the stability of an underground structure. This uncertainty relates to the spatial variability of the mechanical parameters representing the time-dependent behavior of geological rock formation due to the change in its mineralogy. The chosen methodology consists of representing the aleatoric uncertainty of rock properties by random fields, written as correlation functions with respect to the spatial correlation length. The adaptation of the well-known Expansion Optimal Linear Estimationmethod (EOLE) is performed to account for the cross-correlation of the random fields of the viscoplastic parameters of the host rock. Then, the Kriging-based reliability analysis is undertaken with respect to the discretized random fields, which allows elucidating the effect of spatial variability. As an application, the proposed approach is chosen to study the stability in the long-term of a deep double-lined drift within the geological disposal facilities (Cigeo project) conducted by the French National Radioactive Waste Management Agency (Andra). The drift will be excavated in Callovo-Oxfordian (COx) claystone (if the Cigeo project is licensed), considered as a potential host rock for the deep geological nuclear waste disposal in France. The results show that the chosen Kriging metamodel for the reliability analysis can be appropriate for the case of high correlation length represented by a moderate number of random variables (up to about 50) after the discretization of random fields. Further, the consideration of aleatoric uncertainty exhibits a lower probability of exceedance in comparison with the case where spatial variability is ignored. Still, more investigations need to be conducted in the future to conclude this observation.