Permeability evolution and water transfer in the excavation damaged zone of a ventilated gallery

peer reviewedThe fluid transfers occurring around underground galleries are of paramount importance when envisaging the long-term sustainability of underground structures for nuclear waste disposal. These transfers are mainly conditioned by the behaviour of the surrounding material and by its interaction with the gallery air. The hydro-mechanical behaviour of the excavation damaged zone, which develops around galleries due to the drilling process, is thenceforward critical because it is composed of fractures having a significant irreversible impact on flow characteristics and transfer kinetics. Besides, the material interaction with the gallery air may engender water drainage and desaturation. Thus, a gallery air ventilation experiment, preceded by its excavation, is numerically modelled in an unsaturated argillaceous rock to study its influence on hydraulic transfers. The fractures are numerically represented with shear strain localisation bands by means of a microstructure enriched model including a regularisation method. The impact of fracturing on the transport properties is addressed by associating the intrinsic permeability increase with mechanical deformation which is amplified in the strain localisation discontinuities. Such dependence permits us to reproduce a significant permeability increase of several orders of magnitude in the excavation damaged zone, in agreement with available experimental measurements. After the excavation, the hydraulic transfers are studied through the reproduction of a gallery air ventilation experiment that implies drainage and desaturation of the surrounding rock. These transfers depend on liquid water and water vapour exchanges at gallery wall that are introduced through a non-classical boundary condition. The model prediction successfully captures the drainage and desaturation kinetics of undisturbed and damaged rock

Drainage hyperlent

L’étude présentée ici est développée en liaison avec le problème du stockage profond des déchets nucléaires. En raison notamment de mécanismes de corrosion, on s’attend à la formation d’une quantité importante d’hydrogène au sein du stockage. La roche-hôte, étant resaturée par l’eau quand intervient cette production d’hydrogène, l’hydrogène est susceptible de pénétrer dans l’espace des pores en phase gaz en déplaçant l’eau présente dans les pores. Ce déplacement gaz –liquide se distingue des déplacements usuels en raison de son caractère très lent (qui est lié au taux de production très faible du gaz). On s'attend ainsi à une très faible désaturation de la roche hôte. Ceci rend le calcul de ce déplacement à partir du modèle diphasique classique basé sur les équations de Darcy généralisées difficile du fait du comportement particulier (voisinage d'un seuil de percolation) des paramètres phénoménologiques (courbe de rétention et perméabilité relative au gaz) dans la gamme des très grandes saturations. Par ailleurs, le fluide injecté étant beaucoup moins visqueux que le fluide déplacé (de l’eau), la question de la stabilité de l’écoulement se pose, l’apparition d’éventuelles digitations visqueuses pouvant remettre en cause l’utilisation du modèle biphasique classique. Cette question est étudiée dans le cadre de la théorie du drainage, cf. [1], qui indique que le déplacement est de type IPSG (percolation d’invasion dans un gradient stabilisant) et donc compatible avec le modèle diphasique classique. L'étude peut alors se focaliser sur l'étude de la courbe de rétention et des perméabilités relative

The COUPLEX Test Cases: Nuclear Waste Disposal Simulation

International audienceThe models appearing in the COUPLEX benchmark are a set of simplified albeit realistic test cases aimed at simulating the transport of radionuclides around a nuclear waste repository. Three different models were used: The first test case is related to simulations based on a simplified 2D far-field model close to those used for safety assessments in nuclear waste management. It leads to a classical convec-tion diffusion type problem, but with highly variable parameters in space, highly concentrated sources in space and time, very different time scales and accurate results expected even after millions of years. The second test case is a simplification of a typical 3D near-field computation, taking into account the glass dissolution of vitrified waste, and the congruent release of several radionu-clides (including daughter products), with their migration through the geological barrier. The aim of the third test case is to use the results of the near-field computation (COUPLEX 2) to drive the behavior of the nuclide source term in the Far Field computation (COUPLEX 1). The modeling of this last case was purposely left rather open, unlike the previous two, leaving the choice to participants of the way the coupling should be made

Gas transport properties through intact and fractured Callovo-Oxfordian mudstones

A series of controlled water and gas experiments was undertaken on samples of Callovo-Oxfordian (COx) mudstone using a synthetic fluid and helium gas. Data from this study demonstrate that the advective movement of gas through COx is accompanied by dilation of the original fabric (i.e. the formation of pressure-induced microfissures) at gas pressures significantly below that of the minimum principal stress. Flow occurs through a local network of unstable pathways, the properties of which vary temporally and spatially within the mudstone. The coupling of variables results in the development of significant time-dependent effects affecting many aspects of COx behaviour, from the gas breakthrough time to the control of deformation processes. Variations in gas entry, breakthrough and steady-state pressures may result from the arbitrary nature of the flow pathways and/or microstructural heterogeneity. Under these conditions, the data suggest that gas flow is along pressure-induced preferential pathways, where permeability is a dependent variable related to the number, width and aperture distributions of these features. This has important implications for modelling gas migration through low permeability, clay-rich materials

Effect of a partial saturation on the hydromechanical response of a clay rock

The behaviour of claystones and shales ispresently investigated in details in link with their potential use as host rock for radioactive waste disposal at great depth. Claystones are very sensitive to changes in water content, with an increase of their mechanical properties with desaturation under increased suction. Claystones have standard water retention properties but some effect of hydric damage is observed after a drying-wetting cycle that results in enhanced swelling. The microstructure of the Callovo-Oxfordian claystone tested here is characterized by a clay matrix (50% clay fraction) made up of clay platelets that can be assimilated to bricks. The pore size distribution is well organized around a mean diameter of 32nm that provides an idea of the mean thickness of the platelets. A stress release at constant water content results in a swelling and desaturation due to an internal transfer from free inter-platelets water to linked intra-platelets water

An extension of Osher's Riemann solver for chemical and vibrational non equilibrium gas flows

In this paper, we define an extension of Osher's Riemann solver adapted to chemically and vibrationally nonequilibrium flows. Some of its properties are discussed. We give 1D and 2D applications to illustrate the robustness and capability of this new solver. We show on numerical examples that the main properties of the Osher's solver are preserved ; in particular no entropy fix is needed even for hypersonics applications

drainage hyperlent

Dans ce travail, le processus de drainage hyper lent est étudié d'une part à travers une approche théorique issue de la théorie de la percolation, et d'autre part par l'intermédiaire de simulations sur réseau de pores. En utilisant des réseaux modèles, une étude sera présentée montrant comment les courbes de rétention capillaire et de perméabilité relative au gaz doivent être spécifiées dans la gamme des très fortes saturations en eau de façon à obtenir une prédiction optimale à l'aide du modèle diphasique classique

Friction influence on constant volume saturation of bentonite mixed pellet-block samples, a numerical analysis

peer reviewe

Pore size distribution evolution in pellets based bentonite hydration: Comparison between experimental and numerical results

peer reviewedSeveral nuclear waste disposal concept designs take advantage of bentonite based materials to seal underground galleries and shafts. Safety assessment and long-term predictions of the material behaviour have been the main objective of a number of experimental campaigns and of constitutive models development. All these studies have underlined that the multi-porosity bentonite structure affects undeniably the strongly coupled hydro-mechanical processes taking place during water saturation. Due to this, in recent years, many classic experimental tests on unsaturated soils have been performed in conjunction with multi-scale observation techniques (for instance MIP, i.e. mercury intrusion porosimetry analises). Despite the well-known limitations of such observation methods, they provide interesting quantitative measurements in terms of pore diameters families, which differ by several orders of magnitude, and their distribution with respect to different assemblies' types (namely pellets mixtures and compacted bentonite blocks). On the other hand, very few studies have been focusing on the role of such pore size distributions with respect to the hydro-mechanical response, both from an experimental and a numerical point of view. The aim of this paper is to present the experimental campaign and the numerical modelling strategy adopted to analyse the role of different pore size distributions characterising MX-80 bentonite in different forms (i.e. 32 mm pellets mixture, 7 mm pellets mixture and compacted sample surrounded by gap) with same overall dry density during isochoric hydration tests. Taking advantage of multisensor-equipped cells and post-mortem analyses and of the finite element code LAGAMINE, the hydro-mechanical response of these bentonite assemblies is examined. Experimental and numerical outcomes result in good agreement and provide complementary information regarding the features of each assembly type

The role of the stress-path and importance of stress history on the flow of water along fractures and faults; an experimental study conducted on kaolinite gouge and Callovo-Oxfordian mudstone

The flow of water along discontinuities, such as fractures or faults, is of paramount importance in understanding the hydromechanical response of an underground geological disposal facility for radioactive waste. This paper reports four experiments conducted on kaolinite gouge on a 30° slip-plane and on realistic fractures created in Callovo-Oxfordian mudstone (COx) from France. Test histories were conducted that initially loaded the gouge material in step changes in vertical stress, followed by unloading of the sample in similar steps. This loading-unloading history showed considerable hysteresis in hydraulic flow, with only partial recovery of fracture transmissivity. This demonstrates the importance of stress history on fracture flow; consideration of just the current stress acting upon a fracture or fault may result in inaccuracies of predicted hydraulic flow. The stress dependency of fracture flow in both kaolinite and COx can be described by a power-law or cubic relationship, which is likely to be dependent on the fracture roughness, thickness of gouge material, saturation state, permeability of the host material, and clay mineralogy (i.e. swelling potential). The observed response of fracture transmissivity to normal stress in COx is a complex superposition of mechanical response of the fracture and the swelling of clay in the fracture surface. The stress-dependency of flow was also seen to be dependent on orientation with respect to bedding. A fracture perpendicular with bedding accommodates greater compression and results in a lower transmissivity. The orientation dependence is related to the anisotropic swelling characteristics of CO