Observation des micromécanismes de déformation et d'endommagement des argilites sous sollicitation hydrique

La déformation et l'endommagement des argilites sous sollicitation hydrique sont observés au MEB environnemental. La déformation est mesurée par corrélation d'images à l'échelle micrométrique des zones argileuses entre les grains carbonatés et de quartz et révèle un couplage hydromécanique complexe. On montre l'apparition d'une déformation irréversible lors d'un cycle hydrique, ainsi qu'un endommagement sous la forme d'un réseau de microfissures localisées dans la phase argileuse ou aux interfaces grain-matrice, dont la morphologie dépend de la vitesse et du sens du chargement hydrique

Caractérisation par la mesure de perméabilité au gaz de l'endommagement mécanique et hydrique dans l'EDZ des argilites du Callovo-Oxfordien

On the feasibility evaluation of nuclear waste storage in deep formations, the essential issues are as follows: the stability of underground structures over the reversible period, the influence of cavity excavation on geomechanical properties of the wall rock and the variation of those properties during the different phases while storage realization. The work presented here covers the investigations on the variation of geomechanical proprieties of the approximately 500 m deep MHM in France (mudstone in the departments of Meuse/Haute-Marne), chosen as a potential medium for nuclear waste disposal by ANDRA. In order to measure the very low permeability of mudstone and to observe the dependency on saturation, a special test scheme on measurement of gas permeability has been developed. In the scheme, in situ referenced stresses have been chosen as the stresses acting on the solid matrix. The gas permeability has been determined with both analytical and numerical methods. To estimate the mechanical damage of storage induced by the excavation, laboratory tests on gas permeability have been conducted on samples recovered from different locations situated at different distances from the wall of the main access shaft of the MHM (from 0,1 m to 12,5 m). Results of gas permeability obtained under an isotropic stress of 11 MPa vary between 10-21 and 10-22 m² and do not show significant variations between damaged zones (near the wall) and intact zones (sample located 12 m from the wall). The observations in laboratory tests coincide with in situ damage characterizations. The variation of gas permeability under the cycle of loading and unloading is an order less than the initial value under the isotropic stress. Taking into account the precision of the testing system, this variation is not significant. The oviparous intact samples have been imposed different saturations by salt solutions (with a relative humidity from 25 % to 98 %) to form a cycle of de- and re-saturation. The water retention curve, the kinetic dehydration, the dehydration shrink and wetting crack in samples were measured and analyzed. The coefficient of hydraulic diffusion determined from the evolution of mass is about 5.10-10 m².s-1. The gas permeability (k) under the isotropic stress of 5 MPa increases from 10-21 to 10-18 m² when the imposed relative humidity decreases from 98 % to 25 %. A quasi-linear relation between log(k) and Saturation has been observed and mathematically formulated. The gas permeability as a function of deviator stress has also been studied and the results confirm that the effect of the deviator is not evident, even when the deviator excesses the damage threshold. These results coincide with Zhang's observations on the same rock of MHM. The deformation and the acoustics velocity of samples have been traced during the tests and the microstructure of mudstone has been studied with the method of mercury intrusion. The factors influencing the measure of gas permeability, such as the Klinkenberg effect, the variation of saturation during the tests, the dissolution of gas in water and the water transformation due to the gas pressure, especially the leak tightness of the testing system, have been discussed in the study.Dans l'étude de faisabilité d'un stockage géologique profond des déchets radioactifs, les questions essentielles posées aux ingénieurs et chercheurs en géomécanique sont la stabilité des ouvrages pendant la période de réversibilité et la variation des propriétés de confinement de la roche hôte induite par le creusement des cavités et par le stockage lui-même. Le présent travail porte sur l'étude des propriétés de confinement vis-à-vis des gaz des argilites du Callovo-Oxfordien du site de Meuse/Haute-Marne, étudié par l'ANDRA depuis 1995, au voisinage d'un puits d'accès. Un protocole de mesure de la perméabilité au gaz a été élaboré en prenant en compte la très faible perméabilité des argilites et les effets de désaturation. Les trajets de chargement mécanique et de pression de gaz ont été déterminés par rapport aux chargements in situ. Deux méthodes d'interprétation, une fondée sur une solution analytique simplifiée et l'autre sur une solution numérique, ont été utilisées pour déterminer la valeur de perméabilité au gaz. Des mesures de la perméabilité au gaz sur des éprouvettes prélevées à différentes distances, de 0,1 à 12,5 m, de la paroi du puits d'accès au Laboratoire Souterrain de Meuse/Haute- Marne ont été effectuées pour caractériser l'endommagement mécanique induit suite au fonçage du puits. Les résultats obtenus sous une contrainte isotrope de 11 MPa sont compris entre 10-21 et 10-22 m² et ils ne présentent pas de variations significatives entre la zone potentiellement endommagée (éprouvette proximité à la paroi) et la zone présumée intacte (éprouvette à 12,5 m de la paroi). Ce résultat est cohérent avec les caractérisations de l'endommagement menées in situ. Sous chargement déviatorique, la variation des mesures de perméabilité est inférieure d'un ordre de grandeur par rapport à celle mesurée sous contrainte isotrope ; elle est donc peu significative compte tenu de la précision du dispositif de mesure. Des éprouvettes jumelles d'argilites supposés intactes ont subi différents paliers hydriques à l'aide des solutions salines sursaturées (HR variant de 25 % à 98 %) pour constituer un cycle complet désaturation - resaturation. Pour chacune des valeurs du degré de saturation, des mesures de perméabilité ont été effectuées. La cinétique de désaturation, la courbe de rétention, les déformations hydriques ainsi que la fissuration hydrique des éprouvettes ont été mesurées et analysées. La valeur du coefficient de diffusivité hydrique déterminée d'après l'évolution de la masse des éprouvettes est estimée à 5.10-10 m².s-1. La perméabilité au gaz mesurée des éprouvettes, sous un confinement de 5 MPa, augmente de 10-21 m² à 10-18 m² lorsque l'humidité relative à laquelle les éprouvettes ont été conditionnées varie de 98 % à 25 %. La perméabilité au gaz (k) et le degré de saturation (Sr) de la roche présentent une relation quasi-linéaire dans un repère log(k) - Sr. L'évolution de la perméabilité au gaz en fonction d'un déviateur de contraintes a aussi été étudiée pour différents degrés de saturation. Les résultats confirment que l'effet du déviateur de contraintes sur la perméabilité n'est pas significatif, même si le déviateur de contraintes dépasse le seuil d'endommagement et s'il est proche du seuil de rupture. Ces résultats ont été comparés à ceux obtenus par d'autres laboratoires utilisant des techniques différentes (Zhang 2007). La déformabilité et la vitesse des ondes des éprouvettes d'argilites ont été suivies pendant les essais. La microstructure des argilites a été étudiée à l'aide de la porosimétrie au mercure. Les facteurs influençant la mesure de perméabilité tels que : l'effet Klinkenberg, la variation du degré de saturation pendant l'essai, la dissolution du gaz et le déplacement d'eau par le gaz, et surtout l'étanchéité du système, ont été abordés dans cette étude

Experimental study on gas permeability of mudstones

http://www.sciencedirect.com/science/article/B6X1W-4TNWGYD-4/2/41a53368ec5fa6a2743fd2ff6c9f37efInternational audienceThe paper presents the results of gas permeability measurements of mudstones from MHM (Meuse/Haute-Marne) site by the help of measurement facilities in GIG laboratory. The transient pulse technique was used, which can measure very low permeabilities (up to 10−22 m2). Cylindrical 40 mm diameter and about 40 mm length samples come from different distances with respect to the wall of the main access shaft of the MHM URL. The relationship between the permeability and the state of stress is studied and the problem of gas transfer in the saturated sample is analyzed. The relationship between the rupture of sample and the permeability will be also discussed. Experimental (and numerical) results show that the Callovo-Oxfordian stratum of MHM has very low gas permeabilities (not, vert, similar2 × 10−21 m2). The disturbance linked to the excavation of shafts is not significant. Results show that the interstice gradient of gas pressure can induce pore water displacement. Furthermore, it was found that the permeability does not change very much even if a rupture occurs in tests with a confining pressure (11 MPa) equivalent to the lithological pressure at the depth of about 467

Caractérisation par la mesure de perméabilité au gaz de l endommagement mécanique et hydrique dans l EDZ des argilites du Callovo-Oxfordien

PARIS-MINES ParisTech (751062310) / SudocSudocFranceF

Experimental study on gas permeability of the Callovo-Oxfordian argillite of MHM Clays in natural and engineered barriers for radioactive waste confinement

Poster 3rd Int Meeting CLAYS IN NATURAL & ENGINEERED BARRIERS FOR RADIOACTIVE WASTE CONFINEMENT 17-18 Sept 2007 http://www.andra.fr/lille2007/abstract_lille2007/donnees/posters/fs_poster24_art29.ht

Experimental Investigation on Drying and Wetting of GMZ Bentonite Using X-Ray Tomography and Volumetric Digital Image Correlation Technique

To investigate the shrinking and swelling properties of Gao-Miao-Zi (GMZ) bentonite, which has been considered as engineering barriers for high-level radioactive nuclear waste disposal in China, drying and wetting tests were carried out. The microstructure of the material at each hydric state was recorded using X-ray tomography (X-CT). The 3D images of the material were analyzed using digital volume correlation (DVC) technique, and the full-field strains at mesoscale (i.e., in the order of clay aggregate) during drying and wetting were quantified. The results show that the GMZ bentonite exhibits notable swelling characteristics during wetting and the swelling strain of the material is up to 8% at mesoscale. The full-field strain in 3D of the material is heterogeneous, anisotropic, and irreversible during drying and wetting. The corresponding deformation mechanisms are discussed. Two different swelling mechanisms were also identified: traction occurs in the interface between the matrix and inclusions during swelling; pre-existing fissure closes during swelling

Characterization of the hydromechanical behavior of argillaceous rocks with effective gas permeability under deviatoric stress

International audienceThe realization of nuclear waste storage in the deep geological formation will change the hydromechanical properties of the host rocks around the man-made openings due to the stress redistribution during the excavation stage and the variation of the hydraulic conditions during the open drift stage and the closure stage. This paper mainly presents an experimental study on the evolution of effective gas permeability of Callovo–Oxfordient argillaceous rocks during dehydration and rehydration processes and its variation under loading and unloading conditions. The experimental results show that effective gas permeability increases with the diminution of degree of saturation, and the logarithm of effective gas permeability is in quasi linear relation to saturation for the degree of saturation less than 90%. But effective gas permeability is not sensitive to the deviatoric stress, even if the deviator exceeds the damage threshold. The method of mercury intrusion has also been used in studying the microstructure of argillaceous rocks

Thermo-mechanical effect on the anisotropic permeability of a mudstone

Tamusu mudstone is a candidate geological medium for high-level radioactive waste disposal in China. This paper investigates its anisotropic hydraulic properties. Permeability measurements were conducted on two kinds of samples, i.e., along the directions perpendicular and parallel to the beddings, under different stress conditions with heating-cooling cycles (20 → 40→60 → 80→60 → 40→20 °C). The results confirmed the hydraulic anisotropy of Tamusu mudstone. Increasing lithostatic pressure decreases the intrinsic permeability k and hydraulic conductivity K and decreases the hydraulic anisotropy of Tamusu mudstone. The temperature significantly affects the hydraulic properties: the k values exhibit a negative correlation with temperature. A notable variation in parallel hydraulic properties upon temperature elevation indicates that the thermal effect is more crucial in the direction parallel at low lithostatic pressure. A notable effect of thermal paths on the hydraulic properties of Tamusu mudstone was exhibited through the different K or k at the same temperatures before and after heating-cooling cycles

Revisiting the methods for gas permeability measurement in tight porous medium

Permeability is a key parameter to describe fluid transport properties of porous medium; however, the permeability measurement is extremely difficult for tight porous medium, e.g. fine-grained rock or dense soil. In this paper, three methods for gas permeability measurement, i.e. steady state method, pulse decay method (PDM) and pressure oscillation method (POM), are first reviewed and then their advantages and drawbacks are discussed. Both analytical and numerical solutions of gas permeability are presented for the tight porous medium. The results show that the analytical method is relatively simple but only valid under certain conditions, whilst the numerical method is more robust and generic, which can take into account several factors such as porosity, saturation, gas leakage, and unconventional boundary conditions. The influence of the effective porosity on the permeability determination is further analyzed using the proposed numerical method. In this study, new pressure data interpretation procedures for PDM and POM are proposed, and the obtained results can serve as a guidance to define a proper method for permeability measurement of the tight porous medium. Keywords: Permeability measurement, Steady state method, Pulse decay method (PDM), Pressure oscillation method (POM), Effective porosity, Tight porous mediu