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Évolution des propriétés de confinement des roches-couvertures type argilite soumises à des fluides enrichis en CO2 : impact des discontinuités naturelles et artificielles

Guillaume Berthe 1
1 L3MR - Laboratoire de Mesures et Modélisation de la Migration des Radionucléides
SECR - Service d'Etudes du Comportement des Radionucléides : DEN/DPC
Abstract : This research is part of the studies of feasibility of CO2 storage in deep geological strata, focusing more particularly on the evolution of the confinement properties of cap-rocks type argillite subjected to CO2 enriched fluids. The argillite of Tournemire (Aveyron, France) were used as analog rocks, having identified what their weak points could be face to storage, namely their mineralogy, natural fractures filled with calcite and the presence of interfaces cement / argillite expected in filled injection wells. The "through diffusion" experimental setup has been adapted to estimate (i) the possible modification of diffusive transport parameters recorded before and after acid attack for different radioactive tracers (tritium and chlorine-36) and non-radioactive tracers (deuterium and bromide) used to characterize samples of argillite of Tournemire and cement paste and (ii) the evolution of the chemical compositions of the solutions in the upstream and downstream reservoirs of diffusion cells during acid attacks. Finally, the analysis of solids was carried out in part by SEM-EDS, XRD and X-μTomographie. Firstly, for all the samples studied, the values of the transport parameters determined before acid attack (effective diffusion coefficient and porosity) are consistent with those of the literature. In addition, it appears that all materials have reacted strongly to acid attacks. Thus, argillites saw their diffusion parameters increase up to a factor of two, especially for anionic tracers, and, whatever the proportion of carbonate minerals initially present in samples of argillite. The post-mortem observations have led to the identification of a zone of dissolution of carbonate minerals in them, but whose extension (400 microns or less) can not alone explain the significant degradation of the containment properties. Only unobservable phenomena during investigation scale, such as wormhole effects in porous network could be the cause. In addition, the samples of argillite intruded by natural fractures have systematically developed hydraulic short circuits, which can be explained by the introduction of a process of preferential channeling within the fracture observed by SEM-EDS and X. μTomographie In addition, the more close to neutral was the fluid attack the more cementitious materials saw their confinement properties improved. This is related to the appearance of a crust of calcite about 30 microns thick on the front of a dissolution zone, the thickness of which is proportional to the amount of acid in solution. This waterproof crust, clogging the pores, would be responsible for the decrease in diffusion parameters observed across the sample, and as numerical simulations reproduce it. Finally, the interface cement / claystone showed no change in their confinement properties on a global scale, the degradation of clays being offset by aggradation cement via the formation of the crust of calcite.
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Guillaume Berthe. Évolution des propriétés de confinement des roches-couvertures type argilite soumises à des fluides enrichis en CO2 : impact des discontinuités naturelles et artificielles. Sciences de la Terre. Université Paris Sud - Paris XI, 2012. Français. ⟨NNT : 2012PA112420⟩. ⟨tel-00795668⟩

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