This research is part of the studies dealing with the CO2 storage in deep geological formations, focusing on the evolution of the containment properties of argillite type cap-rocks, undergone CO2-enriched fluids. The argillites of Tournemire (Aveyron, France) were used as analogous rocks. Three possible weak points were identified and investigated: their mineralogy, the presence of natural fractures filled with calcite and the presence of cement/argillite interface in the case of refilled injection wells. The "through diffusion" experimental setup was 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 by SEM-EDS, XRD and X-ray tomography.
Firstly, for all the samples studied, the values of the transport parameters determined before acid attack (effective diffusion coefficient and porosity) were consistent with those given in literature. Moreover, all materials strongly reacted to acid attacks. Argillites showed an increase of their diffusion parameters by a factor of two, especially for anionic tracers, independently of the proportion of carbonate minerals initially present in samples. The post-mortem observations led to the identification of a dissolution zone of carbonate minerals, but the low extension of which (less than 400 μm) can not explain the significant degradation of the containment properties. Some further phenomena not observable at our investigation scales have to be proposed, such as wormhole effects in the porous network. The argillite samples crossed by natural discontinuities, systematically developed hydraulic short-circuits, related to the occurrence of a preferential channeling within the fracture, observed by SEM-EDS and X-ray tomography. Regarding the cementitious materials, the more close to neutral pH the attack fluid, the more improved their containment properties. This would be related to the formation of a Caenriched thin crust (of about 30 μm) ahead a dissolution front, the thickness of which being proportional to the amount of acid in solution. This tight crust, clogging the pores, would be responsible for the decrease of the diffusion parameters at the sample scale. Finally, the cement / argillite interfaces showed no change in the containment properties, the degradation of claystone being offset by the cement aggradation related to the formation of the Ca-Enriched crust.