Résumé
Claystone caprocks are often the ultimate seal for CO2 underground storage when residual CO2 gas reaches the reservoir top due to buoyancy. Permeability changes of a fractured claystone due to seepage of CO2-enriched brine and water vapor-saturated CO2 gas are investigated. Results show that brine flow induces a large porosity increase (up to 50%) in the vicinity of the fracture due to dissolution of calcite and quartz, while permeability remains unchanged. Conversely, cyclic flows of CO2-brine and CO2-gas increase the fracture aperture after each gas flow period, producing a progressive decrease of the caprock seal capacity. We demonstrated using atomic modelling that the aperture increase is controlled by decohesion of the clay framework on nano-scale, induced by CO2-gas acidification at the interface.