Changes in seal capacity of fractured claystone caprocks induced by dissolved and gaseous CO₂ seepage

Claystone caprocks are often the ultimate seal for CO 2 underground storage when residual CO 2 gas reaches the reservoir top due to buoyancy. Permeability changes of a fractured claystone due to seepage of CO 2 ‐enriched brine and water vapor‐saturated CO 2 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 CO 2 ‐brine and CO 2 ‐gas increase the fracture aperture abruptly after each gas flow period, producing a progressive decrease of the caprock seal capacity. Aperture increase is controlled by decohesion of the clay framework within a micrometer‐scale‐thick layer induced by CO 2 ‐gas acidification. Results show that hydraulic aperture increases linearly with duration of the preceding CO 2 ‐brine flow period, emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods.