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.