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 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 CO₂-gas acidification. Results show that hydraulic aperture increases linearly with duration of the preceding CO₂-brine flow period, emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods.