Carbon dioxide injection: Reaction and diffusion at the reservoir/shale interface during CO2 Storage: Impact of geochemical kinetics

A reactive diffusion model was applied to investigate what happens to CO₂ injected into a subsurface sandstone reservoir capped by a chlorite- and illite-containing shale seal. The calculations simulate reaction and transport of supercritical CO₂ at 348.15 K and 30 MPa up to 20,000 a. At 5% shale porosity, chemical reactions mostly occurred in the sandstone for the first 2000 a, with some precipitation at the sandstone/shale interface. From 2000 to 4000 a, ankerite, dolomite, and illite began replacing Mg-Fe chlorite at the sandstone/shale interface. Transformation of chlorite to ankerite is the dominant reaction occluding the shale porosity in most simulations. Overall, the carbonates (calcite, ankerite, dolomite), chlorite, and goethite all remain close to local chemical equilibrium with brine. Quartz is almost inert from the point of its dissolution/precipitation. However, the rate of quartz reaction controls the long-term decline in aqueous silica activity and its evolution toward equilibrium.