Experimental study of gravitational mixing of supercritical CO2

CO2 injection into saline aquifers for sequestration will initially result in buoyant supercritical (sc)CO2 trapped beneath the caprock seal. During this period, there is risk of CO2 migration out of the reservoir along wellbore defects or fracture zones. Dissolution of the scCO2 plume into brine results in solubility trapping and reduces this risk, but based on diffusion alone, this mechanism could take thousands of years. Gravitational (density-induced) mixing of CO2-saturated brine is shown to significantly accelerate this process in computational studies, but few experimental efforts have confirmed the phenomenon. Here, constant-pressure, 3-dimensional bench-scale experiments used the mass of added water to quantify the mass transfer of scCO2 into water-saturated porous media at 40-90 °C and 20 MPa, with Rayleigh numbers from 2093 to 16256. Experiments exhibit a period of 7-35X enhancement in mass transfer rates over diffusion, interpreted as gravitational mixing. Convective CO2 flux ranges from 1.6 × 10−2 to 4.8 × 10−3 mol s−1 m−2 in the experiments. Results are used to benchmark a computational model using PFLOTRAN. Experiments show an early diffusive onset period that is shorter with rates much higher than predicted by models and observed in analog experiments. Both experiments and models show convective mixing periods and similar overall rates of CO2 mass transfer.