Long-Term Evolution of Gas Composition in Underground CO ₂ Storage Due to Dissolution by Convection and Diffusion

Underground storage of carbon dioxide will typically involve small amounts of impurities (on the order of a few percent), driven by the economics of separation for a specified purity level. Buoyancy-driven convective flows involved in carbon dioxide (CO2) sequestration in saline aquifers significantly accelerate the overall CO2 dissolution in brine compared to pure diffusion. Convective instabilities in brine-impure CO2 mixtures are controlled by various thermophysical parameters, such as solute solubility, diffusivity, and density effects. These controlling parameters may stabilize the convection in some regimes and accelerate it in others. The effect of temporal fractional composition changes in the free-phase gas mixture is examined as a further mechanism controlling the balance between the possible transport mechanisms and flow regimes. Simplified analytical models are given for the change in gas layer properties in the diffusive regime. These analytical results and Direct Numerical Simulations (DNS) were used to characterize the impact of solubility and diffusivity contrasts on the fractional composition of gas species and convective flow patterns. In the case of a binary mixture of CO2 and methane(CH4), it is shown that the temporal increase in the fractional composition of CH4 in the two-phase region, caused by the CO2 and CH4 solubility contrast, negatively impacts the initiated convective flows. Numerical simulations demonstrate diffusion to convection and convection to diffusion transitions at different fractional compositions of CH4.