The Role of Mixed Convection and Hydrodynamic Dispersion During CO2 Dissolution in Saline Aquifers: A Numerical Study

Dissolution trapping plays a significant role in CO2 geological storage. When CO2 dissolves in aquifer brines, the aqueous solution is heavier than the underlying resident brine. This results in convective instability in the form of CO2-rich fingers, which accelerates dissolution. It was recently shown that background flow affects CO2 dissolution by suppressing the development of convective fingers. However, there was a discrepancy between computational results and measurements, with the computational results demonstrating a decrease in the dissolution rate and the laboratory measurements showing no significant effect. Here, we attempted to understand this difference by exploring the relations between the transport mechanisms that facilitate convective mixing. We investigated the role of background flow by using numerical simulations that were validated by laboratory measurements and analytical solutions. Simulations were generated for the same porous medium and fluid pair used in the experiments, keeping the Rayleigh number constant, while changing the background velocity and the Peclet number. Studying the flux components revealed three distinct regimes characterized by the Peclet to Rayleigh ratio Pe/Ra. When Pe/Ra < 0.77, natural convection dominated, and dissolution remained approximately constant. When the background flow was large (Pe/Ra > 2), dissolution was controlled by pure forced convection and increased with Pe/Ra. In the intermediate range (0.77 < Pe/Ra < 2), both natural convection and forced convection were important.

Automated summary

Dissolution trapping is important in CO2 geological storage, and background flow can suppress the development of convective fingers in CO2 dissolution. The dissolution process can be categorized into three regimes based on the Pe/Ra ratio: natural convection dominant, pure forced convection dominant, and both natural and forced convection important.