Numerical analysis of the effect of heterogeneity on CO ₂ dissolution enhanced by gravity-driven convection

Abstract. Dissolution trapping of CO2 in brine can mitigate the risk of supercritical CO2 leakage during long-term geological carbon sequestration (GCS). The dissolution of overlying supercritical CO2 into brine increases the density of brine in its upper portion, which causes gravity-driven convection (GDC) and thus significantly increases the rate of CO2 dissolution. To date, most studies on GDC-enhanced dissolution are based on homogeneous media, and only few studies exist on the effect of heterogeneity on GDC-enhanced dissolution. Here, we study the effect of heterogeneity and anisotropy on GDC-enhanced dissolution rate using numerical simulations with randomly obtained permeability fields. Dissolution rates calculated by these simulations are related to properties of the permeability field using least-squares regression. We obtained two empirical formulas for predicting the asymptotic GDC-enhanced dissolution rate. In the first formula the dissolution rate is almost linearly proportional to the dimensionless equivalent vertical permeability. In the second one the dissolution rate is linearly proportional to a dimensionless vertical finger-tip velocity. This indicates that the GDC-enhanced dissolution can be predicted using either the equivalent vertical permeability or the vertical finger-tip velocity. Furthermore, both formulas demonstrate that higher-permeability anisotropy results in lower dissolution rates, suggesting that pronounced horizontal stratification can inhibit the dissolution of CO2.