Numerical Analysis of the Effect of Heterogeneity on CO₂ 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-driven dissolution are based on homogeneous media and only few studies exist on the effect of heterogeneity on GDC-driven dissolution. Here, we study the effect of heterogeneity and anisotropy on GDC-driven dissolution rate by using numerical simulations with randomly obtained permeability fields. Dissolution rates calculated by these simulations are related to properties of the permeability field by using least-squares regression. We obtained two empirical formulas for predicting the asymptotic GDC-driven 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-driven dissolution can be predicted using either the equivalent vertical permeability or the vertical finger-tip velocity. Furthermore, both formulas demonstrate that higher anisotropy results in lower dissolution rates, suggesting that pronounced horizontal stratification can inhibit the dissolution of CO₂.