Laboratory experiments and numerical simulation studies of convectively enhanced carbon dioxide dissolution

Abstract

Carbon dioxide (CO2) injected into a permeable rock stratum will be stored via four major mechanisms, (1) bulk containment of the mobile supercritical phase CO2, (2) small-scale trapping by capillary forces, (3) dissolution into the local brine, and (4) chemical reactions with aqueous species and host rock resulting in mineral precipitation. The security and permanence of CO2 storage increases along this pathway from containment to mineralization. After injection into the subsurface, the less-dense free-phase CO2 will tend to rise to the top of the permeable formation and will accumulate beneath a confining layer (cap rock) as a result of buoyancy. Beneath the confining layer, the CO2 will spread out, governed by capillary, buoyant, and viscous forces, forming a relatively horizontal layer at some distance from the injection location. CO2 in contact with local fluids will begin to dissolve into the fluids. The dissolution of CO2 into the brine will result in increasing the brine density. Brine with increased density over less dense brine will result in a fluid dynamics instability such that the heavier brine containing CO2 will tend to flow downward. This will cause lighter brine without dissolved CO2 to move upward, contacting the CO2 plume, dissolving more CO2, and then convecting downward. This dissolution-induced density-driven convection is a desirable process because it can significantly enhance the CO2 dissolution rates, thereby increasing storage security. We have performed laboratory visualization studies in transparent cells and quantitative CO2 absorption tests at elevated pressure to investigate this phenomenon. Numerical modeling of the tests was performed with results comparing favorably with experimental results.