Effects of the geothermal gradient on the convective dissolution in CO₂ sequestration

Convective dissolution is regarded as a stable mechanism for the long-term storage of sequestered CO₂ in deep saline aquifers. The primary focus of this study is to investigate the influence of unstable geothermal gradient on the transport and dynamics of convective dissolution.

We use thermal (Ra_T) and concentration Rayleigh numbers (Ra_S) to measure convection intensity. Direct numerical simulations are conducted in porous media at three Ra_S values from 10³ to 10⁴ with a series of Ra_T up to 300 for each Ra_S.

Simulations reveal that intensified thermal gradient creates large-scale convection rolls, consequently influencing the horizontal distribution and motions of concentration fingers. All simulations eventually reach a shutdown regime, for which we developed a theoretical model to successfully describe concentration flux and volume averaged concentration. The maximum increment of dissolved CO₂ occurs at around unity density ratio for all Ra_S values, which is likely attributed to the two competing effects of the thermal field.

Our results provide a novel perspective on convective dissolution and indicate that the geothermal gradient may play a non-negligible role in many CO₂-sequestration sites.