Effects of the geothermal gradient on the convective dissolution in CO2sequestration

Abstract

Convective dissolution is an important mechanism for long-term CO$_2$sequestration in deep saline aquifers. The presence of an unstable geothermal gradient can affect the process of dissolution. In this paper, we present direct numerical simulations in a three-dimensional porous medium at three different concentration Rayleigh numbers$Ra_S$with a set of thermal Rayleigh numbers$Ra_T$. Simulations reveal that the flow structures alter when${\textit {Ra}}_T$increases for a fixed${\textit {Ra}}_S$. Strong thermal gradient can yield large-scale convection rolls which change the horizontal distribution and motions of concentration fingers. The time evolution of fluxes also has different responses to different${\textit {Ra}}_T$. A theoretical model is developed and successfully describes the evolution of concentration flux and volume averaged concentration during the final shutdown stage. We further calculate the dissolved CO$_2$into the interior over time, which shows non-monotonic variations as${\textit {Ra}}_T$increases. At the end of our simulations, the maximum increment of dissolved CO$_2$occurs when density ratio is around unity for all three concentration Rayleigh numbers we have explored. We apply our results to a typical geological reservoir and discuss their implications.