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

Convective dissolution is an important mechanism for long-term CO2 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 Ra-T increases for a fixed 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 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 CO2 into the interior over time, which shows non-monotonic variations as Ra-T increases. At the end of our simulations, the maximum increment of dissolved CO2 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.