CO₂ dissolution in the presence of background flow of deep saline aquifers

We study the effect of background flow on the dissolution and transport of carbon dioxide (CO₂) during geological storage in saline aquifers, and include the processes of diffusion, advection, and free convection. We develop a semianalytical model that captures the evolution of the dissolution in the absence of free convection. Using the semianalytical solution, we determine scaling relations for the steady rate of dissolution that follow either Jst∼PeR or Jst∼Pe depending on the value of Pe/R, where R represents the ratio of the extent of CO₂ plume to the aquifer thickness and Pe is the Péclet number. Using direct numerical simulations, we provide detailed behavior of the convective mixing during the dissolution. We establish the criteria for forced and mixed (combined free and forced) convection in aquifers that is governed by the background flow. Accordingly, we provide the scaling relations Jst∼PeR and Jst∼RaR representing the forced and free convection asymptotes, respectively, where Ra is a Rayleigh number based on aquifer thickness. The results reveal that the background velocity can delay the onset of free convection and can alter the subsequent mixing. This phenomenon is more profound in the systems subject to strong background flows wherein horizontal component of the velocity field generated by background flow hinders the establishments of vertical component of the velocity field. Finally, by applying the proposed relations to several potential storage sites, we demonstrate the screening process in finding aquifers where the background flow exerts an important influence on the dissolution. Key Points: We investigate effects of aquifer background flow on CO₂ dissolution in brine We report scaling relations that characterize forced and free convection Background flow may greatly affect the evolution of free convection