Scaling CO2 convection in confined aquifers: Effects of dispersion, permeability anisotropy and geochemistry

Solubility trapping of carbon dioxide (CO2) in deep saline aquifers is considered an effective mechanism of carbon storage. The dissolution of CO2 in aquifer brine may result in gravitational instability, which triggers convective mixing (similar to the well-known Rayleigh-Benard convection problem). The process starts with the diffusion of CO2 into the brine, followed by the onset of convection (t(c)), which may be followed by the constant-flux regime, F = F-c, where F is the dissolution flux. Scaling t(c) and F-c with characteristic measures (such as Rayleigh number as a measure of buoyancy to diffusive driving forces) is important for the characterization of CO2 injection at a large scale. Previously, natural complexities such as hydrodynamic dispersion, permeability anisotropy and geochemical interactions have been neglected in the scaling relations of convective mixing. This work explores the effects of such simplifications on the scaling relations of convective mixing using high-resolution 2D numerical simulations. Our results provide new insights from revisiting the convective mixing scaling. We show that the Sherwood number (Sh) depends on the domain height in the presence of the hydrodynamic dispersion. In line with such findings, we show that Sh-Rayleigh scaling prefactor is directly related to longitudinal dispersivity (alpha(L)). Moreover, we show that increasing the permeability in each direction is beneficial for the process by decreasing convection onset time (t(c)) and increasing the dissolution flux (F). Smaller permeability anisotropy ratio (defined as the ratio of vertical to horizontal permeability, l = K-z/K-x) results in earlier onset time and higher dissolution flux, which scales with P-0.54 and P-0.21 compared to convection-diffusion, respectively. Finally, our simulation results show that earlier t(c) and higher F are expected due to carbonate geochemical reactions. The scaling relations of convective mixing in deep saline aquifers guide prioritization and selection of appropriate aquifers for geological CO2 storage and designing efficient injection schemes.