Effect of permeability anisotropy on buoyancy-driven flow for CO2 sequestration in saline aquifers

Solubility trapping of carbon dioxide (CO2) in deep saline aquifers is considered one of the most effective methods for carbon sequestration. Dissolution of CO2 into the brine may create gravitational instabilities that lead to the onset of convection, which greatly enhances the storage efficiency and reduces the possibilities of leakage. Convection appears in the form of downward traveling fingers of relatively dense, CO2-dissolved fluid. Many natural aquifer formations display considerable permeability anisotropy, where the horizontal permeability k(h) may be several times greater than the vertical permeability k(z). It has been previously found that increasing k(h) for a fixed k(z) reduces the critical time t(c) at which onset occurs and the critical wavelength lambda(c) with which the fingers initially form. We extend earlier work by showing how and why this occurs. Our results reveal new insights about lambda(c). We have studied the behavior for times greater than t(c) using high-resolution numerical simulations. We show that the enhanced dissolution from convection can become significant much earlier in anisotropic media. Furthermore, the effects of anisotropy may be sustained for a long period of time. Our results suggest that permeability anisotropy can allow a wider range of aquifer formations to be considered for effective sequestration.