Signature of Geochemistry on Density-Driven CO2 Mixing in Sandstone Aquifers

Density-driven mixing resulting from CO2 injection into aquifers leads to the CO2 entrapment mechanism of solubility trapping. Crucially, the coupled flow-geochemistry and effects of geochemistry on density-driven mixing process for "sandstone rocks" have not been adequately addressed. Often, there are conflicting remarks in the literature as to whether geochemistry promotes or undermines dissolution-driven convection in sandstone aquifers. Against this backdrop, we simulate density-driven mixing in sandstone aquifers by developing a 2-D modified stream function formulation for multicomponent reactive convective-diffusive CO2 mixing. Two different cases corresponding to laboratory and field scales are studied to investigate the effect of rock-fluid interaction on density-driven mixing and the role of mineralization in carbon storage over the project life time. A complex sandstone mineralogical assemblage is considered, and solid-phase reactions are assumed to be kinetic to study the length- and time-scale dependency of the geochemistry effects. The study revealed nonuniform impact of rock-fluid and fluid-fluid interaction in early- and late-time stages of the process. The results show that for moderate Rayleigh (Ra) numbers, rock-fluid interactions adversely affect solubility trapping while improving the total carbon captured through mineral trapping. Simulation results in the range of 1,500 < Ra < 55,000 in the field-scale model showed more pronounced impact of geochemistry for higher Ra numbers, as geochemistry stimulates the convective instabilities and improves the total sequestered carbon. This study gives new insights into the effect of rock-fluid interactions on density-driven mixing and solubility trapping in sandstone aquifers to improve estimation of the carbon storage capacity in deep saline aquifers.