Effects of CO2 Density and Solubility on Storage Behavior in Saline Aquifers

Abstract CO2 sequestration in deep saline aquifers is an essential and quick-remedial measure to reduce CO2 emissions to atmosphere. At conditions of 800 to 4000 meters deep aquifers, CO2 is supercritical and a liquid-like fluid of which density and solubility into water are strong functions of pressure and temperature. In geological sequestration of CO2, behavior of the injected CO2 undergoes multi-phase flow, dissolution into the aqueous phase, and reaction with rock. In aquifers of the closed boundary or open boundary with weak regional groundwater flow, densities of CO2 in its own phase and CO2-dissolved water dominantly influence the extent of horizontal and vertical CO2 migration. In this study, we investigated effects of CO2 and aqueous-phase densities on the migration extent for a long time scale after injection. In order to simulate advection, dissolution, and precipitation processes, we first developed a streamline-based model assuming incompressible and immiscible two-phase flow of CO2 and water. Procedures of the common streamline method were followed. Along streamlines, 1-dimensional flow equations were solved for CO2 in its own phase and CO2 concentration in aqueous phase, where reaction of dissolved CO2 is accounted for in the latter equation. CO2 flow due to gravity was calculated on the underlying grid, and so were permeability changes. After validating the model, we performed simulations of CO2 sequestration in 3-dimensional homogeneous and heterogeneous aquifers. CO2 migration at a long timescale depends on the aquifer pressure and temperature that directly influences density, viscosity, and solubility of CO2 phase. The gravity segregation is controlled equally by aquifer pressure and temperature, and by vertical permeability, while the advective migration is less affected by the pressure and temperature, but more by heterogeneity. As precipitation, that is the ultimate form of sequestration, is directly related to migration extents of CO2 and aqueous phases, CO2 injection schemes need to be appropriately designed in accordance with the aquifer pressure, temperature, and heterogeneity.