(132e) Aspects of Fluid Compressibility, Swelling, and Time-Dependent Boundary Conditions for CO2 Sequestration in Saline Aquifers

A number of theoretical studies have examined the onset and subsequent development of density- driven convective flow for carbon dioxide (CO₂) sequestration in saline aquifers. These studies employ a similar set of assumptions and boundary conditions, some of which may not be justified. For instance, the brine/water is taken to be incompressible (i.e., is described by a divergence-free velocity field), and the interface between the CO₂ and brine/water remains fixed in position and saturated at a constant CO₂ concentration. Our work involves a theoretical study of these specific aspects. We have performed a linear stability analysis to verify that fluid incompressibility may indeed be a good approximation. The stability analysis shows that fluid compressibility has only a small effect on the critical time and critical wavelength which characterize the onset of the density-driven convection. However, the assumption of a fixed interface may not be realistic. Dissolution of CO₂ into water can lead to swelling that may increase the volume of the aqueous phase by as much as seven percent. Furthermore, a fixed interfacial concentration can be achieved only if very large amounts of CO₂ are injected into the aquifer. Doing so can potentially lead to excess pressure buildup and CO₂ leakage from the aquifer. We have performed numerical simulations of experiments where CO₂ is mixed with water in diffusion-convection cells. Our model includes a moving interface where the time-varying CO₂ concentration is computed rigorously from an equation of state and thermodynamic principles. Incorporation of these features allow for closer agreement between our numerical results and experimental pressure evolution curves.