Role of the Capillary Transition Zone on the Dissolution of CO2 into Brine in Saline Reservoirs

Abstract:

Geologic carbon storage in deep saline reservoirs is a promising technology for reducing anthropogenic emissions into the atmosphere. Dissolution of injected CO₂ into resident brines is one of the primary trapping mechanisms generally considered necessary to provide long-term storage security. Given that diffusion of CO₂ in brine is woefully slow, convective dissolution, driven by a small increase in brine density with CO₂ saturation, is considered to be the primary mechanism of dissolution trapping. Previous studies of convective dissolution have typically only considered the convective process in the single phase region below the gas-water contact (GWC) and have ignored the over-lying two-phase region where dissolution actually takes place. Our objective is to improve estimates of the long-term dissolution rate of CO₂ into brine by including the two-phase region above the gas-water contact in model simulations. In the two-phase model, there is a capillary transition zone above the GWC over which the brine saturation decreases with increasing elevation. Our simulations show that when the capillary fringe height is small, which corresponds to very low entry pressure, assuming CO₂-saturated brine in the two-phase region is well-motivated, as has been assumed in analyses of dissolution without the capillary transition. For typical finite entry pressures, the fringe thickness is finite and upwelling convection currents of fresh, un-carbonated brine must extend above the GWC to saturate the brine with CO₂. Our results show the long-term dissolution rate can be enhanced by greater than 3 times the dissolution rates derived from ignoring the capillary transition zone. The single-phase, closed-top dissolution rate is recovered in the limit of vanishing entry pressure.

This material is based upon work supported as part of the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.