Lawrence Berkeley National Laboratory

Rising atmospheric concentrations of CO2, and their role in global warming, have prompted efforts to reduce emissions of CO2 from burning of fossil fuels. An attractive mitigation option under consideration in many countries is the injection of CO2 from stationary sources, such as fossil-fueled power plants, into deep, stable geologic formations, where it would be stored and kept out of the atmosphere for time periods of hundreds to thousands of years or more. Potential geologic storage reservoirs include depleted or depleting oil and gas reservoirs, unmineable coal seams, and saline formations. While oil and gas reservoirs may provide some attractive early targets for CO2 storage, estimates for geographic regions worldwide have suggested that only saline formations would provide sufficient storage capacity to substantially impact atmospheric releases. This paper will focus on CO2 storage in saline formations.Injection of CO2 into a saline aquifer will give rise to immiscible displacement of brine by the advancing CO2. The lower viscosity of CO2 relative to aqueous fluids provides a potential for hydrodynamic instabilities during the displacement process. At typical subsurface conditions of temperature and pressure, CO2 is less dense than aqueous fluids and is subject to upward buoyancy force in environments where pressures are controlled by an ambient aqueous phase. Thus CO2 would tend to rise towards the top of a permeable formation and accumulate beneath the caprock. Some CO2 will also dissolve in the aqueous phase, while the CO2-rich phase may dissolve some formation waters, which would tend to dry out the vicinity of the injection wells. CO2 will make formation waters more acidic, and will induce chemical rections that may precipitate and dissolve mineral phases (Xu et al., 2004). As a consequence of CO2 injection, significant pressurization of formation fluids would occur over large areas. These pressurization effects will change effective stresses, and may cause movement along faults with associated seismicity and increases in permeability that could lead to leakage from the storage reservoir (Rutqvist and Tsang, 2005).