Scoping calculations on leakage of CO2 in geologic storage: The impact of overburden permeability, phase trapping, and dissolution

Abstract

The purpose of this chapter is to examine fundamental aspects of potential leakage of CO 2 from geological sequestration reservoirs. Numerical simulations of fluid and heat flow are conducted to evaluate the rate at which a plume of CO 2 moves upward through the subsurface and the amounts of dissolution and phase (called gas trapping in other chapters) that occur along the way. A quantity of CO 2 is injected into a 1000-m deep, 100-m thick permeable formation saturated with saline water, where it forms an immiscible, supercritical fluid phase and partially dissolves in the aqueous phase. As the supercritical CO 2 moves upward, it smoothly transitions into a gas. Between the injection interval and the ground surface, the overburden is assumed to be homogeneous. For overburden vertical permeabilities of 100 md (∼10 ―13 m 2 ), 10 md (∼10 ―14 m 2 ), and 1 md (∼10 ―15 m 2 ), using a numerical simulator that incorporates hysteretic relative permeability and capillary pressure functions, 1000-year simulations are conducted. For each permeability, simulations are carried out for a range of maximum residual gas saturations (S grmax ), which plays a key role in phase and is poorly known for aqueous/CO 2 systems. The time required for the CO 2 plume to reach the surface increases with decreasing overburden permeability and increasing S grmax . Trade-offs exist between three key mechanisms for CO 2 trapping: stratigraphic trapping, phase trapping, and dissolution trapping. Low overburden permeability promotes stratigraphic but hinders phase or residual gas and dissolution by keeping the CO 2 plume compact. High overburden permeability enables the plume to move upward more readily, but any attendant spreading promotes phase and dissolution trapping. A large value of S grmax promotes phase but hinders dissolution by minimizing contact between brine and immiscible (free-phase) CO 2 . Additional simulations, including a high-permeability conduit in an otherwise low-permeability overburden, provide insights into the effects of geologic heterogeneity, which can greatly shorten the time required to reach the surface.