Integrated modeling of CO 2 storage and leakage scenarios including transitions between super‐ and subcritical conditions, and phase change between liquid and gaseous CO 2

Storage of CO 2 in saline aquifers is intended to be at supercritical pressure and temperature conditions, but CO 2 leaking from a geologic storage reservoir and migrating toward the land surface (through faults, fractures, or improperly abandoned wells) would reach subcritical conditions at depths shallower than 500–750 m. At these and shallower depths, subcritical CO 2 can form two‐phase mixtures of liquid and gaseous CO 2 , with significant latent heat effects during boiling and condensation. Additional strongly non‐isothermal effects can arise from decompression of gas‐like subcritical CO 2 , the so‐called Joule–Thomson effect. Integrated modeling of CO 2 storage and leakage requires the ability to model non‐isothermal flows of brine and CO 2 at conditions that range from supercritical to subcritical, including three‐phase flow of aqueous phase, and both liquid and gaseous CO 2 . In this paper, we describe and demonstrate comprehensive simulation capabilities that can cope with all possible phase conditions in brine‐CO 2 systems. Our model formulation includes: an accurate description of thermophysical properties of aqueous and CO 2 ‐rich phases as functions of temperature, pressure, salinity and CO 2 content, including the mutual dissolution of CO 2 and H 2 O; transitions between super‐ and subcritical conditions, including phase change between liquid and gaseous CO 2 ; one‐, two‐, and three‐phase flow of brine‐CO 2 mixtures, including heat flow; non‐isothermal effects associated with phase change, mutual dissolution of CO 2 and water, and (de‐) compression effects; and the effects of dissolved NaCl, and the possibility of precipitating solid halite, with associated porosity and permeability change. Applications to specific leakage scenarios demonstrate that the peculiar thermophysical properties of CO 2 provide a potential for positive as well as negative feedbacks on leakage rates, with a combination of self‐enhancing and self‐limiting effects. Lower viscosity and density of CO 2 as compared to aqueous fluids provides a potential for self‐enhancing effects during leakage, while strong cooling effects from liquid CO 2 boiling into gas, and from expansion of gas rising towards the land surface, act to self‐limit discharges. Strong interference between fluid phases under three‐phase conditions (aqueous – liquid CO 2 – gaseous CO 2 ) also tends to reduce CO 2 fluxes. Feedback on different space and time scales can induce non‐monotonic behavior of CO 2 flow rates. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd