Integrated modeling of CO2 storage and leakage scenarios including transitions between super‐ and subcritical conditions, and phase change between liquid and gaseous CO2

Abstract

AbstractStorage of CO2 in saline aquifers is intended to be at supercritical pressure and temperature conditions, but CO2 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 CO2 can form two‐phase mixtures of liquid and gaseous CO2, with significant latent heat effects during boiling and condensation. Additional strongly non‐isothermal effects can arise from decompression of gas‐like subcritical CO2, the so‐called Joule–Thomson effect. Integrated modeling of CO2 storage and leakage requires the ability to model non‐isothermal flows of brine and CO2 at conditions that range from supercritical to subcritical, including three‐phase flow of aqueous phase, and both liquid and gaseous CO2. In this paper, we describe and demonstrate comprehensive simulation capabilities that can cope with all possible phase conditions in brine‐CO2 systems. Our model formulation includes: an accurate description of thermophysical properties of aqueous and CO2‐rich phases as functions of temperature, pressure, salinity and CO2 content, including the mutual dissolution of CO2 and H2O; transitions between super‐ and subcritical conditions, including phase change between liquid and gaseous CO2; one‐, two‐, and three‐phase flow of brine‐CO2 mixtures, including heat flow; non‐isothermal effects associated with phase change, mutual dissolution of CO2 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 CO2 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 CO2 as compared to aqueous fluids provides a potential for self‐enhancing effects during leakage, while strong cooling effects from liquid CO2 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 CO2 – gaseous CO2) also tends to reduce CO2 fluxes. Feedback on different space and time scales can induce non‐monotonic behavior of CO2 flow rates. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd