Modeling CO2 leakage scenarios, including transitions between super- and sub-critical conditions, and phase change between liquid and gaseous CO2

Abstract

Abstract Storage of CO 2 in saline aquifers would be made at supercritical pressure and temperature conditions, but CO 2 leaking from a geologic storage reservoir and migrating towards the land surface (through faults, fractures, or improperly abandoned wells) would reach sub-critical conditions above 500-750 m depth for typical temperature and pressure conditions in terrestrial crust. At shallower horizons, subcritical CO 2 can form two-phase mixtures of liquid and gaseous CO 2 , with significant latent heat effects during boiling and condensation processes. Additional strong non-isothermal effects can arise from decompression of gas-like subcritical CO 2 , the so-called Joule-Thomson effect. Evaluation of leakage scenarios requires a capability to model nonisothermal flows of brine and CO 2 at conditions that range from supercritical to subcritical, including three-phase flow of aqueous phase, and liquid and gaseous CO 2 . In this paper we describe the development of comprehensive simulation capabilities that can cope with all possible phase conditions in brine- CO 2 systems. Our model formulation includes • an accurate description of phase properties as function of temperature, pressure, and composition, including the mutual dissolution of CO 2 and H 2 O in aqueous and CO 2 -rich phases; • transitions between super- and sub-critical 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; • 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. The interplay of self-enhancing and self-limiting effects on different space and time scales can induce non-monotonic behavior of CO 2 flow rates. Such behavior may facilitate monitoring of CO 2 storage systems, as it may provide signals that may be more easily distinguished from background noise.