Three-phase non-isothermal flow behavior of CO2-brine leakage from wellbores

Abstract

CO2 leakage from the subsurface is one of the major concerns in relation to CO2 sequestration in geological formations. In this study we propose a transient, three-phase non-isothermal flow model to describe the relevant CO2-water (brine) leakage process through a wellbore. The model is formulated on the basis of the T2WELL program by incorporating the ECO2M equation of state and a three-phase drift-flux model. Three case studies are presented for verification, validation, and application of the proposed CO2-leakage model. In case-1, an analytical solution is derived as a limiting case of the present model, and it allows one to obtain an analytical insight into the flow profile when the leakage scenario is a two-phase steady one without phase transition. Comparison of this analytical solution with the relevant numerical solution shows that both are fully consistent. In case-2, we compare our numerical model with a classic Darcy-law-based CO2-leakage model, demonstrating that the latter one would generate significant deviations because of excluding the inertia effect. In case-3, we use the present model to simulate the potential CO2-leakage profile in the Ordos CO2-sequestration site (China), and demonstrate that very sophisticated flow scenarios could be created, in which three-phase co-existence regime could be present, and highly oscillating flow profile would occur. The temperature of the leaking fluid with high-concentration of CO2 may also drop dramatically because of the Joule-Thomson effect, even leading to formation of ice phase at the wellhead. Our model may be useful for evaluating the potential of CO2 leakage from wellbores, and for assessing the safety issues related to CO2 geological storage.