(126c) A New Three-Phase Equilibrium Calculation Method with Partial Solubility of Selected Components Applied to CO2 Storage in Depleted Reservoirs

There is a significant demand for CO₂ storage in depleted hydrocarbon reservoirs, as an effort to reduce the amount of this greenhouse gas into the atmosphere. Three-phase equilibrium calculations for water-CO₂-hydrocarbon mixtures are required in the compositional simulation of such applications. The very low solubility of hydrocarbon components in water leads to a special mathematical structure of the problem. Several techniques were suggested, such as the free-water flash (FWF) and the augmented the free-water flash (AFWF); in the former, the aqueous phase is pure water, while in the latter only certain components, CO₂ or methane for example, are dissolved in the aqueous phase. However, only the first-order successive substitution method was used in previous approaches, making them unattractive for compositional simulations in which a huge number of phase equilibrium calculations are performed. In this work, a robust and efficient AFWF method is proposed, using hybrid successive substitutions-modified Newton iterations and a new sequential phase stability-flash strategy. The new method is general, allowing partial solubility of any selected component in liquid phases, depending on the specific compositions and operating conditions. A detailed description of second-order methods in a Gibbs energy minimization framework for the general AFWF is presented, for several choices of the independent variables, including the negative flash option. In the AFWF, the dimension of the problem and the number of function evaluations (thus the computation time) are significantly reduced. Moreover, it is mathematically proven that the augmented method always has better convergence properties than its conventional multiphase flash counterpart, in both first- and second-order methods. The new AFWF method is tested for various hydrocarbon-water-CO₂ mixtures and proved to be highly robust and efficient, systematically outperforming the conventional approach. Unlike in previous AFWF formulations, the number of components soluble in water is not limited, leading to an accuracy extremely close to that of a full three-phase equilibrium, even at high pressures and/or large amounts of CO₂. The proposed method is not model-dependent (any thermodynamic model can be used to describe the equilibrium phases). Finally, application of the new AFWF method to a variety of phase equilibrium problems with a somehow similar mathematical structure, such as wax precipitation, presence of mercury in natural gas, asphaltenes, gas hydrates and handling trace components, is discussed.