Abstract

Accurate characterization of fluid phase behavior is an important aspect of CO2 enhanced shale oil recovery. So far, however, there has been little discussion about the nanopore confinement effect, including adsorption and capillarity on the phase equilibrium of water-oil-CO2 mixtures. In this study, an improved three-phase flash algorithm is proposed for calculating the phase behavior of water-oil-gas mixture on the basis of an extended Young-Laplace equation and a newly developed fugacity calculation model. The fugacity model can consider the effect of water-oil-gas adsorption on phase equilibrium. A water-Bakken oil-CO2 mixture is utilized to verify the accuracy of the flash algorithm and investigate the confinement effect. Results show that the confinement effect promotes the transfer of all components in the vapor phase to other phases, while the transfer of water, CO2, and lighter hydrocarbons is more significant. This leads to a large decrease, a large increase, and a small increase in the mole fraction of the vapor, oleic, and aqueous phases, respectively. When the confinement effect is considered, the density difference of vapor-oleic phases decreases, and the interfacial tension of vapor-oleic phases decreases; however, the density difference of vapor-aqueous phases increases, the interfacial tension of vapor-aqueous phases still decreases.