Four-Phase Flash Equilibrium Calculations Using the Peng-Robinson Equation of State and a Mixing Rule for Asymmetric Systems

Summary A technique for predicting one- to four-phase flash equilibrium is presented for multicomponent systems containing water, such as CO2/crude-oil/H2O mixtures that characterize the CO2 miscible flooding of petroleum reservoirs. The Peng-Robinson equation of state (PR-EOS) is used to describe the aqueous and hydrocarbon phases, and an accelerated and stabilized successive-substitution method is used to obtain convergence, even in the near-critical region. In order to describe accurately multiphase equilibria involving water, a recently developed mixing rule for asymmetric systems is incorporated that permits the solubilities of both CO2 in H2O and H2O in CO2 to be modeled. The equation of state (EOS) is also modified to enable the aqueous-phase density to be predicted accurately. A comparison of predicted and experimental three- and four-phase behaviors is made for a CO2/Maljamar crude-oil/H2O mixture at 302.5°C [576.5°F].