Composition Paths in Four-Component Systems: Effect of Dissolved Methane on 1D C02 Flood Performance

Summary New measurements of phase compositions and densities are reported for a quaternary system containing CO2, methane, butane, and decane and related ternary systems at 160°F and 1, 250 psia [71°C and 8620 kPa]. Measured values of phase compositions and vapor densities agreed well with values calculated with the Peng-Robinson equation of state (PREOS), though calculated liquid densities were less accurate. Composition paths for two-phase flow of four-component mixtures were calculated with the method of characteristics and the equation-of-state (EOS) representation of the phase behavior. Analysis of resulting paths indicates why displacement efficiency in ID CO2 floods is insensitive to the addition of dissolved methane to the oil displaced. Methane in the oil partitions so strongly into the more mobile vapor phase that a methane-rich bank forms at the leading edge of the transition zone. The displacing CO2 then encounters hydrocarbon mixtures without methane. Analysis indicates that high displacement efficiency is possible even when two-phase flow occurs throughout the displacement and that high recovery is possible even when a live oil is displaced below its bub-blepoint pressure (BP), if the pressure is above the minimum miscibility pressure (MMP) for the same oil with all methane removed.