Mass Transfer on Multiphase Transitions in Low-Temperature Carbon-Dioxide Floods

Mixtures of reservoir oil and carbon dioxide (CO2) can exhibit complex multiphase behavior at temperatures typically less than 120 degrees F, in which a third CO2-rich liquid (L-2) phase can coexist with the oleic (L-1) and gaseous (V) phases. The three-phase behavior is bounded by two types of critical endpoints (CEPs) in composition space. The lower CEP (LCEP) is a tie line in which the two liquid phases merge in the presence of the V phase, and the upper CEP (UCEP) is a tie line in which the L-2 and V phases merge in the presence of the L-1 phase. Slimtube tests reported in the literature show that low-temperature oil displacement by CO2 can result in the high displacement efficiency of more than 90% when three phases are present during the displacement. The nearly piston-like displacements can be quantitatively reproduced in numerical simulations when the CEP behavior is properly considered. However, it is uncertain how multicontact miscibility (MCM) is developed through the interaction of flow and three-hydrocarbon-phase behavior. This research presents a detailed analysis of mass conservation on multiphase transitions between two and three phases for the limiting three-phase flow, where the L-1 phase is completely displaced by the L-2 phase on the LCEP. The analysis indicates that interphase mass transfer on multiphase transitions occurs in the most-efficient way for MCM development. Simple analytical conditions derived for MCM through three phases are applied to 1D fine-scale simulations of CO2 floods by use of four and more components. Results show that the MCM conditions are nearly satisfied when the effect of numerical dispersion is small. MCM is likely developed through three hydrocarbon phases on the LCEP in the cases studied. This is consistent with analytical solutions of water and gas injection presented in the literature, in which MCM is developed on a CEP for the aqueous, V, and L-1 phases. For MCM cases in this research, the L-2-V two phases are present upstream of the miscible front if the composition path does not go through the UCEP tie line. However, they also can be miscible on the non-L-1 edge of the UCEP tie line if the MCM composition path goes through it. Three-phase flow gradually changes to two-phase flow with varying pressure in the presence of numerical dispersion. It is shown that interphase mass transfer on multiphase transitions becomes less efficient during the change until the three-phase region completely disappears.