Mixtures of reservoir oil and CO2 can exhibit complex multiphase behavior at temperatures typically below 120°F, where a third CO2-rich liquid (L2) phase can coexist with the oleic (L1) and gaseous (V) phases. The three-phase behavior is bounded by two types of critical endpoint (CEP) tie lines in composition space. The lower CEP (LCEP) tie line is where the two liquid phases merge in the presence of the V phase, and the upper CEP (UCEP) tie line is where the L2 and V phases merge in the presence of the L1 phase. Slim-tube tests reported in the literature show that low-temperature oil displacement by CO2 can result in 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 interaction of flow and three-hydrocarbon-phase behavior.
In this research, we analyze mass conservation on multiphase transition between two and three phases for the limiting three-phase flow, where the L1 phase is completely displaced by the L2 phase on the LCEP. The analysis indicates that mass transfer on multiphase transition occurs in the most efficient way for MCM development. Simple analytical conditions derived for MCM through three phases are applied to 1-D fine-scale simulations of CO2 floods using 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, where MCM is developed on a CEP for the aqueous, V, and L1 phases. For MCM cases in this research, the L2-V two phases are present upstream of the miscible front, but can also be miscible on the non-L1 edge of the UCEP tie line.
The limiting three-phase flow does not necessarily occur at the highest pressure for three-phase flow, especially when a heavy oil is displaced by solvent at low temperatures. This is a marked difference from the conventional MCM development through two phases, where the limiting two-phase flow typically occurs at the highest pressure for two-phase flow.
Three-phase flow gradually changes to two-phase flow with varying pressure in the presence of numerical dispersion. We show that mass transfer on multiphase transition becomes less efficient during the change until the three-phase region completely disappear.