Effect of quasi-piston-like flow on miscible gasflood recovery

Multi-contact miscible (MCM) displacements of single-phase reservoir fluids are designed to achieve piston-like flow and optimal recovery of in-situ hydrocarbons. Piston-like displacements occur when the reservoir pressure is greater than the minimum miscibility pressure (MMP) or the gas is enriched beyond the minimum miscibility enrichment (MME). The effect of two hydrocarbon phase reservoir fluids on piston-like flow and recovery in MCM displacements have not been considered in detail in theoretical or experimental research. The consequences of two in situ phases may be significant for displacements in capillary transition zones, vertically-graded compositional reservoirs, surfactant floods, or gas condensate reservoirs. The primary goal of this paper is to explore the ramifications of leading condensate or gas banks on recovery in MCM floods with two-phase initial conditions. We develop detailed method of characteristics (MOC) solutions for 1-D displacement of two-phase initial compositions by gases that are enriched below and above the MME. We consider ternary and quaternary systems to examine the effect of combined condensing/vaporizing (C/V) behavior on recovery. The results show that in MCM displacements, recoveries can be seriously affected by the presence of two-phase initial reservoir fluids for injection times less than 1.0 PVI. The delay in heavy component recovery is substantial for intermediate and large initial gas saturations and could significantly impact the economics of gas injection projects. The results also show that recoveries during dispersion-free MCM flow are affected by relative permeability for injection times less than 1.0 PVI. Because flow is MCM, however, recovery increases rapidly to 100% at 1.0 PVI.