Effect of initial gas saturation on miscible gasflood recovery

Multi-contact miscible (MCM) displacements are designed to achieve piston-like flow and optimal recovery of in-situ hydrocarbons. The primary goal of this paper is to explore the ramifications of leading condensate or gas banks in MCM floods with two-phase initial conditions, a problem which has not been considered in detail in theoretical or experimental research. The consequences of two or more in situ phases may be significant for displacements in capillary transition zones, vertically-graded compositional reservoirs or gas condensate reservoirs. We develop method of characteristics (MOC) solutions for 1-D displacement of two-phase initial compositions by gases that are enriched below and above the minimum miscibility enrichment (MME). We consider ternary and quaternary systems to examine both condensing and condensing/vaporizing (C/V) mechanisms. In the MCM displacements considered, recoveries are dependent on relative permeability functions and can be adversely affected by two-phase initial reservoir fluids for injection times less than 1.0 pore volumes injected (PVI). The delay in heavy component recovery could significantly impact the economics of gas injection projects. Because flow is MCM, however, recovery increases rapidly to 100% at 1.0 PVI in all cases.