Compositional models are commonly used to simulaterecovery processes in which injection gas and reservoir gaseshave distinctly different properties. Such processes includevapourising miscible drives, condensing miscible drives andlow-pressure air injection. Recovery is typically enhanced bychanges in fluid properties and a reduction in interfacialtension, leading to a variation of the relative permeabilityendpoints. These changes are due to mass transfer, inparticular the solution of intermediate hydrocarboncomponents in the oil. Extensions of the black oil model suchas that due to Todd and Longstaff can also treat suchprocesses, although the mechanisms considered are ratherdifferent – in particular the amount of inter-phase mixingwhich occurs.In this paper we attempt to unify these two approaches. Thetraditional black oil model is extended to multiple gases andproperty data constructed which yields a measure of thesurface tension after mass transfer. Once the surface tension isknown, a common end-point shift treatment can be used inboth compositional and extended black oil treatments.Traditional black oil numerical methods based on saturationvariables have problems in treating the effect of partialpressure on the amount of dissolved gas when the gassaturation is low. This may be overcome using mass-basedvariables, but these have typically been less efficient due tothe addition of an extra volume balance equation. A reducedmass variable formulation is described which retains theefficiency of the saturation-based approach whilst being easyto generalize to multiple gas components. The multiple gas black oil and full compositional methods areassessed on some generic examples of gas injection processes.
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