Vapor-liquid equilibrium (VLB) calculations play an important role in the simulation of enhanced oil recovery (EOR) processes. While reservoir simulators in general must treat the VLE of hydrocarbon mixtures over a wide range of pressures and compositions, the simulation of such EOR processes as vaporizing and condensing gas drives require the VLE algorithm to operate at or near the critical point. Conventional VLE calculations based on cubic equations of state such as the Peng-Robinson or Soave-Redlich-Kwong equation fail to converge or are inaccurate in the critical region. In particular, it is well known that these methods assign the same the critical z-factor for all pure chemical species.

The modified Leung-Griffiths model is a non-cubic, corresponding states theory that conforms to modern scaling laws, which successfully correlates, evaluates, and predicts VLE boundaries for fluid mixtures. The model performs well from the critical locus down to about half of the critical pressure. It has produced excellent correlations of many sets of binary hydrocarbon VLE data. We discuss the recent extension of the model to mixtures with three or more components and its application in a general purpose conpositional simulator. This paper examines how a flash computation is performed with this model, how the singlephase region is detected, and how the derivatives required in the solution of the non-linear flow equations are obtained from the flash computation. The model is coded in a form that makes it interchangeable with conventional VLE models in a wide variety of general purpose compositional simulators.

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