A new IMPSAT formulation for compositional simulation is presented. The new formulation is based on explicit solution of variables that are isochoric, i.e., complementary to volumes, instead of explicit solution of phase mole fractions, which is the conventional IMPSAT approach. The paper focuses on performance of the nonlinear iteration and numerical stability.

The use of complementary variables in the new IMPSAT formulation makes the nonlinear system better conditioned. Consequently, fewer nonlinear iteration steps are required. The resulting speed-up more than compensates the added costs of introducing and using the isochoric variables.

The stability criterion introduced with the new formulation is in many cases significantly less conservative than the conventional criterion. However, for cases where there is little or no saturation change between the hydrocarbon phases, e.g., for retrograde gas condensate cases or single hydrocarbon phase cases, the difference between the criteria is insignificant.

The timestep sizes for which instabilities occur with the new and the conventional formulation are practically the same, and no oscillations have been observed unless both the new and the conventional criterion are violated. Consequently, the stability properties are similar, and the new criterion seems to apply to both the new and the conventional formulation.

Our conclusions are supported by numerical results.

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