A major deficiency of current compositional simulators is their failure to model the effect of unstable displacements caused by adverse mobility ratios. Since the injected gas is less viscous than the oil, the gas front becomes unstable and numerous fingers develop and penetrate the oil giving rise to poor displacement and sweep efficiency. This shortcoming restricts the application of current compositional simulators, and engineers often opt for a pseudo-miscible black-oil simulator to model unstable gas displacements. This is certainly unfortunate because a pseudo-miscible simulator does not have all the phase-behavior mechanisms of a compositional simulator.

This paper presents a novel approach for modelling unstable displacements in a fully compositional model. The method distinguishes two flow regions in each reservoir gridblock:

  • a region where complete mixing occurs between the oil in place and the injected gas.

  • a bypassed region where the oil is not contacted by the gas but can still flow because of pressure gradient.

Transfer of components is allowed between the bypassed region and the completely mixed region to simulate the phenomena occurring at the edges of the fingers. The bypassed region shrinks as components are transferred into the mixed region. A two-parameter model is developed to model the transfer rate. Equation-of-state flash calculations are performed in the mixed regions for accurate predictions of phase behavior.

The method is tested against data showing:

  1. Unstable miscible displacement in one dimension.

  2. Unstable miscible displacement in two dimensions.

  3. Interaction between phase behavior and fingering effect in a near-miscible gas injection process.

Comparisons with a compositional model without fingering effect are also presented.

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