Phase Equilibrium for Compositional Simulation Using a Reduced Method Summary

Simulating gas injection processes requires a compositional model to predict the fluid properties resulting from mass transfer between reservoir fluid and injection gas. A drawback of compositional simulation is the long execution time of phase equilibrium calculations. In this research we implemented an efficient reduced method for phase equilibrium calculations in UTCOMP, a compositional IMPEC simulator, to demonstrate the efficiency of the method. The number of variables required in the reduced method is always six for flash calculations, and five for stability calculations, regardless of the number of components (Nc).

The results show that the reduced method can significantly reduce the execution time of phase equilibrium calculations in compositional simulation without loss of accuracy. For example, we observed that implementation of the reduced method in UTCOMP for a 20x20 grid reservoir model results in simulations that are 60% faster than when conventional phase equilibrium calculations are used. The execution time of the reduced phase equilibrium calculations increases only linearly with Nc while those of other methods increase more than quadratically. This is because the size of the Jacobian matrix is Nc x Nc for conventional methods and constant for the reduced method, regardless of Nc used in the simulation.