Robust Multiphase-Split Calculations Based on Improved Successive Substitution Schemes

Successful large-scale compositional reservoir simulations require robust and efficient phase-split calculations. In recent years, there has been progress in three-phase-split calculations. However, there may be convergence issues when the number of equilibrium phases increases to four. Part of the problem is from the poor initial guesses. In phase-split computations, the results from stability provide good initial guesses. Successive substitution (SS) is a key step in phase-split calculations. The method, if efficient, can provide good initial guesses for the final step, the Newton method that has a rapid rate of convergence. In this contribution, we present a robust algorithm with high efficiency and robustness in phase-split calculations in two, three, and four phases. We find that a key step is the SS. The convergence may even be very slow away from the critical point and phase boundaries. A modified SS is used which may reduce the number of iterations many times. In the course of this investigation, we observe some regions often inside the phase envelopes (far from the phase boundary or critical points) with a very high number of SS iterations. The adoption of the improved SS iterations leads to a significant speedup of the multiphase-split computations. In some mixtures, the average reduction is more than 70%.