In thermal compositional reservoir simulators that use energy as a primary variable, thousands to millions of isenthalpic multiphase flash calculations must be performed to calculate temperature, phase splits and compositions for different grid blocks during the simulation. Development of a robust and fast isenthalpic multiphase flash calculation method is necessary to improve the efficiency of such simulations.
A new isenthalpic multiphase flash calculation is described in this paper. The flash calculation method uses a modified Rachford-Rice monotonic objective function and the negative flash concept for phase distribution and phase identification. Therefore phase stability analysis is not necessary. The formulation and algorithm of the new method are presented in detail. This method is able to handle difficult situations such as narrow boiling point regions and phase appearance and disappearance, which are dominant in thermal processes. The current method encounters no difficulty in the latter situations unlike stage-wise isenthalpic flash calculation methods.
After the accuracy of new method was compared and verified against current algorithms used by the industry, it was also tested for robustness and speed. The results show promising performance compared to the current methods. This proposed method is not sensitive to the initial guess for temperature. As a matter for fact, in all of the test cases in this study, the same temperature was used as the initial guess. A poor initial guess for temperature only requires more iterations to reach the solution.