Optimization Based Isenthalpic Flash for Thermal Reservoir Simulations

Abstract

Abstract We present an algorithm for multiphase isenthalpic phase-equilibrium calculations. The algorithm addresses issues related to robustness and performance of isenthalpic flash, which sometimes get aggravated due to narrow boiling conditions. The isenthalpic flash is posed as an optimization of total system entropy for the given input conditions: pressure, system enthalpy, and composition of the mixture. We use a second-order optimization method to maximize entropy, with the individual phase-compositions and phase-enthalpies as the independent variables. At any point during the iterative solution procedure, the direction of the iterative step can be checked by ensuring an increase in entropy. During the iterations we allow phases to be split further into sub-phases to tackle the issue of a phase having multiple roots for the specified enthalpy (the phase is at boiling point temperature). We successfully apply the method to calculate the phase diagrams for many hydrocarbon mixtures. We have shown that the isenthalpic flash with entropy maximization works robustly in all the cases tested, including the narrow-boiling examples. Result of applying the flash to fluids with two hydrocarbon phases and a simplified water and rock phase is also presented.