Equation of State Phase Behavior Modeling for Compositional Simulation

A phase behavior algorithm has been developed for use in compositional simulation which is capable of determining the number and composition of the phases that should be present at equilibrium. The algorithm can determine equilibrium behavior for mixtures forming up to three phases, with the intent to model the low temperature phase behavior of carbon dioxide/hydrocarbon mixtures. An equation of state was used to model the nonaqueous fluid behavior. In the phase behavior algorithm, a sequential procedure was used which combined a phase stability analysis calculation with a phase composition calculation to determine the number and composition of the equilibrium phases. Different methods were implemented and compared for each calculation for twoand three-phase mixtures. The phase behavior algorithm was implemented into a compositional simulator. A method to consistently label phases during simulation was developed and implemented. This method for phase identification was shown to be successful in the single-phase region as well as in the two- and three-phase regions. Phases were also identified consistently during simulation examples where phase mass density inversions occurred. Several simulation examples were presented showing results for mixtures exhibiting complex phase behavior, including mixtures traversing the three-phase region. Results were presented showing an example where the oil/gas/second liquid region had a significant effect and was poorly approximated by two-phase equilibrium calculations. Other simulation results were presented, however, in which the three-phase region had no effect at all. The phase behavior algorithm was evaluated under simulation conditions and problems encountered during phase behavior calculations were presented and discussed.Petroleum and Geosystems Engineerin