Phase equilibrium calculation in compositional reservoir simulation

In a reservoir simulation, it is usually assumed that a instantaneous local thermodynamic equilibrium between all phases is reached within each time step. An equation of state (EOS) is used to calculate the amount and composition of each phase in each block. This approach is computationally slow because the solution is obtained by iteration.In this work, use of local thermodynamic model instead of the EOS to reduce the time of computation in compositional reservoir simulation problems is demonstrated. This method has been implemented in a compositional simulator UTCOMP (Chang, 1990). The natural depletion simulations of three well-defined mixtures are carried out with this new method over a period of 1500 days. The results simulated by the new method are in agreement with those calculated by the Peng-Robinson EOS. But around 40% of the CPU time can be saved by using the new method compared to that required by using the PR EOS for the simulations performed in this work.