Effective phase equilibrium calculation for equation of state compositional reservoir simulation

Field studies based on compositional simulations require a huge number of p hase equilibrium calculations resulting in costly computing times. The most common method for reducing the computing time is to simplify the fluid des cription by combining its components into several pseudo-components. The ma in drawback of such pseudoization is the loss of detailed compositional inf ormation about the reservoir fluids. This study evaluated the accuracy and efficiency of different computational methods for the equation of state incorporated in a compositional simulati on model. The compositional model was formulated by the IMPECS approach. Th e iterative EOS flash calculations were performed by the successive substit ution iteration (SSI) method, a combination of the SSI and Minimum Variable Newton-Raphson methods (SSI + MVNR), and a combination of the direct flash calculation and MVNR methods (DFC + MVNR). These three flash algorithms we re implemented in a generalized Michelsen method, in which the number of th e independent variables was optionally reduced. The flash calculation methods were evaluated by simulating the behavior of a single well in a gas condensate reservoir, in which the reservoir fluid w as grouped into 5, 10, and 16 pseudo-components. The simulation results dem onstrated high effectiveness for the SSI + MVNR and DFC + MVNR combination methods with parameter reduction, resulting in significant decreases in the flash computing time compared to the conventional method, particularly whe n the number of pseudo-components was increased. The computing time decreas ed as the number of parameters is decreased. However, setting all BIP to ze ro is not recommended, because the simulation results were quite different for the case of all zero BIP and the cases of non-zero BIP assigned to more than one component.