During compositional reservoir simulations where underground fluid composition strongly affects the modeling of recovery processes, flash calculations are commonly employed to help determine the correct number of equilibrium phases, the corresponding compositions, and the phase amount of each phase.
Cubic equations of state (EOS) are widely used in the representation of volumetric and phase equilibria due to their simplicity and solvability. Commonly used cubic EOS such as Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) have well known limitations in predicting liquid phase properties for polar compounds.
In this paper, we present a compositional reservoir simulator equipped with the advanced Peng-Robinson EOS and an efficient and robust multiphase flash algorithm that can accurately predict the phase equilibrium. This method utilizes Michelsen's stability test (Michelsen, 1982) and a combination of accelerated successive substitution and a minimum-variable Newton-Raphson (MVNR) method for fast convergence.
The advanced Peng-Robinson (APR) EOS adds volume translation and a flexible attractive temperature-dependent term to the original PR EOS for accurate PVT and saturation property correlation for polar compounds. Examples of pure compounds and mixtures are tested. Computational results show that the developed simulator provides a more detailed description and better understanding of complex dynamic underground fluid phase behavior that may occur during oil recovery processes.