Density Modeling of High-Pressure Mixtures using Cubic and Non-Cubic EoS and an Excess Volume Method

A central issue in the equation of state (EoS) development is to describe accurately fluid density and thus other thermodynamic properties based on it. This study attempts to investigate the density modeling of high-pressure mixtures, particularly related to reservoir fluids, by cubic and non-cubic EoS. A large density database of binary mixtures related to petroleum fluids was established and used to compare some typical cubic and non-cubic EoS, including SRK, PR, PC-SAFT, Soave-BWR, and GERG-2008. For the first four EoS, their volume translated versions were also evaluated. The evaluation results suggest that the EoS form three groups in order of accuracy: GERG-2008 as the first group, Soave-BWR and PC-SAFT in the second, and PR and SRK in the last. Volume translation is more effective for the last group but it does not change the order. A model-to-model comparison was made between SRK and PC-SAFT, and between SRK and PR for 500 binary pairs over a wide range of conditions, showing that the differences in the excess volume are usually small between different models. This observation motivated the introduction of an excess volume method, which combines two EoS in estimating the final thermodynamic properties. The evaluation of this method using the binary density database shows that it can deliver reasonable density estimates using a simple model like SRK. Its limitations were analyzed and its potential application for estimating high-pressure reservoir fluid densities was discussed.