COMPARATIVE-STUDY OF CUBIC EQUATIONS OF STATE FOR PREDICTING PHASE-BEHAVIOR AND VOLUMETRIC PROPERTIES OF INJECTION GAS-RESERVOIR OIL SYSTEMS

Cubic equations of state are commonly used in the petroleum industry, particularly in compositional reservoir simulators, to predict the phase behaviour of petroleum reservoir fluids. Numerous equations of state have been published in the literature and promising new equations keep appearing. A number of comparative studies of these equations have been made with a general conclusion that, although certain equations are superior to others, their capabilities vary for different fluids and conditions. This paper evaluates the reliability of 10 equations of state for predicting the phase behaviour and volumetric properties of hydrocarbon fluids, with particular relevance to North Sea gas injection systems. A number of North Sea oil samples were analysed and functions defining the distribution of components in these fluids were selected. These functions were then used to prepare 25-component synthetic mixtures simulating black and volatile oil samples. Various experiments including swelling, flash separation, multiple contact gas injection and saturation tests were conducted on the above fluids at a wide range of pressures and the compositions, the volumes and the densities of the equilibrated oil and gas phases were measured. The accuracy of the generated experimental data was carefully examined before being used in this comparative study. The capabilities and the weaknesses of the tested equations are discussed and recommendations are made.