Use of Critical Point in Equation of State Modeling for Reservoir Fluids

Abstract

Abstract Several equations of state have been proposed as a supplement or a replacement for cubic equations, but cubic equations have remained an industry standard in the petroleum industry. Cubic equations are bound to correctly predict the critical point of each component in an oil or gas mixture. With a minor correction, the location of the critical point also determines the simulated gas and liquid properties of a pure component at other pressures and temperatures. Also, for multicomponent reservoir fluids, the location of the critical point exerts a large influence on the phase behavior of the fluid in the entire gas and liquid region. The paper presents procedures that, for both reservoir oils and gas condensates, allow a reliable equation of state model to be generated with a fluid composition and an experimental saturation point as the only experimental data. This is possible by using an estimated critical point as an artificial data point. For oils, the estimated critical point is for the entire reservoir fluid, while for gas condensates it is the critical point of the C7+ fraction, since not all gas condensates have a critical point. The method can also be used to quality check existing EoS models or as an additional constraint when developing an EoS model by regression to experimental PVT data. This is particularly useful for heavily lumped EoS models.