An improved equation of state for gaseous hydrocarbon mixtures

Abstract

Performance prediction calculations for petroleum reservoirs existing at high temperatures and pressures require the use of a compositional material balance. The compositional balance in turn requires a method of calculating the phase and volumetric behavior of the oil and gas phases in equilibrium in the reservoir. K-value adequately describe the phase behavior, and an equation of state exists which accurately predicts petroleum liquid volumetric behavior. This work presents a new equation of state for the accurate prediction of the volumetric behavior of complex gaseous hydrocarbon mixtures. The new equation is of the same form as van der Waals' equation, but the constants a and b are considered as functions of temperature. For any pure hydrocarbon, the equation contains only four empirical constants in addition to the universal gas constant. The new equation for pure materials is slightly less accurate than the Benedict equation, but is considerably superior to the Beattie-Bridgeman equation. The new mixture equation describes the volumetric behavior of gas and condensate reservoir fluids with an absolute average deviation of 1.97 percent and a standard deviation of 0.0473, as determined by application of the equation to 776 experimentally determined volumes from 176 complex gas mixtures. This compares with average deviations of 5.87 percent and 4.53 percent, and standard deviations of 0.0711 and 0.0667, resulting from applications of the other two principal techniques available for such calculations to the same samples. The equation is applicable to fluids containing substantial quantities of components heavier than hexane. This group of heavy materials, commonly referred to as heptanes-plus, is characterized in the equation by its specific gravity and molecular weight. The new equation is applicable to mixtures containing high percentages of nitrogen, carbon dioxide and hydrogen sulfide, to which present equations have no application.