Compressibility Factors for High-Molecular-Weight Reservoir Gases

ABSTRACT This paper examines the effect of high concentrations of the heptanes-plus fraction in natural gases on the calculation of gas compressibility (Z) factors. Laboratory measured gas compositions and Z factors are used to evaluate the accuracy of the Standing-Katz chart. It was determined that the chart itself provides satisfactory accuracy; however, Kay's molar average combination rules or comparable gravity relationships for calculating pseudo-critical pressure and temperature result in unsatisfactory Z factors for high molecular weight reservoir gases. The contributions of this paper are two-fold. First, new pseudo-critical property – gas gravity relationships are developed, and second, alternate methods for calculating pseudo-critical properties from composition are established. By utilizing either of these methods to calculate pseudo-critical pressure and temperature, the overall accuracy of Z factors from the Standing-Katz chart is increased almost three-fold.