Analysis shows magnitude of Z-factor error

After four decades, the Standing-Katz Z-factor chart is still widely used as a practical source for obtaining natural gas compressibility factors. The chart is based on experimental data on gas mixtures at pressures up to 10,000 psia. With the advent of computers, however, the need arose to find a convenient technique for calculating Z-factors. Eight popular methods for obtaining the Z-factor for natural gas are compared to the Standing-Katz compressibility chart. Inaccurate Z-factors can lead to erratic gas reserves estimates and produced volume metering errors. Error magnitudes associated with the eight correlations are graphed in 2 figures. To guide designers and experimenters in selecting the best methods for their particular applications, a recommended range of applicability is provided for each method to ensure acceptable compressibility factor values. Method accuracy has been determined based on 5,940 original data points for pseudoreduced temperatures (T{sub r}) and pseudoreduced pressures (P{sub r}) in the ranges of 1.05 {le} T{sub r} {le} 3.0 and 0.2 {le} P{sub r} {le} respectively.