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Gas well and gas reservoir evaluations constitute a major challenge with the need of several gas and reservoir parameters. The gas phase requires special focus other than the oil phase. The major difference is the compressibility and the viscosity. There are charts for viscosity reading and a well-known Standing chart for gas compressibility “z”. In this study, the emphasis is over the compressibility factor. This factor is commonly read from the Standing and Katz Compressibility of Natural Gases Chart(1942) manually. The chart is based on utilization of reduced pressure and temperature. However, for practical and fast utilization a calculation of the compressibility factor was needed. Therefore, this study used the compressibility equations which were driven by Dranchuk and Abou Kassem (1975)to calculate “z” values. In this study, a gas reservoir well pressure data were used as the input pressures. The composition of the gas is not available. Therefore, the calculation of the pseudo-critical pressure and pseudo-critical temperature could be calculated using the specific gravity of the gas. The critical properties were calculated by employing the critical pressure and critical temperature correlations as provided by Standing (1977) using the specific gravity of the gas. The reservoir temperature is 170 F , specific gravity of the gas is 0.65. The calculated pseudo critical pressure is 671 psi and the pseudo critical temperature is 374 R. The corresponding pseudo reduced temperature is 1.68. The pseudo reduced pressure values were calculated for each pressure data in the test. The compressibility factor values were calculated at each test pressure using corresponding to the pseudo reduced properties. The compressibility factors were also read manually using the Standing and Katz Compressibility of Natural Gases Chart(1942). The both results from the calculation and the manual reading were plotted as “z” versus pseudo reduced pressure at constant pseudo reduced temperature of 1.68 (Figure 1). A very good match was observed. The calculated versus read values were plotted to see the linear regression. An R^2 value of 0.98 was calculated (Figure 2). This study validated and confirmed the applicability of the mathematical correlations to calculate the gas compressibility factors. The practical and fast applicability of these equations facilitate engineers to make robust and easier calculations in the presence of big pressure data environment in reservoir and gas well test analysis.
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