Evaluation of correlations for natural gas compressibility factors

Abstract

For more than four decades, several correlations have been used in petroleum calculations to estimate the Natural Gas compressibility factor (Z-factor). This paper compares nine of the most commonly used correlations using error and graphical analysis. A total of 5940 data points were used in the comparison. Prior to correlations evaluation the data were tested for possible discrepancy or inconsistency with other charts, and the results were highlighted. Consequently the data of the lowest two curves Tr=1.05 and 1.10 were smoothed. Among the nine correlations studied, Dranchuk and Abou-Kassem correlation was found to give the least average absolute relative error with respect to original data. On the other hand, Hall and Yarborough correlation gave the least average absolute relative error with respect to smoothed data. In addition, both correlations were found to describe the shape of the actual function very closely.