Development of a New Correlation of Gas Compressibility Factor (Z Factor) for High Pressure Gas Reservoirs

Abstract

Gas compressibility factor or Zfactor for natural gas system can be determined from StandingKatz charts using the pseudocritical gas pressure and temperatures. These charts give accurate values for Zfactors. Reservoir simulation softwares need accurate correlations to estimate the values of Zfactor; one of the wellknown correlations is Dranchuk and AbouKassem (DAK) Correlation. This correlation gives large errors at high gas reservoir pressures, this error could be more than 100%. The error in estimating Zfactor will lead to big error in estimating all the other gas properties such as gas formation volume factor, gas compressibility, and gas in place. In this paper a new accurate Zfactor correlation has been developed using regression for more than 300 data points of measured Zfactor using matlab in addition to other data points at low pressure and temperature from StandingKatz charts and DAK correlation. Old correlations give good estimation of Zfactor at low gas reservoir pressures below 41.37 MPa (6000 psia), at high pressures the error started to appear. The developed correlation is a function of pseudoreduced pressure and temperature of the gas which makes it simpler than the existing complicated correlations. The new correlation can be used to determine the gas compressibility factor at any pressure range especially for high pressures the error was less than 3% compared to the measured data. The developed correlation is very simple to be used, it just needs the gas specific gravity that can be used to determine the pseudocritical properties of the gas and at last the Zfactor can be determined. A new formula of reduced gas compressibility was developed based on the developed Zfactor correlation which in turn can be used to determine the gas compressibility.