Compressibility factors for natural and sour reservoir gases by correlations and cubic equations of state



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Texas Tech University


Compressibility factor (z-factor) values of natural gases are necessary in most petroleum engineering calculations. The most common sources of z-factor values are experimental measurement, equations of state method and empirical correlations. Necessity arises when there is no available experimental data for the required composition, pressure and temperature conditions. Presented here is a technique to predict z-factor values of pure substances, natural gases and sour reservoir gases regardless of the composition of the acid gases at all temperatures and pressures. Eight equations of state have been thoroughly examined and the results suggest that the Lawal-Lake-Silberberg (LLS-EOS) equation of state is capable of predicting z-factor values of both pure substances and mixtures of gases. This equation of state method allows the determination of reduced temperature (TR) and reduced pressure (PR) instead of the pseudo-reduced temperature (TPR) and pseudo-reduced pressure (PPR) both for pure substances and mixtures of gases. This EOS is robust and the results are accurate even if of acid gases present in high concentration. A comparative z-factor prediction result of the various EOS methods for different gas samples is presented fortifying the capability of the LLS-EOS method. Another method of predicting z-factor values is based on the famous Standing-Katz (S-K) Chart (empirical methods). Law of Corresponding States principle has formed the basis to develop a universal adjustable parameter. This developed adjustable parameter forms the basis for using LLS-EOS to be able to use S-K Chart to predict accurate z-factor values of pure substances and mixtures of gases regardless of the concentration of acid gases. In contrast to the existing methods derived from other equations of states (EOS methods) and S-K Chart (empirical methods), this project provides a simple and universal technique for predicting z-factor values for pure substances, natural gases and sour reservoir gases.