Journal of Natural Gas Chemistry 19(2010)189–192 A novel correlation approach for prediction of natural gas compressibility factor Ehsan Heidaryan 1∗ , Amir Salarabadi 2 , Jamshid Moghadasi 3 1. Faculty of Energy, Kermanshah University of Technology, Kermanshah 6717863766, Iran; 2. Ilam Gas Treating Company, Ilam, Iran; 3. Petroleum Engineering Department, Petroleum University of Technology, Ahwaz,Iran [ Manuscript received August 19, 2009; revised October 20, 2009 ] Abstract Gas compressibility factor (z-Factor) is one of the most important parameters in upstream and downstream calculations of petroleum industries. The importance of z-Factor cannot be overemphasized in oil and gas engineering calculations. The experimental measurements, Equations of State (EoS) and empirical correlations are the most common sources of z-Factor calculations. There are more than twenty correlations available with two variables for calculating the z-Factor from fitting in an EoS or just through fitting techniques. However, these correlations are too complex, which require initial value and more complicated and longer computations or have magnitude error. The purpose of this study is to develop a new accurate correlation to rapidly estimate z-Factor. Result of this correlation is compared with large scale of database and experimental data also. Proposed correlation has 1.660 of Absolute Percent Relative Error (E ABS ) versus Standing and Katz chart and has also 3.221 of E ABS versus experimental data. The output of this correlation can be directly assumed or be used as an initial value of other implicit correlations. This correlation is valid for gas coefficient of isothermal compressibility (c g ) calculations also. Key words natural gas; compressibility factor; Standing and Katz chart; correlation 1. Introduction Reservoir engineers need to predict pV T properties of petroleum fluids at given conditions of temperature and pres- sure. This can be determined through pV T analysis of fluid sample tests or can be calculated by using Equations of State (EoS) based on computer codes if the fluid composition is known. This information is often unavailable particularly at the early stage of field development or needs to be verified, supported and supplemented during the course of field devel- opment. It is the task of empirical correlations then to estimate the petroleum fluid properties as a function of the reservoirs readily available characteristics [1]. In the oil and gas industries, gas compressibility fac- tor (z -Factor) is one of the most important parameters in upstream and downstream operations and the importance of z -Factor cannot be overemphasized in material balance, gas reserve evaluation, gas reservoir simulation, gas well testing and gas processing calculations. The experimental measure- ments, EoS and empirical correlations are the most common sources of z -Factor calculations. The purpose of this study is to develop the new accurate correlation for rapid estimating z -Factor. 2. Gas compressibility factor The EoS most often used by petroleum engineers is: pV = znRT (1) Where p is the absolute pressure, V is the volume, T is the absolute temperature, z is the compressibility factor, n is the number of moles of the gas, and R is the universal gas con- stant. The ratio of the actual volume to the ideal volume of real gas “which is a measure of the amount the gas deviates from perfect behavior” is called the compressibility factor. It is also called the gas deviation factor and is denoted by the symbol z . z = V Actual /V Ideal (2) Standing and Katz [2] have developed an accepted chart (SKC) according to the theory of corresponding states for the ∗ Corresponding author. Tel: +98 918 336 0389; E-mail: heidaryan@engineer.com This work was supported by Research Institute of Petroleum Industry-Kermanshah Campus Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60050-5