New correlations to predict natural gas viscosity and compressibility factor Ehsan Heidaryan a, ⁎, Jamshid Moghadasi b , Masoud Rahimi c a Faculty of Energy, Kermanshah University of Technology, Kermanshah 6717863766, Iran b Petroleum Engineering Department, Petroleum University of Technology, Ahwaz, Iran c CFD Research Center, Department of Chemical Engineering, Razi University, Kermanshah, Iran abstract article info Article history: Received 29 September 2009 Accepted 15 May 2010 Keywords: natural gas compressibility factor (z-Factor) viscosity correlation This study proposed two new accurate simple explicit numerical methods for calculating the z-Factor and viscosity of natural gases. Results of these correlations are compared versus experimental data. Proposed correlation for z-Factor has 0.402 and 1.366 of Absolute Average Percent Error (AAE%) respectively versus Standing and Katz chart and experimental data. The output of this correlation can be directly assumed or be used as an initial value of other implicit correlations. In addition, this correlation is valid for gas coefficient of isothermal compressibility (c g ) calculations. The new method for viscosity accounts for the presence of heptane plus and non-hydrocarbon components. This model was derived from 1260 experimental measurement of gas viscosity of eleven different mixtures with AAE% of 2.083. This model is simpler and more efficient than published correlations, and the comparisons indicate superiority of the proposed model over other methods. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Natural gas is a subcategory of petroleum that occurs naturally, and it is composed of complex mixtures of hydrocarbons and a minor amount of inorganic compound. Natural gasses physical properties, and in particular, their variations with pressure, temperature, and molecular weight are of great importance in petroleum and gas engineering calculations. Petroleum engineers need to predict pVT properties of petroleum fluids at given conditions of temperature and pressure. This can be determined through pVT 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 development. Then it is the task of empirical correlations to estimate the petroleum fluid properties as a function of the reservoir's readily available characteristics (Ahmed, 1989). In the oil and gas industries, natural gas compressibility factor (z- Factor) and viscosity are two of the most important parameters in upstream and downstream and their importance cannot be over- emphasized in material balance, gas reserve evaluation, gas reservoir simulation, gas well testing and gas processing calculations. The experimental measurements, EoS and empirical correlations are the most common sources of z-Factor calculations. Occasionally when data become unavailable; viscosity is estimated using empirical correlations as function of gas density, temperature, composition and the gas viscosity at low pressure or pressure. The purpose of this study is to develop the two new accurate simple explicit correlations for rapid estimating z-Factor and viscosity. 2. Natural gas compressibility factor (z-Factor) 2.1. Background The calculation of natural gas volume, density and viscosity at elevated pressures and temperatures requires values of z-Factor. All the Equations of State (EoS) are implicit in terms of the z-Factor; this means that the z-Factor should be solved as a root of the EoS. In general, EoSs are not easy to use, not only because of the numerous calculations involved, but also basically due to the lack of knowledge to calculate critical properties, acentric factors of plus-fraction's components, and the binary interaction parameters. To conquer this, Li and Guo (1991) modified the original Peng and Robinson-EoS to predict properties of natural gases by introducing 33 constants. The modification increases the numerical computations and makes the equation inconvenient for engineering proposes. In addition, EoSs could be used to predict density or physical properties of pure hydrocarbons and hydrocarbon mixtures, whether reliable models or correlations to predict these properties are available. However, Elsharkawy (2004) showed that EoSs have poor ability to predict hydrocarbon gas mixtures volumetric properties. Journal of Petroleum Science and Engineering 73 (2010) 67–72 ⁎ Corresponding author. Tel.: + 98 918 336 0389. E-mail address: heidaryan@engineer.com (E. Heidaryan). 0920-4105/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2010.05.008 Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol