Efficient Methods for Calculations of Compressibility, Density, and Viscosity of Natural Gases
Authors: Elsharkawy A.M. 
Published: March 2010

This study presents two new methods for calculating the properties of natural gases. The first is an efficient empirical model to calculate compressibility and density of natural gases containing high amounts of heptane plus and non-hydrocarbon components. The model is derived from 2,400 measurements of compressibility and density of various gases presented in this study. Accuracy of the model is compared to various equations of state (EOS), corresponding state, and empirical methods. The study shows that the new model is simpler and more efficient than EOS. It eliminates the numerous computations involved in EOS calculations. The new method also eliminates the characterization of the heptane plus fraction and estimation of binary interaction parameters needed for EOS calculations. Experimentally measured densities of several gases have been used to study the validity of the proposed method. These measurements indicate that the new method successfully captures the physical trend of changing gas density as a function of pressure, temperature, and composition.

The second method is a modification of the Lee-Gonzalez- Eakin gas viscosity correlation. The new method accounts for the presence of heptane plus, hydrogen sulfide, and carbon dioxide in natural gases. The proposed method is compared to other EOS-based viscosity models, corresponding state methods, and correlations. The comparison indicates the superiority of the new method over other methods used to calculate the viscosity of natural gases.

Introduction

The compressibility, density, and viscosity of natural gas are necessary in most petroleum and natural gas engineering calculations. Some of these calculations are used for gas metering, gas compression, design of processing units, and design of pipeline and surface facilities. Properties of natural gas are also important in the calculation of gas flow rate through reservoir rock, material balance calculations, evaluation of gas reserves, and reservoir simulations. Usually, the gas properties are measured in the laboratory. Occasionally, experimental data is unavailable and estimations from EOS or empirical correlations becomes a necessity.

In the past three decades, a number of sour natural gas and gas condensates fields have been discovered around the world. Some of these fields have been classified as heavy natural gas. For these gases, the methods available in the literature for the calculation of the properties of heavy gases such as condensates and sour gases produce unsatisfactory results. These methods can be classified into three groups(1). The first group uses gas composition or gas gravity to calculate pseudo-critical properties of gases and predicts gas properties from empirical correlations. In this group, gas density is often used to predict viscosity. Hence, the prediction of viscosity is dependent on the choice of the method that is used to estimate the gas density. The second group uses gas composition to estimate gas properties via the method of corresponding states. The third group, and the most recent one, is based on an equations of state (EOS) approach. The last group has the advantage of using a single equation to calculate k-values, compressibility, density, and viscosity(2–4).