Effect of Non-Hydrocarbon Components on Gas Compressibility Factor VAalues and Correlations

Abstract

Gas compressibility factor is necessary in most natural gas engineering calculations. The most common sources of z-factor values are experimental measurements, equation of state and empirical correlations. There are more than twenty correlations available with two variables for calculating the z-factor from fitting Standing-Katz chart values in EOS or through fitting technique. The theory of corresponding states dictates that the Z-factor can be uniquely defined as function of reduced pressure and temperature. Natural gases frequently contain material other than hydrocarbon components, such as nitrogen, carbon dioxide and hydrogen sulfide. Hydrocarbon gases are classified as sweet or sour depending on the hydrogen sulfide content. Both sweet and sour gases may contain nitrogen, carbon dioxide or both. The compositions of most natural gases are hydrocarbon of the same family (paraffin hydrocarbons), so the correlation of this type is possible but containing non-hydrocarbon on the gases, make the prediction difficult. This paper focuses on evaluating the correlations which calculate gas compressibility factor for natural gas reservoirs contains non-hydrocarbon components. It is found that gas pseudo-critical temperature decreases with the increase of N2 and H2S. Also, it is observed that in the tested gas reservoirs which contain C7+ by Stewart Mixing Rules and Kay’s there are some deviation on z factor between two methods that became negligible by using the correction method for non-hydrocarbon.