Compressibility factor measurement and simulation of five high-temperature ultra-high-pressure dry and wet gases

Abstract

Five groups of high-temperature ultra-high-pressure natural gas samples (two dry gases and three wet gases) are collected from oilfields in northwestern China. The compressibility factors (Z-factors with a maximum uncertainty of 0.005) of these samples at their reservoir temperatures and different pressures are determined using a (high-pressure) pressure–volume–temperature apparatus. The experimental results indicate that the Z-factors of these natural gases under reservoir conditions are considerably higher than those of conventional natural gases. Moreover, the Z-factors decrease as the pressure decreases within the high pressure ranges considered in this investigation. A simple thermodynamic model based on the Soave–Redlich–Kwong equation of state and newly established binary interaction correlations is proposed to simulate the Z-factors of the gas samples investigated herein. Meanwhile, the performances of four Z-factor prediction empirical correlations and the Groupe Européen de Recherches Gazières (GERE) model reported in literature are compared. It is found that the thermodynamic model proposed in this work exhibits the best prediction accuracy; results show that the average absolute deviations in the calculations of the Z-factors of five natural gas samples are only 0.7%, 2.1%, 1.81%, 0.44%, and 0.92%. Furthermore, the high prediction accuracy of this thermodynamic model is verified by comparing the 428 Z-factor experimental data points of high-temperature ultra-high-pressure natural gases reported in literature.