Study the high pressure effect on compressibility factors of high CO2 content natural gas

The aim of this work was to study the effect of high pressure on the compressibility factors of natural gas with a high CO2 content. The obtained research results are important for reservoir and ground engineers. The compressibility factors (Z-factors) of three real natural gas samples (called sample 1, sample 2, and sample 3), and five synthetic gases formed by injecting different amounts of CO2 (10 mol%, 30 mol%, 50 mol%, 75 mol%, 90 mol%) into sample 2 (called sample 4-8) were determined at their reservoir temperatures. The real reservoir temperature and pressure for sample 3 reached 463.15 K and 96 MPa, respectively. The experimental results showed that the Z-factors of all eight gas samples first decreased with decreasing pressure. Then, after the pressure decreased to some degree, the Z-factors began to increase with a further decrease in pressure. Ultra-high temperature and pressure reservoir conditions made the Z-factors of sample 3 much higher than those of sample 1 and sample 2. The injection of CO2 substantially decreased the Z-factors of sample 2. When 90 mol% CO2 was injected, the decrease of the Z-factor at its reservoir pressure reached 24.05%. A thermodynamic model based on an equation of state was further developed to describe the Z-factors of the natural gas samples containing CO2, in which a new correlation for calculating the m parameter in the a(T) function of the Soave-Redlich-Kwong equation of state was proposed. Meanwhile, new interaction correlations between the CO2 and CH4 and the other gas components were also constructed. The calculated Z-factors were in good agreement with the experimental data (116 points) obtained in this work and that (418 points) reported in the literature, with an absolute average deviation of within 1%.