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Abstract
In recent years, many natural gas reservoirs have been discovered with varying CO2 contents, many of which are at supercritical conditions. Calculation of compressibility factors for such reservoirs is important. Therefore, this research presents an extensive review of the various methods to calculate the compressibility factor for different natural gases containing CO2 at various temperatures and pressures. It also provides a comprehensive evaluation of the accuracy of well-known and recently published mixing rules, as well as various Z-factor correlations. Finally, a set of new correlations is presented to calculate the gas compressibility factor with reasonable accuracy. The Z-factor from the proposed correlations, as well as the PR and SRK equations of state are examined against several measured Z-factors for natural gases at supercritical conditions. The proposed correlations have a correlation coefficient of 96% and can be used to calculate compressibility at high pressures and temperatures. Key words: CO2, natural gas mixtures, Z-factor, compressibility factor, correlations.
Highlights
Several natural gas reservoirs containing varying amounts of CO2 have been discovered in different parts of the world at supercritical conditions
This is done by examining the Z-factor calculations obtained through various combinations of mixing rules and Z-factor correlations against already measured Z-factors for the natural gas-CO2 mixtures in the data bank
It is clear from this figure that the calculated Z-factor from the newly proposed correlations is much closer to the experimental value than SRK equation of state, i.e. at 51.8 Mpa the experimentally measured Z-factor is found to be 1.152, whereas, the model presented in this study finds it to be 1.178 while SRK obtains 1.203
Summary
Several natural gas reservoirs containing varying amounts of CO2 have been discovered in different parts of the world at supercritical conditions. Component Temperature (K) Pressure (Mpa) Methane Ethane Propane Butane Pentane Hexane Heptane plus MWC7+ SGC7+ Z-factor Hydrogen Sulfide Carbon dioxide Nitrogen Tpr Ppr. 0.4624 0.0256 0.0088 0.0036 0.0018 0.0010 0.0003 122.4 0.7666 0.8727 0.08261 0.40501 0.00342 1.4683 3.8341 various natural gases with CO2 content ranging from 0% to as high as 94% have been studied in this work. A new method to calculate the Z-factor for natural gas-CO2 mixtures is presented using the data bank for extremely high pressure and temperature systems. The properties of these mixtures were studied at pressures ranging from 0.11 to 144.43 Mpa (16 to 20,948 psia) and temperatures ranging from 286 to 478 K (55 to 402°F). We show that most of the published methods have temperature and pressure limitations, they cannot be used to calculate the compressibility factor, and the density for Tr below 1.0 and reduced Pr above 30
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