In this study, an equation of state has been modified for calculation of phase behavior of fluids and PVT properties of gas condensate. The accuracy of the modified EOS has been proved by estimation of the properties of some hydrocarbons such as densities of methane and condensate gases, vaporization enthalpy, sublimation pressure, compressibility factor and comparison of the obtained results with the results of the present equations of state such as NJ (Nasrifar-Jalali) ZMJL (Zhi- Meiren-Jun-Lee) and PR Peng-Robinson).
Natural gases are important parts of hydrocarbon resources. They contain mostly gaseous hydrocarbons, among which methane is dominant. In addition, they often contain liquid hydrocarbons which are solubilised as a result of the high pressure prevailing in natural gas reservoirs. When the concentration and molecular weight of these liquid hydrocarbons are sufficiently high, the natural gas exhibits liquid-vapor phase split as pressure is decreased at reservoir temperature, and the gas is then classified as condensate gas. As a general feature, the properties of condensate gases are highly sensitive to their composition, particularly to the distribution of liquid hydrocarbons. From the production point of view, the key properties are the compressibility factor, which controls the volumetric behaviour of the fluid, and the dew point pressure below which valuable liquid hydrocarbons would be left in the reservoir. These properties have been modeled extensively through equations of state. Cubic equations of state such as RK (1), SRK (2) and PR (3) are the usual methods for estimation of PVT properties of gas condensate.
In this study, DPTG equation of state(4) has been modified for accurate calculation of the condensate gases. Also, the accuracy of the modified EOS has been studied by estimation of the properties of some hydrocarbons such as densities of methane and condensate gases, vaporization enthalpy, sublimation pressure, compressibility factor and comparison of the obtained results with the results of some known equations of state such as NJ(5), ZMJL(6) and PR(4).
In this study, DPTG(4) is modified firstly and then, it is used for calculation of the PVT properties of gas condensate. The general form of that equation of state is as follows:
Equation (1)(Available in full paper)
The parameters a and b show effects of the intermolecular forces and molecules volume, respectively. These parameters should be corrected for accurate prediction of phase behavior of pure fluids and fluid mixtures. The parameters a and b versus reduced temperature and acentric factor are corrected as below:
Equation (2)(Available in full paper)
Equation (3)(Available in full paper)
Equation (4)(Available in full paper)
Equation (5)(Available in full paper)
In the next section, the accuracy of the modified EOS is studied and then, the PVT properties of gas condensate are predicted by above EOS.
In order to study the accuracy of our modified EOS, different thermodynamic properties are calculated and the obtained results are compared to the results of some other equations of state.