Prediction Of The PVTx And Vapor-Liquid Phase Equilibrium Properties Of Natural Gas Fluid Mixtures With Main Components: A General Helmholtz Free Energy Equation Of State Method

Natural gas is one of important geological fluids in the nature, whose major components include CH4, C2H6, C3H8, CO2 and N2. Its thermodynamic properties, especially the temperature-pressure-volume-composition(PVTx) and vapor–liquid(V-L) phase equilibrium properties, are the basis to determine trapping temperature and trapping pressure in the studies of oil-gas inclusions. Although the past researchers have done a lot of experimental studies, the thermodynamic data are still not enough, which cover a narrow temperature-pressure-volume-composition space. However, equation of state with good predictability developed on the basis of experimental data can calculate these properties, filling up the gap of experimental data.In this work, a new generalized equation of state explicit in dimensionless Helmholtz energy has been developed for the CH4-C2H6-C3H8-CO2-N2 fluid mixtures based on the thermodynamic models of Sun and Ely(2004) and Lemmon and Jacobsen(1999). This equation of state has simple and unified form, high accuracy, and wide range of application. The parameters in the equation of state are fitted from a large number of PVTx and V-L equilibrium experimental data. For the mixtures, the mixing parameters are fitted only by experimental data of binary systems. The calculated results indicate that the equation of state can be used to predict both PVTx and V-L phase equilibrium properties of natural CH4-C2H6-C3H8-CO2-N2 fluid mixtures, with valid temperature-pressure space from the triple point temperature to 650 K, and pressure up to 2000 bar. The calculation accuracy of this EOS is higher than that of Sun and Ely’s Helmholtz energy EOS, and is similar to that of Kunz’s GERG-2004 EOS, but the EOS form is simpler than that of the Kunz’s EOS. Therefore, it can well meet the needs of the geology and chemical industry. If this equation of state is combined with a part of ideal Helmholtz free energy, it can also accurately predict the other thermodynamic properties of the mixtures, such as entropy, enthalpy, heat capacity, chemical potential, activity, fugacity, Gibbs free energy, velocity of sound, etc. The whole calculation program is programmed by Fortran95 language which is an international popular computer language, and it has simple structure and can meet the requirements of corresponding researches.