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An equation of state (EOS) explicit in Helmholtz free energy has been improved to calculate the PVTx and vapor–liquid phase equilibrium properties of CH 4 –CO 2 fluid mixture. This EOS, where four mixing parameters are used, is based on highly accurate EOSs recommended by NIST for pure components (CH 4 and CO 2 ) and contains a simple generalized departure function presented by Lemmon and Jacobsen (1999). Comparison with experimental data available indicates that the EOS can calculate both vapor–liquid phase equilibrium and volumetric properties of this binary fluid system with accuracy close to that of experimental data up to high temperature and pressure within full range of composition. The EOS of CH 4 –CO 2 fluid, together with the updated Gibbs free energy model of solid CO 2 (dry ice), is applied to calculate the CH 4 content ( x CH4 ) and molar volume ( V m ) of the CH 4 –CO 2 fluid inclusion based on the assumption that the volume of an inclusion keeps constant during heating and cooling. V m − x CH 4 diagrams are presented, which describe phase transitions involving vapor, liquid and CO 2 solid phases of CH 4 –CO 2 fluid inclusions. Isochores of CH 4 –CO 2 inclusions at given x CH 4 and V m can be easily calculated from the improved EOS. [ABSTRACT FROM AUTHOR]

Abstract: The IAPWS-95 formulation explicit in Helmholtz free energy proposed by is the best equation of state of water, from which all thermodynamic properties can be obtained over a wide T–p range from 273.16 to 1273 K and from 0 to 1000 MPa with experimental accuracy. This paper reports the applications of the IAPWS-95 formulation in fluid inclusion and mineral-water phase equilibria. A reliable and highly efficient calculation method is presented for the saturated properties of water so that the formulation can be conveniently applied in the study of fluid inclusion, such as calculating homogenization pressures, homogenization densities (or molar volumes), volume fractions and isochores. Meanwhile, the univariant curves of some mineral-dehydration reactions are calculated based on the IAPWS-95 formulation. The computer code of the IAPWS-95 formulation can be obtained from the corresponding author. [Copyright &y& Elsevier]

Abstract: This work reports the application of thermodynamic models, including equations of state, to binary (salt-free) CH4–H2O fluid inclusions. A general method is presented to calculate the compositions of CH4–H2O inclusions using the phase volume fractions and dissolution temperatures of CH4 hydrate. To calculate the homogenization pressures and isolines of the CH4–H2O inclusions, an improved activity–fugacity model is developed to predict the vapor–liquid phase equilibrium. The phase equilibrium model can predict methane solubility in the liquid phase and water content in the vapor phase from 273 to 623K and from 1 to 1000bar (up to 2000bar for the liquid phase), within or close to experimental uncertainties. Compared to reliable experimental phase equilibrium data, the average deviation of the water content in the vapor phase and methane solubility in the liquid phase is 4.29% and 3.63%, respectively. In the near-critical region, the predicted composition deviations increase to over 10%. The vapor–liquid phase equilibrium model together with the updated volumetric model of homogenous (single-phase) CH4–H2O fluid mixtures (Mao S., Duan Z., Hu J. and Zhang D. (2010) A model for single-phase PVTx properties of CO2–CH4–C2H6–N2–H2O–NaCl fluid mixtures from 273 to 1273K and from 1 to 5000bar. Chem. Geol. 275, 148–160), is applied to calculate the isolines, homogenization pressures, homogenization volumes, and isochores at specified homogenization temperatures and compositions. Online calculation is on the website: http://www.geochem-model.org/. [Copyright &y& Elsevier]