Thermodynamic modeling of binary CH4-CO2 fluid inclusions

An equation of state (EOS) explicit in Helmholtz free energy has been improved to calculate the PVTx and vapor-liquid phase equilibrium properties of CH4-CO2 fluid mixture. This EOS, where four mixing parameters are used, is based on highly accurate EOSs recommended by NIST for pure components (CH4 and CO2) 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 CH4-CO2 fluid, together with the updated Gibbs free energy model of solid CO2 (dry ice), is applied to calculate the CH4 content (xCH4) and molar volume (Vm) of the CH4-CO2 fluid inclusion based on the assumption that the volume of an inclusion keeps constant during heating and cooling. Vm−xCH4 diagrams are presented, which describe phase transitions involving vapor, liquid and CO2 solid phases of CH4-CO2 fluid inclusions. Isochores of CH4-CO2 inclusions at given xCH4 and Vm can be easily calculated from the improved EOS.