Accurate Calculation of Thermodynamic Properties of H2O-CO2-CH4 Single and Binary Fluids in P-T Ranges of Geothermal Interest

Duan, et al. (1992a, 1992b) developed an equation of state (EOS), similar to that of Lee and Kesler (1975), that describes P-V-T-X relations in the system H2O-CO2-CH4 over the range 50-1000 C / 0–1000 bar (0-1000 C / 08000 bar for single component fluids). An extensive comparison of values calculated with this EOS and experimental P-V-T-X and solvus data was published by Nieva and Barragán (2003). The equation of state, which will be referred to as DMW EOS, was integrated in order to calculate thermodynamic properties of single and binary fluids in the system H2OCO2-CH4. Calculated values are compared with tabulated data for water, carbon dioxide and methane, as well as their binary mixtures. For single component fluids the ability of the DMW EOS to reproduce tabulated data decreases, as expected, from the nonpolar component to the condensed phase of the more polar component (pressurized liquid water). In a range of interest for the analysis of middle to high temperature hydrothermal systems (125 – 375 C), tabulated specific enthalpy data for methane, carbon dioxide and superheated steam may be reproduced within ± 6, ± 9 and ± 54 KJ/Kg, respectively. Published enthalpy data for H2O CO2 and H2O – CH4 fluids may be reproduced within ± 11 and ± 25 KJ/Kg. In more limited ranges (170 – 370 C for steam and 240 – 370 C for liquid), published enthalpy data for saturated steam and liquid water may be reproduced within ± 76 and ± 45 KJ/Kg, respectively.