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

Abstract

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 oC / 0-1000 bar (0-1000 oC / 0-8000 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 Barrag·n (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 H2O-CO2-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 oC), 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 oC for steam and 240 - 370 oC for liquid), published enthalpy data for saturated steam and liquid water may be reproduced within ± 76 and ± 45 KJ/Kg, respectively.