An equation of state for the CH4-CO2-H2O system: II. Mixtures from 50 to 1000°C and 0 to 1000 bar

An equation of state (EOS) for mixtures in the CH4-CO2-H2O system has been developed. The model is based on the highly accurate endmember EOS presented in the previous article and on an empirical mixing rule. The mixing rule is based on an analogy with high order contributions to the virial expansion for mixtures. Comparison with experimental data indicates that the mixed system EOS can predict both phase equilibria and volumetric properties for the binaries with accuracy close to that of the experimental data for a temperature range from 50 to 1000°C and a pressure range from 0 to 1000 bar (or to 3000 bar with less accuracy). For temperatures below the critical point of water, there is very little PVTX (density) data. However, even for temperatures for which sufficient data exists we found that parameterization from PVTX data alone did not lead to a free energy that would accurately predict liquid-vapor equilibria. On the other hand, using this data alone we obtain a free energy that predicts both liquid-vapor equilibria and the PVTX properties of the binaries with roughly experimental accuracy. There are very few data inside the ternary. However, the mixing rule contains third order parameters, which require evaluation from ternary mixtures. For the single temperature for which we have data, the adjustment of one parameter gives good prediction of phase equilibrium in the ternary. The resulting EOS predicts that the presence of a small amount of CO2 can significantly affect the solubility of CH4. The application of this EOS to the study of fluid inclusions is discussed. The presence of CH4 in CO2rich fluid inclusions can significantly affect the predicted trapping pressure.