A predictive model for the PVTx properties of CO2-H2O-NaCl fluid mixture up to high temperature and high pressure

A predictive thermodynamic model is constructed to calculate the pressure-volume-temperature-composition (PVTx) properties of CO2-H2O-NaCl fluid mixtures by the Helmholtz free energy model of CO2-H2O fluid mixtures. The new model uses no other mixing parameters but those of the CO2-H2O system, because the Helmholtz free energy of H2O-NaCl fluid at a given composition is equivalently converted into that of pure H2O by a scaled temperature redefined at the same pressure. In addition, the parameters developed by Driesner (2007) in the PVTx model of H2O-NaCl fluid system are refitted by the IAPWS-95 formulation. Comparisons with experimental data available show that the model can reproduce the single-phase PVTx properties of H2O-NaCl and CO2-H2O-NaCl fluid mixtures of all compositions from 273 to 1273 K and from 0 to 5000 bar, within or close to experimental uncertainty in most cases (with slightly lower accuracy at 5000-10,000 bar). The isochores of CO2-H2O-NaCl fluid can be obtained from this model by a bisection algorithm, and an application example is given to analyze some natural CO2-H2O-NaCl fluid inclusions in quartz from a wolframite deposit. Computer program code for calculation of molar volume of the CO2-H2O-NaCl fluid as a function of temperature, pressure and composition can be obtained from the corresponding author (maoshide@163.com).