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

Highlights: • A predictive PVTx model is developed for the CO₂–H₂O–NaCl fluid mixtures. • The new model uses no other mixing parameters but those of the CO₂–H₂O system. • Parameters of H₂O–NaCl fluid model are fitted by the IAPWS-95 formulation. • Isochore of CO₂–H₂O–NaCl fluid inclusion can be calculated by a bisection algorithm. - Abstract: 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))