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

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 1273K and from 0 to 5000bar, within or close to experimental uncertainty in most cases (with slightly lower accuracy at 5000–10,000bar). 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).