NaCl-H2O Phase Diagrams and Equations of State for Modeling Geo-hydrothermal Systems

Fluids in seafloor hydrothermal systems and many underground geothermal systems contain certain amount of salts, which have a significant effect on the dynamics of fluid flow and transport processes. Equations of state and phase diagrams of the NaCl-H2O system are needed to determine phase equilibrium and thermodynamic properties of fluids in modeling high-temperature geo-hydrothermal systems, particularly those near mid-ocean ridges. Extant equations of state and phase diagrams in the literature are either applicable to pressure/temperature ranges that are not sufficient to cover those of seafloor hydrothermal systems, or have significant flaws that render successful modeling impractical. To carry out our research projects on modeling seafloor hydrothermal systems, we have developed phase diagrams as described below. Pressure-temperature-salinity and pressure-temperature-density phase boundaries containing the liquid-vapor region of NaCl-H2O system for temperatures from 300 deg C to 801 deg C (the melting point of halite) and pressures from 0.1MPa to above 200MPa were first constructed based on Anderko and Pitzer's [1993] method. These boundaries were then extrapolated to include temperature ranges of 100 deg C-300 deg C to the low end and 801 deg C-1200 deg C to the high end according to data along the critical line and of the saturated phases on the liquid-vapor-halite three-phase boundary in the low- and high-temperature ranges, respectively. Based on the assumption of NaCl-H2O solution as a water-slat assembly with linear contributions from water and salt, the pressure-temperature-enthalpy phase boundary was formed on the basis of pressure, density and salinity data along the boundary.