A Thermodynamic Model for Predicting HCl Partial Pressure above a High Temperature Hypersaline Geothermal Fluid

Abstract

A thermodynamic model has been developed to predict the partial pressure of hydrochloric acid in equilibrium with high salinity fluids at temperatures up to 350°C. The model uses free energy of formation data obtained from heat of solution and electrochemical cell information. The model has been used to provide estimates for the concentrations of HC1 vapor in equilibrium with NaC1 brines at temperatures, concentrations and compositions considered possible within the Geysers geothermal system. At 350°C, the maximum HC1 content of superheated equilibrium steam is 8 ppmw at 5 and 3 molal NaC1. This concentration is an order of magnitude lower than HC1 levels reported for northwest Geysers wells by Hazlip and Treusdell (1988), and suggests that al alternate mechanisms may be needed to explain observed data.