| Abstract |
Chloride (C1) concentrations of 10-120 pprq, have been measured in superheated steam produced by wells at The Geysers, a vapordominated geothermal field in northern California. Corrosion of the well casing and steam-gathering system has been recognized in some parts of The Geysers, and is apparently related to the presence of C1. C1 in the steam is in a volatile form, generated with the steam at reservoir temperatures, and probably travels to the wellhead as HC1 gas. Published experimental data for partial pressures of HC1 in steam over aqueous HC1 solutions and for dissociation constants of HCl were used to calculate distribution coefficients for HC1. Reservoir liquid Cl concentrations capable of generating steam with the observed C1 concentrations were then calculated as a function of pH and temperatures from 250 to 350C. Equilibrium mineral/liquid reactions with the K-mica and K-feldspar assemblage found in the wells limit the reservoir liquid pH values at various C1 concentrations to about 5 to 6 (near neutral at 250 to 350C). Within this pH range, liquid at 250C could not produce steam containing the high C1 concentrations observed. However, liquid at higher temperatures (300 to 350?C) with chloride concentrations greater than 10,000 ppm, could generate steam with 10 to over 200 ppm, C1. There is a positive correlation between pH and the chloride concentrations required to generate a given C1 concentration in steam. The concentration of C1 in superheated steam : constrains not only the reservoir liquid composition, but the temperature at which the steam last equilibrated with liquid. |