Gas pressures in geothermal systems

The partial pressures of CO₂, H₂S and H₂ in geothermal reservoir waters are fixed by temperature-dependent mineral equilibria. The dissolved gases have a significant effect at which depth the rising hot waters start to boil, if their temperature exceeds 250°C and, particularly, when it exceeds 300°C. The following functions describe the temperature dependence of P_CO2 , P_H2S and P_H2 , respectively: log P_CO2 = -2.81 - 5012.7T^-1 - 0.00919T + 6.464 log Tlog P_H2S(s) = -18.75 - 24738.9T^-1 - 0.10133T + 43.170 log Tlog P_H2S(d) = -2.76 - 5758.2T^-1 - 0.00850T + 6.359 log Tlog P_H2(s) = -10.14 - 17763.1T^-1 - 0.04084T + 23.368 log Tlog P_H2(d) = 9.85 + 7290.3T^-1 + 0.07202T - 22.651 log T. where P is partial pressure (in bars abs.); and T is temperature (in kelvins). Two functions are given for each of H₂S and H₂. One set (indicated by subscript d) is valid for all waters < 200°C and waters in the range 200-300°C if chloride concentrations are < 500 ppm. The other set (indicated by subscript s) is valid for all waters > 300°C and waters in the range 200-300°C if containing > 500 ppm Cl^-. It is considered that the temperature functions are valid for waters in basaltic to acidic volcanic rocks and also for some waters, at least, in sedimentary rocks. The following function describes crudely total gas pressure plus P_H2O in geothermal systems: log P_total = 0.00543 - 621.4T^-1.