Gas Chemistry in Geothermal Systems

Abstract

Five new gas geothermometers are introduced. They are useful for predicting subsurface temperatures in water dominated geothermal systems. The geothermometers use data on COz, H2S and H2 concentrations in fumarole steam as well a s CO2/H2 and H2S/H2 ratios. It is demonstrated that the gas composition of fumarole steam may be used with or without drillhole data to evaluate steam condensation in the upflow zones of qeothermal systems. Uncertainty exists, however, in distinguishing between the effects of steam condensation and phase separation at elevated pressures. The gas content in steam from discharging wells and the solute content of the water phase can be used to evaluate which boiling processes lead to "excess steam" in the discharge and at which temperature this "excess steam" is added to the fluid moving through the aquifer and into the well. Examples, using field data, are given to demonstrate a l l the mentioned applications of geothermal chemistry.