Gas Chemistry of Volcanic Geothermal Systems

Abstract

This article focuses on quantifying the processes that govern the concentrations of the major reactive gases (CO2, H2S, H2 and CH4) in fluids in aquifers producing into wet-steam wells in the volcanic geothermal systems of Olkaria (Kenya), Mahanagdong (Philippines) and Nesjavellir, Hellisheidi and Krafla (Iceland). Many of the wells in these areas have “excess” discharge enthalpy (higher than that of vapor-saturated liquid at the aquifer temperature). In two of the wells at Krafla the elevated enthalpy is to a significant extent due to the presence of vapor in the initial aquifer fluid. In general, however, excess enthalpy is mostly caused by segregation of vapor and liquid during their flow through the depressurization zone around discharging wells. The concentrations of H2S and H2 in the initial aquifer liquid are controlled by close approach to equilibrium with specific hydrothermal mineral assemblages. Aquifer liquid CO2 concentrations at Olkaria are also controlled by such equilibria but in the other fields by their supply to the geothermal fluid. The initial aquifer fluids at Hellisheidi and Olkaria contain on average 0.5% and 0.03% vapor by weight, respectively. The vapor content of the aquifer fluid at Nesjavellir and Krafla is considerably higher, on average 0.9% and 1.2%, respectively, if we exclude two of the Krafla wells that have vapor-dominated aquifers in terms of vapor volume (5.6 and 9.6% vapor by weight). A few wells in all these areas discharge slightly degassed fluid due to steam loss in the producing aquifers. The large scatter of the H2 data from Mahanagdong makes estimation of vapor fractions in producing aquifers unreliable. When taking into account the estimated vapor fraction in the aquifer fluids, results indicate that equilibrium between H2, CH4 and CO2 is closely approached at around 300°C and that departure from equilibrium increases with decreasing temperature.