The geochemistry and sequestration of H₂S into the geothermal system at Hellisheidi, Iceland

The geochemistry and mineralization of H₂S in the geothermal system hosted by basaltic rock formation at Hellisheidi, SW Iceland, was studied. Injection of mixtures of H₂S with geothermal waste water and condensed steam into the >230°C geothermal aquifer is planned, where H₂S will hopefully be removed in the form of sulphides. The natural H₂S concentrations in the aquifer average 130ppm. They are considered to be controlled by close approach to equilibrium with pyrite, pyrrhotite, prehnite and epidote. Injection of H₂S will increase significantly the reservoir H₂S equilibrium concentrations, resulting in mineralization of pyrite and possibly other sulphides as well as affecting the formation of prehnite and epidote. Based on reaction path modelling, the main factors affecting the H₂S mineralization capacity are related to the mobility and oxidation state of iron. At temperatures above 250°C the pyrite mineralization is greatly reduced upon epidote formation leading to the much greater basalt dissolution needed to sequestrate the H₂S. Based on these findings, the optimum conditions for H₂S injection are aquifers with temperatures below ~250°C where epidote formation is insignificant. Moreover, the results suggest that sequestration of H₂S into the geothermal system is feasible. The total flux of H₂S from the Hellisheidi power plant is 12,950tonnesyr^-1. Injection into 250°C aquifers would result in dissolution of ~1000tonnesyr^-1 of basalt for mineralization of H₂S as pyrite, corresponding to ~320m³yr^-1.