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 N230 °C geothermal aquifer is planned, where H2S will hopefully be removed in the
form of sulphides. The natural H2S concentrations in the aquifer average 130 ppm. 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 12,950 tonnes yr−1
. Injection into
250 °C aquifers would result in dissolution of ~1000 tonnes yr−1 of basalt for mineralization of H2S as pyrite,
corresponding to ~320 m3 yr