The geochemistry and sequestration of H sub(2S into the geothermal system at Hellisheidi, Iceland)

• Published in: Journal of volcanology and geothermal research Vol. 202; no. 3-4; pp. 179 - 188
• Main Authors: Stefansson, Andri, Arnorsson, Stefan, Gunnarsson, Ingvi, Kaasalainen, Hanna, Gunnlaugsson, Einar
• Format: Journal Article
• Language: English
• Published: 30-05-2011
• Subjects: ANE, Atlantic, Iceland
• ISSN: 0377-0273
• DOI: 10.1016/j.jvolgeores.2010.12.014

The geochemistry and mineralization of H sub(2S in the geothermal system hosted by basaltic rock formation at Hellisheidi, SW Iceland, was studied. Injection of mixtures of H) sub(2)S with geothermal waste water and condensed steam into the > 230 [deg]C geothermal aquifer is planned, where H sub(2S will hopefully be removed in the form of sulphides. The natural H) sub(2)S 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 H sub(2S will increase significantly the reservoir H) sub(2)S 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 H sub(2S mineralization capacity are related to the mobility and oxidation state of iron. At temperatures above 250 [deg]C the pyrite mineralization is greatly reduced upon epidote formation leading to the much greater basalt dissolution needed to sequestrate the H) sub(2)S. Based on these findings, the optimum conditions for H sub(2S injection are aquifers with temperatures below ~ 250 [deg]C where epidote formation is insignificant. Moreover, the results suggest that sequestration of H) sub(2)S into the geothermal system is feasible. The total flux of H sub(2S from the Hellisheidi power plant is 12,950 tonnes yr[super]- 1. Injection into 250 [deg]C aquifers would result in dissolution of ~ 1000 tonnes yr[super]- 1 of basalt for mineralization of H) sub(2)S as pyrite, corresponding to ~ 320 m[super]3 yr[super]- 1.