| Title | Towards Cleaner Geothermal Energy: Subsurface Sequestration of Sour Gas Emissions from Geothermal Power Plants |
|---|---|
| Authors | Edda ARADOTTIR, Ingvi GUNNARSSON, Bjarni JULIUSSON, Bergur SIGFUSSON, Sigurdur GISLASON, Eric OELKERS, Martin STUTE, Juerg MATTER, Sandra SNAEBJORNSDOTTIR, Kiflom MESFIN, Helgi ALFREDSSON, Jennifer HALL, Magnus ARNARSSON, Knud DIDERIKSEN, Wallace BROECKER |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | CO2 sequestration, H2S sequestration, CO2-H2S-water-basalt interaction |
| Abstract | Geothermal energy is a sustainable and clean energy source. Utilization of high-enthalpy geothermal systems is, however, associated with emissions of geothermal gases like CO2 and H2S. H2S emissions are currently the main environmental problem associated with high enthalpy geothermal energy utilization in Iceland as the Icelandic government has issued a strict regulation on atmospheric H2S concentrations. Reykjavík Energy, the largest geothermal power company in Iceland, is developing innovative methods for capturing CO2 and H2S emissions from its power plants and sequestering the gases as carbonate and sulfide minerals in nearby, subsurface basaltic formations. About 350 tons of CO2 and H2S have been injected into two different storage sites near Hellisheidi geothermal power plant in a pilot capture and sequestration phase. The shallower storage formation lies between 400-800 m depth, is 30-80°C warm and consists of porous, relatively fresh basalts. The deeper storage formation is located below 800 m depth, within the high temperature geothermal system, and consists of fractured, hydrothermally altered basalts with aquifer temperatures around 270°C. Rather than injecting captured gases directly into the storage formations, a technology to dissolve the gases in water prior to injection has been developed. Once dissolved, the gases are no longer buoyant compared to pore fluids, improving considerable security of the injection due to decreased leakage risks. We have demonstrated that the developed method leads to geological solubility storage of injected gases in less than 5 minutes. A comprehensive monitoring program based on the use of reactive and conservative tracers has furthermore revealed rapid mineral formation of injected CO2 within the shallower reservoir and mass balance calculations suggest over 85% mineralization within less than one year. H2S mineralization is predicted to be even faster but further monitoring will provide clear indication in that respect. Results obtained so far from the pilot injections are in agreement with results from natural analogs, laboratory experiments and reactive transport model simulations, which all indicate CO2 and H2S mineral sequestration in basalt formations to be a viable option in reducing sour gas emissions from geothermal power plants and thereby bringing high enthalpy geothermal energy closer to zero emission energy production. Plans call for up-scaling sour gas capture and sequestration activities at Hellisheidi geothermal power plant in stages. The first up-scale will commence operation in 2014 and involve capture and re-injection of 8,500 tons of CO2-H2S gas mixture on an annual basis. Additional up-scales and potential further separation of CO2 and H2S will follow in coming years. |