H₂S sequestration traced by sulfur isotopes at Hellisheiði geothermal system, Iceland

Hydrogen sulfide emissions are one of the major environmental problems associated with high enthalpy geothermal energy utilization. Reykjavik Energy is currently undertaking a pilot project at Hellisheidi power plant, SW Iceland, where H₂S is injected together with wastewater to be sequestered in the geothermal reservoir. Multiple sulfur isotope ratios (δ³⁴S and Δ³³S) of injection and reservoir fluids were measured as a potential tracer for subsurface sulfide sequestration processes. The values for δ³⁴S and Δ³³S in the injected H₂S-fluid and wastewater and reservoir geothermal fluids at Hellisheidi were +10.6‰ and +0.047‰, −2.86‰ and −0.005‰ and −2.5 to +0.9‰ and −0.031 and −0.001‰, respectively. Our geochemical and isotope modeling reveals that upon sulfide mineralization the δ³⁴S value of the fluids in the production wells are expected to become lower due to pyrite precipitation, while the value of Δ³³S is expected to be conservative upon mineralization and degassing. It follows sulfur sequestration and mixing process can be monitored by the triple sulfur (³²S, ³³S, ³⁴S) isotope systematics. The total H₂S emissions at Hellisheidi are ˜16 kt per year. Given the size of the geothermal reservoir and the expected production time span of the power plant of about 100–300 years, H₂S sequestration in the geothermal reservoir is expected to be a sustainable solution for the reduction of power plant emissions. Continuous measurements of multiple-sulfur isotope ratios of the reservoir as well as injection fluids will provide a valuable method to monitor sulfur sequestration processes in the geothermal reservoirs.