Hydrogen Sulfide Sequestration and Storage in Geothermal System: New Mitigation Strategy to Reduce H2S from the Atmosphere and Detect its Mineralization with Multiple Sulfur Isotopic Systematics

Abstract:

Hydrogen sulfide (H₂S) is one of the major components in geothermal fluids and is commonly emitted into the atmosphere from geothermal power plants causing potential environmental problems. Among several mitigation methods proposed to reduce the H₂S emissions, is H₂S sequestration into geothermal systems. Reykjavík Energy is undertaking a pilot project at Hellisheidi geothermal system (SW Iceland) called Sulfix project where H₂S is being injected into the geothermal reservoir for permanent sequestration into pyrite. The SulFix project started its operation in June 2014: the soluble geothermal gases are dissolved in geothermal waste water, and injected at 8 bars into the high temperature reservoir (>200˚C) at 750 m below the wellhead. The reactions involving sulfur in the geothermal reservoir may be traced using sulfur fluid chemistry and multiple sulfur isotope systematics (³²S, ³³S, ³⁴S and ³⁶S), including mixing between the reservoir geothermal fluid and the injection fluid, sulfide mineralization and oxidation of sulfide to sulfate.

In this study we investigated the multiple sulfur isotope systematics upon sulfide mineralization under geothermal conditions. High temperature flow through experiments were carried out in basaltic glass at 200-250°C and ~5 mmol/kg H₂S to study the fluid-rock interaction. The results indicate that the sulfide mineralization occurs rapidly under geothermal conditions, highlighting the leaching rate of iron from the basaltic glass as the mineralization rate determining factor. Moreover, the formation of sulfide may be traced using the δ³⁴S-Δ³³S relationship in the fluids and pyrite formation - for example to determine if non-reactive mixing between the injection fluids and reservoir fluids occurs at Hellisheidi. The experimental results have been further supported by geochemical modeling involving multiple sulfur isotope fractionation between aqueous sulfide species and rocks upon basalt dissolution and secondary pyrite formation.