Evaluating calcite as a catalyst for pyrite-hydrogen reactions and hydrogen sulfide generation

In nature, the presence of accessory minerals found in the subsurface have a potential to react with stored hydrogen in process of underground hydrogen storage. Pyrite (FeS₂), the most abundant sulfide mineral on earth and occurring in almost all geological rock formations specially the sedimentary rocks, is an ideal candidate for such reactions. Pyrite is also found in seal rocks in higher quantities and considered a critical mineral while working with subsurface hydrocarbon reservoirs. Although relatively stable at surface conditions, pyrite is still prone to oxidation in the presence of moisture and high temperatures found in subsurface reservoirs, leading to the release of sulfur. This sulfur can readily react with other elements, and in context of hydrogen storage in sedimentary rocks, it can react with hydrogen to form hydrogen sulfide (H₂S) gas. H₂S being extremely toxic, pose significant handling dangers along with potential degradation of the stored hydrogen fuel. Therefore, the presence of reactive and toxic material like H₂S require careful consideration when evaluating subsurface hydrocarbon reservoirs for hydrogen storage. In this study, we examined the reaction of pyrite combined with varying concentrations of calcite (1 %, 5 %, and 50 %), brine water (3.5 wt% NaCl), and hydrogen gas (H₂) under different reservoir conditions (120 °C and 250 °C, and 500 psi). The findings revealed that approximately 12 % H₂S was released, irrespective of the calcite concentrations, and the pyrite did not transform into its secondary mineral, pyrrhotite. These findings highlight the need of paying careful attention to the geochemical interactions within potential hydrogen storage sites to ensure safety and efficacy.