Towards ‘green’ geothermal energy: Co-mineralization of carbon and sulfur in geothermal reservoirs

Geothermal fluid utilization is considered one of the green energy sources. Yet, mitigation strategies must be applied to reduce the associated pollutants like carbon dioxide (CO₂) and toxic hydrogen sulfide (H₂S) emissions. One suggested method is the re-injection of the two gases back into the geothermal reservoir wherein carbon and sulfur are expected to mineralize naturally for a long-term underground storage. However, CO₂ and H₂S mineralization rates for natural systems are not well defined. To address the feasibility of such sequestration, experiments were conducted at 250 °C for several reservoir rock types, ranging from basaltic to silicic. Analysis of solution composition and secondary mineralogy confirmed the precipitation of Fe-Ca carbonates and Fe sulfide for all the rocks within days. The measured mineralization rates indicate that ∼0.2–0.5 t of CO₂, and ∼0.03–0.05 t of H₂S can be sequestrated annually per cubic meter of rock, depending on reservoir lithology and surface area. Calculations show that a total rock sequestration capacity of ∼0.03 km³ would be sufficient to store the annual world CO₂ and H₂S geothermal emissions. These findings indicate efficient abatement of CO₂ and H₂S at field conditions, confirming the strategy potential for at least the typical 50 years-lifetime of geothermal power plants.