CarbFix2: CO₂ and H₂S mineralization during 3.5 years of continuous injection into basaltic rocks at more than 250 °C

Abstract

The CarbFix method was upscaled at the Hellisheiði geothermal power plant to inject and mineralize the plant’s CO₂ and H₂S emissions in June 2014. This approach first captures the gases by their dissolution in water, and the resulting gas-charged water is injected into subsurface basalts. The dissolved CO₂ and H₂S then react with the basaltic rocks liberating divalent cations, Ca^{2+}, Mg^{2+}, Fe^{2+}, increasing the fluid pH, and precipitating stable carbonate and sulfide minerals. By the end of 2017, 23,200 metric tons of CO₂ and 11,800 metric tons of H₂S had been injected to a depth of 750 m into fractured, hydrothermally altered basalts at >250 °C. The in situ fluid composition, as well as saturation indices and predominance diagrams of relevant secondary minerals at the injection and monitoring wells, indicate that sulfide precipitation is not limited by the availability of Fe or by the consumption of Fe by other secondary minerals; Ca release from the reservoir rocks to the fluid phase, however, is potentially the limiting factor for calcite precipitation, although dolomite and thus aqueous Mg may also play a role in the mineralization of the injected carbon. During the first phase of the CarbFix2 injection (June 2014 to July 2016) over 50% of injected carbon and 76% of sulfur mineralized within four to nine months, but these percentages increased four months after the amount of injected gas was doubled during the second phase of CarbFix2 (July 2016 – December 2017) at over 60% of carbon and over 85% of sulfur. The doubling of the gas injection rate decreased the pH of the injection water liberating more cations for gas mineralization. Notably, the injectivity of the injection well has remained stable throughout the study period confirming that the host rock permeability has been essentially unaffected by 3.5 years of mineralization reactions. Lastly, although the mineralization reactions are accelerated by the high temperatures (>250 °C), this is the upper temperature limit for carbon storage via the mineral carbonation of basalts as higher temperatures leads to potential decarbonation reactions