| Summary: | The reduction of carbon dioxide (CO2) emissions in the atmosphere is currently one of the main challenges facing humanity. One solution is carbon capture from concentrated sources and directly from the atmosphere, and long term storage in rocks. Basaltic rocks are rich in divalent cations, Ca2+, Mg2+ and Fe2+, which react with the dissolved CO2 to form stable carbonate minerals. Mineralization of water-dissolved CO2 injected into basaltic rocks at 20–50 °C occurs within two years in field-scale settings. In this study, a high-pressure column flow-through experiment was run to simulate CO2 injection into glassy basaltic rocks at 50 °C. The aim of this experiment was to investigate the proportions of injected dissolved CO2 and high-pH groundwater needed to reach a “sweet spot” in the reacted fluid composition that favors the saturation of carbonates rather than zeolites and clays at pH 5.2–6.5 at 50 °C, as all compete for diva- lent cations and pore space. Results highlighted the importance of initial pCO2 and pH values to obtain a balance between the formation of carbonates versus clays and zeolites. Moreover, modelling indicates that pauses in CO2 injection while still injecting water can result in enhanced large molar volume Ca-Na-zeolite and Mg-Fe-clay formation that consumes pore space within the rocks. Parallel to the laboratory experiment, industrial-scale testing of a CO2-H2S gas mixture injection commenced in 2014 at the Hellisheiði geothermal power plant in Iceland. By the end of 2017, 23,200 metric tons of CO2 and 11,800 metric tons of hydrogen sulfide (H2S) had been injected to a depth of 750 m into fractured, hydrothermally altered basalts at > 250 °C. We collected over 80 water and gas samples from monitoring and injection wells, before and during injection. Major, minor, and trace element geochemi- cal data were compiled to assess the magnitude of carbon and sulfur mineralization in the subsurface in relation to relevant primary and secondary minerals in the geothermal reservoir and to evaluate ...
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