| Summary: | In-situ carbonation of basaltic rocks could provide a carbon storage solution for the long term. Permanence is essential for the success and public acceptance of carbon storage. The aim of this study was twofold, to evaluate and make a first estimate of the theoretical mineral storage potential of CO2 in basaltic rocks, and to characterise the mineralisation process using geochemical data from the CarbFix test site in Hellisheidi, SW-Iceland, which comprises both injection and monitoring wells. Studies on mineral storage of CO2 in basaltic rocks are still at an early stage. Therefore, natural analogues are important for gaining a better understanding of the carbon mineralisation process in basaltic rocks at elevated pCO2. The amount and spatial distribution of CO2 stored as calcite in the bedrock of geothermal systems in Iceland indicate a large storage potential for CO2 in basaltic rocks. These natural analogues were used as a guideline for evaluating the theoretical potential of CO2 storage in basaltic formations. The largest storage potential lies offshore, where CO2 may be stored in minerals for the long term in mid-ocean ridges. The theoretical mineral CO2 storage capacity of the mid-ocean ridges exceeds, by orders of magnitude, the amount of CO2 that would be released by the burning of all fossil fuel on Earth. Iceland is the largest landmass found above sea level on the mid-ocean ridges, about 103,000 km2. It is mostly made of basaltic rocks (~90%), which makes it ideal for demonstration of the viability of this carbon storage method. Two injection experiments were carried out in 2012 at the CarbFix site where 175 tonnes of pure CO2 and 73 tonnes of a CO2-H2S-gas mixture were injected into basaltic rocks at 500-800 m depth with temperatures ranging from 20-50°C. All gases were dissolved in water during their injection. Extensive geochemical monitoring was carried out prior to, during, and after these injections. Sampled fluids from the first monitoring well, HN-04, showed a rapid increase in Ca, Mg, and ...
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