CarbFix I: Rapid CO2 mineralization in basalt for permanent carbon storage

Abstract:

Carbon dioxide mineralization via CO₂-fluid-rock reactions provides the most permanent solution for geologic CO₂ storage. Basalts, onshore or offshore, have the potential to store million metric tons of CO₂ as (Ca, Mg, Fe) carbonates [1, 2]. However, as of today it was unclear how fast CO₂ is converted to carbonate minerals in-situ in a basalt storage reservoir. The CarbFix I project in Iceland was designed to verify in-situ CO₂ mineralization in basaltic rocks. Two injection tests were performed at the CarbFix I pilot injection site near the Hellisheidi geothermal power plant in 2012. 175 tons of pure CO₂ and 73 tons of a CO₂+H₂S mixture were injection from January to March 2012 and in June 2013, respectively. The gases were injected fully dissolved in groundwater into a permeable basalt formation between 400 and 800 m depth using a novel CO₂ injection system. Using conservative (SF₆, SF₅CF₃) and reactive (¹⁴C) tracers, we quantitatively monitor and detect dissolved and chemically transformed CO₂. Tracer breakthrough curves obtained from the first monitoring well indicate that the injected solution arrived in a fast short pulse and a late broad peak. Ratios of ¹⁴C/SF₆, ¹⁴C/SF₅CF₃ or DIC/SF₆ and DIC/SF₅CF₃ are significantly lower in the monitoring well compared to the injection well, indicating that the injected dissolved CO₂ reacted. Mass balance calculations using the tracer data reveal that >95% of the injected CO₂ has been mineralized over a period of two years. Evidence of carbonate precipitation has been found in core samples that were collected from the storage reservoir using wireline core drilling as well as in and on the submersible pump in the monitoring well. Results from the core analysis will be presented with emphasis on the CO₂ mineralization.

[1] McGrail et al. (2006) JGR 111, B12201; [2] Goldberg et al. (2008) PNAS 105(29), 9920-9925.