Rapid permanent carbon dioxide storage via mineralisation in basalt

Dr. Juerg M. Matter (University of Southampton)

Mineralisation of CO₂ via CO₂-fluid rock reactions has been proposed as the most permanent solution for geologic CO₂ storage. However, as of today, it was unclear how fast CO₂ is converted to carbonate minerals in-situ in geologic storage reservoirs. The CarbFix I project in Iceland was designed to verify in-situ CO₂ mineralisation in an unconventional storage reservoir in basaltic rocks. Two injection tests were conducted at a pilot injection site near the Hellisheidi geothermal powerplant, SE of Reykjavik. 175 tons of pure CO₂ and 73 tons of a CO₂+H₂S mixture were injection from January to March 2012 and in June 2012, 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 monitored and detected 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. Mass balance calculations using the tracer data reveal that up to 95% of the injected CO₂ has been mineralized over a period of two years. This is supported by multiple evidence of carbonate precipitation found in core samples and on the submersible pump in the monitoring well.