Quantification of carbon mineralization with reactive and non-reactive tracers in Icelandic basalts at the CARFIX site

Abstract:

The CARBFIX project near Reykjavik in Iceland includes is a field-based pilot injection to study the feasibility of permanent CO₂ (and H₂S) storage in permeable basalt formations [1]. Pure CO₂ and CO₂/H₂S mixtures from the Hellisheidi geothermal power plant were dissolved downhole in groundwater and injected into a permeable confined basalt formation at about 500m depth below ground. We are using non-reactive (sodium fluorescein, SF₅CF₃, and SF₆) and reactive (¹⁴C and ¹³C) tracers in the project to characterize subsurface CO₂ transport and in situ CO₂-water-rock reactions. In January 2012, 170 tons of pure CO₂ tagged with ¹⁴C and SF₆ were injected followed by 73 tons of a CO₂/H₂S mixture starting in June 2012. Samples were collected from injection and monitoring wells in evacuated serum glass bottles with butyl stoppers and analyzed by AMS and mass spectrometry for carbon isotopes, by fluorometry for Na-fluorescein and by gas chromatography for SF₆ and SF₅CF₃. Tracer breakthrough curves obtained from the first downstream monitoring well indicate that the injected water arrives in a fast short pulse and a late broad peak. ¹⁴C/SF₆ and DIC/SF₆ ratios are considerably lower in the monitoring wells as compare to the injection well. Evidence for carbonate precipitation was also found in the pump and on the pump lines in the monitoring well. The ¹⁴C/¹²C ratio of the precipitated carbon is the same as that of the DIC during injection and in the monitoring well, but distinct from ambient groundwater background. Mass balance calculations suggest that about 90% of the injected CO₂ has been mineralized over a period of only 2 years.

[1] Gislason et al. (2010), Int. J. Greenh. Gas Con. 4, 537-545.