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

Monday, 15 December 2014: 5:45 PM
Martin Stute1, Juerg Matter2, Sigurður R Gíslason3, Sandra Snæbjörnsdóttir3, Eric H Oelkers4, Ingvi Gunnarsson5, Edda S Aradottir5, Bergur Sigfússon5 and Helgi A Alfreðsson3, (1)Barnard College, New York, NY, United States, (2)University of Southampton, Southampton, United Kingdom, (3)University of Iceland, Reykjavik, Iceland, (4)CNRS, Toulouse, France, (5)Reykjavik Energy, Reykjavik, Iceland
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.