CO2–water–mineral reactions during CO2 leakage: Geochemical and isotopic monitoring of a CO2 injection field test

• First application during CO2 injection field of multi-isotope approach (C, O, Sr, Li, B, S) in the context of CCS.
• CO2 intrusion as an analogue of fluid leakage in fresh groundwater.
• Evidence of contribution saline/CO2 end members through chemical and isotopic signal.
• Pertinent and efficiency of multi-isotope approach to track source(s) and geochemical processes

A shallow CO2 injection experiment was performed at the CO2FieldLab site near Svelvik, Norway in Sept. 2011. The gas was injected through a 45° inclined well to a target depth of 20 m. Its aim was to test various geophysical and geochemical monitoring tools for integrated near-surface leakage monitoring. A total of 1.67 t of CO2 was injected over a period of six days. Groundwater level was at 60 cm-depth and a complex salinity stratification with brackish water overlying fresh- and saltwater was observed at the beginning of the experiment. Four water sampling boreholes were located at each corner of a square configuration of 8 m with the nominal injection at the center. From baseline conditions to the post-CO2 injection phase, pH, temperature and electrical conductivity of the groundwater were measured, collecting water samples at three different depths (5, 10 and 15 m) for analyses of major, minor, and trace elements (Ca, Na, SO4, Cl, Mg, Al, Ba, Mn, Ni, Co, B, Li), and isotopes (δ11B, δ7Li, δ34SSO4, δ18OSO4, 87Sr/86Sr, δ18OH2O, δ2HH2O). Significant changes in chemical and isotope signatures of water over the duration of the experiment indicated two processes: 1) Binary mixing with seawater and rainwater as the possible end-members due to a near-seashore location, and 2) CO2–water–rock interactions enhanced by dissolved CO2 causing acid conditions and favoring rock dissolution. The relative contribution of those processes was quantified by a mass balance model and equilibrium calculations. This comprehensive geochemical and isotope approach allowed discriminating reactive mechanisms from non-reactive (mixing) processes associated with CO2 leaks, within an aquifer with a strong conductivity stratification and heterogeneous mineralogy and this approach seems promising for investigating CO2 leaks at field scale.