Monitoring permanent CO₂ storage by in situ mineral carbonation using a reactive tracer technique

In situ mineral carbonation provides the most effective and permanent solution for geologic CO₂ storage. Basaltic rocks have the potential to store large volumes of CO₂ as (Ca, Mg, Fe) carbonates [1]. Existing monitoring and verification techniques for geologic CO₂ storage are insufficient to quantitatively characterize solubility and mineral trapping in a geologic reservoir. We developed and tested a new reactive tracer technique for quantitative monitoring and detection of dissolved and chemically transformed CO₂. The technique involves the active tagging of the injected CO₂ with low levels of radiocarbon (¹⁴C) as a reactive tracer in combination with the injection of non-reactive tracers such as sulfurhexafluoride (SF₆) and trifluoromethylsulphur pentafluoride (SF₅CF₃). The tracer technique has been applied at the CarbFix pilot injection site in Hellisheidi, Iceland as part of a comprehensive geochemical monitoring program during two injection phases; Phase III and IV. SF₆ and SF₅CF₃ confirm the arrival of the injected CO₂ and CO₂+H₂S solutions at the first observation well HNO₄, which is 125m west of the injection well at 520 m depth. The initial breakthrough of the migrating dissolved CO₂ front occurred 63 and 62 days after injection began as evidenced by an initial peak in the SF₆, SF₅CF₃, ¹⁴C, and dissolved inorganic carbon (DIC) concentrations. The major increase in the non-reactive tracer concentrations occurred several months after the initial breakthrough, although no major concentration increase has been observed for 14C and DIC suggesting that mineral reactions are dominant during CO₂ injection.