Tracing the movement of CO2 injected into a mature oilfield using carbon isotope abundance ratios: The example of the Pembina Cardium CO2 Monitoring project

During CO2 storage operations in mature oilfields or saline aquifers it is desirable to trace the movement of injected CO2 for verification and safety purposes. We demonstrate the successful use of carbon isotope abundance ratios for tracing the movement of CO2 injected at the Cardium CO2 Storage Monitoring project in Alberta between 2005 and 2007. Injected CO2 had a δ13C value of −4.6 ± 1.1‰ that was more than 10‰ higher than the carbon isotope ratios of casing gas CO2 prior to CO2 injection with average δ13C values ranging from −15.9 to −23.5‰. After commencement of CO2 injection, δ13C values of casing gas CO2 increased in all observation wells towards those of the injected CO2 consistent with a two-source endmember mixing model. At four wells located in a NE-SW trend with respect to the injection wells, breakthrough of injected CO2 was registered chemically (>50 mol% CO2) and isotopically 1–6 months after commencement of CO2 injection resulting in cumulative CO2 fluxes exceeding 100,000 m3 during the observation period. At four other wells, casing gas CO2 contents remained below 5 mol% resulting in low cumulative CO2 fluxes (<2000 m3) throughout the entire observation period, but carbon isotope ratios indicated contributions between <30 and 80% of injected CO2. Therefore, we conclude that monitoring the movement of CO2 in the injection reservoir with geochemical and isotopic techniques is an effective approach to determine plume expansion and to identify potential preferential flowpaths provided that the isotopic composition of injected CO2 is constant and distinct from that of baseline CO2.