Utility of natural and artificial geochemical tracers for leakage monitoring and quantification during an offshore controlled CO2 release experiment

To inform cost-effective monitoring of offshore geological storage of carbon dioxide (CO 2), a unique field experiment, designed to simulate leakage of CO 2 from a sub-seafloor storage reservoir, was carried out in the central North Sea. A total of 675 kg of CO 2 were released into the shallow sediments (∼3 m below seafloor) for 11 days at flow rates between 6 and 143 kg d -1. A set of natural, inherent tracers ( 13C, 18O) of injected CO 2 and added, non-toxic tracer gases (octafluoropropane, sulfur hexafluoride, krypton, methane) were used to test their applicability for CO 2 leakage attribution and quantification in the marine environment. All tracers except 18O were capable of attributing the CO 2 source. Tracer analyses indicate that CO 2 dissolution in sediment pore waters ranged from 35 % at the lowest injection rate to 41% at the highest injection rate. Direct measurements of gas released from the sediment into the water column suggest that 22 % to 48 % of the injected CO 2 exited the seafloor at, respectively, the lowest and the highest injection rate. The remainder of injected CO 2 accumulated in gas pockets in the sediment. The methodologies can be used to rapidly confirm the source of leaking CO 2 once seabed samples are retrieved.