Geological Storage of Carbon Dioxide: Utility of Tracers for Detection and Quantification of CO2 Leakage in the Marine Environment

Carbon capture and storage (CCS) is a key technology to timely, feasibly and substantially reduce carbon dioxide (CO₂) emissions from industrial processes. Although decades of experience in methane storage and CO₂ injection for enhanced oil recovery provide confidence in the performance of gas injection and storage, leakage of the injected CO₂ remains a potential risk. Thus, for geological CO₂ storage to be safe and reliable, robust strategies for CO₂ leak detection, quantification and management are crucial. STEMM-CCS (STrategies for Environmental Monitoring of Marine Carbon Capture and Storage) is an interdisciplinary project which explores monitoring strategies for the offshore, sub-seabed storage of CO₂. To test the utility of different techniques for CO₂ leakage detection and quantification at the seafloor, a controlled CO₂ release experiment was carried out in the central North Sea, whereby CO₂ gas was injected into sub-seabed sediments at different rates over a period of 11 days. Amongst others, the use of tracers for CO₂ leakage detection and quantification at the seafloor was examined. We used a combined approach of natural, inherent tracers (δ¹³C, δ¹⁸O) of injection CO₂ and a set of non-toxic tracer gases (octafluoropropane (C₃F₈), sulfur hexafluoride (SF₆) and krypton (Kr)) that were mixed into the injected CO₂. To detect the impact of CO₂ dissolution in the pore water and water column we collected (i) gas bubbles from seafloor seeps for analysis of CO₂, SF₆, C₃F₈, Kr composition and isotopic signature (δ¹³C_CO2, δ¹⁸O_CO2), and (ii) water from above the bubble stream for analysis of the carbonate system of seawater, isotopic signature (δ¹³C_DIC, δ¹⁸O_H2O) and dissolved tracers (SF₆, C₃F₈, CH₄, Kr). We will present first results on the performance of the tracer approach for the detection and quantification of a CO₂ leak in the marine environment.