An experimental investigation into quantifying CO2 leakage in aqueous environments using chemical tracers

Chemical tracers can be an effective means of detecting, attributing and quantifying any leaks to the surface from geological CO2 stores. CO2 release experiments have found it difficult to ascertain the fate, or quantify the volume of CO2 without the application of tracers. However, a significant proportion of global CO2 storage capacity is located offshore, and the marine environment poses constraints that could limit the success of using tracers. These constraints include uncertainties in the behaviour of tracers in marine sediments and the water column and sampling challenges. However, to date there have been few experimental investigations to address these uncertainties. Here, we used a benchtop experimental setup to explore how effectively methane, a common constituent of captured CO2 and of reservoir fluids, can aid the quantitation of CO2 leakage in aqueous environments. The experiment simulated gas leakage into sediments that mimic the seabed, and we measured the partitioning of co-released gases under different environmental conditions and injection rates. We find that the style of seepage and the fate of the CO2 are affected by the presence of a sand layer and the injection rate. We discuss the implications for leak monitoring approaches, including how tracers may be used to quantify the leak rates and fate of CO2 in aqueous environments. Our work contributes to ongoing efforts to develop robust offshore monitoring system that will assure operators, regulatory bodies and the public of CO2 storage integrity.