The impact of water on CO₂ leak rate measurements for CCS projects

Chemical tracers are a promising technique to detect, attribute and quantify CO₂ leakage from geological CO₂ stores. Indeed, 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. To model some of these challenges, we used a benchtop experimental setup to explore how well methane, a common constituent of captured CO₂ and of reservoir fluids, could 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. This has implications for leak monitoring approaches, including how tracers may be used to quantify the leak rates and fate of CO₂ in aqueous environments.