Chemical monitoring for shallow-water subsea CCS: Anomaly detection and sensor coverage

Chemical monitoring of dissolved carbon dioxide (CO₂) in seawater provides assurance for subsea carbon capture and storage (CCS) but can be confounded by high natural variability, and rapid dilution of any anomalous additions, placing strict limits on leak detection. In this study, we demonstrate how pre-injection baseline monitoring data can be combined with a plume dispersal model to provide guidelines for monitoring design, using a proposed CCS shallow water location on the Australian continental shelf as an example. This avoids the need to accurately simulate background CO₂ variability in an ecosystem model for the purpose of performing CO₂ leak simulations. In the test region, unusually steep spatial concentration gradients associated with the model simulated CO₂ leak plume means that higher frequency chemical sensor sampling is expected to improve anomaly detection. We determine that any leak in the test region should be detectable by chemical monitoring of pH by sampling at a minimum rate of 4 hr^-1 within the area affected by a drop in pH of at least 0.01 over a 60-day monitoring period. The leak-affected area depends on the leak rate and is shown to increase over time due to variable flow direction and decrease with average water velocity due to higher rates of dilution. The dimensions of the leak-affected area are reported for leak rates between 5 and 50 t CO₂d^-1 providing critical information for monitoring design in the region.