Cost assessment of optimal seawater monitoring for unexpected CO2 leakage from an offshore reservoir

Abstract

Carbon Capture and Storage (CCS) has garnered attention as a mitigation strategy for the global warming and a means to achieve carbon neutrality. In Japan, monitoring CO₂ concentration in the sea is mandatory for subsea geological CO₂ storage due to concerns about the potential negative impact on marine environments in the event of accidental CO₂ leakage. However, the monitoring method currently conducted in Japan is not only expensive but also lacks fully scientific grounding. In this study, we aimed to develop a method for optimising the observation point placement to detect biologically significant CO₂ seepage. In the method, observation areas were identified based on the advection-diffusion simulation of dissolved CO₂ concentration emitted from every location in a possible seepage area when the CO₂ seepage rate was varied from 1000 to 25000 tonnes/year. Then, within the observation area, optimal observation locations were determined using a greedy algorithm. Finally, monitoring costs were calculated based on the number of observation points, the travel distance of an observation boat, and necessary observation days. The results showed a negative nonlinear relationship between detectable seepage rates and monitoring costs, which decreases rapidly up to seepage rates of 5000 tonnes/year and then remains almost constant above 10000 tonnes/year. This study suggests that monitoring policies could be determined according to risk tolerance and budget constraints for seepage that must be detected considering biological impacts.