DETECTABLE AREA OF ANOMALOUS pCO<sub>2</sub> IN THE SEA DUE TO CO<sub>2</sub> LEAK FROM SUB-SEABED CO<sub>2</sub> STORAGE

Monitoring CO2 concentration in the sea is mandatory at offshore CO2 storage sites in Japan. This paper discusses whether anomalous partial pressure of CO2 (pCO2) due to CO2 leak could be detected by observing pCO2, based on a simulation of passive tracers in Hidaka Bay off Tomakomai. The area where anomalous pCO2 due to CO2 leak could be detected is much larger in summer than in winter, and the area where the probability of detection is higher than 25% is estimated to be roughly at most 0.4 km2 even in summer if the observation is sporadic. This implies that approximately 90 observation points are necessary within 6 km by 6 km while 12 points were arranged in the Tomakomai CCS Demonstration project. It is suggested that CO2 leak could be hardly detected by a sporadic observation of pCO2 unless unrealistically dense observation points were deployed, and that few of the observed pCO2 data that exceed the threshold would be due to CO2 leak. If monitoring CO2 concentration in the sea is required, it should be conducted at a few important areas such as fishing grounds and aquaculture farms around the CO2 storage site to verify that CO2 concentration is within the natural variability, rather than to detect CO2 leakage.