DETECTABLE AREA OF ANOMALOUS pCO₂ IN THE SEA DUE TO CO₂ LEAK FROM SUB-SEABED CO₂ STORAGE

Abstract

 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.