Influence of Duration of Co2 Leakage on the Increment of Co2 Concentration in the Sea

Abstract

Some studies showed that CO2 continues to leak out to the sea for a long time in the event of CO2 leakage at offshore CO2 storage sites. These results make us wonder whether CO2 concentration in the sea continues to increase until CO2 leakage stops. To elucidate this, a numerical simulation was conducted for the area off Tomakomai, Japan. In the simulation, 4 passive tracers regarded as leaked CO2 in the dissolved form began to be released every 3 months, at 1st May, 1st August, 1st November and 1st February, respectively, at a constant rate. The concentration of the second, third, and fourth tracers became almost the same as that of the first one within at most 10 days at the release grid point. It is implied that tracer concentration is influenced by the season (or stratification/destratification) rather than the duration of the release. The concentration was high in summer and low in winter. Conversion of the concentration to increment of partial pressure of CO2 revealed that anomalous pCO2, which may imply CO2 leakage, is much harder to detect in winter than in summer. The model results showed that all CO2 leakage at 50,000 tonnes/year could not be detected in winter while CO2 leakage at 10,000 tonnes/year could be surely detected in summer in an area within about 400 m from the leakage point.