[HSC08-P04] An option for marine monitoring at offshore CO2 storage sites: observing pCO2 in the sea
Keywords:offshore storage, CCS, partial pressure of CO2, dissolved oxygen, marine monitoring
Carbon dioxide (CO2) capture and storage (CCS) is a promising option to reduce CO2 emissions and consequently to mitigate global warming. Since reservoirs capable of storing CO2 stably are selected, the risk of CO2 leakage is extremely low. There is, however, concern that CO2 might leak out. To verify that CO2 is not leaking, as well as to detect CO2 leakage if leakage occurs, monitoring is important. In offshore storage, since leaked CO2 would go out into the sea out of the seabed, monitoring in the sea is necessary. However, there seems to be no monitoring method that is useful in all sea areas or for all situations. In the practical monitoring, it is necessary to combine a few methods according to the situation and the sea area. Partial pressure of CO2 (pCO2) in the sea is one of items to monitor since pCO2 in the sea would increase by leaked CO2. However, it is difficult to distinguish high pCO2 values due to CO2 leakage from those due to natural variability in some areas. In the present study, we discuss a method to assess anomalously high values of pCO2 using not only pCO2 but also dissolved oxygen. As an example, we analyzed data observed in Osaka Bay. We have shown that the method using both pCO2 and DO is effective in the eastern (innermost) part, where stratification is relatively strong throughout the year. However, the method is less effective in the western part of Osaka Bay, where water is relatively well mixed vertically due to strong tidal currents. We have concluded that observing pCO2 and assessing it based on both pCO2 and DO is potentially a useful option for marine monitoring although this method is not effective in all sea areas.