Keywords:offshore CO2 storage, marine monitoring, Leak detection
Carbon dioxide (CO2) Capture and Storage (CCS) is expected to play an important role in reducing CO2 emissions, which is essential for mitigating global warming. CO2 is believed to be stored safely and stable if storage sites and reservoirs are properly selected and managed. However, monitoring to detect CO2 leakage is essential. First, it is because the monitoring is believed to help win public acceptance for CCS. Second, it is because the monitoring is mandatory in many countries and regions. In Japan, CO2 storage under the seabed, that is offshore CO2 storage, is an important option, so that monitoring methods and strategies in the marine environment should be developed. In this paper, we outline the methods and strategies that we have developed. CO2 takes the form of bubbles if it leaks out from the seabed of shallow sea. Since CO2 bubbles are relatively easy to dissolve in seawater, there are two approaches to detect CO2 leakage; one is to detect CO2 bubbles in the water column, and the other is to detect increment of dissolved CO2 in seawater. To detect CO2 bubbles, we propose to use side-scan sonar (SSS). We have conducted an in situ experiment where CO2 bubbles were released at the seabed of shallow sea and searched for with SSS towed by a vessel. The experiment has shown the minimum release rate of bubbles and the farthest distance from SSS that SSS can detect. To detect increment of dissolved CO2 in seawater, we have studied a threshold method to judge the measured pCO2 (partial pressure of CO2) to be normal or anomalous. We have proposed a pCO2-DO covariance method. We emphasize that it is difficult to make a threshold that never makes both false-positives and -negatives. There remains a challenge in the monitoring: where to monitor? Even if a good method is used in the monitoring, CO2 leakage would never be detected if it occurred outside the monitoring area. Hence, we propose to conduct what is called a deep-focussed monitoring which is the monitoring of reservoirs and overburden before the marine monitoring (which is a shallow-focussed monitoring). To conduct the marine monitoring in the area where CO2 would leak out, the possibility of the leak detection becomes high.