Development of detection and monitoring techniques of CO2 leakage from seafloor in sub-seabed CO2 storage

Carbon dioxide capture and storage (CCS) in sub-seabed geological formations is currently being studied as a potential option to mitigate the accumulation of anthropogenic CO₂ in the atmosphere. To investigate the validity of CO₂ storage in the sub-seafloor, development of techniques to detect and monitor CO₂ leaked from the seafloor is vital. Seafloor-based acoustic tomography is a technique that can be used to observe emissions of liquid CO₂ or CO₂ gas bubbles from the seafloor. By deploying a number of acoustic tomography units in a seabed area used for CCS, CO₂ leakage from the seafloor can be monitored. In addition, an in situ pH/pCO₂ sensor can take rapid and high-precision measurements in seawater, and is, therefore, able to detect pH and pCO₂ changes due to the leaked CO₂. The pH sensor uses a solid-state pH electrode and reference electrode instead of a glass electrode, and is sealed within a gas permeable membrane filled with an inner solution. Thus, by installing a pH/pCO₂ sensor onto an autonomous underwater vehicle (AUV), an automated observation technology is realized that can detect and monitor CO₂ leakage from the seafloor. Furthermore, by towing a multi-layer monitoring system (a number of pH/pCO₂ sensors and transponders) behind the AUV, the dispersion of leaked CO₂ in a CCS area can also be observed. Finally, an automatic elevator can observe the time-series dispersion of leaked CO₂. The seafloor-mounted automatic elevator consists of a buoy equipped with pH/pCO₂ and depth sensors, and uses an Eulerian method to collect spatially continuous data as it ascends and descends.Hence, CO₂ leakage from the seafloor is detected and monitored as follows. Step 1: monitor CO₂ leakage by seafloor-based acoustic tomography. Step 2: conduct mapping survey of the leakage point by using the pH/pCO₂ sensor installed in the AUV. Step 3: observe the impacted area by using a remotely operated underwater vehicle or the automatic elevator, or by towing the multi-layer monitoring system.