Detection and monitoring of leaked CO2 through sediment, water column and atmosphere in a sub-seabed CCS experiment

Abstract

Carbon capture and storage in sub-seabed geological formations (sub-seabed CCS) is currently being studied as a realistic option to mitigate the accumulation of anthropogenic CO2 in the atmosphere. In implementing sub-seabed CCS, detecting and monitoring the impact of the sequestered CO2 on the ocean environment is highly important. The first controlled CO2 release experiment, Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS), took place in Ardmucknish Bay, Oban, in May–September 2012.We applied the in situ pH/pCO2 sensor to the QICS experiment for detection and monitoring of leaked CO2, and carried out several observations. The cabled real-time sensor was deployed close to the CO2 leakage (bubbling) area, and the fluctuations of in situ pH and pCO2 above the seafloor were monitored in a land-based container. The long-term sensor was placed on seafloor in three different observation zones. The sediment pH sensor was inserted into the sediment at a depth of 50cm beneath the seafloor near the CO2 leakage area. Wide-area mapping surveys of pH and pCO2 in water column around the CO2 leakage area were carried out by using an autonomous underwater vehicle (AUV) installed with sensors. Atmospheric CO2 above the leakage area was observed by using a CO2 analyzer that was attached to the bow of ship of 50cm above the sea-surface.The behavior of the leaked CO2 is highly dependent on the tidal periodicity (low tide or high tide) during the CO2 gas release period. At low tide, the pH in sediment and overlying seawater decreased due to strong eruption of CO2 gas bubbles, and the CO2 ascended to sea-surface quickly with a little dissolution to seawater and dispersed into the atmosphere. On the other hand, the CO2 bubbles release was lower at high tide due to higher water pressure, and slight low pH seawater and high atmospheric CO2 were detected. After stopping CO2 gas injection, no remarkable variations of pH in sediment and overlying water column were observed for three months.