[H-SC08] [JJ] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC08-05] Acoustic sonar detectability of gas bubbles from seafloor for environmental monitoring at offshore CO₂ storage sites

*Takamichi Nakamura¹, Makoto Nishimura¹, Keisuke Uchimoto¹ (1.CO2 Storage Research Group, Research Institute of Innovative Technology for the Earth)

Keywords:offshore environmental monitoring, offshore CO2 geological storage, acoustic sonar, leakage detection

Geological CO₂ storage is proposed in sedimentary formations. Because storage sites are selected deliberately to minimize the risk of leakage, CO₂ is assumed to be stable in the reservoirs. CO₂ leakages from geological storage systems are unlikely, but still possible. At offshore CO2 storage site, CO₂ could leak out from the seafloor into the seawater. CO₂ leakage may lead to significant effects on the local environment. From the public and stakeholders, concerns about the risk of in situ leakage and ecological impacts are emerging. Therefore, it is the most important verification for geological CO₂ storage project that there is no detectable leakage or migration of CO₂ by environmental monitoring. Appropriate environmental monitoring methods and public communications are lead to secure social license and also needed to progress of safety geological CO₂ storage project. Further, in Japan, operators of offshore CCS are required to plan monitoring programs, as stated in the Act for the Prevention of Marine Pollution and Maritime Disasters. In the monitoring plan, an operator has to be able to determine the location and extent of any CO₂ leakage. Consequently, it is necessary to develop detection methods of CO₂ leakage in the sea.
For offshore environmental monitoring, acoustic methods were expected that could direct detection of CO₂ bubbles in the seawater. It was described that effectiveness of acoustic methods for seafloor and water column monitoring in IEAGHG special report (IEAGHG, 2012). In practice, it was used in seafloor survey of several offshore geological CO₂ storage projects. For example, in Sleipner, side scan sonar (SSS) and multibeam echo sounder systems (MBES) were used for seafloor observations on storage site (www.eco2-project.eu). In QICS project (Blackford et al., 2014) that CO₂ controlled release experiment from shallow seafloor, clearly images of both gas plume within the water column and pockmarks on the seafloor obtained using MBES (Cevatoglu et al., 2015). Furthermore, gas flux quantification using hydrophone, passive acoustic method was shown (Bergès et al., 2015). However, there were no data about flow rates of released CO₂ and MBES image data was used as check the location of releasing point. Detectability of acoustic methods was unclear.
This paper focuses specifically on the detectability of active acoustic method. Controlled compressed air release experiments were carried out in shallow inner bay at the depth of 5 to 6 m. Observations using active acoustic instruments such as multibeam sonar (MBS) and SSS were deployed to assess its detectability for gas bubbles stream.Analysis of the image acquired by active acoustic instruments led to the following conclusions:
·Images of MBS data could detect gas bubbles stream. The survey using MBS have the potential to be effective method for localization of leakage point in narrow area.
·Images of SSS data could detect gas bubbles stream. Images of SSS data showed high detectability of gas bubbles stream The survey using SSS have the potential to be effective method for detection of CO₂ leakage and localization of leakage point.
·Image processing and analyzing of SSS data, the quantification data of gas bubbles stream were obtained. Flow rates from any image data could estimate within order of magnitude, with relative high quantification accuracy.
·The survey using SSS have the potential to be effective method for not only leakage detection but also leakage scale grasp. The broad seafloor survey using active acoustic instruments such as SSS is crucial point of efficient leakage detection and localization.

Bergès, B.J.P., Leighton, T.G., White P.R. (2015) Passive acoustic quantification of gas fluxes during contlloed gas release experiments. Int. J. Greenh. Gas Cont. 38, 64-79.
Blackford, J., Stahl, H., Bull, J.M., et al. (2014) Detection and impacts of leakage from sub-seafloor deep geological carbon dioxide storage. Nat. Clim. Chang. 4, 1011–1016.
Cevatoglu, M., Bull, J.M., et al. (2015) Gas migration pathways, controlling mechanisms and changes in sediment acoustic properties observed in a controlled sub-seabed CO₂ release experiment. Int. J. Greenh. Gas Cont.38, 26-43.
IEAGHG. (2012) Quantification Techniques for CO₂ Leakage. Report. 2012/02.