This innovative study, published in Nature Climate Change, was designed to understand how marine life on the seabed and in the water above might react to a real-life leakage, as well as determine methods for detection and monitoring of a small-scale carbon dioxide (CO2) leak event.
The research found that, for a leak of this scale, the environmental damage was limited; restricted to a small area and with a quick recovery of both the chemistry and biology.
The University of Southampton contributed to the active and passive acoustic monitoring, environmental chemistry, as well as the response of marine ecosystems. The National Oceanography Centre, Southampton had a major role in chemical monitoring and in planning the overall experiment.
Dr Rachael James worked with Professor Jon Bull and Dr Chris Hauton, together with post-doctoral researcher Dr Liz Morgan and PhD studentMelis Cevatoglu, from Ocean and Earth Sciences at the University of Southampton. She said: “These findings are contributing to the growing knowledge base necessary for deployment of CCS to alleviate climate change; in particular for the regulatory requirement for monitoring. The results show that small-scale leakage will not be catastrophic, although we do caution that impacts are likely to increase if a larger amount of CO2 is released. Water movement in the area is also important; impacts are estimated to be less and recovery quicker in environments with stronger water mixing so that the CO2 is dispersed more rapidly.”
The experiment, which took place in Scotland in 2012, saw the injection of 4.2 tonnes of CO2 (less than the annual CO2 emission of a gas-heated UK home*) over 37 days through a borehole to the release site, 350 meters from the shore and 11 meters below the seabed (see illustration below). A combination of chemical sensors and active and passive acoustic techniques were shown to provide the optimal monitoring technology to detect leakage or give assurance of no leakage.
The impact of this simulated leak shows that the impact of escaped CO2 on a similar scale would be limited. CO2-induced chemical changes occurred towards the end of the CO2 release but impacts including changes to environmental chemistry returned to background levels within 17 days of turning off the CO2 release.
No biological effect was observed during the early stages of the release. At the end of the release period and early in the recovery period, there was a change in seabed-dwelling communities as well as the gene expression of microbes. These impacts were not catastrophic or long-lasting and full recovery was seen in weeks.
The passive acoustic technique, developed by Professor Tim Leighton andPaul White at the University’s Institute of Sound and Vibration Research(ISVR) and deployed at sea by their PhD student Benoit Berges, successfully quantified the gas flux released from the seabed into the water column. Professor Leighton said: “I am particularly delighted that the monitoring technique we proposed in 2012** proved to be so practical to use. It worked here with standard off-the-shelf equipment, which makes its uptake by industry and other researchers far easier than if they had to build specialist equipment.
“Moreover, our passive acoustic technique managed remotely to measure the gas release continuously over seven days. Divers made one measurement during that time, collecting the gas with bottles. This measurement provided a useful data point against which we could confirm the accuracy of our passive acoustic system, but it showed the power of passive acoustics: its continuous measurements revealed how the amount of gas released from the seabed into the water correlates with the tide: the lower the water in the tidal cycle, the more gas was released.”
This study did not address the integrity of storage in reservoirs situated 1km or more below the sea floor, but addressed the “what if” scenario of leakage at the seabed. Leakage of CO2 from storage reservoirs is thought to be unlikely.
Recommendations for CCS operators developing risk strategies are:
• CCS site selection should be below dynamic bodies of water to promote dispersal of CO2 in the unlikely event of leakage.
• A comprehensive baseline study, encompassing sediment structure and content, sea water chemistry, biological community structure and ambient noise, is required to maximise monitoring efficiency.
• A combination of chemical and bubble-listening sensors will maximise early leakage detection or alternately provide assurance that leakage is not occurring.
* the typical annual energy use in a UK gas heated home equates to approximately 4.8 tonnes of CO2 emissions (Ofgem 2010).
** Leighton, T.G. and White, P.R. (2012) Quantification of undersea gas leaks from carbon capture and storage facilities, from pipelines and from methane seeps, by their acoustic emissions, Proceedings of the Royal Society A, 468, 485-510
