World-first experiment on a controlled sub-seabed CO2 leak demonstrates minimal environmental impact and rapid recovery

The Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS) project was led by Plymouth Marine Laboratory’s (PML) Jerry Blackford and funded by the Research Councils UK, the Natural Environment Research Council, the Scottish and Japanese Governments. The University of Southampton and National Oceanography Centre Southampton both made a substantial contribution to the experimental controlled release of CO2 undertaken in Ardmucknish Bay, (near Oban) Scotland in 2012, together with a number of UK and Japanese institutes; the experiment was co-ordinated by the Scottish Association for Marine Science (SAMS). The University of Southampton contributed particularly 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.

4.2 tonnes of CO2 (less than the annual CO2 emission of a gas-heated UK home*) was injected over 37 days from a land-based lab via a borehole drilled through rock to the release site, 350 meters from the shore and 11 meters below the seabed (see illustration below). Scientists initially monitored how the CO2 moved through the sediment and the 12 meters of water above. Over the following 12 months the impact on the chemistry and biology of the surrounding area was assessed using a combination of techniques, including chemical sensors, listening for bubbles and diver-mediated sampling.

A combination of chemical sensors and active and passive acoustic techniques are 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 and Paul White at the Institute of Sound and Vibration Research 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 7 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.”

Professor White of ISVR Said “This experiment allowed us to demonstrate how acoustics can play a critical role in the carbon capture and storage: a technology which has the potential to assist in controlling human impacts on climate change and in which the UK is world leading”. Prof White co-supervised, with Prof Leighton, their PhD student Benoit Berges, who said “Going up to Oban, living in a caravan in winter for few weeks and then going out to sea with rough weather conditions to deploy the instruments, proved to be an enjoyable physical and mental challenge. During this project, I had the opportunity to collaborate with inspirational scientists from various institutes that work in other disciplines from mine. Overall, it was a fabulous opportunity to me, and it added tremendous value to my PhD in terms of personal enjoyment to carry out work in an outdoor environment”.

Dr Rachael James worked with Prof Jon Bull and Dr Chris Hauton, together with post-doctoral researcher Dr Liz Morgan and PhD student Melis Cevatoglu, in 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”.

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