THE CARBON CONUNDRUM

It’s becoming a race against time to deal with the billions of metric tonnes of carbon dioxide we produce around the world every year – and, in doing so, curtail temperature increases that cause severe environmental damage.

Capturing and storing CO₂, then locking it away offshore, is one climate change mitigation strategy that will become big business in the coming years. But can we give the public confidence that this method is safe and will make a significant contribution to reducing carbon emissions?

A team of researchers in Southampton played a key role in a major European project to determine the viability of safely and securely storing carbon offshore. This is one method that will soon be employed to meet global CO₂ reduction requirements.

The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project was funded by Horizon 2020 and drew on expertise from 14 institutions across Europe. The project ran from 2016 to 2020.

The University of Southampton was awarded £2.4 million for its involvement in STEMMCCS. Jon Bull, Professor of Geophysics and Associate Dean (Research) for the Faculty of Environmental and Life Sciences, and Rachael James, Professor of Geochemistry, led a team of more than 20 researchers from Southampton, including a large contingent of Early Career Researchers.

Jon said: “Our main role in the project was the development of the concept, and contributions to sensing and verification methods. We conducted acoustics experiments as well as chemical sensing.”

Southampton researchers developed the concept for STEMM-CCS, alongside the National Oceanography Centre (NOC). Jon, Professor Douglas Connelly, Associate Director for Research at NOC, and Professor Matthew Mowlem, Head of the Ocean Technology and Engineering Group at NOC, led on the initial concept and proposal.

Jon explained: “One of the biggest things to happen in the next 20 years will be the largescale roll-out of carbon capture and storage under the seabed, several hundred metres down. A key issue is public confidence in CCS, so it is important to be able to give certainty that in the unlikely event stored CO₂ escapes, it can be detected.

“STEMM-CCS was focused on demonstrating that technologies exist which can monitor, measure and verify the source of any CO₂ escaping across the seabed.”

Bubble monitoring

The passive acoustics experiments, led by the Southampton team, involved monitoring bubbles from a controlled release of CO₂ at the seabed. The experiments were conducted in the exclusion zone around the Goldeneye platform, in the North Sea. An autonomous underwater vehicle, named Gavia, surveyed the seabed and sub-seabed before and during the experiment.

“We put large tanks of pressurised CO₂ on the seabed, about 120 metres below the water surface,” explained Jon. “Then we drilled a hole to release the CO₂ about three metres beneath the seabed, and then monitored where we could see the CO₂, quantified it, and checked it was the CO₂ we had injected and not natural CO₂.

“When you release CO₂ in the sub-surface, some will emerge as bubbles, some will dissolve, and some will stay trapped below the seabed. We determined the relative amount that came out as opposed to that trapped beneath the surface.

“There were lots of different ways we monitored it. One was to listen to the bubbles that came out of the seabed – we used hydrophones to detect the sound of the bubbles, and from that we could determine the quantity of CO₂ that was bubbling out.”

Chemical detection

Injected CO₂ that dissolved into the sediment pore waters and in the water overlying the seabed was detected using different sensors, including a pH sensor, as well as highly precise analysis of inorganic carbon compounds in seawater samples. However, as CO₂ is produced naturally in the marine environment, it was important to verify that the chemical changes observed were caused by the injected CO₂.

Rachael said: “We labelled the injected CO₂ with tiny quantities of non-toxic chemicals. Analyses on board the ship showed that water with high CO₂ also contained the chemical labels, confirming that we sampled the injected CO₂. If you are operating a CCS site, this is really important because you don’t want to be held responsible for somebody else’s leak!

“This was the first time that these chemical labels had been used in the marine environment, and they worked really well.”

Meeting EU requirements

Through these experiments, the team was able to prove it is possible to detect small CO₂ leakages from sub-surface reservoirs – making carbon capture and storage a viable solution to help meet EU targets to tackle climate change.

The EU’s CCS Directive lays down requirements for CO₂ storage, including the requirement to be able to detect, quantify and verify where any escaped CO₂ has come from.

“What we demonstrated is that we can detect and quantify and verify the CO₂ coming out in the way that’s required by the governmental directive,” said Jon. “That gives confidence in carbon capture and storage.”

The monitoring techniques tested by STEMMCCS will enable large energy companies to plan their monitoring strategies for CCS.

“These monitoring techniques need to be deployed over the lifetime of an offshore CCS site, at a depth of between 50 metres and 200 metres,” explained Jon. “The issue is the lifespan of these sites, as each site will be injected with CO₂ for about 20 to 30 years. Sites will need to be constantly monitored to ensure CO₂ remains captured. We have shown the technology to work, but the long-term operation of this in a cost-effective manner is the issue we are now working on.”

In support of this, the team is now looking at the possibility of using autonomous underwater vehicles to collect chemical and acoustic data.