Method puts carbon-capture and storage 'leaks' to test

Researchers at Edinburgh University said they have used the technique to rule out a suspected leak from a CCS storage site in Canada.

It could provide added assurance for future plants and eventually be integrated into plant architecture as part of a remote, closed-loop monitoring system.

‘Geologically speaking, if it [CO₂] is put in the right place, we’re convinced we can keep it there. We’ve got evidence from previous work for 40–60 million years storage from CO₂-rich oil and gas fields,’ said project collaborator Dr Stuart Gilfillan of Edinburgh.

‘There’s always going to be a risk that a small amount of leakage could occur, but it’s a risk that can be managed; what we’re doing with this test is producing a way that you would be able to detect that leakage very early to take steps to remediate it.’

The team’s method is based on the analysis of noble gases such as helium, neon and xenon, which naturally co-occur with CO₂ in defined proportions depending on the original source of the gas.

Specifically, deeper CO₂ sources tend to be marked by the presence of helium-3 and helium-4 that were trapped within the mantle when the Earth formed.

The team performed validation studies at St John’s Dome on the border of Arizona and New Mexico in the US. The site has two sources of naturally occurring CO₂ — a deep geological reservoir and shallower dissolved gas in underground water.

Surprisingly, the team found much more helium-4 in the samples from the springs and groundwater wells than would be expected. It demonstrated that this was due to a fault line in the region allowing deep CO₂ and helium-4 to escape to the shallower waters.

Using this principle it established that high CO₂ levels reported at a farm in Saskatchewan, Canada, arose from wetlands rather than leakage from a CCS site at nearby Weyburn Oil Field.

Discussing how the technique might be implemented in the future, Gilfillan said: ‘In some cases it may be possible to use what’s already in existence in a storage site, just because it might contain a fairly unique signature at depth anyway.’

’In other cases you could inject different traces in with the CO₂, such as xenon, which is relatively cheap, and we now know that it will travel up with the CO₂ should it leak out of the storage site — you could measure for that and conclusively show that you have a problem.’

At present, the actual experimental set-up requires fairly bulky mass spectrometry equipment, but if the aforementioned artificial signatures are used it may be possible to design a remote-monitoring sensing system specifically for the signatures.