Underground carbon capture

Carbon dioxide in the Earth’s crust is stored mainly by dissolution into underground water, rather than by being chemically bound up in minerals, according to new research.

Capturing carbon dioxide from industrial sources and storing it in deep, depleted oil and gas reservoirs is one potential way to lessen the impact of this greenhouse gas on global climate.

But a major concern with the technique is whether the carbon dioxide will stay underground, or simply leak back into the atmosphere.

The answer depends to a large extent on how carbon dioxide is naturally held underground in these reservoirs.

To find out whether it is primarily chemically bound in minerals, or dissolved in groundwater, a British-Canadian research team measured the ratios of the isotopes of carbon, helium and neon in samples from oil and gas reservoirs in the US, China and Europe.

The isotopes act as tracers of carbon dioxide’s subterranean behaviour.

‘At sites throughout the world, we found that the major way CO2 is stored is by dissolution into the underground water, rather than by being chemically bound up in minerals,’ said Barbara Sherwood Lollar, a University of Toronto geochemist, whose NSERC-supported lab analysed the carbon isotope ratios.

Importantly, the team discovered that these underground reservoirs of naturally carbonated water have been geologically stable for tens of thousands to millions of years.

The carbon dioxide in these waters accumulated naturally from deep within the earth.

‘What’s really compelling about what we found is not just that the carbon dioxide dissolves into the ground water, but that this is overwhelmingly the dominant storage mechanism,’ said Lollar.

The research was led by Stuart Gilfillan as his PhD research at Manchester University, and completed while he was at Edinburgh University.