Less carbon dioxide than expected may actually turn to rock to be locked away for ever by current carbon sequestration techniques

Carbon sequestration less efficient than expected study hints

American researchers have found that less carbon dioxide injected into the ground as part of carbon capture and storage techniques could be permanently locked in the rock than previously thought.

The finding, published in the latest issue of the journal Proceedings of the Royal Society A, casts a shadow over current prospects to use carbon capture and storage to remove up to 90 per cent of the CO2 produced by coal-fired power plants before it enters the atmosphere and thus slow down global warming.

“The expectation was that most of the carbon dioxide would become solid mineral. Our work suggests that significantly less will precipitate,” said Yossi Cohen, a postdoctoral researcher at the Massachusetts Institute of Technology, who led the study.

Current geologic carbon-sequestration techniques aim to inject carbon dioxide into the subsurface more than 2,000 metres below the Earth’s surface. At such depths, carbon dioxide may be stored in deep-saline aquifers - large pockets of brine that can chemically react with carbon dioxide to solidify the gas.

The researchers modelled the reaction involved in carbon sequestration in a computer simulation and found that contrary to previous expectations only a small fraction of the CO2 involved turns to rock, which would lock it away forever.

“If it turns into rock, it’s stable and will remain there permanently,” Cohen said. “However, if it stays in its gaseous or liquid phase, it remains mobile and it can possibly return back to the atmosphere.”

The majority of the greenhouse gas remains gaseous because of a sort of clogging effect that takes place when carbon dioxide dissolved in water reacts with brine, the study found. Instead of solidifying throughout the bulk, CO2 only turns into rock at the interface, creating a solid wall that prevents the rest of the gas from reacting with the brine.

“This can basically close the channel, and no more material can move farther into the brine, because as soon as it touches the brine, it will become solid,” Cohen said.

The researchers said their findings need to be verified in an experiment in order to draw any conclusions.

“Experiments would help determine the kind of rock that would minimise this clogging phenomenon,” Cohen said. “There are many factors, such as the porosity and connectivity between pores in rocks that will determine if and when carbon dioxide mineralises. Our study reveals new features of this problem that may help identify the optimal geologic formations for long-term sequestration.”

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