Cap rock keeps a tight lid on greenhouse gases.
Carbon capture and storage (CCS), in which carbon dioxide (CO2) produced by power stations is captured, compressed, and pumped into reservoirs in the rock more than a kilometre underground, is one possible way to reduce carbon emissions and meet climate change targets. But concerns have been raised that the greenhouse gas could leak back out.
To figure out how secure this storage method is, researchers assessed natural reservoir in Utah, USA, where CO2 released from deeper formations has been trapped for around 100,000 years. This reservoir provided a good test because its CO2 has been buried much longer than the 10,000 years required to avoid impacts on climate.
Writing in the journa Nature Communications, the researchers find no evidence that CO2 has leaked out of the natural reservoir, suggesting that this method of storage is much safer over long periods of time than previously thought.
The critical component of geological carbon storage is the relatively impermeable layer of "cap rock" that retains the CO2.
"Some earlier studies, using computer simulations and laboratory experiments, have suggested that these cap rocks might be progressively corroded by the CO2-charged brines, formed as CO2 dissolves, creating weaker and more permeable layers of rock several metres thick and jeopardising the secure retention of the CO2," explains lead author Mike Bickle of the University of Cambridge, in a press release.
To determine the extent to which the acidic carbonated water had corroded the minerals in the cap rock and how this affected its ability to keep the CO2 trapped, the researchers analyzed the mineralogy and geochemistry of cap rock, bombarding samples with neutrons to identify changes that may have occurred in its pore structure and permeability.
They found that the CO2 had very little impact on corrosion of the cap rock minerals. “Our study demonstrates that geological carbon storage can be safe and predictable over many hundreds of thousands of years," Bickle says, adding that with “careful evaluation, burying carbon dioxide underground will prove very much safer than emitting CO2 directly to the atmosphere."
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