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Carbon Dioxide Stored Underground May Escape Back to the Atmosphere

First Posted: Feb 12, 2016 02:27 PM EST

Carbon dioxide that's stored underground may often escape. Scientists have found that when this gas is stored, it can find multiple pathways to the surface due to chemical reactions between carbon dioxide, water, rocks and cement from abandoned wells.

In this latest study, the researchers took a closer look at the properties of porous rocks into which carbon dioxide is injected. These rocks, known as host rocks, function like containers for the carbon dioxide. In this case, the researchers looked at limestone and sandstone, which have different chemical properties.

"We were interested in examining these rocks because they are widely found underground, but there have been concerns that carbon dioxide may escape once it's injected underground," said Li Li, one of the researchers, in a news release. "Even if it doesn't escape to Earth's surface, there are concerns that it may leak into groundwater drinking aquifers."

Carbon dioxide that's stored underground can contact and dissolve into saltwater deposits. When this happens, the carbon dioxide increases the acidity of the saltwater. The high-acidity salter-carbon dioxide mixture can then dissolve certain types of rocks, such as limestone, in addition to cement casings on abandoned oil and gas wells.

"If this plume of carbon dioxide-saturated brine reaches an abandoned well, it will react with the cement," said Zuleima Karpyn, one of the researchers. "This may open up cracks in the cement depending on the conditions, which would increase the likelihood of carbon dioxide escaping. We were trying to assess what would happen in the process if the host rock itself were to react with the carbon dioxide-saltwater mixture."

What did they find? In limestone interactions, the rock becomes the dominant medium for the dissolution reaction while cement was the secondary reactant. In other words, wellbores are more likely to stay intact if they have limestone. However, dissolving limestone can create leakage pathways.

The findings highlight the complexity of underground carbon sequestration.

The findings are published in the International Journal of Greenhouse Gas.

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