Carbon dioxide stored underground can find multiple ways to escape
When carbon dioxide is stored underground in a process known as geological sequestration, it can find multiple escape pathways due to chemical reactions between carbon dioxide, water, rocks and cement from abandoned wells, according to Penn State researchers.
The researchers investigated the properties of porous rocks into which carbon dioxide is injected. These rocks, known as host rocks, function like containers for the carbon dioxide. The team looked at two abundant host rocks, 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, associate professor of petroleum and natural gas engineering. "Even if it doesn't escape to the Earth's surface, there are concerns that it may leak into groundwater drinking aquifers."
In addition to encountering host rocks, carbon dioxide stored underground may also contact and dissolve into saltwater deposits. When this happens, the carbon dioxide increases the acidity of the saltwater. The high-acidity saltwater-carbon dioxide mixture can dissolve certain types of rocks, such as limestone, as well as 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, associate professor of petroleum and natural gas engineering and Quentin E. and Louise L. Wood Faculty Fellow in Petroleum and Natural Gas Engineering. "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."
To recreate natural settings, the researchers conducted an experiment by flowing carbon-dioxide-rich saltwater into two different systems -- cement embedded in a sandstone host rock, and cement embedded in limestone. They monitored the chemical reactions that took place and measured changes occurring in the host rocks and cement. Their findings, published in the current issue of the International Journal of Greenhouse Gas Control, indicate that the host rocks can create different types of escape pathways.
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Abandoned well image via Shutterstock.