Carbon dioxide stored underground can find multiple ways to escape

http://www.eurekalert.org/pub_releases/2016-02/ps-cds021116.php

Public Release: 11-Feb-2016
Carbon dioxide stored underground can find multiple ways to escape
Penn State

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.

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"In the limestone interactions, the rock itself becomes the dominant medium for the dissolution reaction while the cement was the secondary reactant," said Karpyn. "This means that wellbores are more likely to stay intact if you have limestone. But dissolving the limestone can create leakage pathways, for example, by forming finger-like channels of dissolved rock."

The researchers found the opposite to be true for the sandstone sample. Rather than dissolving the sandstone, the solution degraded the cement. The sandstone lost very little mass, and the cement lost mass and became more porous.

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