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Carbon-Capturing Rock

Geologists discover that certain rock formations could sequester large amounts of carbon dioxide.
November 4, 2008

Chemical reactions that pull carbon dioxide out of the atmosphere and store it in the form of solid rock inside geological formations could offset billions of tons of carbon-dioxide emissions each year, according to researchers at Columbia University, in New York. The scientists say that research done on large rock formations in Oman suggests new ways to sequester carbon-dioxide emissions to help lessen global warming.

Carbonated rock: A type of rock rich in magnesium, called peridotite, is laced with veins containing magnesium carbonate that form when the rock comes into contact with carbon dioxide from the atmosphere.

The researchers have shown that rock formations called peridotite, which are found in Oman and several other places worldwide, including California and New Guinea, produce calcium carbonate and magnesium carbonate rock when they come into contact with carbon dioxide. The scientists found that such formations in Oman naturally sequester hundreds of thousands of tons of carbon dioxide a year. Based on those findings, the researchers, writing in the current early edition of the Proceedings of the National Academy of Sciences, calculate that the carbon-sequestration rate in rock formations in Oman could be increased to billions of tons a year–more than the carbon emissions in the United States from coal-burning power plants, which come to 1.5 billion tons per year.

The Columbia researchers’ strategy is attractive because of the very large potential to store vast amounts of carbon dioxide, says Marco Mazzotti, the head of the Separation Processes Laboratory at the Swiss Federal Institute of Technology, in Zurich. Typically, today’s strategy for carbon sequestration involves pumping it underground, where it is trapped in porous aquifers. Since the Columbia researchers’ approach would store carbon dioxide in the form of rock, it would eliminate the chance that the carbon dioxide would leak out, Mazzotti says.

The researchers found that the natural peridotite formations in Oman captured carbon dioxide in a network of underground veins. Peridotite contains large amounts of olivine, a mineral composed of magnesium, silicon, and oxygen. As groundwater reacts with the olivine, the water becomes rich in dissolved magnesium and bicarbonate, with the latter effectively increasing the carbon concentration in the water by about 10 times. As this water seeps deeper into the rock and stops reacting with the air, the magnesium, carbon, and oxygen precipitate out of solution and form magnesium carbonate, also called magnesite. Dolomite, which contains calcium, magnesium, carbon, and oxygen, also forms. As the magnesite and dolomite form, they increase the total volume of the rock by about 44 percent, causing cracks to appear throughout it, which creates a network of fractures as small as 50 micrometers across. This opens up the rock and allows water to penetrate further. “It’s a little bit like setting a coal seam on fire,” says Peter Kelemen, a professor of earth and environmental studies at Columbia University. “You’re taking rocks that haven’t been exposed to the atmosphere, and you’re oxidizing them very fast.”

The researchers calculate that the natural process can be sped up dramatically. Using techniques commonly employed by the oil industry to increase oil production, the rock could be fractured further, increasing the surface area for the reactions. Carbon dioxide captured from power plants could then be pumped into the rock, where it would trigger the formation of carbonates. Heating the rock would increase the rate of the reactions. What’s more, because the reactions themselves generate heat, once they reach a certain rate, they will be self-sustaining. Initiating this self-sustaining reaction would require heating the rock to 185 °C, the researchers say, which could be done during the process for fracturing the rock. They calculate that in such a system, one cubic kilometer of rock would store a billion tons of carbon dioxide per year.

Mineral springs: Calcium carbonate rock that forms as alkaline, calcium-rich water seeps from underground springs in Oman.

The researchers propose a carbon-sequestration strategy that would eliminate the need to transport carbon dioxide, as well as the need to heat up the rock. In this scenario, they would access rock formations in shallow ocean waters off the coast of Oman and elsewhere by drilling into them and fracturing the rock using existing oil-industry techniques. The researchers would drill two holes. Into one, they’d pump cool seawater. Rock temperature increases with depth, so as the water is pumped into the holes, it will get hotter, until it reaches nearly 185 °C. Carbon dioxide naturally dissolved in the water would then precipitate out of the solution. The hot water would eventually make its way through the fractured rock to the second drilled hole, where it would rise to the surface via convection. This seawater would quickly absorb more carbon dioxide, since shallow waters and surf mix well with the atmosphere. Because “the atmosphere transports carbon dioxide all over the world for free,” Kelemen says, this approach, if deployed on a grand scale, could be used to lower worldwide levels of carbon dioxide.

This scenario would be limited by the concentration of carbon dioxide in seawater, so a cubic kilometer of rock would only sequester about a million tons of carbon dioxide a year. But since it wouldn’t be necessary to transport carbon dioxide or pay to heat the rock, Kelemen says, it would be possible to work with much larger areas of rock, and thereby reach a rate of billions of tons of carbon dioxide per year.

“From a conceptual point of view, all they say makes sense,” says Mazzotti. Yet questions remain about whether the methods will work in practice. For one thing, the self-sustaining reactions depend on the magnesium carbonate and other precipitates continuing to fracture the rock to expose more of it. The researchers have observed that this has happened in the geology in Oman, but it’s not a given that it would continue in the scenarios that they propose. The researchers’ concepts should now be complemented with large-scale tests, Mazzotti says.

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