In-Situ Carbonation of Peridotite Offers Large Scale Capacity for Permanent Storage of CO2

7 November 2008

Researchers at Columbia University’s Lamont-Doherty Earth Observatory have concluded that the in situ carbonation of peridotite, a type of rock found at or near the surface in Oman and other areas around the world, could consume more than 1 billion tons of CO₂ per year in Oman alone, affording a low-cost, safe, and permanent method to capture and store atmospheric CO2.

Their studies show that the rock reacts naturally at surprisingly high rates with CO₂ to form solid minerals, and that the process could be speeded by multiple orders of magnitude with simple drilling and injection methods. The study appears in this week’s early edition of the Proceedings of the National Academy of Sciences.

Peridotite comprises most or all of the rock in the mantle, which undergirds earth’s crust. It starts some 20 kilometers or more down, but occasionally pieces are exhumed when tectonic plates collide and push the mantle rock to the surface, as in Oman.

Geologists already knew that once exposed to air, the rock can react quickly with CO₂, forming a solid carbonate like limestone or marble. However, schemes to transport it to power plants, grind it and combine it with smokestack gases have been seen as too costly and energy intensive. The researchers say that the discovery of previously unknown high rates of reaction underground means CO₂ could be sent there artificially, at far less expense.

Geologist Peter Kelemen and geochemist Jürg Matter made the discovery during field work in the Omani desert, where they have worked for years. Their study area, a Massachusetts-size expanse of largely bare, exposed peridotite, is crisscrossed on the surface with terraces, veins and other formations of whitish carbonate minerals, formed rapidly in recent times when minerals in the rock reacted with CO₂-laden air or water.

Up to 10 times more carbonates lie in veins below ground. The subterranean veins were previously thought to be formed by processes unconnected to the atmosphere, and to be nearly as old as the 96-million-year-old rock itself. However, using conventional carbon isotope dating, Kelemen and Matter showed that the underground veins are also quite young&mdsah;26,000 years on average—and are still actively forming as CO₂-rich groundwater percolates downward. Many underground samples were conveniently exposed in newly constructed road cuts.

All told, Kelemen and Matter estimate that the Omani peridotite is naturally absorbing 10,000 to 100,000 tons of carbon a year—far more than anyone had thought. Similarly large exposures of peridotite are known on the Pacific islands of Papua New Guinea and Caledonia, and along the coasts of Greece and the former Yugoslavia; smaller deposits occur in the western United States and many other places.

The scientists say that the process of locking up carbon in the rocks could be speeded 100,000 times or more simply by boring down and injecting heated water containing pressurized CO₂. Once jump-started in this way, the reaction would naturally generate heat. That heat would in turn hasten the reaction, fracturing large volumes of rock, exposing it to reaction with still more CO₂-rich solution. Heat generated by the earth itself also would help. (The exposed Omani peridotite extends down some 5 kilometers.)

The scientists say that such a chain reaction would need little energy input after it was started. Accounting for engineering challenges and other imperfections, they assert that Oman alone could probably absorb some 4 billion tons of atmospheric carbon a year—a substantial part of the 30 billion sent into the atmosphere by humans, mainly through burning of fuels. With large amounts of new solids forming underground, cracking and expansion would generate micro-earthquakes—but not enough to be readily perceptible to humans, says Kelemen.

We see this as just one of a whole suite of methods to trap carbon. It’s a big mistake to think that we should be searching for one thing that will take care of it all.>

—Peter Keleman

Matter has been working on a separate project in Iceland, where volcanic basalt also shows promise for absorbing CO₂ produced by power plants. Trials there are set to begin in spring 2009, in partnership with Reykjavik Energy, and the universities of Iceland and Toulouse (France).

Resources

• Peter B. Kelemen and Jürg Matter (2008) In situ carbonation of peridotite for CO₂ storage. PNAS doi: 10.1073/pnas.0805794105