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How to stash carbon dioxide: Turn it into stone.

Dissolved in water and pumped underground, CO2 becomes limestone.

By Victoria SchlesingerContributor / February 5, 2009

Experiment site: This is an undated photo of the Hellisheidi Geothermal Plant in Iceland, taken in winter. Next month, scientists from Iceland, France, and the US will inject carbon dioxide emitted from the plant into a rock formation 1,300 feet underground.

Stefan Mar Petursson/Courtesy of Reykjavik Energy

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A long-known but little-tested means of permanently storing carbon dioxide (CO2) underground, mineral carbonation, is now the subject of study in Iceland and Oman.

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The good news is that mineral carbonation promises to lock away CO2 – a powerful greenhouse gas – in a much more stable form than simply pumping it into an underground geologic formation. The bad news: Sequestering CO2 in this way is resource-intensive.

Scientists the world over are exploring ways to capture carbon dioxide and pump it deep underground as a compressed gas. But what’s to keep the buoyant gas from working its way up through fractured rock and reentering the atmosphere?

Ensuring that such a leak never occurs could require monitoring the injection site for a very long period of time – from hundreds to tens of thousands of years.

“How long can you afford, or how long can you plan on a government being in place to be able to watch over something?” says Travis McLing, a technical lead researcher and geochemist at the Idaho National Laboratory in Idaho Falls. “Name a government that’s been stable or intact for even 1,000 years. Long-term stewardship is a real issue.”

One potential way to prevent CO2 from leaking out after injection is to quickly turn the gas into minerals that also occur naturally, such as calcium carbonate, which is the main component in limestone. Many questions remain about the mineralization process, from how quickly it would occur to the cost of required resources and infrastructure, but a handful of scientists are bent on unearthing the answers.

Next month in Reykjavík, Iceland, a pilot project will, for the first time, inject carbon dioxide-saturated water into a formation of basalt rock more than 1,300 feet below ground. The project includes Icelandic, French, and American scientists in partnership with Reykjavík Energy, a geothermal energy company. They hope to learn how quickly the carbon dioxide and water mixture reacts with the basalt rock to form calcium carbonate. Under laboratory conditions, mineralization began within four to six weeks and had occurred extensively within months.

In most sequestration projects, compressed carbon dioxide is injected into a geologic formation already filled with water. In order for mineralization to start, the gas must first dissolve into the water. Scientists don’t know how long that natural process will take, but it is considered the limiting factor, according to Dr. McLing, and the CO2 could leak out during that time. Scientists at Reykjavík want to bypass the slow process of CO2 absorption into water and the risks it poses by mixing the gas with water before injecting it underground.

“How fast you can convert CO2 into a mineral – that is critical,” says Juerg Matter, a geochemist at Columbia Uni­ver­sity’s Lahmont-Doherty Earth Obser­vatory in New York. “We have to expedite it because of the global-warming issue.”

Dr. Matter is involved in another similar study of peridotite, an igneous rock, where it occurs in a large formation in the Middle Eastern country of Oman. Like basalt, peridotite chemically reacts with carbon dioxide-saturated water to quickly form minerals, some of which contain carbon. Matter and his colleague, geologist Peter Kelemen, estimate that with accelerated mineralization, the 16,000 cubic-mile formation could absorb some 4 billion tons of CO2 a year, about 12 percent of the world’s annual CO2 emissions – if it could be collected and transported there.

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