Storing Carbon Dioxide under the Ocean

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As worries over the impact of carbon dioxide emissions on global climate change soar, researchers are increasingly searching for ways to rid the atmosphere of the greenhouse gas. But, so far, industrial-scale projects have been limited. Notable among them, oil giant BP and GE recently announced a project to build power plants in Scotland and California that derive hydrogen from fossil fuels and sequester the carbon dioxide by-product. And Statoil in Stavanger, Norway, separates excess carbon dioxide in natural gas extracted in its North Sea mining operations and injects it into underground reservoirs. While these reservoirs are under the ocean, they are under too little water, and too deep below the sea floor to use the mechanisms described by Schrag and his colleagues.

The most prominent storage method nowadays (Statoil's project is an example) involves depositing carbon dioxide in underground geologic formations such as depleted oil fields. Here the dynamics between carbon dioxide and surrounding fluids are different than those in the sea floor, where the ocean keeps the fluids cool. Rather, these formations are heated by the earth's crust, and the high temperature make the carbon dioxide less dense than the water in the surrounding rock, making it prone to rising to the surface, Harvard's Schrag says.

Sea-floor injections also offer an immense amount of storage capacity. If all the known geologic reservoirs for conventional storage were useable, they could store all the carbon dioxide currently produced each year, and continue doing so for 80 years at current emission rates. In contrast, sea-floor storage around the United States alone could store thousands of years worth of U.S. carbon dioxide production, the researchers estimate.

Robert Socolow, co-director, of Princeton University's Carbon Mitigation Initiative, notes that the sea-floor injection method has the advantage of being intrinsically secure. But he says that well-mapped reservoirs, away from seismically active areas, can be effectively capped to prevent the greenhouse gas from escaping, and therefore these methods will continue to have a place.

Indeed, the costs for the new sea-floor method will vary, Schrag says, but will probably be slightly more than for land-based storage. It could, however, be more economical for areas near the ocean, especially those far from a known geological reservoir. "If you're sitting right next to a big basin, it's probably slightly more expensive. If you're in New Jersey, and you have to pump the carbon dioxide 300 miles to get to such a basin, then I would say no." He notes that the cost for any method of large-scale sequestration is still unclear.

"The need for robust, potentially inexpensive carbon sequestration schemes is enormous," says Nathan Lewis, a professor of chemistry at Caltech. While it still requires more experimental validation, he says Schrag's work "is potentially very important. It ought to be considered very seriously."