The Key to Safe and Effective Carbon Sequestration

Some rock acts as a natural stopper to buried carbon dioxide.

By Jeremy Jacquot|Friday, February 29, 2008

To the dismay of environmentalists, coal is still king in the U.S. electricity market. Nearly 50 percent of the electric power in this country comes from burning coal to create steam that drives electricity-generating turbines. Coal-burning power plants in the United States emit about 2.1 billion tons of carbon dioxide each year—nearly 17 percent of worldwide coal emissions—and finding technologies that reduce those emissions in the United States and China, which burns even more coal than we do, is crucial to combating global warming. One oft-cited but little-used solution is to catch carbon dioxide as it is released from smokestacks and pump it underground into rocks capped by impermeable shale, a process called carbon capture and storage. The worry is that the injected material could leak and bubble to the surface, negating the whole point of the process.

Now, a British geologist’s study suggests sandstone could rapidly absorb the gas, potentially providing a safe, leakproof reservoir. Last year, Bruce Yardley, a professor at the University of Leeds in England, was monitoring oil extraction at a BP oil field in the North Sea. To speed the oil’s flow to the surface, seawater had been pumped to the bottom of the wells. When Yardley analyzed a sample of the injected water, he found it rich in silica. That signaled that the water and minerals in the surrounding sandstone had reached a chemical equilibrium with the injected seawater far more quickly than anticipated—in two years rather than a century.

Past studies had shown that when carbon dioxide is injected into sandstone, it dissolves common carbonates in the rock, changing the chemistry of sitting water and making a carbonic acid that eats holes in the rock. This can lead to CO2 leakage. Based on the speed of the silicates’ reaction with seawater, Yardley believes that when CO2 is injected into high-silicate minerals like feldspar, it too will quickly react, making clays and carbonates that clog the pores of the rock and trap the gas.

Porous, feldspar-rich sandstone formations are abundant worldwide—in saline aquifers below freshwater bodies, in coal seams, and in aging oil and gas fields. Industrial-scale carbon capture facilities like the Great Plains Synfuel Plant in Beulah, North Dakota (which pipes CO2 to Canada, where it is injected into oil wells to improve oil recovery), already exist, and leaks have never been detected. Although carbon capture and storage has attracted a growing number of advocates, including environmental groups like the Natural Resources Defense Council, it has also attracted its fair share of detractors, such as Greenpeace, and skeptics including the U.S. Geological Survey’s Yousif Kharaka (pdf), who has shown that leaking CO2 can make surrounding water acidic, mix with brine and leach metals, and pose potential health risks to people and wildlife. Yardley’s finding could alleviate these worries.

Yardley’s study is one of the most comprehensive of its kind, says Kharaka, but he favors more research. While new data may come from FutureGen, a $1.8 billion prototype “zero emissions” coal-fired plant funded in part by the U.S. Department of Energy, it is not likely to open before 2012. Yardley’s study gives hope that this technology is feasible now, and in a world whose coal lust is unlikely to diminish, quick reactions—in sandstone or by coal plants—may be just what is needed.

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