CO2 STORAGE: Captured carbon dioxide can be pumped deep underground for permanent storage, as has been done with CO2 extracted from natural gas at the Sleipner field in the North Sea. The artist's rendering here depicts the extraction well for the natural gas as well as the injection pipe for CO2. Image: Alligator film / BUG / StatoilHydro
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Editor's Note: This is the third in a series of five features on carbon capture and storage, running daily from April 6 to April 10, 2009.
For more than a decade, Norwegian oil company Statoil Hydro has been stripping climate change–causing carbon dioxide (CO2) from natural gas in its Sleipner West field and burying it beneath the seabed rather than venting it into the atmosphere.
The company estimates that since 1996 it has stored more than 10 million-plus metric tons of CO2 some 3,300 feet (1,000 meters) down in the sandstone formation from which it came—and all of it has stayed put, which means storage may be the simplest part of the carbon capture and storage (CCS) challenge.
The basics of carbon dioxide storage are simple: the same Utsira sandstone formation that has stored the natural gas for millions of years can serve to trap the CO2, explains Olav Kaarstad, CCS adviser at Statoil. An 800-foot (250-meter) thick band of sandstone—porous, crumbly rock that traps the gas in the minute spaces between its particles—is covered by relatively impermeable 650-foot (200-meter) thick layer of shale and mudstone (think: hardened clay). "We aren't really much worried about the integrity of the seal and whether the CO2 will stay down there over many hundreds of years," Kaarstad says.
The company monitors its storage through periodic seismic testing, a process that is not unlike a sonogram through the earth, says hydrologist Sally Benson, director of the global climate and energy project at Stanford University. That monitoring indicates that between 1996 and this past March, the liquid CO2 has spread to occupy some three square kilometers, just 0.0001 percent of the area available for such storage.
"We're not going into a salt cavern, we're not going into an underground river. We're going into microscopic holes," explains geologist Susan Hovorka of the University of Texas at Austin, who has worked on pilot projects in the U.S. "Add it up and it's a large volume" of storage space.
How large? The U.S. Department of Energy (DoE) estimates that the U.S. alone has storage available for 3,911 billion metric tons of CO2 in the form of geologic reservoirs of permeable sandstones or deep saline aquifers, according to a 2008 DoE atlas. These reservoirs are more than enough for the 3.2 billion metric tons of CO2 emitted every year by the roughly 1,700 large industrial sources in the country. Most of that storage is near where the majority of coal in the U.S. is burned: the Midwest, Southeast and West. "There are at least 100 years of CO2 sequestration capacity and probably significantly more," Benson says.
The storage seems to be long-term as well; the sequestered CO2 doesn't just sit in the rock waiting for a chance to escape. Over decades it forms carbonate minerals with the surrounding rock, or it dissolves into the brine that shares the pore space, Hovorka notes. In fact, when she tried to pump CO2 out of her test site south of Dayton, Tex. using natural gas extraction techniques, the attempts failed completely.