Energy & Environment

Locked Up

On a ranch outside Cranfield, Mississippi, workers for the state's largest oil and gas operator are shooting a dense liquid 10,300 feet into the earth. The...
by | August 31, 2008

On a ranch outside Cranfield, Mississippi, workers for the state's largest oil and gas operator are shooting a dense liquid 10,300 feet into the earth. The liquid is a supercritical form of carbon dioxide that serves a valuable purpose for Denbury Resources Inc. Oozing through porous rock, the CO2 mixes with oil and pries it out from underground nooks that otherwise would be hard to reach. Denbury pumps the slimy blend back out of the ground, sells the oil and sends the carbon dioxide back down, repeating the cycle until a well runs dry.

What geologists want to know is what happens to the CO2 that's left behind. For each barrel of oil produced using this method of "enhanced oil recovery," Denbury says that perhaps 2,500 times as much carbon dioxide remains in the ground. If that proves true, variations of this long-used technique for producing more fossil fuel also holds promise for reducing greenhouse gas emissions.

Minimizing the risks of climate change is not really the goal for Texas-based Denbury. The focus at its Mississippi operations is oil, not global warming. But 500 miles away, geologist Susan Hovorka is paying close attention to the oil company's work and its environmental implications. Hovorka is one of the foremost experts on the emerging science of carbon sequestration, and Denbury has allowed her and her team of researchers from the Texas Bureau of Economic Geology to rig the company's wells with subterraneous monitors that beam data back to their computers in Austin. Every 10 minutes, Hovorka can see how the injections build pressure in underground formations, how the carbon dioxide is moving and behaving, and whether it poses a threat to drinking water near the well sites two states away. "If we can understand what's happening in this rock," Hovorka says, "then we can scale up with confidence."

Hovorka's work is one of more than two dozen research projects around the country focused on carbon sequestration. About half of them piggyback on oil projects, while others are more purely experimental. What all of them are testing is the ability of different geological formations, called "sinks," to absorb carbon dioxide and hold it -- hopefully forever.

Geologists in Illinois and New Mexico recently injected small amounts of carbon dioxide into coal seams that are so deep in the earth they can't be mined. In Michigan, the target was a saline formation 3,500 feet below the surface. Researchers also are testing the ability of forests, farm soil and wetlands to hold carbon, a process known as "terrestrial sequestration."

Most of the "geologic sequestration" tests so far have been tiny in scale. A large-scale sequestration project tied to the U.S. Energy Department's "FutureGen" plant was scrapped in January because of cost overruns. But the department believes its other research experiments, including the one in Mississippi, are ambitious enough to determine whether long-term sequestration is viable.

The potential is certainly there. According to an Energy Department survey, underground pockets in the United States and Canada might be able to store between 1.2 trillion and 3.6 trillion metric tons of carbon dioxide. That's more than 200 times the 5.9 billion metric tons of CO2 emissions the United States currently produces every year. But not every state is a candidate to store carbon dioxide. Minnesota, New Hampshire and Vermont, among others, simply don't have the right underground geology.

In states with a lot of potential sinks, there's a growing interest in where sequestration science is headed. Some are funding research through state agencies, universities and geological surveys. A few legislatures have begun writing laws to handle the legal and environmental aspects of pumping large quantities of carbon dioxide into the ground. State utility regulators also have been looking at carbon storage for several years, ever since electric utilities began proposing to build dozens of coal-fired power plants around the country. That sequestration is still an unproven technology is one reason many states have been denying permits for new coal plants -- mostly in favor of natural gas plants, which emit less greenhouse gas but costs more.

Most proposals for dealing with global warming focus on using less fossil fuel. That's not what sequestration is about. The premise behind storing carbon underground is that it would allow consumers and industry to continue using cheap-and-dirty fuels such as coal for years to come, without guilt. "This is a critical technology to help us solve the problem of using an important energy source and using it in a more environmentally benign manner," says Howard Herzog, a chemical engineer who heads a carbon sequestration initiative at the Massachusetts Institute of Technology. "I'm not saying it's a 100 percent slam dunk that it will work. But the chance of success is high and we should be investing in making it happen."

Critics say sequestration research is a distraction from the pursuit of clean-energy alternatives, such as wind and solar power. The goal of sequestration, after all, is to enable our addiction to fossil fuels, rather than break it. In any case, carbon storage for the purpose of reducing emissions is probably a decade or more away. The big hang-up is in developing efficient and affordable methods for capturing carbon dioxide as it comes out of power plants. For now, most of the CO2 used in both enhanced oil recovery and the sequestration pilot tests comes from naturally occurring sources rather than smokestacks.

Safety is another concern. When it comes to storing vast amounts of carbon dioxide in the ground, scientists and policy makers alike want more certainty -- about whether the CO2 might harm drinking water supplies, whether so much pressurized gas in the ground might cause small earthquakes, and whether they can confidently say that carbon dioxide, once sent underground, will stay there forever. In working to solve the environmental disaster of climate change, the last thing anyone wants to do is create another environmental disaster.

Sequestration Science

Susan Hovorka is confident that all of those obstacles can be overcome. She's been working in environmental geology for two decades, and several years ago led a pioneering study of carbon sequestration near Houston. "We know a lot more than most people in government are aware," Hovorka says. "Even people who are well informed often think that the earth is a big exciting mystery. But those of us who are geoscientists have a lot of confidence about what's down there. The expertise is substantial."

Scientists have more confidence about some sequestration sites than others. One ideal type of geological formation, and an abundant one at that, is a deep, porous sandstone that is saturated naturally with briny water. This is what lies below Denbury's oil field in Mississippi. When injected into such a formation, supercritical carbon dioxide moves through underground pores like water through the soil of a flowerpot. Over time, the gas dissolves into the saline. Eventually, some of it hardens into rock.

If sites are picked carefully and monitored closely over time, there's little risk of leaks. Although carbon dioxide is buoyant and wants to rise, the ideal saline formation for sequestration would have a layer of impervious "cap" rock such as shale lying above it. That seals in the pressurized gas. As Hovorka puts it, "It's not hard to pick a relatively easy place where you can inject CO2 with a high confidence that it will go where you think it's going to go, do what you think it will do, and stay down, not come out, to be a net benefit to the atmosphere."

Still, there are unknowns. The biggest is a question of scale. Making a real dent in the global warming problem would require stashing several Great Lakes' worth of carbon dioxide underground. How subterranean reservoirs will respond to such massive volumes remains to be seen. Another problem is less a matter of science than logistics. Power plants aren't always located in places with ideal geological characteristics for sequestration. If carbon is ever to be stored underground in massive quantities, vast networks of pipelines will need to be built to move it around.

The pipeline problem is one reason why near-term progress in this field is likely to be driven by oil companies. They have the resources to build pipelines, and with high oil prices, they have a strong financial incentive to build them. Denbury Resources already operates 400 miles of CO2 pipelines in Mississippi and Louisiana, and is planning on building 400 miles more. "Enhanced oil recovery is the place to get this started," says Tracy Evans, senior vice president for reservoir engineering with Denbury. "We build the pipelines, and not only do we lower the cost of carbon capture and storage but we also produce an economic benefit, which is more domestic energy."

Nuts, Bolts and Incentives

In the meantime, state officials can help push carbon sequestration along. California, for example, has passed a law against importing electricity unless it is produced from power plants that emit no more than a modest amount of greenhouse gas. The maximum emission level is set at a point that essentially rules out coal plants -- unless those plants were to apply carbon capture and storage technologies to handle the emissions. A similar incentive may come from the carbon cap-and-trade systems under development in the Northeast, West, Midwest and Florida (see " Carbon Goes to Market "). By putting a price on carbon emissions and creating a market for "offsets," those systems may improve the economics of sequestration to the point that companies other than oil interests would see dollar signs in it.

Other policy choices are at a more nuts-and-bolts level. States need to settle some tricky property rights questions, such as who owns the pore spaces -- as much as two miles underground -- where the carbon dioxide would settle. Environmental policies need to be put in place to make sure that injections are done properly and monitored closely, and to make sure that the CO2 doesn't push saline and other contaminants into drinking water. (The U.S. Environmental Protection Agency published draft regulations regarding carbon sequestration and drinking water in July.) There also are questions about liability. Who would be responsible if the CO2 actually were to leak? And what if that happens hundreds of years from now?

Lawrence Bengal believes it's important for laws to treat carbon dioxide as a natural resource to be managed and regulated by the states, much as oil and gas are. Bengal is director of the Oil and Gas Commission of Arkansas. He also headed a task force of the Interstate Oil and Gas Commission that developed a model carbon sequestration statute for legislatures to consider. He cautions states not to treat carbon dioxide as a hazardous waste to be disposed of. That could create a public relations problem similar to the one that continues to plague nuclear waste. "If you call it a waste, hazardous substance or a pollutant, it makes it harder to deal with," Bengal says.

A handful of states, including Illinois, Kansas, North Dakota, Texas and Washington State, have taken a crack at writing carbon storage regulations. So has Wyoming, whose legislature passed a pair of sequestration bills earlier this year. One of them settles the ownership question there by declaring the underground pore spaces to be the property of whoever owns the land above them -- as opposed to those who may own underground mineral rights. The other bill sets up a regulatory framework for sequestration, including permitting requirements and monitoring standards.

State Representative Tom Lubnau sponsored both of the Wyoming bills. Lubnau is a Republican from Gillette, which sits in the middle of a big coal-producing region. If carbon regulation creates a demand for carbon storage, he says, Wyoming will at least have the legal infrastructure for projects to get going quickly. From Lubnau's perspective, this is more a matter of economic survival than it is about global warming. "Ten percent of the nation's energy comes from my county," he says. "If people aren't burning coal, oil or gas, a lot of people will be out of work here. So anything we can do to make sure our product remains economically and environmentally viable, we'd be fools not to do."


More from Energy & Environment