Chemistry matters. Join us to get the news you need.

If you have an ACS member number, please enter it here so we can link this account to your membership. (optional)

ACS values your privacy. By submitting your information, you are gaining access to C&EN and subscribing to our weekly newsletter. We use the information you provide to make your reading experience better, and we will never sell your data to third party members.



Driving CO2 Underground

Despite many unknowns, hopes abound that carbon sequestration can stanch global warming

by Bette Hileman And Jeff Johnson
September 24, 2007 | APPEARED IN VOLUME 85, ISSUE 39

Credit: Tim Ford/NETL
DOE contractors drill 1,100 feet into a coal seam near Kenmare, in northwest North Dakota to extract cores for study of carbon dioxide sequestration.
Credit: Tim Ford/NETL
DOE contractors drill 1,100 feet into a coal seam near Kenmare, in northwest North Dakota to extract cores for study of carbon dioxide sequestration.

Restructuring the world's energy supply to cut carbon dioxide emissions is like turning a ship the size of Wyoming or West Virginia. It can be done, engineers and scientists say, but they warn that the ship's pilot better have a clear plan, a highly experienced crew, and ample time to bring the big ship around in what are likely to be rough seas.

Like Wyoming and West Virginia, the big ship is full of coal. Most of the world's electricity and more than one-third of its anthropogenic CO2 atmospheric emissions come straight from coal. Coal-fired power plants generate a bit more than half of the U.S.'s electricity and an ever-growing amount of the world's. Coal is integral to the climate-change problem as well as key to any climate-change solution.

To avoid catastrophic global warming, most climate scientists believe the atmospheric level of carbon dioxide cannot be allowed to rise above 550 parts per million. They calculate that if CO2 levels pass this threshold, the world will experience runaway climate change and a host of disasters. Among them, much of the Greenland ice sheet will likely melt over a century, raising sea levels by meters. Currently, the atmospheric CO2 concentration is 383 ppm, 37% above the preindustrial level of 280 ppm.

To keep atmospheric CO2 below 550 ppm, these scientists say, global emissions must start falling by 2020 or 2030 at the latest, a seemingly impossibly short time to overhaul the world's energy supply.

A massive reduction of carbon emissions can happen through some combination of vast improvement in vehicle efficiency and overall energy efficiency; widespread and rapid deployment of renewable energy sources, such as wind and solar power; much more nuclear power; and a huge reduction in emissions from coal-fired power plants and other CO2-emitting industrial point sources.

Many scientists are betting that the fastest and cheapest way to curb CO2 is to effectively sweep it under the rug. The solution, they say, is to capture the gas at industrial emission point sources, compress it, and inject it deep into Earth, where it will be sequestered and isolated forever. Preliminary studies estimate that global geologic reservoirs could hold some 9.5 trillion metric tons of CO2, more than 300 times the 26 billion metric tons of CO2 the world vents to the atmosphere each year from all anthropogenic sources.

There are more than 8,000 industrial point sources around the world, emitting 60% of global CO2. Coal-fired power plants make up more than half of those point sources. These coal plants emit about 10 billion metric tons of CO2 annually, 38% of the world's yearly CO2 emissions. However, the world depends heavily on these same plants for electricity. And while the U.S. is now only slowly adding to its complement of coal-fired power plants, China is building more than one plant a week, experts say.

CO2 sequestration is a gigantic undertaking, and like the enormous volumes of CO2 to be captured and held, the stakes in sequestration's success or failure are huge. If successful, sequestration offers a path to avoiding a global climate-change disaster while retaining coal as an energy source. It could even allow the coal industry to expand. If a quest to implement sequestration proves unsuccessful, however, the result would be a waste of valuable time and resources that could have been vigorously directed toward non-carbon-emitting energy sources and extreme energy efficiency, rather than relying on a false solution that protects a historically powerful industry.

Credit: Battelle
Credit: Battelle

When quizzed about the merits of carbon sequestration versus those of nuclear, wind, or solar energy technologies, James J. Dooley, senior staff scientist with Battelle and the Joint Global Change Research Institute, stops the discussion to underscore the size of the problem.

"We are talking about a transformation here," he stresses. "We are going to be transforming the global energy system from one that is optimized around venting greenhouse gases to the atmosphere to one that is a multi-decadal effort to rebuild the global energy system and one that is optimized to deliver all the services it did before but with far, far fewer atmospheric emissions. This is not tinkering around the edges of the economy. This will transform how we make energy."

Dooley is a big supporter of carbon capture and sequestration (CCS). He notes that the oil industry has for years injected pressurized CO2 into depleted oil deposits to recover oil, that some 3,000 miles of CO2 pipeline already exists in North America, and that CCS technology is understood, although it has never been used or even tested on this scale.

Indeed, real data on costs, monitoring requirements, or the fate of CO2 after years or centuries of being held in an underground repository are absent and won't become available any time soon. Development of CCS will take decades, as CO2 continues to build up in the atmosphere.

Despite this lack of experience with CCS, the strategy has a host of supporters. The coal industry and utilities back it, and if implemented on a large scale, it would allow the utilities to keep burning coal while reducing CO2 emissions to the atmosphere. It is popular with the oil and gas industry because it would alleviate a shortage of pressurized CO2 that is used for enhanced oil recovery. The Department of Energy says a flood of CO2 could quadruple the country's recoverable oil reserves.

And many environmental groups are on the CCS bandwagon. The Natural Resources Defense Council and Environmental Defense, for example, see it as a bridge to a carbon-free energy future that relies much more on solar and wind power and energy-efficient buildings. Environmentalists now demand that no new coal-fired power plants be constructed in the U.S. unless they are designed for CO2 capture. California, Oregon, and Washington have mandated such a policy.

But Dooley and other climate scientists worry that the world, and particularly the U.S., lacks a comprehensive plan and the experience and time to cut global CO2 emissions enough to make a difference for the future. He stresses that there is no driver for CCS.

CCS is a carbon-mitigation technology, he stresses. It takes energy to run it. "There is no reason for a power provider to reduce the output of its plant absent a belief that we are going to address climate change. Without a climate policy that says we are going to reduce emissions, you are not going to get that first [CCS] plant built," Dooley says.

He is joined in his concern by several scientists interviewed by C&EN who, like Dooley, helped write a defining report by the United Nations' Intergovernmental Panel on Climate Change (IPCC) on CO2 capture and storage. The report was published in 2005 but remains an important publication in this debate.

Credit: Courtesy of Howard Herzog
Credit: Courtesy of Howard Herzog
Credit: U Of Calgary
Credit: U Of Calgary

Dooley and other IPCC panel members Howard J. Herzog, program manager of the Carbon Sequestration Initiative at Massachusetts Institute of Technology, and David Keith, director of the Institute for Sustainable Energy, Environment & Economy at the University of Calgary, Alberta, warn that the world and the U.S. better pick up the pace for research and deployment of CCS technologies if Earth is to avoid the more drastic impacts of climate change.

A medium-sized 500-MW coal plant produces about 3 million tons of CO2 per year, an average of 2.4 tons of CO2 for every ton of coal, Herzog says. "There are the equivalent of more than 500 medium-sized coal power plants in the U.S."

Although the volume of CO2 is huge, he notes that it is comparable in scale to other industrial activities. For instance, if 60% of the CO2 produced from U.S. coal-fired utilities were captured and compressed to a liquid, the volume would equal the nation's yearly oil consumption, or 7.3 billion barrels, Herzog calculates.

Dooley envisions a future in which hundreds, if not thousands, of new CCS power plants would be operating around the world. He believes coal would continue to be a primary source of baseload electricity, but CCS would be required for that to happen. And he argues that other industrial sectors must also be included for a carbon-limiting system to work.

"The atmosphere is completely indifferent to where a CO2 molecule comes from," he adds. He lays out a future in which the location of a new coal-fired power plant would be largely determined by its proximity to a geological reservoir that could hold a lifetime of a plant's CO2 emissions, as well as to rail lines, electrical distribution, and water.

CO2 could be captured from such a plant in basically three ways: amine separation of CO2 from the exhaust gases of conventional coal plants; coal combustion in pure oxygen to produce an almost pure stream of CO2; and coal gasification, converting coal to CO2 and hydrogen, which then can be used as a fuel or to make electricity.

Credit: IPCC
Credit: IPCC

Currently, integrated gasification combined cycle technology (IGCC) is the leading candidate for future carbon capture from coal-fired power plants due to its higher efficiency and greater ease of capture than conventional power plants. A few IGCC electrical generation demonstration plants are in operation and more are planned. But none capture CO2.

After capture, CO2 would be compressed to a supercritical state and transported to a storage or injection site. Primarily the same types of pipelines that are suitable for natural gas can transport CO2, and construction costs would be similar, about $200,000 per mile. Hauling CO2 by ships, trucks, and railcars has been ruled out due to costs, says John T. Litynski, program manager of the Environmental Projects Division at DOE's National Energy Technology Laboratory (NETL).

IPCC estimates that capturing and compressing CO2 would draw 10 to 40% of the energy of a conventional plant. More than half the costs of CCS would be to capture CO2, Dooley says, estimating a total cost of $50 a ton of sequestered CO2 for the first power plants. Estimates of the increased cost per kilowatt-hour of electricity range from a penny to more than a dime.

In 1992, when scientists and engineers began research on carbon sequestration in geological repositories, they considered injecting CO2 into deep saline sandstone and basalt formations under land or under the seafloor, storing it in depleted oil and gas reservoirs, placing it on the seafloor where vast pressures presumably would keep it in place, and pumping it into unminable coal seams.

Now, as a result of both research and public opposition, the options have been winnowed down considerably. Only three are being actively developed: storage in deep saline aquifers in sandstone, injection into basalt formations, and injection into depleted oil and gas reservoirs.

Among those dropped was deep-sea placement. Research suggested that after diffusion, CO2 would return to the ocean surface and eventually to the atmosphere. And sea life would be killed under and near a pool of CO2 due to its acidity and oxygen deprivation.

But the primary reason for dropping ocean disposal was public opposition, DOE officials say, and that holds a message for the overall success of carbon sequestration, no matter where it's tried.

In 2002, an international consortium that included DOE planned to inject 60 tons of liquid CO2 into the deep ocean off the coast of Kona, Hawaii. When environmentalists attempted to block research ships from leaving the port, the consortium backed down. "We decided we were going to run into so much public opposition that it would not be worthwhile," says David J. Wildman, focus area lead for geological and environmental sciences at NETL.

Deep Storage
Credit: IPCC
Potential methods for sequestering CO2 in deep underground geological formations include (1) storage in depleted oil and gas reservoirs, (2) use in enhanced oil recovery, (3) storage in deep saline formations (a) offshore and (b) onshore, and (4) use in enhanced coal-bed methane recovery.
Credit: IPCC
Potential methods for sequestering CO2 in deep underground geological formations include (1) storage in depleted oil and gas reservoirs, (2) use in enhanced oil recovery, (3) storage in deep saline formations (a) offshore and (b) onshore, and (4) use in enhanced coal-bed methane recovery.

DOE also cut back research on CO2 storage in unminable coal seams. Results show that Eastern coal, which is bituminous, reacts with CO2 and swells, limiting storage and blocking injection, says Wildman. DOE is continuing small levels of research and plans to inject CO2 into unminable western seams of subbituminous coal in the San Juan Basin in New Mexico.

The best hopes today are deep saline aquifers and basalt formations. Such formations have the capacity to store hundreds of years of global CO2 emissions without the apparent leakage and permeability problems of other options.


Research is primarily centered on 5,000- to 8,000-foot-deep aquifers in sandstone, Wildman says. Many DOE scientists consider sandstone formations ideal because they are less likely than basalt to be fractured, they are widely distributed across the U.S., and they are close to numerous coal-fired power plants and industrial facilities. "Saline aquifers will either make or break this technology," says Herzog.

The IPCC report explains how CO2 storage in a saline aquifer would work. Supercritical CO2 would be injected through concrete-lined wells, past multiple geologic strata and below drinking water aquifers. There, deep in a sandstone saline aquifer, CO2 is relatively buoyant compared with sandstone and formation brine. It would rise to the capstone, an impermeable rock layer at the top of the reservoir. The CO2 would dissolve into the aquifer's fluids, and over hundreds to thousands of years, the CO2-laden water would become dense, sinking into the formation and precipitating to a solid carbonate mineral. IPCC says this could possibly take thousands, if not millions, of years.

The largest scale sequestration projects are injecting CO2 at two natural gas facilities: Statoil's Sleipner facility in the North Sea off the coast of Norway and BP's In Salah gas field in Algeria. At both sites, CO2 is being stripped from natural gas and injected into saline aquifers at a rate of about 1 million tons per year. BP and Statoil are collecting some data on the behavior of the CO2 in the aquifers, but the commercial operations are not designed for research.

Starting in 2008, DOE researchers in partnership with universities and industry will inject some 1 million tons of CO2 annually into saline aquifers at as many as a half-dozen sites. They hope to determine how quickly CO2 dissolves in brine, where it circulates within the aquifer, and how much an aquifer can hold. They also hope to determine monitoring needs and whether large plumes of supercritical CO2 can trigger earthquakes.

"Anytime you inject fluids," Herzog says, "earthquakes are always a concern. To understand the pressure issues, we need to be able to measure pressure feedbacks. That is why we need to begin looking at injections at a very large scale."

The first step in the growing research program will be to drill and extract very deep 5,000- to 8,000-foot cores in the aquifers where future injections are planned, says Wildman.

Drilling that deeply into aquifers is going into unknown territory, notes Wildman. "We haven't done any characterization of these aquifers in terms of their capacity, porosity, and permeability," he explains. Samples of the cap rock and the brine will be brought back to NETL in Pittsburgh, where NETL scientists will study how CO2 reacts by re-creating the same temperatures and pressures found at the greater depths.

"When we do experiments, we'll try to figure out what will happen as a function of time with CO2 injected into saline reservoirs," Wildman says. "We'll have to look at how fast those reactions take place and try to project them out over thousands of years.

"The other concern we have is how to monitor the surface for CO2 leaks as the carbon sequestration demonstrations get larger and larger," Wildman adds. "We need techniques that do not require a great deal of manpower. One method being considered is helicopters equipped with highly sensitive CO2 sensors to detect leaks over large portions of the surface.

"We also are looking at developing small, inexpensive sensors that we can put out in a large array. These would feed back to a computer network," Wildman explains. "Then if we see a spike in CO2 concentrations at a particular sensor, we would want to go out and investigate. One problem with CO2 is there are natural emissions from vegetation and organic matter in the soil. So background studies of CO2 emissions are necessary before injection begins."

Scientists have assumed that injected CO2 will not be immobilized until it is mineralized into carbonates, taking many thousands of years. But several other processes to immobilize CO2 on a much shorter timescale are being examined.

Escape Routes
Credit: IPCC
At least seven mechanisms can enable CO2 to escape during or after sequestration. (A) CO2 gas pressure exceeds capillary pressure and the gas passes through siltstone. (B) Free CO2 leaks from A into a fault in the upper aquifer. (C) CO2 escapes through a gap in the caprock into a higher aquifer. (D) Injected CO2 migrates up a dip and increases resrvoir pressure an dthe permeability of a fault. (E) CO2 escapes via a poorly plugged, old abandoned well. (F) The natural flow of the aquifer dissolves CO2 at the CO2/water interface and transports it out of the repository. (G) Dissolved CO2 escapes to the atmosphere or the ocean.
Credit: IPCC
At least seven mechanisms can enable CO2 to escape during or after sequestration. (A) CO2 gas pressure exceeds capillary pressure and the gas passes through siltstone. (B) Free CO2 leaks from A into a fault in the upper aquifer. (C) CO2 escapes through a gap in the caprock into a higher aquifer. (D) Injected CO2 migrates up a dip and increases resrvoir pressure an dthe permeability of a fault. (E) CO2 escapes via a poorly plugged, old abandoned well. (F) The natural flow of the aquifer dissolves CO2 at the CO2/water interface and transports it out of the repository. (G) Dissolved CO2 escapes to the atmosphere or the ocean.

Along with industrial partners, DOE is also examining sequestration in depleted oil and gas wells, particularly when the process is combined with enhanced oil recovery. One of the largest demonstration projects is pumping CO2 205 miles from a North Dakota synfuels plant to an oil field in Saskatchewan, where about 1 million tons of CO2 is injected annually in oil recovery operations. About half that amount is expected to remain sequestered.

Enhanced oil recovery has been held up as an option that could generate energy and value through oil extraction, rather than one that simply takes energy and money. However, the global storage capacity in these reservoirs is far smaller than the potential capacity in deep aquifers, Wildman says. What's more, Dooley adds, if CO2 capture is successful, great amounts of CO2 will become a common commodity that no one is likely to buy.

Although petroleum companies have had many years of experience exploring and drilling for oil and gas, there are still unknowns associated with storing large quantities of CO2 under high pressure in these formations. Just the seemingly simple process of finding and capping old gas and oil wells presents problems. Hundreds can be found in many old fields, and all must be capped to prevent leaks of CO2 to the surface and to maintain supercritical pressures required for dissolving the oil that remains in the wells.

In the U.S., there are many depleted oil fields, some near towns and settlements, where CO2 injections could enable the extraction of much more oil. For example, Rick Hammack, research geochemist at NETL, is helping a company find thousands of wells on a large depleted western oil field, where drilling went on for 100 years. But many of the old wells are hard to detect because they are unlined or lined with wood, so the company was only able to locate about two-thirds of the wells.

However, Hammack found some 500 more by using a helicopter equipped with methane and magnetic sensors. Over time, small settlements have been built on the oil field, and some wells were found in unsafe locations, such as under mobile homes. The company is now capping all of the wells as it proceeds with enhanced oil recovery. Conventional drilling has removed at most 35% of the oil. With enhanced oil recovery, the take should reach 80%, Hammack says.

DOE is also looking at CO2 injection in basalt formations and is beginning field tests in Wallula, Wash., along the Columbia River. Pete McGrail, a geochemist with Pacific Northwest National Laboratory, is leading the project, which is cosponsored by Edison Mission Energy, a California energy provider.

Edison Mission hopes to locate an IGCC plant at the site if the repository is adequate, McGrail says. The basalt formation, made up of a pancake of lava flows laid down six million to 12 million years ago, has the possibility of storing large amounts of CO2. Due to the mineral makeup of formation liquids, McGrail says, carbonate mineralization of CO2 in basalt can be sped up, taking place in hundreds, rather than thousands of years.

Overall, Herzog estimates the DOE sequestration budget to be on the order of $100 million. "It should be a billion-dollar program," he says.

Considering the huge number of unknowns and with so much riding on success and failure, Dooley, Keith, and Herzog all recommend a much more aggressive research and development program, more in line with the scale of the problem.

Herzog believes the government needs a decadelong demonstration program injecting million-ton amounts of CO2 before the technology can be used on a large scale to sequester coal emissions. "To the best of our knowledge, it would work, but if you go through a 10-year demonstration, there may be some surprises," Herzog says.

Keith adds that there must be a start-up of many commercial-scale CCS projects. "We need to build some real plants," he says, with industry leading the way and learning as it builds.

To encourage energy providers to explore options and take over research, Keith says, the world needs a carbon tax or some other driver. He'd like to see DOE's research role be absorbed by the private sector, which has a bigger stake in success or failure.

"In a carbon-constrained world, CCS is needed to save coal," he says, but adds that other technologies will also be on the table. He believes wind and nuclear energy will increasingly be competitive with coal as a baseload electricity provider when CCS is figured into the price tag over the next two decades. Even solar power, which he says is far too expensive today, may get a foothold.

The course of the future, he adds, is about to undergo a huge change and will be determined by a new set of decisionmakers and under conditions very different from those of today. "Today's discussion about coal and carbon sequestration's role in the future of energy is like talking about the future of petroleum in 1869," Keith says. "More ideas and technological change are sure to come."

More On This Topic



This article has been sent to the following recipient:

Leave A Comment

*Required to comment