POSTED 22 MARCH 2007
What happens to carbon dioxide underground?
Before we pump billions of tons of carbon dioxide underground, it might help to understand what will happen to it. Will the carbon dioxide just leak from the ground, squandering money but solving nothing? Will it cause deadly eruptions like the one in Cameroon?
H Photo: USGS
The issues are complex, but some answers are beginning to emerge about the various underground formations now being considered for carbon storage: deep coal mines, expired oil fields, and especially the widespread, deep saline aquifers (layers of porous minerals filled with salty water).
Carbon dioxide is less viscous -- and therefore faster moving -- than the salty water in an aquifer. But the fate of the waste gas is also controlled by temperature, pressure, chemistry, rock structure and permeability, and the presence of deep drill holes or earthquake faults.
Potential leakage routes and possible countermeasures
for CO2 injected into saline aquifers
Several chemical and physical processes can slow or stop carbon dioxide in the deep rocks. The gas may dissolve in subsurface water, adhere to coal or oil, or get snagged between rock grains. Some may be trapped by "anticlinal structures," that look and act like the underside of a giant umbrella. (Similar anticlines act as "roof rocks" that prevent the escape of underground oil and gas). Eventually, carbon dioxide can react with underground minerals to form carbonate minerals -- rock. That is a stable and desirable end point for this waste disposal technique.
But long before much mineralization occurs, a large amount of trapping may happen in the host rock, according to two recent studies.
The Blob battles CO2
A 2006 report (see #5 in the bibliography) by Ruben Juanes, a professor of civil and environmental engineering at MIT, predicts that a good deal of carbon dioxide will be trapped inside pores between rock grains in saline aquifers. Because the injected carbon dioxide is buoyant, it will rise through the permeable rock, displacing water from the pores.
However, the surface of the rock attracts water, so the grains will remain coated by a film of water. This thin film of water will begin to swell and block off the pores, and that "chops up the carbon dioxide into tiny blobs," Juanes says. These blobs are not buoyant enough to continue rising, so they remain trapped in the pores. "We have shown that this is a much safer way of disposing of CO2 than previously believed, because a large portion -- maybe all -- of the CO2 will be trapped in small blobs in the briny aquifer."
Courtesy Ruben Juanes, MIT.
Or at least, that's what the computer model says. Further testing has begun, Juanes says, but for now, the take-home message is this: "Based on experiments and on the physics of flow and transport, we know that the flow of the CO2 is subject to a safety mechanism that will prevent it from rising up to the top just beneath the geologic cap." (And as mentioned, carbon storage experts expect the cap rock to trap the carbon dioxide if the blob flops.)
A second study, of geologic injection beneath 2.5 kilometers of ocean water, shows a different mechanism that may trap the injected CO2. In this cold, extremely pressurized zone, carbon dioxide becomes a liquid that is dense enough to sink through water-saturated rock. Because temperature rises with depth in the earth, the sinking, liquid CO2 will eventually expand and become less dense than water. That will cause the liquid CO2 to rise, returning it to cooler rock, where it will again gain density. Like a yo-yo, the CO2 may oscillate up and down, but it cannot rise past 400 meters below the ocean floor, says Kurt Zenz House, a graduate student in the department of earth and planetary sciences at Harvard University. So if the CO2 were to be injected below 400 meters, it "could rise to that point, and then not rise any further."
The study (see #6 in the bibliography) shows that this protective mechanism is dictated by the physics of carbon dioxide. "It is only a function of pressure and temperature," he says. "Once the CO2 is beneath the seafloor, there is no way it can rise into the ocean."
If carbon dioxide does escape a reservoir on land, don't count on it being detoxified by the soil and subsoil, says Curtis Oldenburg of Lawrence Berkeley National Laboratory. "Carbon dioxide coming up through the ground, even at fairly low rates, is capable of producing quite high concentrations in the soil, and not very much dissipates in the soil," says Oldenburg. Once the carbon dioxide escapes into the air, he says, "it mixes or disperses very rapidly through wind or turbulence. ... But soil cannot be counted on for very much attenuation."
At this early date, carbon-storage researchers seem optimistic that carbon dioxide will stay trapped for the long term. "We can more or less guarantee that it will not leak... or that a leak will not be catastrophic," says Martin Blunt, a professor of petroleum engineering at Imperial College London. The demonstration projects will need monitoring, probably through expensive seismic studies, he adds. But if the technology proves reliable, cheaper monitors, such as carbon dioxide "sniffers," may be enough to spot leaks.
Beyond the technological challenges of carbon storage, the system also has political and economic considerations. Coal burners need an incentive to spend money and energy to capture and store carbon dioxide, and a regulatory framework is essential to keep storage claims honest. "The institutions have to be built up," says Howard Herzog of MIT's Laboratory for Energy and the Environment, "so you know what permits you need and where you get them. There are some issues. Exactly how do you do the long-term stewardship? There is no clear consensus on this."
Like others working on carbon storage, the need for speed weighs on Herzog: "I think it will take a decade to run through the demonstrations." But he stresses the issue of motivation. "The economic incentives are not there to have this start happening sooner. It will probably take a price signal [a carbon tax, a cap-and-trade system, or another mechanism to raise the cost of venting carbon dioxide] of $30 per ton of CO2. The chances of that happening in the United States before 10 years are not very high. So let's use that time to do some demonstrations, so we are ready to run."
Time to get cracking?
Herzog contributed to the 2007 "The future of coal" study (see #7 in the bibliography) which did not exactly describe carbon capture and storage (CSS) as "ready to run." (Coal is critical to global warming because it is the most widespread fossil fuel, especially in China, Australia and the United States).
The "Future" study was unenthusiastic about the carbon-storage record of the U.S. Department of Energy: "To date, the DOE CSS program has not been pursued with an urgency to establish the key enabling science and technology needed for increased coal use in a carbon-constrained world. ... Establishing sequestration as a practical large-scale activity requires work across the board, including science, technology, infrastructure design, regulation and international standards. None of the key technical and public acceptance issues have been addressed with sufficient intensity [emphasis ours]. The program ... does not grapple with answers to the hard questions."
The MIT report had this to say about FutureGen, DOE's flagship CSS project:
"Ambiguity about objectives leads to confusion and incorporation of features extraneous for commercial demonstration of a power plant with CCS."
The international partnership could "further muddle the objectives."
"The effort to satisfy all constituencies runs the risk of undermining the central commercial demonstration."
At this point, according to the MIT researchers (and many others), it's time to capture a major hunk of carbon dioxide -- megatons per year -- from a coal-fired power plant, inject the gas underground, and see what it costs and how well it works. And then to do it again, five or 10 times more in other locations using other technology.
Doing this will take years, and cost gigabucks. But compared to the human, economic and environmental cost of steady or even accelerated global warming, it seems a reasonable price to pay.
Carbon storage is not the only cool suggestion for dealing with a feverish Earth, and perhaps not the best one, but it might be foolish to write it off without some serious research. "There is a growing realization that the world is facing a very serious problem, and you don't try to tackle a serious problem with only one technology," says Martin Blunt of Imperial College London. "You try all the possible solutions, nuclear, renewable, efficiency, carbon capture and storage, and see which one works out. You will probably find that some of those solutions will be better than others, but until you have treated them all seriously, you will not know."
Check out our non-fattening, low-carbon bibliography.