A bold undertaking to store one million metric tonnes (1.1 million short tons) of carbon dioxide in a sandstone reservoir 1.3 miles (2.1 km) below Decatur, Illinois, is well under way. The project began last November, and has so far injected more than 75,000 tons of carbon dioxide, almost one tenth of the target. The University of Illinois, which is leading the Illinois Basin - Decatur Project (IBDP), hopes that the scheme will demonstrate the safety and effectiveness of carbon sequestration, as well as raise public awareness of the process's potential environmental benefits.

What is carbon sequestration, again?

Carbon sequestration is quite simply the physical storage of carbon dioxide having captured it from industrial sources such as carbon fuel power stations or from the atmosphere. By storing carbon dioxide underground that would otherwise be in the atmosphere, atmospheric carbon dioxide levels are reduced, and so is the gas's contribution to the greenhouse effect and global warming.

Modes of carbon sequestration

The IBDP employs geological sequestration, which involves literally injecting compressed carbon dioxide into an underground layer of porous rock. Sandstone is ideal. Conveniently, Illinois has ready access to the Illinois Basin - an 80,000-sq mile (207,000-sq km) Paleozoic layer of sandstone. Above it are several layers of shale, which cap the reservoir to prevent the stored carbon dioxide from escaping.

Map of Illinois showing Decatur near to the center (Image: Shutterstock)

Sandstone sequestration is merely one option for subterranean storage, as carbon dioxide may also be injected into oil and gas reservoirs. Unmineable coal has been proposed as another possible store. The ocean depths have also been mooted as an impermanent store of carbon dioxide, but carbon dioxide reacts with water to create carbonic acid, which, it is suggested, could have a detrimental effect on ocean life.

Atmospheric carbon dioxide can also be reduced by preserving and enhancing natural carbon stores such as peat bogs and forests. By increasing the amount of carbon stored in the biomass at any given time, the amount of carbon dioxide that will eventually make its way back into the atmosphere (via the processes of the carbon cycle) is reduced. Of course, all living things die, so such this approach requires the long-term preservation (or better still, growth) of habitats and biomass.

Has this been attempted before?

The IBDP's one million tonnes of carbon dioxide is, at first glance, made to look rather insignificant when one considers that the United States currently injects between 30 and 50 million tonnes of carbon dioxide every year into oil fields. This isn't an entirely altruistic gesture, since the process aids the recovery of additional oil.

Numerous pilot schemes exist that inject between one and and 50 tonnes (up to 55 tons) of carbon dioxide per day - but only commercial, fossil fuel-oriented schemes inject hundreds or thousands of tonnes in the same timeframe. Salt Creek (USA), In Salah (Algeria), Sleipner (Norway) and Weyburn (Canada) all inject between 3,000 and 6,000 tonnes of carbon dioxide daily, in commercial ventures associated with either oil or gas extraction. It is thought that the additional fuel extracted ends up emitting more carbon dioxide than is captured in the process - better than nothing, perhaps, but hardly a greenhouse effect panacea.

So what makes IBDP special?

What differentiates the IBDP is that it appears to be purely for the benefit of carbon dioxide capture alone. Though it is by no means unique in this regard, it may just be the first pilot scheme on such a grand scale. Similar pilot schemes typically capture carbon dioxide in the tens of thousands of tonnes, so it bears repeating that the Decatur Project seeks to capture one million tonnes of carbon dioxide.

Map of the thickness of the Mt. Simon sandstone in the Illinois Basin (Image: Midwest Geological Sequestration Consortium)

It is also the first "large-scale" sequestration project in the USA to take carbon dioxide from biofuel production (or indeed any man-made source) - in this case ethanol. Over three years the IBDP will capture carbon dioxide from the Archer Daniels Midland ethanol fermentation processing plant in Decatur, and inject it into the ground at a rate of 1,000 tonnes per day - a rate in the same order of magnitude as the fossil fuel projects mentioned above.

The safety of carbon storage

IBDP director Robert Finley is adamant that carbon capture and storage has a vital role to play in managing atmospheric carbon dioxide levels. "If you're going to achieve some of the reductions in emission by 2050 that have been set forth by international agencies, you can't come close to those targets without carbon capture and storage being a part of the process," he said. "For us to perfect this in a site that we believe to be safe and effective is very important. We can create a test case that demonstrates the best practices."

"A site that we believe to be safe" - a curious choice of words? Perhaps not, when you consider that a same issue of water contamination that applies to ocean sequestration could conceivably apply to geological sequestration were carbon dioxide able to leak from its sandstone host into freshwater aquifers.

A 2010 study published in Environmental Science & Technology simulated the slow release of carbon dioxide over a period of more than 300 days, slowly exposing it to water taken directly from a number of USA's aquifers including Illinois' Mahomet Aquifer. The study found that the exposure caused a drop in pH of between one and two units, which in turn increased the erosion of rock in the samples, releasing elements contained therein. In some cases these levels exceeded the maximum levels imposed by the Environmental Protection Agency.

The likelihood of such leaks in real-world geological sequestration is not clear, and safety considerations are among the questions we have posed the University of Illinois (we await answers). That said, investigating the safety of large scale sequestration appears to be part of the IBDP's core purpose, and as such the project is being carefully monitored.

Ears underground

The whole process is monitored using geophysical surveying tools. According to the University of Illinois, the reflections of "energy pulses" sent down into the earth are recorded. "It's essentially like taking a sonogram of the earth," said Illinois State Geological Survey sequestration communications coordinator Sallie Greenberg. "Using geophysical technology allows us to create a time-lapse view of how the carbon dioxide is distributed in the sandstone reservoir." A second verification well on the site is used to monitor pressure and fluid chemistry. It's by tracking pressure levels that any leaks would become evident.

"The research that we're doing is very much on the subsurface geologic environment, to make sure that we can do this safely and effectively, and that we can monitor the CO2," Finley told the LA Times. "So we're using our research dollars to answer these important questions about safety and effectiveness, and we don't have to use our Department of Energy-funded dollars to just try to get our flow of CO2."

On questions of safety it may simply be case of not knowing without trying, and in this respect there's inherent value in the IBDP. Perhaps the larger questions surround carbon sequestration's role in combatting global warming. Will it coexist with other carbon reduction measures, or might it serve as a comfort blanket, inadvertently reinforcing humankind's fossil fuel habit?

Sources: University of Illinois, Midwest Geological Sequestration Consortium, Scientific American, LA Times