Natural gas production and carbon sequestration may be headed for an underground collision course.

That is the message from a new study finding that many of the same shale rock formations where companies want to extract gas also happen to sit above optimal sites envisioned for storing carbon dioxide underground that is captured from power plants and industrial facilities.

The problem with this overlap, the researchers found, is that shale-gas extraction involves fracturing rock that could be needed as an impenetrable cover to hold CO2 underground permanently and prevent it from leaking back into the atmosphere.

"Shale gas production through hydraulic fracturing can compromise future use of the shale as a caprock formation in a CO2 storage operation," said Michael Celia, a civil and environmental engineering professor at Princeton University and a co-author of the study.

"There is an obvious conflict between the two uses," the study says.

Celia's work with colleague Thomas Elliot, a postdoctoral research associate, will be published in an upcoming paper version of Environmental Science & Technology.

The two reported that 80 percent of the potential area to store CO2 underground in the United States could be restrained by shale and tight gas development. The numbers held when they examined potential CO2 storage sites close to the nation's largest greenhouse gas emitters, such as coal plants.

Natural gas is extracted from shale via hydraulic fracturing, in which rock is cracked so that injected fluids can flow through the rock more easily to extract gas. The process is designed to increase permeability of the rock over a long distance. That cracking of the shale rock is what could make it inappropriate for use as a stable, impervious rock layer blocking upper migration of injected CO2, the researchers said.

Drilling into potential storage sites
The study raises issues that would play out in the future, since carbon capture and sequestration (CCS) in deep rock formations or saline aquifers currently has never been proved at scale in the power sector. It envisions separating the greenhouse gas from power stacks and piping the CO2 to an underground storage spot to prevent release in the atmosphere.

There is a large CCS pilot project at an ethanol plant in Illinois, but otherwise the energy industry is testing the concept in nonintegrated, small pieces in the United States. Many other projects have stalled because of cost concerns. The concept is considered pivotal for coal's survival in a carbon-constrained world, since the fossil fuel releases about a third of the nation's greenhouse gas emissions.

Shale gas production also has not reached full scale.

Celia and Elliot looked at a carbon sequestration database of potential storage spots for underground CO2 created by the National Energy Technology Laboratory known as the National Carbon Sequestration Database and Geographic Information System, or NATCARB.

That analysis was then overlaid with a Department of Energy database of large shale regions such as the Marcellus formation stretching from Ohio to northern New York and the Barnett formation in Texas. The researchers considered areas already known to be rich in shale gas, in addition to regions likely to yield future production. They also considered tight gas, another uncoventional natural gas source where the fuel sits tightly in impenetrable rock.

Nationally, there is an estimated storage capacity underground for CO2 of 1,711 to 20,402 gigatons. When gas plays were added to the picture, though, the range fell to between 217 and 2,885 gigatons of space -- an 80 percent reduction. The researchers said the numbers were "an upper bound" estimate, but "appear to be compelling."

A potentially different picture
An even higher degree of overlap occurred when the two mapped large stationary sources of CO2 in the United States, such as coal plants, and considered available storage spots for CO2 within 20 miles of those emitters. In that case, the researchers concluded that shale and tight gas production could affect 85 percent of potential storage spots for stored CO2.

One reason the overlap between shale gas and sequestration regions is so great, they said, is that shale formations typically have very low permeability in the first place. That gives them an obvious dual purpose -- ideal for gas extraction, but also for protecting underground storage spots for CO2.

But Bruce Hill, a geologist at the Clean Air Task Force, said the study offered a simplistic, two-dimensional view of the landscape underground.

Similarly, Susan Hovorka, a sequestration expert and senior research scientist at the University of Texas, Austin, said sedimentary rock is very thick. There typically would be layers and layers of rock offering protection against leakage from a CO2 storage spot 800 meters beneath the ground, she said.

That means that if a rock seal in one spot were broken by hydraulic fracturing, or "fracking," it wouldn't usually create an issue at all, she said. Other layers of impermeable rock underneath the fractured area would block migration of the gas, she said.

"We need to pay attention to this, but I don't think you can conclude that 85 percent of the resource for underground storage of CO2 is crossed out," said Hill.

If additional three-dimensional imaging of the subsurface were done, it would show the overlap number could be much smaller, because of the other protective layers, said Hill.

Additionally, the potential area for underground storage of CO2 in saline aquifers is vast, he said. The database from the federal government reports that the overall storage capacity for injected CO2 could be as high as 20,000 gigatons of carbon dioxide. In comparison, annual CO2 emissions from fossil fuel combustion are a tiny fraction of that, or about 5.7 gigatons.

Future room to maneuver
The size of the resource indicates there will still be plenty of room for CO2 injections in the future, despite the overlap, said Hill. Currently, federal law requires carbon sequestration operators to undergo a vigorous permitting process under the Safe Drinking Water Act.

The Class VI injection well program for carbon sequestration under federal law, for example, requires CCS developers to do thorough seismic measurements of the subsurface and ensure a stable overhead rock before obtaining a permit to shoot CO2 underground. It also requires continual monitoring of underground plumes of gas.

The scenario raised by the study would be mainly relevant in one scenario, where gas producers wanted to come into an area after CO2 injections, several analysts said.

An already-fractured caprock is not going to win approval for CO2 injection in the first place, said Robert Van Voorhees, a counsel at the law firm Bryan Cave.

If gas producers did become interested in the same formation holding CO2, there would be an extensive record of the injections of the greenhouse gas from the Class VI program, making it known where to avoid, said Van Voorhees. Additionally, gas producers do not want to fracture a whole formation because it would impede gas production, he said.

"The two can co-exist if the geology works," he said about CCS and shale gas production.

Because CCS is still in a very early state, gas production is likely to come first in most cases as well, he said. That reduces the likelihood of the "CO2 first" scenario, he said.

Additionally, Hill said, much of the potential for carbon capture and sequestration involves a process not considered by the study -- enhanced oil recovery. In enhanced oil recovery, captured carbon dioxide is piped to an oil field, where the gas is injected under pressure into the reservoir to push out more crude.

The carbon capture industry's focus on enhanced oil recovery has grown because, unlike saline aquifers, it gives captured CO2 a commercial value. Oil companies do not have access to enough CO2 to fully expand, putting the greenhouse gas in demand for them. Some estimates say there is enough capacity with enhanced oil recovery to store about a sixth of the U.S. annual total output of carbon dioxide (ClimateWire, July 13, 2011).

However, many analyses indicate that saline aquifers will have to play a major part in carbon sequestration from coal plants eventually. Many large emission sources are not close to oil patches. The International Energy Agency has said that carbon sequestration should provide 19 percent of emissions reductions by 2050 for greenhouse gas emissions to be cut in half by then.

In an email from Germany, Celia said he was not prepared to comment at this time on needed changes to regulatory structure. "Our main message is that there needs to be broader awareness," he said.

The study was funded with a grant from U.S. EPA, the Carbon Mitigation Initiative at Princeton University and the Natural Science and Engineering Research Council of Canada.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500