UNDERGROUND CONFLICT: Fracking for natural gas may require shattering geologic formations that efforts to store CO2 permanently underground require to be impermeable. Image: Wikimedia Commons/Richard Bartz
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."