COLUMBUS, OHIO, Sept. 15, 2014 -- Results from a new study conducted by a team led by a researcher at Ohio State University and researchers at Duke, Stanford, Dartmouth, and the University of Rochester, have found that deteriorating cement well linings and casings at hydraulic fracturing (fracking) sites are likely the source of most natural gas contamination in drinking water wells associated with fracking. As such, improved construction standards potentially serve as a viable solution to overcoming this challenge.
The team recently devised a new method of geochemical forensics to trace how methane migrates under the earth. The study identified eight clusters of contaminated drinking water wells in the states of Pennsylvania and Texas. Most important among their findings is that neither horizontal drilling nor fracking of shale deposits seems to have caused any of the natural gas contamination.
"There is no question that in many instances, elevated levels of natural gas are naturally occurring, but in a subset of cases, there is also clear evidence that there were human causes for the contamination," said study leader Thomas Darrah, assistant professor of earth sciences at Ohio State. "However, our data suggests that where contamination occurs, it was caused by poor casing and cementing in the wells."
During fracking, water is pumped underground to break up shale at a depth far below the water table, he explained. The long vertical pipes that carry the resulting gas upward are encircled in cement to keep the natural gas from leaking out along the well. The study suggests that natural gas that has leaked into aquifers is the result of failures in the cement used in the well.
"Many of the leaks probably occur when natural gas travels up the outside of the borehole, potentially even thousands of feet, and is released directly into drinking water aquifers," said Robert Poreda, professor of geochemistry at the University of Rochester. Further, Avner Vengosh, professor of geochemistry and water quality at Duke, added, "These results appear to rule out the migration of methane up into drinking water aquifers from depth because of horizontal drilling or hydraulic fracturing, as some people feared."
The method that the researchers used to track the source of methane contamination relies on the basic physics of the noble gases (which happen to leak out along with the methane). Noble gases such as helium and neon are so called because they don't react much with other chemicals, although they mix with natural gas and can be transported with it.
That means that when they are released underground, they can flow long distances without getting waylaid by microbial activity or chemical reactions along the way. The only important variable is the atomic mass, which determines how the ratios of noble gases change as they tag along with migrating natural gas. These properties allow the researchers to determine the source of fugitive methane and the mechanism by which it was transported into drinking water aquifers.
The researchers were able to distinguish between the signatures of naturally-occurring methane and stray gas contamination from shale gas drill sites overlying the Marcellus shale in Pennsylvania and the Barnett shale in Texas. The researchers sampled water from the sites in 2012 and 2013. Sampling sites included wells where contamination had been debated previously; wells known to have naturally high level of methane and salts, which tend to co-occur in areas overlying shale gas deposits; and wells located both within and beyond a one-kilometer distance from drill sites.