The likely source of most natural gas contamination in drinking-water wells associated with hydraulic fracturing is not what people think it is and it is fixable, a new study said.
The problem lies in construction standards. If there is improved construction standards for cement well linings and casings at hydraulic fracturing sites the issue could and should go away, according to research at Ohio State University.
Using a new method of geochemical forensics, scientists were able to trace how methane migrates under the earth, according to a team led by a researcher at The Ohio State University and consisting of researchers at Duke, Stanford, Dartmouth, and the University of Rochester devised. The study identified eight clusters of contaminated drinking-water wells in Pennsylvania and Texas.
Most important among their findings, is neither horizontal drilling nor hydraulic fracturing, or fracking, of shale deposits seems to have caused any of the 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.”
In hydraulic fracturing, water pumps underground to break up shale at a depth far below the water table, he said. The long vertical pipes that carry the resulting gas upward end up encased 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.
“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,” said Avner Vengosh, professor of geochemistry and water quality at Duke.
“This is relatively good news because it means that most of the issues we have identified can potentially be avoided by future improvements in well integrity,” Darrah said.
“In some cases homeowner’s water has been harmed by drilling,” said Robert B. Jackson, professor of environmental and earth sciences at Stanford and Duke. “In Texas, we even saw two homes go from clean to contaminated after our sampling began.”
The method 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 don’t react much with other chemicals, although they mix with natural gas and can end up transported with it.
That means when they release 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 ended up 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 within and beyond a one-kilometer distance from drill sites.
As hydraulic fracturing starts to develop around the globe, including countries South Africa, Argentina, China, Poland, Scotland, and Ireland, Darrah and his colleagues are continuing their work in the United States and internationally. And, since the method the researchers employed relies on the basic physics of the noble gases, it can work anywhere. Their hope is their findings can help highlight the necessity to improve well integrity.