A team of researchers from five universities has found natural gas in drinking water wells in areas of shale gas production in Pennsylvania and Texas. They blamed the gas migration, in part, on nature but also on on faulty gas well casings. Hydraulic fracturing was not the culprit, they said.
"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 The Ohio State University. "However our data suggests that where contamination occurs, it was caused by poor casing and cementing in the wells."
Scientists from Duke, Stanford, Dartmouth, and the University of Rochester also participated in the research.
They relied upon 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 Pennsylvania and Texas. "Most important" among the findings, which were published this week in the Proceedings of the National Academy of Sciences, is that neither horizontal drilling nor hydraulic fracturing of shale deposits seems to have caused any of the natural gas contamination, they said.
The researchers said that where there is a problem, it could be easily fixed with the application of improved construction standards for cement well linings and casings. "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 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 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, the researchers said. These properties allowed them to determine the source of fugitive methane and the mechanism by which it was transported into drinking water aquifers.
The researchers sampled water from Marcellus and Barnett shale well 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.
The research was funded by the National Science Foundation, Duke University, and a gift from Duke alumni Fred and Alice Stanback to the Nicholas School of the Environment.