Fugitive methane found in water wells above the Marcellus and Barnett shales came from poor well construction, not hydraulic fracturing, a study the National Academy of Sciences released on Sept. 15 concluded.
Scientists from 5 higher education institutions—Duke, Ohio State, Stanford, Dartmouth College, and the University of Rochester—used noble gas and hydrocarbon tracers to differentiate naturally occurring methane from contamination.
Their 2013 study, which they submitted for peer review on Nov. 27, examined 8 clusters of water wells overlying the 2 major US shale gas formations, including declining water quality over time in the Barnett.
“Gas geochemistry data implicate leaks through annulus cement (4 cases), production casings (3 cases), and underground well failure (1 case) rather than gas migration induced by hydraulic fracturing deep underground,” the study’s abstract concluded.
Determining the contamination mechanisms will improve the safety and economics of shale gas extraction, it added.
“We found 8 clusters of wells—7 in Pennsylvania and 1 in Texas—with contamination, including increased levels of natural gas from the Marcellus shale in Pennsylvania and from shallower, intermediate layers in both states,” said Thomas H. Darrah, an assistant earth science professor at Ohio State who led the study while he was a research scientist at Duke.
Contamination’s source
“Our data clearly show that the contamination in these clusters stems from well-integrity problems such as poor casing and cementing,” he noted.
“These results appear to rule out the possibility that methane has migrated up into drinking water aquifers because of horizontal drilling or hydraulic fracturing, as some people feared,” added Avner Vengosh, a geochemistry and water quality professor at Duke.
The combination of noble gas and hydrocarbon tracers enabled the researchers to identify and distinguish between the signatures of naturally occurring methane and stray gas contamination from shale gas drill sites.
They analyzed gas content in 113 drinking water wells and one natural methane seep overlying the Marcellus, and in 20 wells overlying the Barnett 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 1-km distance from drill sites.
“This is the first study to provide a comprehensive analysis of noble gases and their isotopes in groundwater near shale gas wells,” said Darrah, who is continuing the analysis in his lab at Ohio State.
Pinpoint sources, pathways
“Using these tracers, combined with the isotopic and chemical fingerprints of hydrocarbons in the water and its salt content, we can pinpoint the sources and pathways of methane contamination, and determine if it is natural or not,” he explained.
Robert B. Jackson, an environmental and earth sciences professor at Stanford and Duke; Nathaniel R. Warner, a post-doctoral fellow at Dartmouth College; and Robert J. Poreda, an earth and environmental sciences professor at the University of Rochester, co-wrote the study. Thure E. Cerling, a geology and geophysics professor at the University of Utah, was its editor.
Scott Anderson, a senior advisor with the Environmental Defense Fund’s US Climate and Energy Program in Austin, said the study’s findings are “yet another reason why more attention must be focused on well integrity issues.”
Many states have updated oil and gas well integrity regulations in recent years, he conceded. “But this research is a call for increased action,” Anderson said. “EDF has and will continue to work with state officials to develop effective regulatory frameworks that will help ensure that wells are constructed and maintained properly—minimizing the number of incidents reported of contaminated groundwater.”
Contact Nick Snow at [email protected].
