Cabot Oil & Gas Corp. and GSI Environmental Inc. are questioning a Duke University study that found a link between hydraulic fracturing and increased cases of gas migration in northeastern Pennsylvania.
Their survey of more than 1,700 water wells in Susquehanna County, PA, before natural gas drilling began in the vicinity, found methane concentrations to be "ubiquitous," particularly in wells in low-lying areas. "The correlation of methane concentrations with elevation indicates that, on a regional level, elevated methane concentrations in groundwater are a function of geologic features, rather than shale gas development," a team of five geologists from the two companies wrote in a December 2011 report.
As part of its standard pre-drilling operations in Pennsylvania, meant to protect operators from "presumed liability" in contamination cases, Cabot collected 1,713 samples from water wells in Susquehanna County between 2008 and 2011. "The results of the extensive 'pre-drill' water well sampling and background survey show methane to be nearly ubiquitous in water wells in this region, with over 78% of the water wells exhibiting detectable methane concentrations," the authors concluded in their report.
That rate rises in low-lying areas, Cabot said. Although only 51% of the sample wells were in valleys, those wells represented around 88% of the wells containing dissolved methane above normal levels.
The report suggests that the methane in the water wells could be coming from thermogenic gas-charged sandstones in the Catskill formation, or biogenic methane produced from decomposition underground.
Cabot is a major producer in Susquehanna, where it has been accused of contaminating water wells; the company has long argued that sampling data proved its innocence (see Shale Daily, Oct. 20, 2010). The Cabot report specifically challenges a May 2011 study by Duke University showing a correlation between hydraulic fracturing and gas migration into nearby water wells (see Shale Daily, May 11, 2011).
The Duke researchers sampled 68 private water wells across six counties in the Marcellus country of northeastern Pennsylvania and New York. While they found methane in 85% of the samples, the levels were, on average, 17 times higher in wells located within a kilometer of "active hydrofracking sites." That study, however, found no signs of contamination from the chemical-laced hydraulic fracturing fluids used to improve recovery rates, or from wastewater extracted out of of the wells after completion.
Using Pennsylvania Department of Environmental Protection (DEP) data that suggest a way to distinguish between gas from the Marcellus and gas from the Upper and Middle Devonian overlying it, Cabot concluded that the "methane samples analyzed in the Duke study could have originated entirely from shallower sources above the Marcellus that are not related to hydraulic fracturing activities."
Because the Duke study didn't include the location of its sampled wells with respect to topography or geology, Cabot wasn't able to directly tests its theories about low lying wells or the Catskill formation. Duke recently returned to the region to collect more information (see Shale Daily, Aug. 10, 2011).
DEP Secretary Michael Krancer call the original Duke study "biased science," and claimed the researchers stonewalled his requests for information about the sampling locations (see Shale Daily, June 8, 2011). The researchers called that claim "simply not true" in a Philadelphia Inquirer editorial, but added that they couldn't release sampling information without the permission of homeowners.