About 40 percent of the oil and gas wells in parts of the Marcellus shale region will probably be leaking methane into the groundwater or into the atmosphere, concludes a Cornell-led research team that examined the records of more than 41,000 such wells in Pennsylvania.

In research published today (June 30) in the journal Proceedings of the National Academy of Sciences, the researchers examined Pennsylvania Department of Environmental Protection inspection records that show compromised cement and/or casing integrity in more than 6 percent of the active gas wells drilled in the Marcellus region of Pennsylvania. This study shows up to a 2.7-fold higher risk for unconventional wells – relative to conventional wells – drilled since 2009 in the northeastern region of the Marcellus in Pennsylvania.

“These results, particularly in light of numerous contamination complaints and explosions nationally in areas with high concentrations of unconventional oil and gas development and the increased awareness of the role of methane in ... climate change, should be cause for concern,” said the researchers in the paper. Anthony Ingraffea, Cornell professor of civil and environmental engineering, an expert on drilling and hydraulic fracturing, led the study.

Both conventional and unconventional oil and gas wells are 8.5 times more likely to leak methane in northeastern Pennsylvania than throughout the rest of the state, according to the study.

The researchers examined 75,505 publicly available compliance reports for 41,381 oil and gas wells in Pennsylvania from 2000 to 2012 to determine whether the well casing or the cement used was impaired. The shale gas wells were six times more likely to leak, compared with conventional wells.

Typically in wells, holes of various diameters are drilled and steel casing – of smaller diameters – are inserted. Water is mixed with cement powder and then pumped into the gaps between the casing layers to form a shield.

When not properly mixed, the impaired cement sheaths can be caused by excessive water loss in the cement, high permeability in the cement slurry, casing cracks due to pressure fluctuations, poor bonding or normal deterioration with age.

“In a typical well, hundreds of bags of cement are mixed and injected,” said Ingraffea. “If the water-to-cement mixture ratio isn’t right, you have problems. With too much water, the cement shrinks. With too little water, the mixture dries too fast.”

Other authors for the paper, “Assessment and Risk Analysis of Casing and Cement Impairment in Oil and Gas Wells in Pennsylvania, 2000-2012,” were Martin Wells, Cornell professor of statistical sciences; Renee Santoro of Physicians, Scientists and Engineers for Healthy Energy, Ithaca, New York; and Seth Shonkoff, University of California, Berkeley. The researchers used no federal funding.