People who live among the fracking fields of Pennsylvania should expect considerable leaking of methane from natural gas wells into the groundwater and atmosphere, according to new research by a professor who has been a consistent critic of the boom in hydraulic fracturing.

A research team led by Anthony Ingraffea of Cornell University reached this conclusion after examining state inspection records of more than 41,000 wells drilled from 2000 through 2012 throughout Pennsylvania.

Because of flaws detected by inspectors in the concrete or casing of the wells, up to 40 percent of the oil and gas wells in some parts of the state may end up leaking methane, they reported in the Proceedings of the National Academy of Sciences.

Problems with the wells were especially likely to be found when the drilling was done in recent years and in northeastern parts of the state, trends that the researchers said they could not fully explain. The drilling boom has been especially intense there, and many residents have complained of leaks of methane, a potent global warming gas that also contaminates water supplies. It's possible, the paper said, that the industry might have cut corners or that inspectors have not looked closely enough at wells installed during the boom.

Researchers seeking to explain the presence of methane in groundwater, and regulators who want to control its escape, now have evidence of one possible culprit, the authors said: "compromised structural integrity of casing and cement in oil and gas wells."

Methane, they noted, "is a strong greenhouse gas. The identification of mechanisms through which methane may migrate to the atmosphere as fugitive emissions is important to understand the climate dimensions of oil and gas development."

Ingraffea, a 40-year veteran engineer specializing in complex dynamics of how materials wear out and break, has been warning for years that it is dangerous to rely on natural gas as a replacement for dirtier coal as long as a significant amount of methane escapes during the production of gas, offsetting reductions of carbon dioxide emissions. Methane is a much more potent greenhouse gas in the short term, although it does not persist as long as CO2 in the atmosphere.

The jury is still out on how to measure, let alone balance, the contributions of these two main greenhouse gases to global warming.

Ingraffea and two of his coauthors are affiliated with Physicians, Scientists and Engineers for Healthy Energy, an advocacy group critical of fracking.

"This study makes it clear that oil and gas development regulators need to inspect a larger quantity of wells more frequently, and not give up on inspecting aging wells," said one of the authors, Seth B.C. Shonkoff, executive director of PSE, in a press release from the group.

The main thrust of the research was not to predict precisely how much leakage will occur, but rather to provide statistical evidence for why leaks might happen.

"Pennsylvania State inspection records show compromised cement and/or casing integrity in 0.7-9.1% of the active oil and gas wells drilled since 2000, with a 1.6-  to 2.7-fold higher risk in uncontrolled walls spudded since 2009 relative to conventional well types," they reported. "Hazard modeling suggests that the cumulative loss of structural integrity in wells across the state may actually be slightly higher than this, and upward of 12% for unconventional wells drilled since January 2009."

This wide range of estimates, they said, is influenced by the exceptionally high rate of compromised wells in the northeastern counties of Pennsylvania, an area "with predicted cumulative hazards exceeding 40%."

Similar studies in the past, they suggested, may have underestimated the problem by counting only inspection records that resulted in notices of violation from the state. Their study uses a specialized hazard model to also predict future risks.

These results "should be cause for concern," they wrote, especially in light of numerous methane contamination complaints and explosions nationally in drilling areas "and the increased awareness of the role of methane in anthropogenic climate change."

In an interview, Ingraffea said he was not surprised at the extent of compromised wells. It's well known in the industry, he said, that when there is a rapid expansion of drilling one can expect more problems of this kind.

"A significant percentage of wells leak initially, and that percentage grows as wells age," he said.

One surprise, he added, was that the researchers could not find any inspection reports at all for 8,000 wells, even though the state is supposed to inspect them all once a year.

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