There are 300,000 to 500,000 abandoned oil and gas wells in Pennsylvania, and some of them might be leaking significant quantities of the potent greenhouse gas methane, according to a new study in the Proceedings of the National Academy of Sciences.

That abandoned wells may leak methane—which is 86 times as bad for the climate as CO2 on a 20-year time scale—has so far flown under the radar of regulators and industry. Emissions from such wells are not included in most government databases of the oil and gas sector.

"There definitely are leaky abandoned oil and gas wells out there, and we should really consider including them in greenhouse gas inventories," said Mary Kang, a researcher currently at Stanford University who did the study during graduate school at Princeton University.

Abandoned wells are ones that are no longer in production.

Doing a rough calculation, the researchers found that abandoned wells in Pennsylvania may have contributed 4 to 7 percent of the total man-made methane emissions from all sectors (agriculture, wastes, oil and gas, and others) in 2010.

Wells found unplugged

The study began while Kang was researching rock formations that can store carbon dioxide as part of carbon capture and sequestration projects. She found that abandoned gas wells could serve as potential pathways for CO2 to leak back into the atmosphere. She theorized that methane, which is the primary component of natural gas, could use the same pathway and escape into the atmosphere through abandoned well bores.

The scientists chose 19 abandoned wells in western Pennsylvania, a region that has been drilled since 1859, to study in greater detail. Five of them were plugged, a process where workers pour cement into a retired well to trap gases inside. Most were unplugged.

In 2013 and 2014, the scientists measured methane near the wellhead and compared that with background measurements of ambient air 2 feet away. They found that abandoned wells were significantly leaky, emitting about 56 milligrams per hour on average. The comparable measurement in ambient air was close to zero.

The scientists found that both plugged and unplugged wells were emitting methane. It is possible that the plugs may have failed over time because cement has an average lifetime of 30 years. Some of these wells were more than half a century older.

Of the 19 wells, three were unusually high emitters, Kang said. The scientists do not know why some wells were emitting more than their neighbors.

Understaffed agency monitors old wells

Scientists are yet to resolve how much methane is leaking across the state from such abandoned wells.

To resolve the scope of the problem, they did a rough calculation by assuming their subset of 19 was representative of wells across Pennsylvania. If so, they found that abandoned wells would have emitted 0.5 metric ton of methane in 2010, which was 4 to 7 percent of the emissions recorded in the state in 2010.

Put another way, the wells could have emitted about 0.5 percent of the total natural gas produced in the state in 2010.

Further research is needed to learn how big a problem these wells are, Kang said.

The Pennsylvania Department of Environmental Protection has an active program to plug abandoned wells—although not to replug wells that were previously plugged. But the agency is also short-staffed and not equipped to handle hundreds of thousands of wells.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500