Abandoned oil and gas wells are a source of greenhouse gases, particularly methane.

Yet there are so many scattered across the United States that stopping the leaks—and even determining which wells are leaking—presents a challenge.

Scientists from Princeton, Stanford, and Ohio State universities, as well as the Lawrence Berkeley National Laboratory, have identified the specific attributes of abandoned wells that leak significant amounts of methane. The technique could help state governments prioritize which wells to repair.

Combining measurements of methane flow rates; concentrations of noble gas, ethane, propane and n-butane; and data from historical documents and field investigations, the researchers found that wells with high emissions were either unplugged gas wells or those that were plugged but vented, which is a safety measure for old wells in areas containing coal.

Methane escaping from abandoned wells makes up 5%-8% of Pennsylvania's greenhouse gas emissions. Image credit: Michael Celia, Princeton Methane escaping from abandoned wells makes up 5%-8% of Pennsylvania's greenhouse gas emissions. Image credit: Michael Celia, Princeton The scientists, led by Stanford postdoctoral researcher Mary Kang, focused on abandoned wells in western Pennsylvania because the state has the nation's longest history of oil and gas operations. Among their findings:

· Methane escaping from abandoned wells makes up 5%-8% of the state's greenhouse gas emissions.

· Wells releasing large amounts of greenhouse gases represent about 10% of all wells but produce about 90% of emissions.

· Repeated measurements over two years show that flow rates of high emitters are sustained through time.

"Our goal in this work was to identify well characteristics that would help identify high emitters and thus provide an opportunity to target them for mitigation and avoid the cost of plugging abandoned wells with low or no emissions" says Denise Mauzerall, professor of civil and environmental engineering at Princeton. "We hope this approach can be used across the United States and abroad to identify high emitters and target them for remediation."

The researchers also concluded that the number of abandoned wells in Pennsylvania is likely to be far higher than those tracked by state regulators.

Pennsylvania's Department of Environmental Protection, for example, has records for 31,676 abandoned wells as of October 2015. The researchers estimate that the actual number ranges from 470,000 to 750,000 wells.

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