When fracking in tight shale deposits began to open vast new regions of the United States to oil and gas development at the start of the decade, a question troubled researchers: Just how much of the unearthed gas was leaking on its way from deep wells to delivery points? 

A new article in the journal Science, synthesizing five years of research by more than 140 scientists at 40 institutions, offers the first comprehensive answer on a national scale.

It estimates that the U.S. oil and gas industry emitted 13 million metric tons of methane in 2015 across the supply chain — from new and old wells, pipelines, processing equipment and gas lines beneath city streets.

The lost gas amounts to 2.3 percent of total U.S. gas production — below the leak rate that is generally understood to preserve the climate benefits of burning natural gas for electricity instead of coal, but more than 60 percent higher than the official U.S. Environmental Protection Agency inventory reports.

Methane is the primary component of natural gas and is a powerful greenhouse gas in the short term.

The amount of lost methane documented in the study “is large enough to really have a material impact on the climate footprint of natural gas,” said the study’s lead author, Ramón Alvarez, associate chief scientist at the Environmental Defense Fund. “It is large enough to roughly double the climate impact of natural gas use over a 20-year period.”

The study integrated measurements taken at wells and pipelines with those captured by sensors on airplanes and towers between 2012 and 2016 — combining the specificity of ground-based measurements at individual sites with the broad picture presented by aerial surveys.

It drew data from studies in six oil and gas-production basins, including the Appalachian Basin with its prolific Marcellus and Utica shale wells and scores of operating and abandoned shallower wells.

The study found that 85 percent of emissions across the supply chain come from producing, gathering and processing oil and gas — meaning that leaks tend to be concentrated where gas is drilled more than where it is delivered — although the study relied on inventory estimates to count emissions from the local distribution end of the system because data there are scarce.

Peter DeCarlo, a Drexel University atmospheric scientist who has studied methane levels in the Marcellus Shale region but was not an author of the paper, said it provides a valuable point of comparison — both for future studies and for gauging how leakage rates may change as infrastructure ages.

“There is still some uncertainty in all this, but now we have a point of comparison that is very broad-looking and covers much of the U.S. production,” he said.

The paper found that the U.S. EPA’s official inventory is generally not that far off in terms of the emissions formulas it uses to estimate how much each piece of equipment leaks.

The large discrepancy seems to come from what the EPA’s count simply misses: high emissions caused by unusual operating conditions, like equipment malfunctions.

Earlier studies had demonstrated that outliers — so-called super-emitters — are to blame for a disproportionate amount of total leakage.

The good news is that “a lot of the leaks, these abnormal ones, are not that hard to fix,” said Allen Robinson, one of the study’s co-authors and a professor of engineering and public policy at Carnegie Mellon University. “The challenge is identifying them efficiently.”

Another co-author, Pennsylvania State University atmospheric and climate science professor Kenneth Davis, pointed out that the national 2.3 percent leakage rate is “not at all” uniform across basins.

A study that his team did using aerial surveys in the northeastern Pennsylvania region of the Marcellus Shale, for example, found only 0.4 percent emissions — the lowest emissions as a percentage of production of any basin in the country.

But the official EPA inventory of Pennsylvania’s oil and gas emissions is even further away from actual measurements than the 60 percent underestimate nationally, he said.

“So while the leakage rate is really small, our understanding of where it’s coming from is even worse.”

An EPA spokesman said the agency is “looking forward to reviewing this study.”

Companies have a natural incentive to cut down on methane leaks, since the more product they keep in the system, the more they can sell. The lost methane reported in the new study is worth about $2 billion a year, the researchers said.

A report by the National Energy Technology Laboratory last month found that an industry collaborative of major companies dedicated to curbing methane leaks across the supply chain, known as the ONE Future Coalition, had lower than average emissions rates, demonstrating that better practices make a difference.

The average lifecycle methane emission rate for the coalition was 0.67 percent, NETL found, compared to 1.6 percent nationally.

Pennsylvania regulators recently released new permits that will directly control methane emissions from new shale gas wells for the first time. They are scheduled to go into effect in August.

Laura Legere: llegere@post-gazette.com.

First Published: June 21, 2018, 9:02 p.m.