Methane emissions study stirs questions

Posted: Feb 19, 2012

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Natural gas drilling

An energy expert is challenging some of the assumptions in a recently published study that suggests natural gas operations emit more methane than previously believed.

Michael Levi, energy policy fellow at the Council on Foreign Relations, criticized the study from the Journal of Geophysical Research -- Atmospheres in a blog post this week.

Gabrielle Pétron, lead author of the study and researcher at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory, responded to the challenge in comments to Greenwire.

Both Levi and Pétron agreed the new study cannot be easily compared to earlier ones on methane leakage, such as a hotly debated study out of Cornell University on the subject. In the NOAA study, Pétron measured methane levels in the air over Colorado's Northern Front Range during the summer of 2008. She correlated the chemical signature of the methane to the chemical signature of gases vented by gas operations on the Denver-Julesburg Basin (Greenwire, Feb. 14).

Her paper found a rate of leakage from 2008 gas operations in the basin between 2.3 and 7.7 percent, with an average of 4 percent.

In his post, Levi questioned the study's assumptions about the composition of the leaked gas. Natural gas contains primarily methane, and other gases including propane and butane depending on the well. Levi specifically questioned the scientists' use of the ratios of methane-to-propane present in vented gas.

In response, Pétron said the methane-to-propane ratios were derived from raw data from 77 oil and gas wells surveyed by the Colorado Oil and Gas Conservation Commission.

"We don't just take numbers off the shelves," she wrote. "We calculate these uncertainties."

Levi also questioned whether the researchers can be sure the emissions are from gas wells. To pinpoint gas operations as the source of methane, the NOAA scientists matched the chemical signature of the sampled air to emissions from tanks storing condensate, or natural gas liquids, that operators keep on their fields.

Levi wrote that this method introduces uncertainty about whether the methane comes from natural gas wells, or from wells that yield natural gas liquids.

Pétron replied that the methane in the air over the Northern Front Range, which is the only region her study deals with, certainly came from the Denver-Colorado Basin. Almost all gas operations on the basin extract natural gas liquids, which are "liquid gold" to operators in the region, she said.

Levi also wrote that since some shale plays in the United States are purely gas wells, the NOAA results cannot be extrapolated to these.

He further questioned the researchers' use of uncertainty in the data. The study found a range of 2.3 to 7.7 percent of leakage, and the study's lower limit overlaps with rates of leakage calculated elsewhere, he wrote. Cornell University's research found leakage at 2.2 to 3.8 percent, and U.S. EPA has calculated a rate of 2.8 percent.

"It's not clear why the upshot of this paper ... isn't that leakage rates are probably in this lowish area," he wrote.

Pétron responded that a small overlap is "not really reassuring." The statistics simply mean that the true value of leakage lies somewhere between the range of 2.3 and 7.7 percent, she said. Determining whether it is closer to the 2.3 percent would require more research, which the scientists are doing.

Finally, Levi differentiated between the Cornell University study and the NOAA study, saying the two are "apples and oranges." They cannot be compared directly since they study wells at different stages of their life, he wrote.

In the Cornell study, scientists estimate leakage from drilling a well, when a lot of methane is released, followed by production. In the NOAA study, the scientists looked at overall emissions for the entire basin, including new and old wells, producing or being drilled, as well as sources such as leaks from compressors and gathering pipelines.

Pétron agreed that the two studies cannot be directly compared.

While Levi praised the paper for its "fantastic observational data," he wrote that, for now, "I'm not ready to rely on its results." He also wrote that he "may end up submitting a technical 'Comment' to JGR," so his comments may seem oblique, because "I'm going to have to be careful not to preempt myself here."

Pétron said she and her colleagues look forward to the peer-reviewed comments and would be happy to take Levi on a tour of the Denver-Julesburg Basin so he can clarify some of his doubts about the origin of the methane.

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