Oil and gas basins in North Dakota and East Texas leaked around 10 percent of natural gas they produced to the atmosphere between 2006 and 2011. Natural gas is composed primarily of methane, a potent greenhouse gas that exacerbates climate change.

Considering that North Dakota's Bakken Shale produced 485 million cubic feet per day of gas in September 2011, a 10 percent leak adds up to a whole lot of greenhouse gas emissions. The Eagle Ford of East Texas produced 1,232 million cubic feet of gas per day in 2011. The leakage rates were published this month in the journal Earth's Future.

The leakage rate in the study is the largest reported so far for the energy industry, and it challenged the industry's and U.S. EPA's perception of operations as relatively clean.

The study finds that the Bakken and Eagle Ford basins leaked between 3 percent and 17 percent of the natural gas produced between 2009 and 2011, with the Bakken most likely emitting 10.1 percent and the Eagle Ford most likely emitting 9.1 percent.

For the study, scientists used satellite measurements and a unique methodology to compute the methane leakage rate, which is one of the key unknowns about energy usage in the United States today.

The United States is in the middle of an oil and gas boom since 2005, and the Obama administration has promoted natural gas as a bridge fuel to a post-carbon future. The reasoning is that gas emits only half as much carbon dioxide as coal when burned in a power plant.

But scientists have pointed out that gas may also be bad for the climate, not because of its carbon dioxide emissions but because of its methane emissions. The greenhouse gas is 86 times as potent as carbon dioxide for the climate on a 20-year time scale. The gas leaks from wellheads, valves, tanks, pipes, processing plants and other parts of the energy supply chain.

A methane leakage rate above 3.2 percent may negate the fuel's climate benefits in the power plant, scientists say. And in such a case, gas will be as bad as coal.

The Earth's Future study puts leakage well above that.

The death of an effective spy in the sky

The study used a European remote-sensing instrument, SCIAMACHY (affectionately referred to as "Scia" by scientists), to get a picture of emissions from the Bakken, the Eagle Ford and the Marcellus Shale in Pennsylvania. The instrument, aboard the Envisat satellite, captured a range of atmospheric conditions between 2006 and 2012. Scientists led by Oliver Schneising, a researcher at the University of Bremen in Germany, spent years deciphering the signature of methane from the satellite data.

The data stopped in 2012 because Envisat -- and Scia -- died of power failure that year, said Russell Dickerson of the Department of Atmospheric and Oceanic Science at the University of Maryland and co-author of the study. "Scia was indeed, she was important, but she was never animate," he eulogized.

But by then, the Eagle Ford, Bakken and Marcellus were already in boom times with the advent of hydraulic fracturing and directional drilling. The techniques unlocked vast new reserves in shale reservoirs.

Schneising and his colleagues were able to compare methane emissions in the period of 2006-08 with the emissions from 2009-11. They found more methane was emitted after 2009, probably due to increased drilling, in all three basins. They then calculated the amount of methane leaking into the atmosphere over this period in the Bakken and the Eagle Ford.

The benefit of using a satellite is that it provides a comprehensive picture of emissions over a long period of time. Other studies have found high leakage rates; for example, a National Oceanic and Atmospheric Administration study found 9 percent of natural gas produced in the Uinta Basin of Utah leaks. But these studies relied on measurements made from aircraft flying over oil fields on particular afternoons, and as such, they provide only a snapshot in time.

"The satellite can give you years of information over a broad area," Dickerson said.

The scientists were extremely conservative in their calculations, which is why they estimated a broad range of leakage rates, from 2 to 17 percent. But even at the low end of this range, the rate is higher than the rate calculated by EPA of 1.2 percent.

Manvendra Dubey, a scientist at Los Alamos National Laboratory who was not affiliated with the study, said the study shows the importance of the SCIAMACHY measurements. Dubey and colleagues used the space-based data set for a similar study that found high emissions from the San Juan basin in New Mexico (ClimateWire, Oct. 10).

Dubey cautioned, however, that the scientists did not "ground truth" their results by measuring emissions directly on the ground and comparing that with the space measurements, which is why their error bars are so large.

Shale production leaks are 'substantially greater'

Space observations cannot be used to pinpoint the source of the leaks, Dubey said. A small subset of wells could be responsible for observed high leakage rates, he said. But overall, both studies show satellites are an important tool, he said.

"Let's measure more, let's get better satellites up there so that we can continue to monitor [emissions]," he said.

Robert Howarth, a professor of ecology at Cornell University who was not involved in the study, said satellites are great for integrating information over space and time.

"This study finds the highest levels of methane emission reported yet for shale gas, confirming that shale gas emissions are substantially greater than from conventional natural gas," he said via email.

Other studies have found much lower leakage rates. For instance, David Allen, a researcher at the University of Texas, Austin, and the Environmental Defense Fund measured leaks from equipment at individual well pads. They found leakage was just 0.42 percent of the methane produced (EnergyWire, Sept. 17, 2013).

The Allen study could be indicative of the industry at its best. Allen and his colleagues measured well pads with the cooperation of the industry and did not include emissions from gathering systems, compressors and other equipment known to be leaky.

The industry "provided him access to a select set of oil and gas plays and operations that one could imagine were a showcase of the best possible engineering," Dickerson said. "So what I like about David Allen's paper is that it is indicative of what can be achieved with careful engineering.

"I'm not sure it is representative of the entire system."

Kelsey Robinson, a spokeswoman at EDF, said: "It should be noted that the [University of Texas] study only estimated leakage rates from the production sector, whereas this one estimates emissions from the entire supply chain. That could be a reason for the disparity."