A small number of natural gas wells are responsible for the majority of the methane gas being released into the atmosphere during production, but at higher levels than previously estimated by the U.S. Environmental Protection Agency, according to a new study.
Researchers from the University of Texas at Austin, benefiting from unprecedented direct access to gas well sites across the United States, found in one test that methane releases into the atmosphere were the lowest in the Rocky Mountain region but the highest along the Gulf Coast.
With natural gas exploration expected to increase over the next decade, the researchers said it is important to get a better understanding since methane emissions amounting to just a percentage of natural gas use “can change the greenhouse gas footprint of natural gas.”
David Allen, the principal investigator for the study and a chemical engineering professor at the Cockerell School of Engineering at UT Austin, compared the impact of the methane emissions from a small number of drilling sites to the small subset of cars that contribute to pollution.
“This is not a new idea,” Allen said. “Over the past decade, 10 percent of the cars on the road have been responsible for the majority of the automobile exhaust pollution.”
Matt Watson, the Environmental Defense Fund’s national policy director, said the study is significant since its conclusions are based on direct measurements made in the field and not from calculations that assume how equipment is operating. The EDF was one of the study’s sponsors.
“These pollution-reduction strategies are highly cost effective for methane alone, but when you consider the other things this reduces it is that much more bang for the buck,” Watson said.
The UT Austin-led field study is being published in the Environmental Science & Technology journal today. It is the second phase of the team’s 2013 study, which involved the sponsorship of several energy companies, including Pioneer Natural Resources in Irving and Fort Worth-based XTO Energy, a subsidiary of Exxon Mobil.
The research team from the Austin campus examined two major sources of methane emissions — liquid unloadings and pneumatic controller equipment — at wells pads across the country. Together, they make up 40 percent of gross production emissions, the study found.
In all, the researchers took measurements at 268 wells across the country.
The study found that 19 percent of the pneumatic devices accounted for 95 percent of the emissions. These devices use gas pressure to control the opening and closing of valves and emit gas when they operate. These emissions are estimated to be among the largest sources of methane gas emissions in the natural gas supply chain, the study said.
But while the EPA reports that there are about 500,000 of these devices in use throughout the United States, or about one per well site, the UT study found that there were almost three at each site they visited, increasing the opportunity for emissions. The UT team actually measured the emissions at 377 controllers at 65 pad sites with 161 wells that had been hydraulically fractured.
The average methane emissions per controller in the study are 17 percent higher than the average emissions estimated in an 2012 EPA study released earlier this year. About two-thirds of the high-emitting devices were not operating properly and may need to be repaired or replaced, Allen said.
Liquid unloading is a method used by operators to clear wells of accumulated liquids to increase their production. Since older wells typically produce less gas as they near the end of their life, unloadings happen more often than in new wells, the study states.
The research team measured emissions from wells at 107 natural gas production sites and found that 20 percent of the wells with unloading emissions venting into the air accounted for up to 83 percent of the methane released into the atmosphere, the study found.
The team found a statistical correlation between the age of wells and the frequency of liquid that is unloaded, and that the amount of emissions was directly tied to how many times this occurred, according to the study.
Because of the large number of wells with frequent unloadings that vent into the air, the Rocky Mountain region accounted for about half of the overall methane emissions, researchers found.
Fixing the problem
Ed Ireland, executive director of the Barnett Shale Energy Education Council, an industry-sponsored group, found it noteworthy that the amount of gas escaping at well sites dropped from 0.42 percent to 0.38 percent when compared to an earlier study by this team. And that occurred while natural gas production was increasing, he said.
“I think it is a key statement that most of the wells they have surveyed had low to no methane emissions,” Ireland said, adding, “The majority of wells are not emitting methane.”
“Methane emissions are lower and on the right trajectory,” he said.
In that earlier study, the UT-led team found that “green” completion equipment captures methane emissions on new natural gas wells. The EPA already requires drillers to either capture or flare methane and, starting next year, the gas must be captured. Two years ago, the agency estimated about half of the new wells had green completion devices or flared methane.
Watson agreed that fixing the problem at the well sites is one of the most cost-effective things to do. He said producers can get a 40 percent reduction in emissions for about 1 cent per thousand cubic feet of natural gas.