A small group of natural gas wells in the U.S. onshore may account for a majority of the methane emissions linked to two major sources -- pneumatic controllers and liquid unloadings -- researchers at the University of Texas at Austin (UT) said Tuesday.

The Environmental Defense Fund (EDF), with an assist from 10 of the largest U.S. natural gas producers, backed the study, the second phase of an in-depth investigation in which researchers were given direct access to hydraulically fractured well sites. The research, overseen by an independent scientific advisory board, is published in the journal Environmental Science & Technology. The first phase was published last year (see Shale Daily, Sept. 17, 2013).

UT researchers from the Cockrell School of Engineering and environmental testing firm URS measured emissions from 377 gas actuated (pneumatic) controllers at natural gas production sites and a small number of oil production sites throughout the country. They found that a "small subset" of gas wells was responsible for most of the emissions from the two major sources at the production sites.

Nineteen percent of the pneumatic devices accounted for 95% of the emissions from pneumatic devices, and 20% of the wells with unloading emissions that vent to the atmosphere accounted for 65-83% of those emissions.

"To put this in perspective, over the past several decades, 10% of the cars on the road have been responsible for the majority of automotive exhaust pollution," said principal investigator David Allen, chemical engineering professor at the Cockrell School. "Similarly, a small group of sources within these two categories are responsible for the vast majority of pneumatic and unloading emissions at natural gas production sites."

EDF Vice President Mark Brownstein said the results of the study "speak to the value of those kind of leak-detection and repair requirements...The idea that you've got a certain subset of pneumatic devices that are responsible for a large proportion of emissions in the study suggests proper operation and maintenance is an important strategy for keeping emissions low overall."

Pneumatic devices use gas pressure to control the opening and closing of valves and they emit gas as they operate. Pneumatic emissions are estimated to be among the larger sources of methane emissions from the gas supply chain. The U.S. Environmental Protection Agency (EPA) has reported that 477,606 pneumatic (gas actuated) devices are in use at gas production sites throughout the United States, UT said. EPA and the Department of Interior are considering regulations and voluntary programs to control methane emissions at well sites, initial steps that could be announced this month.

"Our team's previous work established that pneumatics are a major contributor to emissions," Allen said. "Our goal here was to measure a more diverse population of wells to characterize the features of high-emitting pneumatic controllers."

The researchers sampled all identifiable pneumatic controller devices at each well site, more comprehensive than random sampling previously conducted. The average methane emissions/pneumatic controller reported were 17% higher than the average emissions/pneumatic controller in the 2012 EPA greenhouse gas national emission inventory released this year, "but the average from the study is dominated by a small subpopulation of the controllers," researchers sa.

"Specifically, 19% of controllers, with measured emission rates in excess of 6 standard cubic feet per hour (scf/h), accounted for 95% of emissions. The high-emitting pneumatic devices are a combination of devices that are not operating as designed, are used in applications that cause them to release gas frequently or are designed to emit continuously at a high rate."

For the pneumatic devices, research also confirmed regional differences first reported last year, with methane emissions highest on the Gulf Coast and lowest in the Rocky Mountains. Some of the regional differences in emission rates were attributed to the difference in controller type, such as continuous vent versus intermittent vent, among regions.

After observing variable emissions for liquid unloadings for a limited group of well types in the 2013 study, researchers conducted more measurements and confirmed that most of the emissions came from a "small fraction" of wells that vent frequently.

"Although it is not surprising to see some correlation between frequency of unloadings and higher annual emissions, the study's findings indicate that wells with a high frequency of unloadings have annual emissions that are 10 or more times as great as wells that unload less frequently."

The field study measured emissions from unloadings from wells at 107 gas production wells throughout the country, representing the most extensive measurement of emissions associated with liquids unloadings so far, according to UT.

"A liquid unloading is one method used to clear wells of accumulated liquids to increase production," UT said. "Because older wells typically produce less gas as they near the end of their life cycle, liquid unloadings happen more often in those wells than in newer wells."

A statistical correlation was found between the age of wells and the frequency of liquid unloadings. The "key identifier for high-emitting wells is how many times the well unloads in a given year," researchers said.

Liquid unloadings may use a variety of liquid lifting mechanisms, and the study's results reflected differences in emissions between wells that used two different types -- wells with plunger lifts and those without lifts.

Emissions for unloading events for wells without plunger lifts averaged 21,000 scf to 35,000 scf. For wells with plunger lifts that vent to the atmosphere, emissions averaged 1,000-10,000 scf of methane per event.

"Although the emissions per event were higher for wells without plunger lifts, these wells had, on average, fewer events than wells with plunger lifts," UT said. "Wells without plunger lifts averaged fewer than 10 unloading events per year, and wells with plunger lifts averaged more than 200 events per year. Overall, wells with plunger lifts were estimated to account for 70% of emissions from unloadings nationally."

The Rocky Mountain region has a large number of unloadings frequently venting to the atmosphere, and those emissions accounted for about half of overall total from liquid unloadings, researchers said.

Producers that cooperated in the research and provided financial support for the 30-month methane research series were Anadarko Petroleum Corp., BG Group plc, Chevron Corp., ConocoPhillips, Encana Oil & Gas (USA) Inc., Pioneer Natural Resources Co., SWEPI LP (Shell), Statoil ASA, Southwestern Energy Co. and ExxonMobil Corp. subsidiary XTO Energy.

"While we are still reviewing the study, it provides further support for the findings of other credible researchers -- that greater use of natural gas reduces greenhouse gas emissions," said America's Natural Gas Alliance Vice President Erica Bowman, who handles research and policy analysis. "Natural gas producers have reduced methane emissions by 25% since 1990, even as production has grown 37%.

"The study shows emissions to be lower than current EPA estimates and that a small number of sources are responsible for the majority of emissions. We remain committed to working with stakeholders to ensure that as an industry, we continue the substantial progress we have made through ongoing innovation."