A new study led by Colorado State University found wide variations in the amount of methane being emitted at U.S. natural gas gathering facilities and processing plants. Researchers at CSU, Carnegie Mellon University and Aerodyne Research took samples at 114 gathering stations and 16 processing plants across 13 states in the most comprehensive field study to date on these little-researched natural gas sectors.

Their findings indicate facility-level methane emissions ranged from less than 1 kilogram per hour to 698 kilograms per hour, while loss rates ranged from less than 0.01 percent to greater than 10 percent. (Loss rates are the ratio of methane being emitted to the amount flowing through a facility.) All of the stations with loss rates greater than 10 percent were smaller facilities with lower natural gas throughput.

"This is an exciting study because it is, by far, the largest and most comprehensive data set ever collected on direct methane emissions from the gathering sector," said Anthony Marchese, a CSU professor of mechanical engineering who led the study.

"The results point to the gathering sector likely being a notable source of emissions and identify areas where emission reductions can be achieved."

The team's findings were published today in the journal of Environmental Science and Technology. Researchers are using the measurement data to develop an estimate of total methane emissions from all gathering and processing facilities in the country. The results are expected to be published in the coming months.

Other key findings include:

+ Significant variations were found among facilities of similar size and type and appear to be driven by differences in inlet and outlet pressure and abnormal process conditions. This resulted in a few sites contributing to a majority of the emissions. For example, approximately 30 percent of the gathering facilities accounted for nearly 80 percent of the methane emissions measured.

+ One-sixth of the gathering facilities (with compression and/or dehydration) were classified as having an abnormal process condition that resulted in larger than expected methane emissions at liquid storage tanks.

+ Emissions at these "abnormal" facilities were, on average, around 300 percent higher than similar facilities functioning normally.

+ Measurements indicate methane loss rates at processing plants were much lower than at gathering facilities. None of the processing plants had loss rates higher than 0.6 percent.

"The results of this study and the direct measurements obtained, suggest the majority of emissions can be attributed to a relatively small number of facilities," said Allen Robinson, the Lane Professor of Mechanical Engineering at Carnegie Mellon who led the field measurements for the study. "That means we may be able reduce the overall methane emissions from the gathering and processing sectors by focusing on sources at these higher-emitting sites."

The CSU study is one of 16 organized by the Environmental Defense Fund and industry partners to better quantify the amount of methane escaping into the atmosphere from the natural gas supply chain. It was sponsored by EDF, Access Midstream, Anadarko Petroleum Corp., Hess Corp., Southwestern Energy Co. and Williams. The company sponsors provided financial support for the study and access to facilities. DCP Midstream provided access to one of its processing plants.

Methane, the primary component of natural gas, is a potent greenhouse gas, and natural gas and petroleum systems are the largest single source of man-made methane emissions in the United States, according to the Environmental Protection Agency.

Facilities in the gathering and processing sectors "collect" natural gas from production wells, remove impurities, separate natural gas liquids and then deliver the product to the interstate pipeline network or local distribution lines.

The difference between gathering and processing plants is not surprising, Marchese said.

"Processing plants are generally much larger and permanently staffed, and are required to report methane emissions to the EPA," he said. "They also are required by federal law to repair any leaks within five days of detection. Most gathering facilities aren't subject to those federal regulations."

Only gathering facilities that exceed the 25,000-metric-ton carbon dioxide-equivalent emissions threshold are required to report any methane emissions to the U.S. Environmental Protection Agency and are not subject to the same leak detection and repair regulations.

The researchers say the comprehensive field study has provided the most methane emissions data to date for the gathering and processing sectors.

"Prior to our study, most of the information we had on methane emissions from these facilities originated from studies in the early 1990s," Marchese said.