A 2-year study by analysts at the U.S. Department of Energy’s National Energy Technology Laboratory (NETL), in which they synthesized new methane emission data from a series of ground-based field measurements, shows that 1.7 percent of the methane in the U.S. natural gas supply chain is emitted between extraction and delivery. Identifying the magnitude and sources of methane emissions will allow producers to prioritize opportunities to reduce emissions of the potent greenhouse gas. Results of the study have been published in the Journal of Cleaner Production.
The study identified gathering systems and pneumatic controllers at production sites as top contributors to the emission of methane in the natural gas supply chain. Gathering systems, which play a key role connecting production and processing, represented 22 percent of methane emissions. Production pneumatics, which accounted for 10 percent of methane emissions, intentionally release methane into the atmosphere as part of their function to reduce control line pressure.
Another emission category, termed “unassigned emissions” comprises a complex blend of sources. Unassigned emissions make up 19 percent of methane emissions from natural gas systems, but are not a single, unified emission source. Instead, the category includes multiple sources—a small number of production sites with atypically high emission rates, production equipment that requires maintenance, intermittent wellhead maintenance events, or any combination thereof. Future efforts may attribute unassigned emissions to specific emission sources.
For its synthesis report, NETL used new data from studies funded by the Environmental Defense Fund that were representative of the U.S. natural gas supply chain from production through distribution. These studies measure, model, and report methane in different ways, creating a need to compile the results in a clear and consistent manner. NETL made use of its own natural gas life cycle analysis tool to synthesize this new data.
The study has shed new light on methane emissions in U.S. natural gas supply chain, as well as revealing the knowledge gaps and sources of uncertainty that must be addressed through further study. Future research may include geographically diverse measurement studies that can provide a better understanding of regional variability and may validate emission measurements by using a combination of component- and site-level measurements.