It’s a question that’s puzzled policymakers, researchers and regulatory agencies for years: How much methane is being emitted from natural gas operations across the U.S.? And why have the two main estimation methods applied across U.S. oil and gas basins always seem to disagree?
A new study, published October 29 in Proceedings of the National Academy of Sciences and led by Colorado State University, was based on a detailed examination of the infrastructure in Arkansas’ Fayetteville Shale Gas Play, by scientists working alongside industry employees who explained the purpose and operation of each component.
“Our study is the first of its kind, in its scope and approach,” said Dan Zimmerle, senior author of the PNAS study, and a senior research associate at the CSU Energy Institute. “It utilized concurrent ground and aircraft measurements and on-site operations data, and as a result reduces uncertainties of previous studies.”
A principal finding: episodic releases of methane occurring mostly during daytime-only maintenance operations may explain why emissions estimates based on the two traditional accounting techniques have not agreed in past analyses.
The Basin Methane Reconciliation Study included scientists from CSU, Colorado School of Mines, NOAA, CIRES, and the National Renewable Energy Laboratory. Other scientific partners were University of Wyoming, Aerodyne, AECOM, Scientific Aviation and GHD. The field campaign took place in 2015 in Arkansas’ Arkoma Basin.
The research team set out to investigate the persistent gap between two widely used methods of estimating methane emissions from natural gas operations. “Bottom-up” estimates, such as those used in the EPA Inventory of U.S. Greenhouse Gas Emissions and Sinks, are developed by measuring emissions from a representative sample of devices, scaled up by the number of devices or emission events. In contrast, “top-down” measurements can be performed at a regional scale, such as flying an aircraft upwind and downwind of a study area to derive total emissions from methane entering or leaving a basin.
Most aircraft-based, basin-scale emissions estimates have been statistically higher than estimates based on bottom-up accounting.
The campaign, a project of the nonprofit Research Partnership to Secure Energy for America, involved more than 60 researchers making coordinated facility- and device-level measurements of key natural gas emissions sources. The campaign also included a series of aircraft flyovers to collect measurements during the same period when researchers were taking measurements on the ground. The flights took place when meteorological conditions allowed accurate regional emissions estimates.
“The key to our efforts was having everyone out in the same field, at the same time,” said Gabrielle Petron, a research scientist from CIRES at CU Boulder and NOAA who was the principal investigator for the top-down measurement team.
“By comparing the merits and pitfalls of multiple methods of measurement, we were able to paint a much more comprehensive, and we believe accurate, picture of the methane emissions landscape for natural gas infrastructure.”
This story has been adapted from a Colorado State University press release. Click here to read the full version.