Fayetteville Study: Basin Reconciliation
Prior methane measurement studies or analyses had indicated that top-down, aircraft-based methods have consistently reported higher total emissions or emissions intensity rates than bottom-up, inventory-based methods. The Basin Methane Reconciliation Study was designed to understand, and potentially reconcile, the persistent gap between top-down and bottom-up methane emissions estimates for production regions. To minimize the potential shortcomings of prior studies, the Basin Methane Reconciliation Study was designed as a first of its kind to conduct contemporaneous measurements at the device, facility and regional scales, with site access and activity and emission data input from local natural gas operators.
Measuring methane emissions is accomplished through two general approaches. A “bottom-up” approach takes direct measurements of representative emissions from individual sources, for example, flanges, valves and pneumatic controllers at a facility, and rolls up those estimates based on population of potential sources and other factors to reach an extrapolated total.
The other approach is “top-down,” in which the airborne concentration of methane is measured up- and down-wind of potential sources and then analyzed to estimate the emissions rate and to attribute emissions to certain sources. An example of this approach at the regional scale is a “mass balance” measurement performed by comparing aircraft measurements of methane concentration up- and down-wind of a basin or group of facilities. An example of this approach at the facility scale is to estimate emissions using the tracer release method.
According to prior studies, top-down, regional estimates of methane emissions from natural gas systems via atmospheric observations are generally higher than bottom-up emissions inventories extrapolated from various points in the natural gas supply chain (production, gathering, processing, etc.). The airborne, top-down estimates average are up to six times higher than inventory-based bottom-up emissions estimates. Reconciling this discrepancy was the chief motivation behind the Basin Methane Reconciliation Study.
The study was a first-of-its-kind, multi-agency research project that drew from the scientific expertise of Colorado State University (CSU), Colorado School of Mines (CSM), University of Colorado-Boulder (CU), the National Oceanic and Atmospheric Administration (NOAA), and the National Renewable Energy Laboratory (NREL). The University of Wyoming, AECOM, Aerodyne and GHD Engineering also participated in the study.
A four-week field campaign took place in September/October 2015 in Arkansas’ Fayetteville Shale Play, which is located in the Arkoma Basin. With support of industry partners, teams of researchers took contemporaneous measurements of methane emissions using both bottom-up and top-down approaches at both facility scales and basin scales. Incorporating all the data collected by each team, a CSU team developed a bottom-up model that aligned the spatial and temporal characteristics of the methane emissions and showed distinct agreement between the bottom-up and top-down estimates.
First, the field teams were given unprecedented access by gas facility operators, who provided escorts and site access, as well as data and insight about how their systems operate and what was occurring at facilities when measurements were made. Without this cooperation, the study would not have been possible. Site access or operational data were provided by Southwestern Energy, XTO, CenterPoint, Enable, Kinder Morgan and BHP Billiton.
Second, the methods and measurements used during the campaign were done contemporaneously – that is, on-site, direct measurements and downwind facility measurements were made at nearly the same time as the top-down measurements. This was an unusually coordinated effort.
Third, the CSU team’s bottom-up model, which pulls together all the data, accounts for both spatial and time variability of all operations taking place in a natural gas basin – a level of accuracy that has never been achieved before.
The natural gas supply chain provides clean-burning fuel that is of fundamental importance for U.S. energy and economic security. Methane, the main component of natural gas, is a short-lived greenhouse gas with a global warming potential 34 times that of CO2 over a 100-year timeframe. The natural gas supply chain is one of the largest contributors of anthropogenic methane emissions. Throughout the nation’s natural gas production, gathering, processing, transmission and storage network, some methane is lost to the atmosphere via either unintended leaks or vented as part of normal operations. Regulators, policymakers, and industry are interested in better understanding methane emissions to develop appropriate programs and policies to minimize product loss and reduce climatic impacts from methane.
Several measurement teams were deployed in the study area in September/October 2015.
- Aircraft: One team from Scientific Aviation, CU Boulder and NOAA performed several different types of aircraft-based measurements in the study area
- Tracer: Two Aerodyne teams – one for production facilities, one for gathering facilities – utilized well-established dual-tracer flux methods to make downwind measurements at individual facilities.
- OTM33a: One team from University of Wyoming utilized EPA “Other Test Method 33a” to estimate emissions at production facilities.
- Onsite: Two teams from industry, and one additional team from an industry partner, performed device-level measurements at production and gathering facilities.
- Distribution/gathering lines: One team split time between surveying gathering lines, and measuring distribution systems
- Mobile survey: One team from NOAA performed mobile surveys of ethane/methane ratios in the study area.
In addition to measurement teams, CSU provided study observers who monitored the measurement methods, coordinated the campaign and assured data integrity during the campaign. CSU was also responsible, with industry cooperation, for selecting facilities for measurement and collecting operational data at measured facilities.
- Vaughn, TL, Bell, CS, Pickering, CK, Schwietzke, S, Heath, GA, Pétron, G, Zimmerle, DJ, Schnell, RC, Nummedal, D (2018) Temporal variability largely explains top-down/bottom-up difference in methane emission estimates from a natural gas production region. Proc Natl Acad Sci USA 115: 11712-11717. https://doi.org/10.1073/pnas.1805687115
- Mielke-Maday, I., Schwietzke, S., Yacovitch, T., Miller, B., Conley, S., Kofler, J., Handley, P., Thorley, E., Herndon, S.C., Hall, B., Dlugokencky, E., Lang, P., Wolter, S., Moglia, E., Crotwell, M., Crotwell, A., Rhodes, M., Kitzis, D., Vaughn, T., Bell, C., Zimmerle, D., Schnell, R. and Pétron, G., 2019. Methane source attribution in a U.S. dry gas basin using spatial patterns of ground and airborne ethane and methane measurements. Elem Sci Anth, 7(1), p.13. DOI: http://doi.org/10.1525/elementa.351
- Bell, C. S., Vaughn, T. L., Zimmerle, D., Herndon, S. C., Yacovitch, T. I., Heath, G. A., … Soltis, J. (2017). Comparison of methane emission estimates from multiple measurement techniques at natural gas production pads. Elem Sci Anth, 5, 79. DOI: http://doi.org/10.1525/elementa.266
- Zimmerle, D. J., Pickering, C. K., Bell, C. S., Heath, G. A., Nummedal, D., Pétron, G., & Vaughn, T. L. (2017). Gathering pipeline methane emissions in Fayetteville shale pipelines and scoping guidelines for future pipeline measurement campaigns. Elem Sci Anth, 5, 70. DOI: http://doi.org/10.1525/elementa.258
- Vaughn, T. L., Bell, C. S., Yacovitch, T. I., Roscioli, J. R., Herndon, S. C., Conley, S., … Zimmerle, D. (2017). Comparing facility-level methane emission rate estimates at natural gas gathering and boosting stations. Elem Sci Anth, 5, 71. DOI: http://doi.org/10.1525/elementa.257
- Yacovitch, T. I., Daube, C., Vaughn, T. L., Bell, C. S., Roscioli, J. R., Knighton, W. B., … Herndon, S. C. (2017). Natural gas facility methane emissions: measurements by tracer flux ratio in two US natural gas producing basins. Elem Sci Anth, 5, 69. DOI: http://doi.org/10.1525/elementa.251
- Stefan Schwietzke, Gabrielle Pétron, Stephen Conley, Cody Pickering, Ingrid Mielke-Maday, Edward J. Dlugokencky, Pieter P. Tans, Tim Vaughn, Clay Bell, Daniel Zimmerle, Sonja Wolter, Clark W. King, Allen B. White, Timothy Coleman, Laura Bianco, and Russell C. Schnell. Improved Mechanistic Understanding of Natural Gas Methane Emissions from Spatially Resolved Aircraft Measurements. Environmental Science & Technology 2017 51 (12), 7286-7294 DOI: https://doi.org/10.1021/acs.est.7b01810
- Anna M. Robertson, Rachel Edie, Dustin Snare, Jeffrey Soltis, Robert A. Field, Matthew D. Burkhart, Clay S. Bell, Daniel Zimmerle, and Shane M. Murphy. Variation in Methane Emission Rates from Well Pads in Four Oil and Gas Basins with Contrasting Production Volumes and Compositions. Environmental Science & Technology 2017 51 (15), 8832-8840. DOI: https://doi.org/10.1021/acs.est.7b00571
Funding Provided by:
This multi-million dollar project was commissioned by the National Energy Technology Laboratory (NETL), a division of the U.S. Department of Energy (DOE), through the Research Partnership to Secure Energy for America (RPSEA) program. Additional funding for the project was provided by the Colorado Energy Research Collaboratory, and by matching funds from participating research institutions. Industry sponsors included Southwestern Energy, XTO Energy, Equinor, Chevron and the American Gas Association.