Measuring methane loss in Texas' Barnett Shale
New study finds natural gas leaks in the region higher than some estimates, similar to others
About 170,000 pounds (76,000 kg) of the greenhouse gas methane leak per hour from the Barnett Shale region of Texas, including the urban areas of Dallas and Fort Worth, according to a new study led by NOAA/ESRL and Cooperative Institute for Research in Environmental Sciences (CIRES) researchers.
The researchers measured regional methane emissions by aircraft, and the new leak rate estimate agreed with the U.S. EPA's national estimate, but was higher than estimates reported in other commonly-used inventories, they reported in the current issue of Environmental Science & Technology. The scientists found methane leakage rates (leak per unit of natural gas produced) between 1.3 and 1.9 percent, which is lower than estimates from some airborne studies of different oil and gas basins, but higher than those from others.
"The total amount of methane emitted is similar to what we found in Utah's Uintah Basin in 2012, but production in the Barnett Shale region is so much higher, so that means the leak rate is lower," says Anna Karion, a CIRES scientist working at ESRL's Global Monitoring Division (GMD). Karion is lead author of the new study, which includes researchers across ESRL, the University of Michigan, Purdue University, the University of California, and others.
The new estimate of the region's methane leak rate, 1.3 to 1.9 percent, is close to the EPA's national average leakage rate, about 1.5 percent of production, which is based on the agency's inventory of annual natural gas emissions from all components of the natural gas supply chain and annual natural gas production figures. The new study finds that other estimates, including the EPA's Greenhouse Gas Reporting Program (based on self reports from large producers), and the Emission Database for Global Atmospheric Research (EDGAR), likely underestimate methane emissions in the Barnett Shale region—by factors of three and almost five, respectively.
Texas' Barnett Shale is one of the country's largest gas production basins, accounting for 8 percent of the total U.S. natural gas production. Methane is the primary constituent of natural gas and a potent greenhouse gas, so having an accurate estimate of methane emissions from the region is important for better understanding the greenhouse gas footprint of U.S. onshore shale gas production.
For this study, the researchers used measurements taken during eight aircraft flights crisscrossing the dense gas production and urban areas in the Barnett Shale region of Texas, in two seasons (March-April and October 2013). "The sheer amount of data collected sets this study apart from others that have used a similar method to calculate an estimate of regional methane," Karion said.
To calculate methane emissions for the region, researchers in an instrumented aircraft followed an air mass as it moved into a region and then flowed out, and looked at the difference in methane levels between those two places; this technique is called mass balance calculation.
Once they had the overall methane emission estimate for the region, the researchers used airborne ethane measurements to split those methane emissions into parts that can be attributed to oil and gas production, or to non-fossil fuel sources like landfills and agricultural activities. Because emissions from non-fossil fuel sources do not contain ethane, the researchers could look for a chemical signature of methane and ethane together to determine what proportion of methane emissions came from oil and gas operations.
Karion and her colleagues noted that their flights included the cities of Dallas and Fort Worth, where municipal distribution of natural gas to homes may be one of several sources of the methane emissions. "So we were able to capture more of the natural gas supply chain than we've captured before, including leaks from homes, pipelines, and other parts of the natural gas distribution system," Karion said.
This new paper, along with several other airborne emission studies conducted by NOAA and CIRES in recent years, may help policymakers determine how to meet the goals of the President's Climate Action Plan, said Russ Schnell, Deputy Director of GMD. "To meet those goals, the natural gas and oil sector would need to reduce its methane emissions by 40 percent below 2012 levels in the next decade," Schnell said. "To achieve this, we must understand where large leaks occur and why some fields leak 5 to 10 times more than others."
This study was funded primarily by the Environmental Defense Fund.
The 21 authors of "Aircraft-based estimates of total methane emissions from the Barnett Shale Region," part of special issue published in the current issue of Environmental Science & Technology, are from CIRES, NOAA, the University of Michigan, Purdue University, the University of California, Carbon Now Cast, Aerodyne Research, the Environmental Defense Fund, and Picarro. Their paper is one of 11 in a journal special edition on "Measuring oil and gas methane emissions from oil and gas in the Barnett Shale."