Methane Emissions from the Natural Gas Supply Chain
The primary component of natural gas, methane is a greenhouse gas many times more potent than carbon dioxide when released into the atmosphere unburned. The nation’s vast natural gas infrastructure – which includes wells, compressor stations, processing plants, pipelines, and storage facilities – constitutes the largest industrial source of methane emissions in the United States. The gathering and processing study is part of a series of methane emissions studies organized by Environmental Defense Fund, and represents the largest campaign to measure the U.S. natural gas infrastructure to date.
Natural Gas Gathering and Processing
Gathering facilities collect and compress natural gas from multiple wells, and then send the gas to a transmission line or a processing plant where ethane and natural gas liquids are removed. In addition to large natural gas-powered compressors, both gathering and processing facilities include equipment such as liquid separators, dehydration systems, acid gas removal systems and liquid storage tanks. Processing plants also include additional systems to remove ethane and/or natural gas liquids. While all processing plants are required to report methane emissions under EPA’s Greenhouse Gas Reporting Program, very few gathering facilities are currently required to report methane emissions.
The study found that 0.47 % of the methane produced domestically is lost during gathering and processing operations. According to the study, methane emissions from gathering systems are equivalent to 30 percent of overall methane emissions from natural gas operations, which the current U.S. EPA Greenhouse Gas Inventory estimates to be 6,186 Gg.
The researchers also identified opportunities to improve the two federal programs that track methane emissions from the natural gas supply chain. These programs are the EPA Greenhouse Gas Reporting Program (GHGRP) and the EPA Greenhouse Gas Inventory (GHGI). The GHGI does not explicitly categorize gathering facilities as being separate from field production. The CSU study team, which also included researchers from Carnegie Mellon University and Aerodyne Research, analyzed the current production inventory to determine the appropriate fraction of emissions to assign to gathering, and the results were eight times higher than the best interpretation of EPA data for gathering facilities.
The majority of the methane emissions from gathering and processing operations were attributed to normal operations of gathering facilities. Researchers found that methane emissions from gathering facilities are substantial – 1,697 Gigagrams (Gg) per year — while emissions from processing plants are less than half that amount — 505 Gg per year. These emissions are equivalent to the amount of natural gas consumed annually by 3.2 million U.S. homes and represent a potential loss of $390 million in revenue to producers.
Processing plant methane emissions are reported to the GHGRP. Depending on the type, many processing plants are also subject to regulations that require companies to periodically monitor their facilities for leaks and make a first attempt to repair any leaks found within five days. Gathering facilities are not presently subject to those regulations, and only a small fraction of gathering facilities report methane emissions to the GHGRP.
The study compared processing plant emissions to the emissions listed in the GHGRP and found methane emissions from processing plants were three times higher than those in the GHGRP. Currently, methane emissions reported from gathering facilities are very low (less than 1 Gg per year), because very few of these facilities are required to report methane emissions according to current EPA rules. The EPA has proposed a rule that would require gathering systems to report methane emissions.
The CSU team did not measure emissions from natural gas gathering lines in this study. Future studies need to be conducted to accurately determine the total number of gathering pipelines in the U.S. and their methane leakage rates.
Who sponsored the study?
The Colorado State University-led gathering and processing study is sponsored by the Environmental Defense Fund and five energy companies, four of which have substantial operations in the gathering and processing segment of the natural gas industry. It is one of 16 studies organized by EDF and industry partners to quantify methane emissions across the country’s entire natural gas supply chain.
How many sites did the team visit?
The CSU-led research team, which also included researchers from Carnegie Mellon University and Aerodyne Research, measured methane emissions at 114 gathering facilities and 16 processing plants across 20 natural gas basins in 13 states. It is believed to be the first large-scale study to measure emissions at gathering facilities, and is the largest study to date on the gathering and processing sectors.
How were the sites selected?
Partner companies provided the research team with a list of all their gathering facilities and processing plants. Industry partners also provided site access and detailed facility data (natural gas throughput, gas type, gas composition, equipment inventories, compressor power, age, and inlet/outlet pressures). The gathering facilities that were sampled were selected at random from the combined partner inventory of more than 700 facilities. The 16 processing plants that were sampled represented all accessible partner processing plants. The origin of the gas entering the gathering facilities and processing plants included shale gas, coal bed methane, conventional gas and tight gas.
Can you tell me the location of the sites that were selected and which company operates each one?
That information is not publicly available. The partner companies involved in the study provided the research team with access to sites on the condition that those locations were not made public.
How were measurements taken?
The research team used two independent and complementary measurement techniques to quantify the methane emissions at each of the sites. These techniques were infrared gas imaging and downwind dual-tracer flux. The methods were performed simultaneously so that researchers could compare the quantitative results from the dual-tracer flux, which measured the total emissions rate, to the qualitative results from the infrared imaging, which identified the major sources of leakage at each facility.
Why use two sampling methods?
The downwind dual-tracer flux method accurately captures the total emissions from each facility but does not pinpoint the source. The infrared imaging can pinpoint the source of a major methane leak, but it does not produce a quantitative measurement of how much methane is leaking. The two measurements are complementary.
When were the measurements taken?
Measurements were taken between October 2013 and April 2014.
What is infrared gas imaging?
With this method, leak sources are identified using real-time infrared imaging.
What is downwind dual-tracer flux measurement?
Research personnel released specific “tracer gases” (acetylene and nitrous oxide) at carefully controlled rates from each facility. A second group of research personnel measured concentrations of the tracer gases, along with methane, ethane, carbon dioxide and other gases, downwind of the site using high-time resolution instruments mounted in a mobile laboratory. Measurements of the tracer gases are correlated with the measurements of methane to estimate the amount of methane being emitted at the facility. Measurements of other gases, such as ethane, allow the researchers to distinguish between methane emitted from the facility versus methane emissions originated from other nearby sources. This method is used to measure the magnitude of overall methane emission for a facility.
How many samples were taken at each site?
The amount of data collected at each facility varied by the type of the facility, its size, its accessibility, and other conditions. Multiple tracer gas plumes (10 to 100) were acquired from each facility. Only those facilities where a sufficient number of plumes passed the method’s quality control criteria (greater than 3, and typically 10 to 20) were included in the final 130 facility dataset.
Are raw data publicly available?
Yes, raw and interpreted data from the onsite measurements and analyzed data from the tracer flux measurements will be made available on this website under the “Research Results” section upon publication of each manuscript.
Will the study team publish other papers on methane emission in the gathering and processing sectors of the natural gas supply chain? If so, when?
The study team has published three papers to date. The first paper focuses on measurement methods and was published in Atmospheric Measurement Techniques on May 7, 2015. The second paper focuses on the experimental results from the field campaign and was published in the journal of Environmental Science and Technology on February 10, 2015.
A third paper focuses on developing a national estimate of total methane emissions from gathering and processing, and was published in Environmental Science and Technology on August 18, 2015.