Over the past two years large variations in the Environmental Protection Agency (EPA) estimates of the volume of methane released during natural gas production have been used by organizations arguing respectively that natural gas is cleaner than or dirtier than coal.
The data variation is confusing to the public as well as Washington, D.C., policy makers who could choose to restrict or encourage natural gas production based on its assumed environmental impact.
The confusion is justified: It is not simple to estimate how much methane escapes from half a million wells that use varied completion and production techniques.
It is especially difficult to assure the existing small sample of wells reflects the universe of well-completion operations such as flowback and liquids unloading. These operations are poorly sampled and show highly most variable methane emission volumes – more about this later in the article.
The best news is the technologies to reduce fugitive methane emissions, such as green completions, are improving and more widely used.
Also encouraging is the fact that additional studies are expected to define and constrain some of the poorly documented statistics for flowback and well unloading.
A timeline of methane emission studies may help explain how the discrepancies developed:
The EPA launched its greenhouse gas (GHG) Inventory, a national-level estimate of large and small emitters, 20 years ago.
The GHG Reporting Program (GHGRP), which collects data from the largest GHG emitting facilities in the United States, was launched in response to 2008 legislation.
Most sources, including petroleum refineries, started reporting 2010 emissions to the GHGRP in 2011. Petroleum and natural gas systems and CO2 injection projects (for enhanced oil recovery or geologic sequestration) reported emissions for the first time in September 2012 (for 2011 emissions).
Only facilities that emit over 25,000 metric tons of CO2 equivalents (MMCO2e) are required to submit reports.
EPA received 2011 emissions reports from 1,880 petroleum and natural gas facilities, including natural gas production, processing, transmission, distribution, storage and LNG facilities. These were used to estimate the 2011 Inventory that was released in April 2013.
EPA’s April 2012 GHG Inventory Report for the first time used Natural Gas Star data (industry reports that bragged about methane emissions reduction successes for 8,800 wells) as a surrogate for 2010 emissions from 488,000 wells.
Compared to prior years, the 2012 report revised the estimated 2010 emissions from natural gas systems up by about 11 percent, to 215.4 MMCO2e.
In response to the high EPA estimate, the American Petroleum Institute and America’s Natural Gas Alliance (API/ANGA) surveyed industry to collect data from nearly 91,000 wells, which projected that methane emissions from natural gas systems were 102.6 MMCO2e – about half the EPA estimate.
More importantly, the API/ANGA September 2012 report pointed out the need for additional sampling of well unloadings, which are poorly sampled but account for 51 percent of methane emissions from natural gas systems in EPA’s 2012 report.
EPA released the 2011 inventory in April 2013. The inventory revised the estimate of 2010 emissions for natural gas systems downward to 143.6 MMCO2e, a 33 percent reduction from the prior year’s report.
This change evidently reflects consideration of the API/ANGA survey results.
David T. Allen, the Melvin H. Gertz Regents Chair in Chemical Engineering at the University of Texas (UT), and 13 other researchers reported in the September 2013 Proceedings of the National Academy of Sciences on their study of 190 natural gas sites. The study, which was supported by the Environmental Defense Fund, yielded national estimates similar to that in EPA’s 2013 report on the 2011 inventory: methane emissions were 0.42 percent of gross gas production, versus 0.47 percent for the EPA 2011 inventory.
The Allen group plans additional studies to better define the emissions profile of pneumatic pumps – the largest source of methane emissions in their initial study – and liquids unloading, a technology defined by few measurements.
The University of Texas Environmental Defense Fund Study made direct measurements of 150 production sites that included 489 wells that were hydraulically fractured, 27 well-completion flowbacks, nine well unloadings and four workovers.
Well-completion flowbacks, which clear liquids from the wellbore to allow gas production, showed methane emissions from 0.01 million grams or metric ton (Mg) to 17 Mg, compared with an average of 81 Mg per event in the EPA 2011 national emission inventory, reported in April 2013.
The lower UT results reflect the growth in green completions, in which methane is captured or controlled – UT samples were collected in 2012, but the EPA data is from 2011.
Well unloading technologies vary, but the ones of interest divert gas production from the separator, reducing the backpressure and allowing more gas to flow, which lifts liquids out of the wellbore and improves gas flow.
The UT group monitored nine unloading events and the API/ANGA survey used by EPA included several thousand wells. Both studies showed a large variation in emissions levels between wells, while a small number of wells accounted for the majority of emissions.
Pneumatic pumps and controllers showed higher emissions than in the EPA 2011 inventory, as did equipment leaks.
The UT report highlights weaknesses in all the existing data sets – a fact that should encourage restraint by policy makers and advocates for and against natural gas development.