Methane is of particular concern because it is about 20 times more potent a greenhouse gas than carbon dioxide. Because methane is found in natural gas systems, some claim that methane released by wells makes natural gas equal to coal from a greenhouse gas standpoint.
The bad news
Last month, the Carnegie Institution for Science released a study of the total U.S. methane emissions that suggested that government estimates are roughly 50 percent shy of the actual amounts produced. The study, published on Nov. 25 in the online edition of Proceedings of the National Academy of Sciences, used atmospheric methane observations from across the continent taken in 2007 and 2008.
Overall, the study found that total methane emissions in the United States appear to be from 1.5 to 1.7 times higher than the amounts previously estimated by the Environmental Protection Agency and by the Emissions Database for Global Atmospheric Research (EDGAR), respectively. The study measured existing methane in the atmosphere, while the EPA and EDGAR calculate total emissions based on estimates of the total amount of methane typically released per cow or per unit of coal or gas.
The study found large differences between the actual methane amounts and government estimates in the south-central United States, where methane emissions were 2.7 times greater than reported. The study suggested that about half of the difference could be linked to emissions from oil and gas drilling and processing.
However, the study also pointed the finger at ruminants and manure, where methane levels were twice the magnitude of existing estimates.
The study concluded that methane produced by both the animal-husbandry and fossil-fuel industries have larger greenhouse gas impacts than previously indicated, and suggested that the EPA’s recent decision to downscale its estimate of natural-gas emissions by 25 percent to 30 percent might be premature.
On the other hand
The second study was a cooperative effort led by the University of Texas-Austin with experts from the Environmental Defense Fund, Anadarko Petroleum Corp.;, BG Group plc, Chevron; Encana Oil & Gas (USA) Inc., Pioneer Natural Resources Co., SWEPI LP (Shell), Southwestern Energy, Talisman Energy USA and XTO Energy, an ExxonMobil subsidiary.
The data in the second study was obtained in 2011, compared with the first study which used data from 2007 and 2008.
This study focused on the production aspect of natural gas by measuring actual emissions at gas-production facilities and equipment. Thanks to the collaboration with the energy companies, researchers had unprecedented access to natural-gas production wells and equipment, which made it possible to acquire direct measurements of methane emissions from wells.
The study measured methane emissions directly at 190 onshore well pads during completion operations (including hydraulic fracturing). The sampling was designed to be representative of operations in the Gulf Coast, Mid-Continent, Rocky Mountain and Appalachian regions. Measurements of active equipment at 150 production sites with 489 wells, 27 well-completion flowbacks, nine well unloadings and four well workovers were included in the study. The types of sources measured account for two-thirds of methane emissions that occur during natural-gas production, as estimated in the most recent national greenhouse gas inventory.
The study found that the EPA was much closer to the target overall, with average total methane emissions from natural-gas production found to be from 757 gigarams (Gg) to 1,157 Gg, compared with the EPA estimate of approximately 1,200 Gg. When scaled to a nationwide estimate, total methane emissions were found to be approximately 2,300 Gg compared with the EPA’s estimate of 2,545 Gg.
Building better wells
The study indicated that the EPA overestimated methane emissions in fracked wells, probably because industry has advanced methane-reducing modifications and equipment. The study determined that the majority of the hydraulically fractured wells had equipment in place that reduced methane emissions by 99 percent – and that total methane emissions from well completions were 97 percent lower than the EPA’s calendar year 2011 national emission estimates.
“The way in which wells are drilled and brought into production has been evolving,” said David Allen, professor of chemical engineering at UT’s Cockrell School of Engineering and principal investigator for the study. “The net emissions for completion flowbacks is significantly lower than previous estimates, indicating the type of emission control activities observed during these events are very effective.”
In addition, the study found significant differences in different parts of the country, with emissions per controller in the Rocky Mountain region the lowest by far.
“This study tackles one of the most hotly debated issues in environmental science and policy today,” said Mark Brownstein, associate vice president and chief counsel of the U.S. Climate and Energy Program, Environmental Defense Fund, noting that when producers work at it, methane emissions can be dramatically reduced. “The study also demonstrated, however, that certain methane emissions are larger than previously thought, indicating that there are many further opportunities to reduce emissions.”
The UT-EDF’s study found a methane-leakage rate of only about 1.5 percent, if not less than that. Previous studies had determined that methane leakage needed to be below 2 percent for shale gas to be a superior substitute for coal in terms of greenhouse gases produced.
The significance of the study’s findings are twofold: first, it put to rest the claim that fracking’s methane emissions negated any climate advantages of the natural gas produced by the method.
Second, new technologies to collect and contain methane at the wellsite are effective. Progress made in curtailing methane emissions are reflected in the EPA’s own Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011, which shows an overall methane decline of 8 percent between 1990 and 2011.