Study Aims To ‘Reduce Uncertainties’ In Methane Emissions Measurements
A new study released today could give us a better understanding of methane emissions from oil and gas activity.
Dr. Dan Zimmerle and Dr. Tim Vaughn of Colorado State University (CSU) were the lead authors of a new Basin Methane Reconciliation Study published in the Proceedings of the National Academies of Science (PNAS), which attempts to address a problem that has long vexed researchers on methane emissions: why do some studies show dramatically different results than others?
“This study is the first to spatially and temporally align top-down and bottom-up methane emission estimates for a natural gas production basin, using multi-scale emission measurements and detailed activity data reporting. We show that episodic venting from manual liquid unloadings, which occur at a small fraction of natural gas well pads, drives a factor-of-two temporal variation in the basin-scale emission rate of a US dry shale gas play,” Vaughn and Zimmerle, et al. wrote.
In a press release, Zimmerle outlined the study’s aim—to reduce uncertainties in the data.
“It utilized concurrent ground and aircraft measurements and on-site operations data, and as a result reduces uncertainties of previous studies,” Zimmerle said.
Authors of the paper included scientists at the University of Colorado Boulder’s Cooperative Institute for Research in Environmental Sciences; the National Oceanic and Atmospheric Research Administration’s Earth System Research Laboratory in Boulder, Colo.; the National Renewable Energy Laboratory in Golden, Colo.; and a former director of the Colorado Energy Research Institute at the Colorado School Mines.
The authors argue that “top-down” (TD) approaches, a more recent method for measurements using aircraft to fly over areas of oil and gas areas, have produced emissions estimates “typically 1.5 times larger” than bottom-up methane emission estimates, which are measurements conducted on-site. The authors admit that both methods have relative shortcomings.
“A better understanding of the TD—BU discrepancy is needed to improve confidence in emission estimates from both approaches,” they write.
“However, it is well-known that emissions from [oil and gas] infrastructure vary both temporally and spatially and that aggregated and annualized activity and emission factors often deviate substantially from local emissions occurring during shorter time periods,” the authors write.
“Study area total emissions exhibited significant variability throughout the day. On both days of the study period, modeled emissions peaked during mid-afternoon hours due to manual liquid unloadings (MLUs) performed and recorded by production facility operators,” the study noted.
In other words, emissions from oil and gas operations vary by hour of the day, by days of the week, and over the course of the year. Studies that try to extrapolate measurements during mid-day activities like routine maintenance from manual liquid unloadings (MLUs) or depressurization of equipment (“blowdowns”) may end up overstating emissions. Such maintenance activities “are often triggered by human operators during daytime work-week hours and may produce high emission rates for short durations.”
Taking measurements during the short-duration events during the day and spreading those readings out over a 24/7/365 model may not produce the best input for “understanding oil- and gas-related methane emissions. Careful consideration of all factors influencing methane emissions—including temporal variation—is necessary in scientific and policy discussions to develop effective strategies for mitigating [greenhouse gas] emissions from natural gas infrastructure,” according to the paper.
The authors call for high-resolution space and time activity data and additional contemporaneous measurements from both methods to deliver best practices and render scientific studies that inform the policy debate over methane and other emissions.
That includes a June study in the journal Science, led by the Environmental Defense Fund and including several co-authors, such as Anthony Marchese, a CSU Professor of Mechanical Engineering, and Zimmerle.
The study purported to find that methane emissions from oil and gas could be as much as 60 percent higher than current Environmental Protection Agency estimates, using TD methods.
The study published today came as the result of public and private funding under contract with the Department of Energy’s National Energy Technology Laboratory and was also co-sponsored by the Colorado Energy Research Collaboratory, NOAA, the American Gas Association, Chevron, XTO Energy, Equinor, and Southwestern Energy.