Reconciling divergent estimates of oil and gas methane emissions

Abstract

Published estimates of methane emissions from atmospheric data (top-down approaches) exceed those from source-based inventories (bottom-up approaches), leading to conflicting claims about the climate implications of fuel switching from coal or petroleum to natural gas. Based on data from a coordinated campaign in the Barnett Shale oil and gas-producing region of Texas, we find that top-down and bottom-up estimates of both total and fossil methane emissions agree within statistical confidence intervals (relative differences are 10% for fossil methane and 0.1% for total methane). We reduced uncertainty in top-down estimates by using repeated mass balance measurements, as well as ethane as a fingerprint for source attribution. Similarly, our bottom-up estimate incorporates a more complete count of facilities than past inventories, which omitted a significant number of major sources, and more effectively accounts for the influence of large emission sources using a statistical estimator that integrates observations from multiple ground-based measurement datasets. Two percent of oil and gas facilities in the Barnett accounts for half of methane emissions at any given time, and high-emitting facilities appear to be spatiotemporally variable. Measured oil and gas methane emissions are 90% larger than estimates based on the US Environmental Protection Agency's Greenhouse Gas Inventory and correspond to 1.5% of natural gas production. This rate of methane loss increases the 20-y climate impacts of natural gas consumed in the region by roughly 50%. methane emissions | oil and gas emissions | greenhouse gas footprint | natural gas supply chain | Barnett Shale M ethane (CH 4 ), the principal component of natural gas, is a powerful greenhouse gas. Although natural gas emits less carbon dioxide (CO 2 ) per unit of energy than coal or oil when burned, CH 4 losses during the production, processing, transportation, and use of natural gas reduce its climate advantage compared with other fossil fuels. For example, if CH 4 losses are large enough (e.g., ∼3% of production), new natural gas power plants can cause greater climate damage than new coal plants for decades or longer (∼1% when comparing natural gas to diesel freight trucks) (1).The lack of current data on CH 4 emissions, magnified by intense public concern over the broader environmental implications of shale gas development, has stimulated significant research to improve estimates of CH 4 emissions (2-18). A recurring theme in recent literature is that "top-down" (TD) approaches produce estimates that are significantly higher than those from "bottomup" (BU) approaches. Concerns about available inventories and divergent TD and BU estimates create confusion regarding policy formulation and leave room for conflicting claims about the greenhouse gas implications of increased use of natural gas.TD approaches for estimating total CH 4 emissions at the regional or larger scale include airborne mass balance (2-4, 19), atmospheric transport models (5,6,(20)(21)(22)(23), and enhancem...

The global methane budget 2000–2012

“…Most recent studies M. Saunois et al: The global methane budget 2000709 al., 2014Olivier and Janssens-Maenhout, 2014;Jackson et al, 2014b;Howarth et al, 2011;Pétron et al, 2014;Karion et al, 2013) albeit not all Cathles et al, 2012;Peischl et al, 2015) suggest that methane emissions are underestimated by inventories and agencies, including the USEPA. For instance, emissions in the Barnett Shale region of Texas from both bottom-up and top-down measurements showed that methane emissions from upstream oil and gas infrastructure were 90 % larger than estimates based on the USEPA's inventory and corresponded to 1.5 % of natural gas production (Zavala-Araiza et al, 2015). This study also showed that a few high emitters, neglected in the inventories, dominated emissions.…”

Emissions from oil and gas operations in the United States and their air quality implications

“…In contrast, a bottom-up analysis of emissions for the region led to an estimate of 46,200 kg/hr of methane from oil and gas operations (48,400 kg/hr including other minor geogenic sources), which is 61-63% of the region's emissions. When super-emitters were accounted for, however, the top-down and bottom-up emission estimates converged (Zavala et al, 2015c).…”

“…One prominent recent example of a large unplanned emission event is the well blowout in a natural gas storage facility in California. This large leak had emissions that reached a peak of 60 metric tons of methane per hour (Conley et al, 2016), a methane emission rate, from a single point, that is roughly equivalent to the routine emissions from tens of thousands of wells in the Barnett Shale Zavala et al, 2015c). This leak persisted for several months and is estimated to have released 97,100 metric tons of methane (97.1 Gg; 2.4 million metric tons of CO 2 e); an amount equivalent to 1% of the annual emissions from the oil and natural gas supply chains (Conley et al, 2016).…”

Mobile measurement of methane emissions from natural gas developments in northeastern British Columbia, Canada

“…Using MDLs for our study, we can reasonably estimate the minimum likely emissions inventory, because it is expected that infrastructural sources with larger emission rates cumulatively contribute the majority of CH 4 emissions (Frankenberg et al, 2016;Mitchell et al, 2015;Rella et al, 2015;Subramanian et al, 2015;Zavala-Araiza et al, 2015). According to a distribution of emissions at a US oil and gas site in the Four Corners region, emissions < 0.2 g s −1 did not significantly contribute to the overall CH 4 flux rate (Frankenberg et al, 2016).…”