Quantifying methane emissions from natural gas production in north-eastern Pennsylvania

Abstract

Abstract. Natural gas infrastructure releases methane (CH 4 ), a potent greenhouse gas, into the atmosphere. The estimated emission rate associated with the production and transportation of natural gas is uncertain, hindering our understanding of its greenhouse footprint. This study presents a new application of inverse methodology for estimating regional emission rates from natural gas production and gathering facilities in north-eastern Pennsylvania. An inventory of CH 4 emissions was compiled for major sources in Pennsylvania. This inventory served as input emission data for the Weather Research and Forecasting model with chemistry enabled (WRFChem), and atmospheric CH 4 mole fraction fields were generated at 3 km resolution. Simulated atmospheric CH 4 enhancements from WRF-Chem were compared to observations obtained from a 3-week flight campaign in May 2015. Modelled enhancements from sources not associated with upstream natural gas processes were assumed constant and known and therefore removed from the optimization procedure, creating a set of observed enhancements from natural gas only. Simulated emission rates from unconventional production were then adjusted to minimize the mismatch between aircraft observations and model-simulated mole fractions for 10 flights. To evaluate the method, an aircraft mass balance calculation was performed for four flights where conditions permitted its use. Using the model optimization approach, the weighted mean emission rate from unconventional natural gas production and gathering facilities in northeastern Pennsylvania approach is found to be 0.36 % of total gas production, with a 2σ confidence interval between 0.27 and 0.45 % of production. Similarly, the mean emission estimates using the aircraft mass balance approach are calculated to be 0.40 % of regional natural gas production, with a 2σ confidence interval between 0.08 and 0.72 % of production. These emission rates as a percent of production are lower than rates found in any other basin using a top-down methodology, and may be indicative of some characteristics of the basin that make sources from the north-eastern Marcellus region unique.

Estimating Methane Emissions From Underground Coal and Natural Gas Production in Southwestern Pennsylvania

“…From the results of this study, we estimate emissions from UNG production and gathering facilities in SWPA to be equivalent to 0.5 ± 0.3% of production, in agreement with published top‐down emission estimates from northeast PA (0.36 ± 0.09%) and SWPA (0.0–3.5%) and site‐level measurements at well sites in northeast PA (0.44 ± 0.15%) and SWPA (0.57 ± 0.23%; Alvarez et al, ; Barkley et al, ; Omara et al, ; Ren et al, ). These emission rates as a percent of production are lower than rates found from top‐down studies performed in other gas basins, with the next lowest rate measured in the Haynesville shale at 1.3% (Alvarez et al, ; Karion et al, ; Peischl et al, ; Pétron et al, ; Schwietzke et al, ; Smith et al, ).…”

“…These emission rates as a percent of production are lower than rates found from top‐down studies performed in other gas basins, with the next lowest rate measured in the Haynesville shale at 1.3% (Alvarez et al, ; Karion et al, ; Peischl et al, ; Pétron et al, ; Schwietzke et al, ; Smith et al, ). The low fractional emission rates in this region are likely due to Marcellus wells having the highest production per well in the United States (Barkley et al, ; U.S. Energy Information Administration, ), requiring fewer components to produce large amounts of gas and thus lowering the potential for leaks (Brantley et al, ; Mitchell et al, ; Omara et al, ). However, though the emission rate is low compared to top‐down estimates performed in other regions, it is significantly higher than the inventory estimate reported by the Pennsylvania Department of Environmental Protection for the year 2015 (Pennsylvania Department of Environmental Protection, ).…”

Methane Emissions from the Marcellus Shale in Southwestern Pennsylvania and Northern West Virginia Based on Airborne Measurements

“…Our mean CH 4 emission rate as a percentage of natural gas production was estimated to be 1.1%. This emission percentage is greater than the emission rate of 0.18–0.41% by Peischl et al () and 0.08–0.72% by Barkley et al () estimated for the Marcellus Shale region in northeastern Pennsylvania, but is similar in magnitude to the loss rates estimated by a number of other studies (Table ), including a study that measured facility‐level CH 4 emissions in southwestern Pennsylvania and northern West Virginia with a mean emission rate of 1.35% (a median emission rate of 11% for conventional wells and 0.13% for unconventional wells; Omara et al, ). One possible reason for the different emission rates between southwestern and northeastern Pennsylvania is that most wells in northeastern Pennsylvania are newer and unconventional (i.e., hydraulic fractured and horizontally drilled), while there are many older and conventional wells in southwestern Pennsylvania and northern West Virginia.…”

Quantifying Methane and Ethane Emissions to the Atmosphere From Central and Western U.S. Oil and Natural Gas Production Regions

“…The mass balance technique (White et al, ) used here to quantify emissions has been extensively used to estimate CH 4 and C 2 H 6 emissions from O&NG‐producing regions in the United States, including the Uinta Basin of Utah (Karion et al, ), the Denver Basin of northeastern Colorado (Pétron et al, ), the Marcellus shale region of northeastern Pennsylvania (Barkley et al, ; Peischl et al, ), the Fayetteville shale region of Arkansas (Peischl et al, ; Schwietzke et al, ), the Haynesville shale region of northwestern Louisiana and eastern Texas (Peischl et al, ), the Barnett shale region of Texas (Karion et al, ; Smith et al, ), the Bakken shale region of North Dakota (Kort et al, ; Peischl et al, ), and the San Juan region of New Mexico and Colorado (Smith et al, ). A similar mass balance technique has been used to quantify CH 4 and other emissions from portions of O&NG‐producing regions and from large point sources (Caulton et al, ; Conley et al, ; Lavoie et al, ).…”