Download Hi-Res ImageDownload to MS-PowerPointCite This:Environ. Sci. Technol. 2018, 52, 21, 12915-12925

Methane Emissions from Natural Gas Production Sites in the United States: Data Synthesis and National Estimate

Mark Omara*
Mark Omara
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
*Phone: +1-512-691-3432; e-mail: [email protected]
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http://orcid.org/0000-0002-8933-1927

Naomi Zimmerman
Naomi Zimmerman
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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Melissa R. Sullivan
Melissa R. Sullivan
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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Xiang Li
Xiang Li
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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http://orcid.org/0000-0001-6797-7340

Aja Ellis
Aja Ellis
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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Rebecca Cesa
Rebecca Cesa
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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R. Subramanian
R. Subramanian
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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Albert A. Presto
Albert A. Presto
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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http://orcid.org/0000-0002-9156-1094

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Allen L. Robinson
Allen L. Robinson
Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
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http://orcid.org/0000-0002-1819-083X

Cite this: Environ. Sci. Technol. 2018, 52, 21, 12915–12925

Publication Date (Web):September 26, 2018

https://doi.org/10.1021/acs.est.8b03535

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Abstract

We used site-level methane (CH₄) emissions data from over 1000 natural gas (NG) production sites in eight basins, including 92 new site-level CH₄ measurements in the Uinta, northeastern Marcellus, and Denver-Julesburg basins, to investigate CH₄ emissions characteristics and develop a new national CH₄ emission estimate for the NG production sector. The distribution of site-level emissions is highly skewed, with the top 5% of sites accounting for 50% of cumulative emissions. High emitting sites are predominantly also high producing (>10 Mcfd). However, low NG production sites emit a larger fraction of their CH₄ production. When combined with activity data, we predict that this creates substantial variability in the basin-level CH₄ emissions which, as a fraction of basin-level CH₄ production, range from 0.90% for the Appalachian and Greater Green River to >4.5% in the San Juan and San Joaquin. This suggests that much of the basin-level differences in production-normalized CH₄ emissions reported by aircraft studies can be explained by differences in site size and distribution of site-level production rates. We estimate that NG production sites emit total CH₄ emissions of 830 Mg/h (95% CI: 530–1200), 63% of which come from the sites producing <100 Mcfd that account for only 10% of total NG production. Our total CH₄ emissions estimate is 2.3 times higher than the U.S. Environmental Protection Agency’s estimate and likely attributable to the disproportionate influence of high emitting sites.

Introduction

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Natural gas (NG) extracted from shale and tight oil reservoirs has transformed the U.S. energy landscape resulting in rapid increases in total NG production and consumption. (1) While NG combustion emits less than half the carbon dioxide (CO₂) of other fossil fuels, (2) it is primarily composed of methane (CH₄), which produces 86 times more radiative forcing than CO₂ over a 20-year time frame. (3) Therefore, CH₄ emitted from the NG system represents wasted resources, lost revenue, and erodes the potential climate benefits of NG relative to other fossil fuels.

There has been a major effort over the last five years to quantify CH₄ emissions from the oil and NG supply chain. Dozens of recent measurement-based studies (4−32) have exposed the magnitude and scope of the CH₄ emissions problem, highlighting the following common themes: (i) Government inventories often significantly underestimate CH₄ emissions, (ii) a small fraction of high-emitting sites or sources account for a disproportionately large fraction of total CH₄ emissions, and (iii) there are significant basin-to-basin differences in production-normalized CH₄ emissions (i.e., CH₄ emissions expressed as a fraction of CH₄ produced).

We focus on CH₄ emissions from NG production sites. The U.S. EPA (33) attributes two-thirds of the 6.5 Tg of total CH₄ emissions from the NG supply chain to the NG production sector, which includes 2 Tg of CH₄ emissions associated with NG production from more than 400 000 NG wells. Herein, we define NG production sites to include any NG-producing well pad with one or more wellheads and ancillary surface equipment (e.g., NG separators, pneumatic pumps/controllers, and/or storage vessels). Such onsite processing equipment are often significant sources of elevated CH₄ emissions. (4,8,29) These high emissions are often the result of abnormal process conditions (e.g., equipment malfunctions); they can be persistent or episodic and are difficult to predict. (4,27−29) The stochastic characteristics of high-emitting sites appear to contribute, at least in part, to the orders-of-magnitude variability in measured absolute site-level CH₄ emissions (Figure 1). Furthermore, top-down aircraft studies report widely varying estimates of basin-level, production-normalized CH₄ emissions. (19−24) The causative factors for site- and basin-level variability in CH₄ emissions are not well-understood. It has been suggested that differences in the composition of the extracted NG (e.g., dry versus wet gas), operator practices, and/or differences in NG production rates may be important. (11,24,29) Additionally, several states have recently proposed or enacted oil and NG CH₄ regulations, (34−38) which may yield pronounced differences in future regional/basin-level CH₄ emissions when compared against regions/basins without similar regulatory actions.

Figure 1. Site-level measurement data synthesized in this study. Numbers in parentheses indicate the number of sites with emissions data for each basin; the citations to the original studies are indicated as superscripts after basin names. New measurements are indicated with an (a) in the Denver-Julesburg (DJB; n = 18, or 17% of all DJB sites), Marcellus (NE PA unconventional sites, n = 45, or 57% of all Marcellus data), and Uinta (n = 29, or 50% of all Uinta sites). The boxes represent the 25th and 75th percentiles, while the whiskers extend to 1.5× the interquartile range, and values outside this range are the outliers, marked with red crosses. The black horizontal line inside each box represents the median while means are shown in purple. The notches visually depict the 95% confidence interval on the median. For Eagle Ford, measurement data for the four sites are represented with an error bar indicating the minimum and maximum. For Fayetteville, the notch extends beyond the 25th percentile as a result of the sample size and the data spread in this basin. “All Abs.” and “All Norm.” represent combined data set for all absolute and production-normalized CH₄ emission rates (n = 1009), respectively.

To better understand CH₄ emissions among NG production sites and across multiple basins, we compiled and analyzed recently measured site-level CH₄ emissions data for more than 1000 NG production sites in eight U.S. basins (4−11) (Figure 1). We use this large data set of site-level CH₄ emissions (both absolute and production-normalized emissions) and site-level NG production to test the hypothesis that there are no significant basin-to-basin differences in the distribution of site-level CH₄ emissions and that, on average, site-level emissions correlate with their NG production characteristics. We then combine these site-level CH₄ emissions data with a national database on NG production site characteristics (i.e., site-level NG production rate) to estimate (i) total CH₄ emissions from U.S. NG production sites, (ii) major sources and distributions of CH₄ emissions, including CH₄ from high-emitting sites, and (iii) variability in CH₄ emissions among basins.

Materials and Methods

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Overview of Site-Level Measurement Data

We analyzed measured site-level CH₄ emissions data from 1009 NG production sites located in eight different basins. This includes recently published data from eight independent studies, (4−11) supplemented with new data for 92 additional sites sampled in the Denver-Julesburg (DJB), Uinta, and Marcellus (northeastern PA, Figure 1, Supporting Information (SI) Section S1). These new measurements targeted production regions with unique site-level production characteristics, that is, unconventional dry gas production sites with high site-level production rates in northeastern Marcellus and low-producing, mixed oil and gas sites in Uinta and DJB. The new measurement data help diversify site-level production characteristics and measured production regions/basins in the consolidated data set.

We focus on routinely producing sites with known NG production and assumed that measured CH₄ emissions resulted from routine operations (e.g., equipment leaks, venting from pneumatic controllers and storage tanks) or were unplanned (e.g., unintended emissions from malfunctioning equipment). Thus, the combined data set does not include CH₄ emissions from completion flowback (8) or liquids unloadings. (11) Additionally, site-level CH₄ emissions rates were unavailable for storage or coalbed CH₄ well sites, and emissions from these sites were not assessed in the present study. For measured sites with reported NG production rates, the CH₄ measurements were performed between 2010 and 2016 and used a variety of onsite and downwind ground-based site-level CH₄ measurement techniques that can be broadly grouped into three categories:

(a)	Direct onsite measurements (henceforth, “onsite measurements”), which involved optical gas imaging for leak identification followed by direct quantification of all identified leaks. (5) These component-specific measurements are then summed to estimate site-level emission rate. Onsite measurements accounted for 28% of all site-level measurement data.
(b)	Downwind tracer flux (TF) measurements of downwind plumes of CH₄ and intentionally released tracers (e.g., acetylene and nitrous oxide). (6,8) TF sites accounted for 6.7% of the total data.
(c)	Downwind CH₄ plume measurements combined with inverse Gaussian modeling. This includes both downwind stationary measurements using EPA’s Other Test Method (OTM-33A (4,11)), and downwind mobile measurements followed by Gaussian modeling (MM-Gaussian (7,9,10)). Sites sampled using these techniques accounted for 65% of the total sites.

There are limitations with each method. For example, onsite measurement (method (a)) requires site access and proper operation and performance of both the plume imaging (for leaks survey) and leak rate measurement devices. (5,39) Even with site access, some onsite emission sources may be present at locations that are not safely accessible for leak rate quantification (e.g., see Subramanian et al. (16)). Additionally, all stationary and mobile downwind measurements require sites with downwind road access and favorable meteorological conditions. These measurement methods have different uncertainties, which range from approximately ±20% to ±60% (TF and OTM-33A) to a factor of 3 for the MM–Gaussian approach. (4−11)

We use the site-level production-normalized CH₄ emissions as reported by Robertson et al. (11) and Omara et al. (8) For Rella et al., (10) Yacovitch et al., (9) and Lan et al. (7) we use the site-level CH₄ emissions, NG production, and production-normalized CH₄ emissions as consolidated and reported by Zavala-Araiza et al. (29) For onsite measurements in the Barnett, (5) we calculated site-level production-normalized CH₄ emissions based on the study’s reported site-specific CH₄ mole fraction in NG and site-level NG production; sites without NG production rates were excluded from this analysis. For Brantley et al. (4) and measurements performed as part of the present study, we estimated site-specific production-normalized CH₄ emissions based on the average county-specific or region-specific CH₄ mole fractions from the EPA’s Oil and Gas Tool. (40)

We excluded data for eight sites with production-normalized CH₄ emissions >100%. These eight sites were sampled offsite using the downwind plume measurements approaches utilizing Gaussian plume inverse modeling. It is possible that the measured CH₄ emissions exceeding 100% of CH₄ production was due to offsite CH₄ sources (e.g., biogenic CH₄ source, CH₄ from collocated equipment such as abandoned well, etc.). The exclusion of these eight sites from the consolidated data set does not change our results: if we include them in our analysis, the total production CH₄ emissions increases by <7%, well within the overall 95% confidence interval.

Overall, the 1009 measured sites were located in the Barnett (n = 554 sites), Denver-Julesburg (DJB, n = 107), Pinedale (n = 106), Marcellus (n = 79), Uinta (n = 58), Upper Green River (n = 51), Fayetteville (n = 50), and Eagle Ford (n = 4) basins (Figure 1). Site-specific NG production rates ranged from 0.4 Mcfd (1 Mcfd = 1000 cubic feet per day) to 78 000 Mcfd. Analysis of production data from Drillinginfo (41) (further description below) indicates that in 2015, 94% of U.S. NG production sites had site-level NG production rates that fell within this range. Additional site information (e.g., number of wells onsite, gas processing and emissions control equipment in use, conventional or unconventional well type, and site age) were generally unavailable or not reported.

National Activity Data

We used well-level NG production data reported by Drillinginfo (DI Desktop (41)), a commercial platform that aggregates publicly available and proprietary well-level data, including monthly NG production, first reported production date, drilling configuration, operator name, and location. Using geospatial analysis with ArcGIS, we aggregated well-level information into site-level (well-pad) information (see SI Section S2). In total, 498 000 NG producing well pad sites were identified, with total 2015 NG production of approximately 27 Tcf (trillion cubic feet). As of March 2017, Drillinginfo did not report 2015 well-level production data for wells in Kentucky, Tennessee, Missouri, Oregon, Illinois, and Indiana. These states were not included in our analyses. The EIA (42) estimates that these states contributed <0.5% of total national NG production. Using the EPA’s county-specific and basin-specific estimates of mean CH₄ content in NG, (40) we estimated total CH₄ production of 23 Tcf from these sites in 2015. The distribution of sites based on their NG production characteristics is shown in Table 1.

Table 1. Ten NG Production Bins Used in the Nonparametric Model (Developed Based on Deciles of NG Production for the Measured Sites) And Their Estimated Mean Site-Level Production-Normalized CH₄ Emissions

production bin (Mcfd)	<0.4–31	31–73	73–147	147–254	254–390	390–616	616–1047	1047–1699	1699–3342	>3342
no. measured sitesa	101	101	101	101	101	101	101	101	101	100
national sites (%)b	65%	15%	8.3%	3.9%	2.1%	1.7%	1.4%	0.90%	0.83%	0.81%
national prod. (%)c	3.2%	4.2%	5.2%	4.5%	3.97%	4.9%	6.7%	7.2%	11.7%	48.4%
mean (% CH₄)d	20	5.4	2.8	1.6	1.9	1.4	0.89	1.2	0.23	0.17
lower bound on mean (% CH₄)	16	3.3	1.6	1.1	0.96	0.7	0.38	0.45	0.14	0.12
upper bound on mean (% CH₄)	25	7.9	4.4	2.2	3.3	2.5	1.7	2.2	0.34	0.24

^{^a}

Denotes the number of measured sites in each production bin (total = 1009).

^{^b}

Total U.S. NG production sites = 498 000.

^{^c}

NG production for these 498 000 sites was 83 billion cubic feet per day (Bcfd).

^{^d}

The mean production-normalized CH₄ emission rate in each bin was obtained by randomly drawing, with replacement, an emission rate from the empirical distribution until a randomly sampled emission rate was assigned to each of the sampled sites. This was repeated 10 000 times and the mean obtained from the average of averages of each similution, while the 2.5th and 97.5th percentiles characterized the lower and upper bounds on the mean, respectively (SI Figure S18).

Extrapolation of Measured Site-Level Emissions to Total Population of Sites

We used two methods for extrapolating the measured site-level CH₄ emissions to the total population of sites: (i) a robust regression model, and (ii) a nonparametric model. These two approaches allow us to explore the influence of high-emitting sites on predicted total CH₄ emissions; whereas the first approach downweights their contribution, the second approach fully incorporates them. As described further in detail below, both of these approaches utilize the site-level production-normalized CH₄ emissions data. In the first approach, we estimated site-level CH₄ emissions for each of the 498 000 NG production sites in 2015 by fitting a robust weighted least-squares quadratic regression model of the production-normalized CH₄ emission as a function of NG production. The robust fit was performed using a MATLAB Statistics Toolbox algorithm that uses an iteratively reweighted least-squares approach with a bisquare weighting function, (43) wfun (wfun = (abs(r) < 1) × (1 – r²)²; r = resid/(tune × s × (1 – h)^0.5);

× median absolute deviation of the residuals (resid) from the median; h is a vector of leverage values for the least-squares fit and tune is a tuning constant = 4.685). Thus, for each site, its production-normalized CH₄ emission rate (%) was estimated based on the fit obtained from the robust regression, which is a function of the site’s NG production rate. The site’s absolute CH₄ emission rate (kg/h) was then calculated by multiplying its production-normalized CH₄ emission rate with its CH₄ production rate.

In the second approach, we estimate site-level CH₄ emissions using nonparametric bootstrap resampling methods in order to adequately characterize the asymmetrical distributions of the empirical data. We first developed 10 empirical production-normalized CH₄ emissions distributions by grouping measured emissions into 10 bins based on deciles of NG production for the 1009 sites with emissions data (Table 1). We then grouped all 498 000 U.S. NG production sites into the same 10 bins based on the measured site-level NG production deciles. Among sites with emissions data, the site-level NG production rates ranged from 0.4 Mcfd to 78 000 Mcfd; however, among the total population of U.S. NG production sites, site-level NG production ranged from 0.001 Mcfd to 138 000 Mcfd (SI Figure S21). In grouping the national population of sites, the 28 900 sites (5.8% of all sites) producing <0.4 Mcfd were placed in the same bin as the sites producing 0.4 to 31 Mcfd, which describes the first decile. Similarly, the six sites (0.0012% of all sites) that produced >78 000 Mcfd were placed in the last bin as the sites producing 3342 to 78 000 Mcfd. As shown in Table 1, the mean production-normalized CH₄ emissions in each bin decreases consistently with increases in site-level NG production. Our analysis shows that this grouping for national sites with NG production outside of the measured production is robust: if, for example, sites producing <0.4 Mcfd were assigned a production-normalized CH₄ emissions of 100%, the resulting national CH₄ emissions would increase by only 0.33%. Furthermore, we find that the distribution of site-level NG production rates for the sampled sites is statistically similar to that for the national population of sites across all production bins, except for the low production sites in the first bin which are undersampled (SI Figure S17).

For each site in each production bin, we estimate its site-level CH₄ emissions (kg/h/site) by randomly drawing, with replacement, a production-normalized CH₄ emission rate from the bin-specific empirical distribution. We then multiply this randomly sampled production-normalized CH₄ emission rate with the site-specific CH₄ production rate, repeat this process for every site, and then sum across all sites. We repeat this simulation 5000 times for each site in order to estimate the mean total CH₄ emissions; the 2.5th and the 97.5th percentiles were then used to characterize the 95% confidence interval on mean total CH₄ emissions.

Our overall estimated uncertainty on mean total CH₄ emissions, obtained from the 2.5th and 97.5th percentiles based on the nonparametric resampling, was +40%/ −36% and was dominated by variability in mean site-level CH₄ emissions. These are influenced by study-specific sample sizes, site representativeness, and/or method accuracy. There are also uncertainties associated with activity data from Drillinginfo (41) but they are difficult to quantify as these data are aggregated from publicly available sources that may be subject to reporting errors. These uncertainties include uncertainties in well location and production data. All Drillinginfo data were used as reported without any modifications. Finally, there are uncertainties associated with county/basin-level CH₄ mole fractions from the EPA’s Oil and Gas Tool. (40) However, the impact of these uncertainties on estimated total CH₄ emissions are expected to be small. For example, in the Appalachian Basin, we used an average CH₄ content of 83%. (40) Increasing this to 95% or decreasing it to 75% yields results that are within 15% of the mean estimated total CH₄ for this basin, well within the overall method uncertainty of +40%/–36%.

Two-Sample Kolmogorov–Smirnov Tests

We compared the distributions of site-level absolute and production-normalized CH₄ emissions among different basins using the two-sample Kolmogorov–Smirnov (K–S) test, with significance established at p < 0.01. These statistical comparisons were performed using MATLAB.

Results and Discussion

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Variability in Empirical Site-Level and Basin-Level Methane Emission Rates

Figure 1 shows that both the absolute and production-normalized CH₄ emission rates are highly variable. Nearly three-quarters (74%) of sites exhibited site-level CH₄ emissions between 0.1 and 10 kg/h but, overall, site-specific absolute CH₄ emission rates varied by more than 5 orders of magnitude, ranging from 0 to 300 kg/h. There are basin-to-basin differences in emissions with mean basin-specific absolute CH₄ emission rate ranging from 0.61 kg/h/site (95% confidence interval, henceforth CI: 0.33–0.95) to 7.9 kg/h/site (CI: 4.9–12) in the Fayetteville and Marcellus basins, respectively. The overlap, or lack thereof, in the length of the boxplot notches in Figure 1 suggests basins cluster into three groups with statistically different measured median absolute CH₄ emission rates: (i) A group of basins with high site-level emissions: Marcellus (median = 3.4 kg/h/site), Pinedale (2.3 kg/h/site), Uinta (2.2 kg/h/site); (ii) A group of basins with moderate site-level emissions: Barnett (0.65 kg/h/site) and Denver-Julesburg (DJB) (0.64 kg/h/site); and (iii) Fayetteville, with low site-level emissions (0.13 kg/h/site). The Upper Green River basin falls between the high and moderate emissions groups with a median emissions of 1.3 kg/h/site.

Site-specific production-normalized CH₄ emission rates ranged from 0% to 91%, while the mean basin-specific production-normalized CH₄ emission rates ranged from 0.34% (CI: 0.07–0.74%) to 11% (CI: 6.9–16%) in the Fayetteville and Uinta, respectively (SI Table S8). Within 95% confidence intervals, four groups of basins have measured median production-normalized CH₄ emissions that are statistically different (SI Figure S19): (i) DJB (median = 1.6%) and Uinta (3.5%) have high production-normalized emissions; (ii) Marcellus (0.27%), Pinedale (0.65%), and Upper Green River (0.50%) have moderate production-normalized emissions; (iii) Barnett (0.15%) and (iv) Fayetteville (0.031%) both have low emissions. Thus, at the basin level, the measured mean or median site-level production-normalized CH₄ emissions vary by one to 2 orders of magnitude, while the measured mean or median site-level absolute CH₄ emissions vary by at least an order of magnitude.

Comparison of Empirical Site-Level Methane Emissions Distributions among Basins

Figure 2 shows the empirical cumulative distributions of both the absolute and production-normalized CH₄ emissions sorted by basins. The site-level emissions are highly skewed; for example, among all sampled sites in the Barnett, the top 5% of high-emitting sites accounted for 66% of cumulative absolute CH₄ emissions. The skewness of CH₄ emissions distributions are determined, in part, by the sample size. (18) A concern is representativeness of the sample population, including the magnitude and frequency of extreme emitters. (44) Representative distributions are difficult to capture in part because of the stochastic characteristics of site-level CH₄ emissions and the logistical limitations of common site-level measurement techniques (see Materials and Methods). To compare CH₄ emissions distributions among basins, we attempted to control for these potential sampling artifacts by stratifying basin-specific CH₄ emissions based on the median site-level NG production rate (390 Mcfd) and, in each group, limiting our comparison to only include basins with n ≥ 50.

Figure 2. Comparison of CH₄ emissions distributions among basins. (a) absolute and (b) production-normalized emissions. Only basins with n ≥ 50 sites were evaluated. For the <390 Mcfd bin, comparisons were made among the Barnett (n = 245), DJB (n = 95), Pinedale (n = 50), and Uinta (n = 50) Basins. For the >390 Mcfd bin, comparisons were made among the Barnett (n = 309), Pinedale (n = 56), and the Marcellus (n = 57) Basins. The lightly colored lines indicate the 10 000 bootstrap distributions obtained by randomly sampling 50 sites, with replacement, from the empirical distributions (shown in bold solid lines). Differences in distributions were assessed using the 2-sample Kolmogorov–Smirnov test with significance established at p < 0.01.

We used the two-sample Kolmogorov–Smirnov test (i.e., six paired tests for the <390 Mcfd group and three paired tests for the >390 Mcfd group) to compare the different basin-specific emissions distributions shown in Figure 2 (SI Table S13). The absolute CH₄ emissions distributions are generally statistically different among basins at the 1% significance level (Figure 2a). Indeed, the two-sample K–S test suggests only the absolute site-level CH₄ emissions distributions for the <390 Mcfd sites in the Pinedale and Uinta basins came from similar continuous distributions (p = 0.24).

In contrast, the distributions of production-normalized CH₄ emissions were generally statistically similar among all paired basins (p > 0.01, SI Table S13), except for the Barnett sites producing <390 Mcfd (Figure 2b). The Barnett data for the <390 Mcfd sites are skewed low compared to the other basins partly because 20% of all the sampled sites in this bin had nondetectable emissions, (10) and nearly one-third were obtained from short-term onsite measurements. (5) The sites with reported zero emissions were sampled using mobile downwind plume measurements but had no independent measure to verify sites with undetectable plumes (e.g., the use of an intentionally released tracer from the site). Additionally, component- level measurements, (5) which are typically completed in minutes, may be biased low as periodic emission events (e.g., tank flashing) may be missed. Given this uncertainty, it is possible that high-emitting sites are underrepresented among sites producing <390 Mcfd in the Barnett, thus increasing the uncertainty in the scaled-up CH₄ emissions. This potential sampling artifact was not observed among sites producing >390 Mcfd (Figure 2b). However, the 10 000 bootstrap distributions (recreated from the empirical distribution) for the Barnett indicate overlap with the Denver-Julesburg (DJB), Pinedale, and Uinta CH₄ distributions, particularly at the high end of the distribution (Figure 2).

Relationship between Measured Site-Level Methane Emissions and NG Production

Recent studies report weak relationships between absolute CH₄ emissions and site-level characteristics, including NG production, oil production, water production, and/or site age. (4,8,11,29) Given the limited information for individual sites, we can only examine the relationship between site-level CH₄ emissions and NG production.

Figure 3 shows the site-level absolute and production-normalized CH₄ emissions as functions of NG production. Figure 3b shows a strong trend of decreasing production-normalized CH₄ emissions with increases in site-level NG production. To quantify the trend, we fit the entire data set with quadratic robust weighted least-squares regression with bisquare weighting (see Materials and Methods):

On average, low NG producing sites emit a larger fraction of their CH₄ production than high NG producing sites and up to 74% of the variability is explained by variability in NG production rates (Figure 3). This implies that basins in which total NG production are dominated by high NG production sites are likely to have lower production-normalized CH₄ emissions and vice versa.

Figure 3. Relationship between site-level CH₄ emissions and NG production. (a) absolute and (b) production-normalized CH₄ emissions. Solid cyan lines show quadratic robust weighted least-squares regressions with bisquare weighting (see Materials and Methods) performed on the entire data set. Measured site-level CH₄ emissions were also binned by deciles of their site-level NG production, which are numbered sequentially on the top x axis. The notched box plots (outliers not shown) visually depict the data spread in each production decile. The black horizontal line in each notched box shows the median. The triangular purple symbols show the mean CH₄ emission rate in each production decile and the solid red lines show the polynomial fit through the mean CH₄ emission rate in each decile. These regression equations are (a) log₁₀[CH₄ (kg/h)] = 0.30 ± 0.14 × log₁₀[Prod (Mcfd)] – 0.23 ± 0.38; r_adj² = 0.72) and (b) log₁₀[%CH₄(^kg/h/_kg/h)] = −0.71 ± 0.15 × log₁₀[Prod(Mcfd)] + 2.0 ± 0.41; r_adj² = 0.93) for the absolute and production-normalized CH₄ emissions, respectively.

Figure 3 also shows modest increases in absolute CH₄ emissions with site-level NG production (quadratic robust weighted least-squares regression with bisquare weighting:

High NG production sites (e.g., > 1000 Mcfd/site) are generally newer facilities (SI Figure S3); they may have optimally performing equipment and components, and are likely subjected to more frequent on-site inspection and maintenance than old, low producing sites. (8) Because of their high NG production rates, exceptionally high CH₄ emissions (e.g., > 10% of site-level CH₄ production) at these sites would likely be audible and/or visible, increasing the possibility for detection and repair if routine inspections are performed.

Both Figure 1 and Figure 3 highlight the significant scatter in CH₄ emissions within basins and within NG production bins; this underscores the stochastic character of emissions at any given site, which may result from sources that include malfunctions (e.g., separator dump valve stuck open), operational errors (e.g., storage tank venting from thief hatch accidentally left open), and/or process and design issues (e.g., overpressurized separators). (27−29) Therefore, the fits in Figure 3 predict the emissions of an average site as a function of production but do not predict the emissions for any specific site. Fortunately, trends in average emissions are what is needed to develop national or basin-level emission estimates, which integrate an average emission factor with activity data reflecting the total number of well sites.

Influence of High-Emitting Sites on Total Methane Emissions

Within each basin (Figure 1) and production bin (Figure 3, notched box plots), the mean CH₄ emission rate is higher than the median because of the disproportionate influence of low frequency, high emitting sites. The high CH₄ emitters are commonly referred to as “super emitters”; since we lack information on the site-level CH₄ sources, we denote them simply as “high emitters” and identify them as the top 5% of sites based on the cumulative fraction of CH₄ emissions. Figure 4a shows, empirically, that the top 5% of high-emitting sites account for 57% (CI: 40–70%) of cumulative CH₄ emissions, with each of these sites having site-level CH₄ emissions >13 kg/h/site. Furthermore, their cumulative CH₄ emissions are equivalent to 1.6% (CI: 1.1–2.2%) of their total CH₄ production. This result is consistent with the observation by Brandt et al. (44) that the largest 5% of leaks from NG systems typically contribute over 50% of total leakage volume. Overall, our results suggest that CH₄ emission models (or CH₄ emission factors) that do not adequately capture the disproportionate contribution of high emitters may significantly underestimate total emissions.

Figure 4. Total CH₄ emissions are dominated by a small fraction of high-emitting sites. (a) Site-level absolute CH₄ emissions distribution plotted in descending rank-order. Empirically (n = 1009), the top 5% of sites contribute 57% of total absolute CH₄ emissions (solid blue line); their cumulative CH₄ emissions are equivalent to 1.6% of their total CH₄ production (solid red line). The light blue and orange bands visually depict the 95% confidence intervals on the cumulative fraction of absolute and production-normalized CH₄ emissions, respectively. The dotted green line shows the predicted CH₄ distribution for all 498 000 U.S. onshore NG production sites as obtained from the nonparametric model–the top 5% of sites account for 50% of total CH₄ and have mean site-level CH₄ emissions of 17 kg/h/site (CI: 10–25). (b) Comparison of estimated total U.S. production CH₄ emissions based on (i) nonparametric model, (ii) total CH₄ emission estimate for all production sources reported by Alvarez et al., (45) (iii) a regression model approach, and (iv) total onshore CH₄ emissions from the 2017 EPA GHGI (see Main Text). The top bubble plots visually depict the differences in production-normalized CH₄ emissions (see SI Section 2.2).

To quantify the importance of accurate accounting of high-emitters on national emission estimates such as the EPA GHGI, (33) we estimated, for the year 2015, the total U.S. CH₄ emissions from NG production using the two approaches previously described (see Materials and Methods). In the first approach, we use a robust regression model that captures the average site-level CH₄ emissions behavior and simulates a bottom-up inventory approach in which the full effect of extreme CH₄ emitters are not accounted for. Second, we use a nonparametric model that fully incorporates the disproportionate influence of high-emitting sites. For this analysis, we use the combined production-normalized CH₄ emissions data given the strong similarities in basin-specific distributions (Figure 2b) and robust trend with site-level NG production (Figure 3b). We include site-level production-normalized CH₄ data from all basins, including those where small sample sizes precluded a comparative assessment of their CH₄ distribution. We acknowledge that this and other potential sampling artifacts discussed above likely increase the overall uncertainty in total estimated CH₄ emissions.

When combined with activity data from Drillinginfo’s DI Desktop, (41) the regression model approach estimates total CH₄ emissions of 330 Mg/h (95% CI on mean: 260–410; or 0.67 kg/h/site and equivalent to production-normalized emissions of 0.59%; Figure 4b) in 2015. In contrast, the nonparametric model yields an estimate that is more than two times the regression model results, that is, total CH₄ emissions of 830 Mg/h (CI: 530–1200; or 1.7 kg/h/site and equivalent to production-normalized emissions of 1.5%).

The regression model approach is similar, in principle, to the bottom-up inventory methods in which an average CH₄ emission factor is applied to activity data, which may include count of wells, components, and/or equipment at a site or region. This is the approach typically used in government inventories such as the EPA GHGI. (33) Our estimated total CH₄ emissions based on the regression model (330 Mg/h) is similar to the 360 Mg/h of total CH₄ emissions for onshore oil and NG production sites as reported in the 2017 EPA GHGI for 2015 emissions (Figure 4b). Furthermore, the EPA GHGI estimate is similar to our mean estimate of 420 Mg/h (CI: 300–570; or 0.88 kg/h/site (CI: 0.61–1.2)) for the lowest-emitting 95% of all U.S. sites (based on cumulative fraction of emissions) as obtained from the nonparametric model (Figure 4b). The robust regression fit (cyan line in Figure 3b) essentially passes through the median CH₄ emission rate in each production bin as the effect of extreme CH₄ outliers are downweighed in favor of tracing the underlying trend through the bulk of the data. Thus, total emissions estimates based on the robust regression model and the results for the lowest-emitting 95% of sites do not fully account for the CH₄ emissions from the exceptionally high emitters.

Our nonparametric model incorporates high emitters (i.e., the top 5% of sites). Its estimate of the total CH₄ emissions is 2.3 times higher than the EPA GHGI’s estimate for total CH₄ from the oil and NG production sites (Figure 4b). Similar discrepancies between bottom-up inventories and measurements have been reported in recent studies. (17,18,25,26,30,45) Herein, our results suggest that CH₄ emissions from the high emitters (national mean: 17 kg/h/site (CI: 10–25); range: 7.2–1100), which account for 50% (CI: 32–75%) of cumulative emissions (Figure 4a), are primarily responsible for the discrepancy between our predicted total CH₄ and the EPA GHGI estimate. That is, the nonparametric model results match the EPA GHGI only when we exclude the contribution of the top 5% of high emitting sites. As reported in the 2017 GHGI, (33) the 2015 national CH₄ emissions for the NG gathering and processing (2.8 Tg), transmission and storage (1.3 Tg), and distribution (0.44 Tg) sectors already incorporate data from skewed emissions distributions obtained in recent sector-specific measurement-based campaigns. (14,17,18) Our analysis supports a similar adjustment of CH₄ emissions for the NG production sector.

Distribution of Methane Emissions among Natural Gas Production Sites

Figure 5 summarizes the total CH₄ predictions stratified by production level. We define low, intermediate, and high NG production sites as sites producing <100 Mcfd, 100 to 1000 Mcfd, and >1000 Mcfd, respectively. Low NG production sites account for 85% of the total number of sites but only 9.6% of total NG production (Figure 5a,b). Although their mean site-level CH₄ emissions are low (0.46 and 2.1 kg/h/site for the <10 Mcfd and 10 to 100 Mcfd sites, respectively; SI Table S10), their very large number makes them an important source of CH₄ emissions nationally, contributing nearly two-thirds (63% (CI: 45–83%) of the total CH₄ emissions. In contrast, high NG production sites (>1000 Mcfd/site) contribute two-thirds of total U.S. NG production and have higher mean site-level CH₄ emissions (8.3 kg/h/site; SI Table S10). However, they are few in number (i.e., they account for only 2.6% of the total number of sites) and contribute only 13% (CI: 7–21%) of the total CH₄ emissions.

Figure 5. Distribution of sites, NG production, and CH₄ emissions based on four cohorts of site-level NG production. (a) Distribution of U.S. NG production sites in 2015 (n = 498 000). (b) Distribution of their NG production (total = 83 Bcfd). (c) Distribution of their estimated CH₄ emissions (total = 830 Mg/h). (d) CH₄ emissions from the high-emitting sites (none of the 220 000 sites producing <10 Mcfd was estimated to be a high emitter). High-emitting sites are defined as the top 5% of U.S. sites (based on the cumulative fraction of site-level CH₄ emissions (n = 25 000)) and that emit >7.2 kg/h/site. Total CH₄ from high-emitting sites were estimated to be 420 Mg/h (95% CI: 260–630 Mg/h).

We estimate that none of the 220 000 sites (45% of all sites) producing <10 Mcfd/site are high emitters; these low-production sites had mean site-level CH₄ emission estimate of 0.46 kg/h/site (CI: 0.35–0.58; SI Table S10). In contrast, we estimate that 47% of the total CH₄ from high-emitting sites came from 8.3% of sites producing 10–100 Mcfd/site. High emitters in this cohort accounted for 23% of the total CH₄ from all sites (Figure 5d, SI Table S10). The remainder of the emissions from high emitters are contributed by sites with intermediate (16% of total CH₄ from all sites) and high NG production (11% of total CH₄ from all sites, Figure 5d).

Spatial Distribution of Methane Emissions

Figure 6 shows the estimated spatial distribution of site-level CH₄ emissions calculated from the nonparametric model, as previously discussed (i.e., the site-level CH₄ emissions distributions for all 498 000 NG production sites plotted in Figure 6 is the same distribution shown in Figure 4a (dotted green line)). Using site-specific location data from Drillinginfo, (41) the estimated emissions were then geo-spatially joined to 35 km × 35 km grid cells and summed to give the grid-specific total CH₄ (Figure 6).

Figure 6. Spatial distribution of CH₄ emissions, plotted on 35km × 35km grid cells. Percentages and numbers in parentheses above and below basin names indicate the basin-level production-normalized and average site-level CH₄ emission rates, respectively. Pie chart labels indicate the predicted mean total CH₄ emissions (kg/h); percentages above the labels indicate the production-normalized CH₄ emissions for the top five states, while percentages inside each pie indicate the predicted fraction of total U.S. CH₄ contributed by that state. Additional data can be found in SI Tables S7, S9, and S11 and in the provided Google Earth kmz file. The oil and gas basin boundaries are from the U.S. EIA. (46) Map data source: ArcUSA, U.S. Census, and ESRI. The map was created using ArcGIS software by ESRI (www.esri.com) and used herein under license.

We predict production CH₄ hotspots in the liquids-rich fairway of the Appalachian Basin (southwestern Pennsylvania and northern West Virginia), and in northwestern (San Juan Basin) and southeastern (Permian Basin) New Mexico, and in Weld County, Colorado (Denver Basin; Figure 6). Methane hotspots are not necessarily areas with high NG production. For example, Weld County (CO) was the eighth largest NG producing county in 2015 but is predicted to be the highest CH₄ emitting county in 2015 (26 Mg/h (CI: 16–34 Mg/h)). The emissions in Weld County are four times greater than that from Susquehanna (PA), the highest NG producing county. This is due to the very large number of sites with relatively low NG production in Weld County (13 000 sites) compared to Susquehanna (400 high producing sites).

Our analysis predicts wide variability in CH₄ emissions among states and among basins. For example, we predict that Texas contributes approximately one-third (32% (95% CI: 20–48%); Figure 6, SI Table S11) of total CH₄ emissions from NG production sites, roughly equivalent to the contribution of the combined CH₄ emissions from NG production sites in Oklahoma, New Mexico, and Pennsylvania (28% (95% CI: 19–39%)). Additionally, predicted basin-specific mean absolute CH₄ emissions per site ranged from 0.91 kg/h/site (CI: 0.62–1.2) in the Appalachian to 2.9 kg/h/site (CI: 1.6–3.9) in the Greater Green River Basin (SI Table S9). Mean production-normalized CH₄ emissions ranged from 0.88% (CI: 0.48–1.2%) in the Greater Green River Basin to 4.5% (CI: 3.3–6.6%) in the San Juan Basin (Figure 6; Table S9). These trends are caused by differences in the distributions of both the number of sites and their NG production characteristics. For example, the Appalachian Basin has the highest basin-level CH₄ emissions (140 Mg/h (CI: 95–180 Mg/h) or 17% of total CH₄ emissions (Figure 6). However, the Appalachian and Greater Green River Basins have the lowest estimated production-normalized CH₄ emissions of approximately 0.90% (Figure 6). NG production in both of these basins are dominated by high-producing sites with relatively low estimated production-normalized emission rates. These sites account for 94% and 72% of total NG production in the Appalachian and Greater Green River Basins, respectively. Similarly, the high estimated production- normalized CH₄ emissions of 4.5% (CI: 2.9–6.0%) in the San Juan Basin (Figure 6; SI Table S9) reflects the large contribution to total NG production (90%) from sites producing <1000 Mcfd/site in this basin.

Comparison with Previous Literature Estimates

Two previously published studies used site-level CH₄ emissions data that are part of the consolidated data set in the present study to estimate basin-level (26) or state-level CH₄ emissions. (8) Zavala-Araiza et al. (26) reported 2013 site-level CH₄ emissions of 1.8 (CI: 1.3–2.5) kg/h/site for the Barnett; the 2015 site-level CH₄ emissions estimates in the present study of 2.4 (CI: 1.4–3.6) kg/h/site for the Fort Worth Basin are in good agreement with their study. Similarly, our estimate of 115 Mg/h (CI: 78–150) for 2015 CH₄ emissions for NG producing sites in Pennsylvania and West Virginia overlaps with a previous estimate by Omara et al. (8) for these sources in 2014 (144 Mg/h (CI: 70–190)). Finally, our national estimate for total CH₄ emissions from NG production sites (830 Mg/h (CI: 530–1200)) compares well with recent estimates by Alvarez et al. (45) (870 Mg/h (CI: 680–1080, Figure 5b) that were based on site-level measurements but utilized a different extrapolation approach incorporating parametrized nonlinear models. (26)

Recent aircraft studies estimated the total CH₄ emissions from different NG production regions. (19−24,31,32) These studies report widely varying mean production-normalized CH₄ estimates, ranging from approximately 0.3–9% in northeastern PA (Marcellus) and Uinta Basin, respectively. Our analysis predicts that the distribution of sites and their NG production levels are important contributors to these trends. For example, basins in which low producing sites dominate site count and NG production (e.g., San Juan) have much higher production-normalized CH₄ emissions than basins where NG production are dominated by high NG producing sites (e.g., Greater Green River). We compared our new bottom-up estimates with these top-down studies (SI Section 3.6; Figure S20), which estimate CH₄ emissions from all oil and NG sources. Our predictions explain, on average, 58% of the airborne top-down CH₄ emissions (20–129% of basin-specific airborne CH₄ emissions (SI Table S12)). There are uncertainties in both estimates. For aircraft measurements, CH₄ source attribution and mass balance closure are uncertain, while the uncertainties in our bottom-up estimates were dominated by variability in study-specific mean site-level emissions.

Other factors (beyond number of sites and site production characteristics) such as new state/local regulations (34−38) or voluntary emissions reductions programs performed by specific operators, likely also contribute to basin-to-basin variability, but we could not assess those factors in this analysis. Additionally, our approach assumes that the large and diverse ensemble of sites considered here reproduces the distribution of emissions across the NG production system at any given point in time. However, there are uncertainties on CH₄ emissions distributions that are difficult to quantify based on available data. Future studies are needed to specifically address these factors.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b03535.

A Google Earth kmz file showing the data presented in Figure 6, and documentation that describes the measurement results obtained in the present study, characteristics and distribution of natural gas production sites, and additional study results, figures, and tables (PDF, ZIP)

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Corresponding Author
- Mark Omara - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States; Present Address: Environmental Defense Fund, 301 Congress Avenue, Austin, Texas 78701, United States; http://orcid.org/0000-0002-8933-1927; Email: [email protected]
Authors
- Naomi Zimmerman - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States; Present Address: Department of Mechanical Engineering, University of British Columbia, 2054–6250 Applied Science Lane, Vancouver, British Columbia, Canada, V6T 1Z4
- Melissa R. Sullivan - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
- Xiang Li - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States; Present Address: Department of Soil, Water, and Climate, University of Minnesota, St Paul, Minnesota 55108, United States; http://orcid.org/0000-0001-6797-7340
- Aja Ellis - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
- Rebecca Cesa - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
- R. Subramanian - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States
- Albert A. Presto - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States; http://orcid.org/0000-0002-9156-1094
- Allen L. Robinson - Center for Atmospheric Particle Studies, Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States; http://orcid.org/0000-0002-1819-083X
Notes
The authors declare no competing financial interest.

Acknowledgments

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Funding for this study was provided by the National Oceanic Atmospheric Administration Climate Program Office, Award No. NA14OAR4310135 and by the Department of Energy, National Energy Technology Laboratory, funding opportunity #DE-FOA-0000894. N.Z.’s work was supported in part by the NSERC postdoctoral fellowship, No. PDF-487660-2016. X.L’s work was supported in part by the NASA Earth and Space Science Fellowship Program, Grant 15-EARTH15F-181. We thank Dr. Eben Thoma for providing the OTM-33A site-level measurement data presented in Brantley et al. (4) The views expressed herein are solely those of the authors and do not necessarily reflect those of the project funders or Environmental Defense Fund.

References

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CH4 emissions from oil and gas producing well pad facilities in the Barnett Shale region of Texas, measured with an innovative ground-based mobile flux plane (MFP) system, as part of the Barnett Coordinated Campaign are described. Using only public roads, CH4 emissions were measured from nearly 200 well pads over 2 wk in Oct. 2013. The population of measured well pads was split into those with detectable emissions (N = 115) and those with emissions below the MFP system detection limit (N = 67). For those well pads with non-zero emissions, the distribution was highly skewed, with a geometric mean of 0.63 kg/h, a geometric std. deviation of 4.2, and an arithmetic mean of 1.72 kg/h. Including the population of non-emitting well pads, the arithmetic mean of sampled well pads was 1.1 kg/h. This distribution implied that 50% of emissions is due to the 6.6% highest emitting well pads; 80% of emissions is from the 22% highest emitting well pads.
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Robertson, A. M.; Edie, R.; Snare, D.; Soltis, J.; Field, R. A.; Burkhart, M. D.; Bell, C. S.; Zimmerle, D.; Murphy, S. M. Variation in methane emission rates from well pads in four oil and gas basins with contrasting production volumes and composition. Environ. Sci. Technol. 2017, 51, 8832– 8840, DOI: 10.1021/acs.est.7b00571
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Allen, D. T.; Torres, V. M.; Thomas, J.; Sullivan, D. W.; Harrison, M.; Hendler, A.; Herndon, S. C.; Kolb, C. E.; Fraser, M. P.; Hill, A. D.; Lamb, B. K.; Miskimins, J.; Sawyer, R. F.; Seinfeld, J. H. Measurements of methane emissions at natural gas production sites in the United States. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 17768– 17773, DOI: 10.1073/pnas.1304880110
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12
Measurements of methane emissions at natural gas production sites in the United States
Allen, David T.; Torres, Vincent M.; Thomas, James; Sullivan, David W.; Harrison, Matthew; Hendler, Al; Herndon, Scott C.; Kolb, Charles E.; Fraser, Matthew P.; Hill, A. Daniel; Lamb, Brian K.; Miskimins, Jennifer; Sawyer, Robert F.; Seinfeld, John H.
Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (44), 17768-17773,S17768/1-S17768/77CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Engineering ests. of CH4 emissions from natural gas prodn. led to varied projections of national emissions. This work reports direct measurements of CH4 emissions at 190 on-shore natural gas sites in the US (150 prodn. sites, 27 well completion flow-backs, 9 well unloadings, 4 work-overs). For well completion flow-backs, which clear fractured wells of liq. to allow gas prodn., CH4 emissions were 0.01-17 Mg (mean, 1.7 Mg; 95% confidence interval bounds, 0.67-3.3 Mg) vs. an av. of 81 Mg/event in the 2011 USEPA national emission inventory (Apr. 2013). Emission factors for pneumatic pumps/controllers and equipment leaks were comparable to and higher than national inventory ests. If emission factors from this work for completion flow-backs, equipment leaks, and pneumatic pumps/controllers were assumed to be representative of national populations and were used to est. national emissions, total annual emissions from these source categories were calcd. to be 957 Gg CH4 (with sampling and measurement uncertainties estd. at ±200 Gg). The est. for comparable source categories in the USEPA national inventory is ∼1200 Gg. Addnl. measurements of unloadings and work-overs are needed to produce national emission ests. for these source categories. The 957 Gg emissions for completion flow-backs, pneumatics, and equipment leaks, in conjunction with USEPA national inventory ests. for other categories, led to an estd. 2300 Gg CH4 emissions from natural gas prodn. (0.42% of gross gas prodn.).
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Allen, D. T.; Sullivan, D. W.; Zavala-Araiza, D.; Pacsi, A. P.; Harrison, M.; Keen, K.; Fraser, M. P.; Hill, A. D.; Lamb, B. K.; Sawyer, R. F.; Seinfeld, J. H. Methane emissions from process equipment at natural gas production sites in the United States: pneumatic controllers. Environ. Sci. Technol. 2015, 49, 633– 640, DOI: 10.1021/es5040156
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Methane Emissions from Process Equipment at Natural Gas Production Sites in the United States: Pneumatic Controllers
Allen, David T.; Pacsi, Adam P.; Sullivan, David W.; Zavala-Araiza, Daniel; Harrison, Matthew; Keen, Kindal; Fraser, Matthew P.; Daniel Hill, A.; Sawyer, Robert F.; Seinfeld, John H.
Environmental Science & Technology (2015), 49 (1), 633-640CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Pollutant emissions from 377 gas-actuated (pneumatic) controllers were measured at natural gas prodn. sites and several oil prodn. sites throughout the US. A small subset of devices (19%), with whole gas emission rates >6 std. ft3/h (scf/h) accounted for 95% of emissions. More than half the controllers recorded emissions of ≤0.001 scf/h during a 15-min measurement. Pneumatic controllers in level control applications on separators and in compressor applications had higher emission rates than controllers in other types of applications. Regional emission differences were obsd.; lowest emissions were measured in the Rocky Mountains, highest emissions were measured at the Gulf Coast. Av. reported CH4 emissions/controller were 17% higher than av. emissions/controller in the 2012 USEPA greenhouse gas national emission inventory (2012 GHG NEI, released in 2014). The av. of 2.7 controllers/well obsd. in this work was higher than the 1.0 controllers/well reported in the 2012 GHG NEI.
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Lamb, B. K.; Edburg, S. L.; Ferrara, W. W.; Howard, T.; Harrison, M. R.; Kolb, C. E.; Townsend-Small, A.; Dyck, W.; Possolo, A.; Whetstone, J. R. Direct measurements show decreasing methane emissions from natural gas local distribution systems in the United States. Environ. Sci. Technol. 2015, 49, 5161– 5169, DOI: 10.1021/es505116p
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Direct Measurements Show Decreasing Methane Emissions from Natural Gas Local Distribution Systems in the United States
Lamb, Brian K.; Edburg, Steven L.; Ferrara, Thomas W.; Howard, Touche; Harrison, Matthew R.; Kolb, Charles E.; Townsend-Small, Amy; Dyck, Wesley; Possolo, Antonio; Whetstone, James R.
Environmental Science & Technology (2015), 49 (8), 5161-5169CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Fugitive losses from natural gas distribution systems are a significant source of anthropogenic methane. Here, we report on a national sampling program to measure methane emissions from 13 urban distribution systems across the U. S. Emission factors were derived from direct measurements at 230 underground pipeline leaks and 229 metering and regulating facilities using stratified random sampling. When these new emission factors are combined with ests. for customer meters, maintenance, and upsets, and current pipeline miles and nos. of facilities, the total est. is 393 Gg/yr with a 95% upper confidence limit of 854 Gg/yr (0.10% to 0.22% of the methane delivered nationwide). This fraction includes emissions from city gates to the customer meter, but does not include other urban sources or those downstream of customer meters. The upper confidence limit accounts for the skewed distribution of measurements, where a few large emitters accounted for most of the emissions. This emission est. is 36% to 70% less than the 2011 EPA inventory, (based largely on 1990s emission data), and reflects significant upgrades at metering and regulating stations, improvements in leak detection and maintenance activities, as well as potential effects from differences in methodologies between the two studies.
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Mitchell, A. L.; Tkacik, D. S.; Roscioli, J. R.; Herndon, S. C.; Yacovitch, T. I.; Martinez, D. M.; Vaughn, T. L.; Williams, L. L.; Sullivan, M. R.; Floerchinger, C.; Omara, M.; Subramanian, R.; Zimmerle, D.; Marchese, A. J.; Robinson, A. L. Measurements of methane emissions from natural gas gathering facilities and processing plants: measurement results. Environ. Sci. Technol. 2015, 49, 3219– 3227, DOI: 10.1021/es5052809
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Measurements of Methane Emissions from Natural Gas Gathering Facilities and Processing Plants: Measurement Results
Mitchell, Austin L.; Tkacik, Daniel S.; Roscioli, Joseph R.; Herndon, Scott C.; Yacovitch, Tara I.; Martinez, David M.; Vaughn, Timothy L.; Williams, Laurie L.; Sullivan, Melissa R.; Floerchinger, Cody; Omara, Mark; Subramanian, R.; Zimmerle, Daniel; Marchese, Anthony J.; Robinson, Allen L.
Environmental Science & Technology (2015), 49 (5), 3219-3227CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Facility-level methane emissions were measured at 114 gathering facilities and 16 processing plants in the United States natural gas system. At gathering facilities, the measured methane emission rates ranged from 0.7 to 700 kg per h (kg/h) (0.6 to 600 std. cfm (scfm)). Normalized emissions (as a % of total methane throughput) were less than 1% for 85 gathering facilities and 19 had normalized emissions less than 0.1%. The range of methane emissions rates for processing plants was 3 to 600 kg/h (3 to 524 scfm), corresponding to normalized methane emissions rates <1% in all cases. The distributions of methane emissions, particularly for gathering facilities, are skewed. For example, 30% of gathering facilities contribute 80% of the total emissions. Normalized emissions rates are neg. correlated with facility throughput. The variation in methane emissions also appears driven by differences between inlet and outlet pressure, as well as venting and leaking equipment. Substantial venting from liqs. storage tanks was obsd. at 20% of gathering facilities. Emissions rates at these facilities were, on av., around four times the rates obsd. at similar facilities without substantial venting.
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Subramanian, R.; Williams, L. L.; Vaughn, T. L.; Zimmerle, D.; Roscioli, J. R.; Herndon, S. C.; Yacovitch, T. I.; Floerchinger, C.; Tkacik, D. S.; Mitchell, A. L.; Sullivan, M. R.; Dallmann, T. R.; Robinson, A. L. Methane emissions from natural gas compressor stations in the transmission and storage sector: measurements and comparisons with the EPA Greenhouse Gas Reporting Program protocol. Environ. Sci. Technol. 2015, 49, 3252– 3261, DOI: 10.1021/es5060258
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Methane Emissions from Natural Gas Compressor Stations in the Transmission and Storage Sector: Measurements and Comparisons with the EPA Greenhouse Gas Reporting Program Protocol
Subramanian, R.; Williams, Laurie L.; Vaughn, Timothy L.; Zimmerle, Daniel; Roscioli, Joseph R.; Herndon, Scott C.; Yacovitch, Tara I.; Floerchinger, Cody; Tkacik, Daniel S.; Mitchell, Austin L.; Sullivan, Melissa R.; Dallmann, Timothy R.; Robinson, Allen L.
Environmental Science & Technology (2015), 49 (5), 3252-3261CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Equipment- and site-level methane emissions from 45 compressor stations in the transmission and storage (T&S) sector of the US natural gas system were measured, including 25 sites required to report under the EPA greenhouse gas reporting program (GHGRP). Direct measurements of fugitive and vented sources were combined with AP-42-based exhaust emission factors (for operating reciprocating engines and turbines) to produce a study onsite est. Site-level methane emissions were also concurrently measured with downwind-tracer-flux techniques. At most sites, these two independent ests. agreed within exptl. uncertainty. Site-level methane emissions varied from 2-880 SCFM. Compressor vents, leaky isolation valves, reciprocating engine exhaust, and equipment leaks were major sources, and substantial emissions were obsd. at both operating and standby compressor stations. The site-level methane emission rates were highly skewed; the highest emitting 10% of sites (including two superemitters) contributed 50% of the aggregate methane emissions, while the lowest emitting 50% of sites contributed less than 10% of the aggregate emissions. Excluding the two superemitters, study-av. methane emissions from compressor housings and noncompressor sources are comparable to or lower than the corresponding effective emission factors used in the EPA greenhouse gas inventory. If the two superemitters are included in the anal., then the av. emission factors based on this study could exceed the EPA greenhouse gas inventory emission factors, which highlights the potentially important contribution of superemitters to national emissions. However, quantification of their influence requires knowledge of the magnitude and frequency of superemitters across the entire T&S sector. Only 38% of the methane emissions measured by the comprehensive onsite measurements were reportable under the new EPA GHGRP because of a combination of inaccurate emission factors for leakers and exhaust methane, and various exclusions. The bias is even larger if one accounts for the superemitters, which were not captured by the onsite measurements. The magnitude of the bias varied from site to site by site type and operating state. Therefore, while the GHGRP is a valuable new source of emissions information, care must be taken when incorporating these data into emission inventories. The value of the GHGRP can be increased by requiring more direct measurements of emissions (as opposed to using counts and emission factors), eliminating exclusions such as rod-packing vents on pressurized reciprocating compressors in standby mode under Subpart-W, and using more appropriate emission factors for exhaust methane from reciprocating engines under Subpart-C.
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Marchese, A. J.; Vaughn, T. L.; Zimmerle, D. J.; Martinez, D. M.; Williams, L. L.; Robinson, A. L.; Mitchell, A. L.; Subramanian, R.; Tkacik, D. S.; Roscioli, J. R.; Herndon, S. C. Methane emissions from United States natural gas gathering and processing. Environ. Sci. Technol. 2015, 49, 3219– 3227
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Zimmerle, D. J.; Williams, L. L.; Vaughn, T. L.; Quinn, C.; Subramanian, R.; Duggan, G. P.; Wilson, B.; Opsomer, J. D.; Marchese, A. J.; Martinez, D. M.; Robinson, A. L. Methane emissions from the natural gas transmission and storage system in the United States. Environ. Sci. Technol. 2015, 49, 9374– 9383, DOI: 10.1021/acs.est.5b01669
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Methane Emissions from the Natural Gas Transmission and Storage System in the United States
Zimmerle, Daniel J.; Williams, Laurie L.; Vaughn, Timothy L.; Quinn, Casey; Subramanian, R.; Duggan, Gerald P.; Willson, Bryan; Opsomer, Jean D.; Marchese, Anthony J.; Martinez, David M.; Robinson, Allen L.
Environmental Science & Technology (2015), 49 (15), 9374-9383CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Recent growth in natural gas prodn. and utilization offers potential climate benefits which depend on CH4 (primary natural gas component and greenhouse gas) life cycle emissions. This work estd. CH4 emissions from transmission and storage (T&S) sector of the US natural gas industry using data collected in 2012, including 2292 on-site measurements, addnl. emissions data from 677 facilities, and activity data from 922 facilities. The largest emission sources were fugitive emissions from compressor-related equipment and super-emitter facilities. Total CH4 emissions estd. from the T&S sector was 1503 (1220-1950) Gg/yr (95% confidence interval) vs. the 2012 USEPA Greenhouse Gas Inventory (GHGI) est. of 2071 (1680-2690) Gg/yr. While the overlap in confidence intervals indicated the difference is not statistically significant, this is due to several significant, but offsetting, factors. Factors which reduce the study est. include: a lower estd. facility count, a shift away from engines toward lower-emitting turbine and elec. compressor drivers, and redns. in use of gas-driven pneumatic devices. Factors which increase the study est. relative to the GHGI include: updated emission rates in certain emission categories and explicit treatment of skewed emissions at component and facility levels. For T&S stations required to report to the EPA Greenhouse Gas Reporting Program (GHGRP), this study estd. total emissions to be 260% (215-330%) of reportable emissions for these stations, primarily due to inclusion of emission sources not reported under GHGRP rules, updated emission factors, and super-emitter emissions.
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Caulton, D. R.; Shepson, P. B.; Santoro, R. L.; Sparks, J. P.; Howarth, R. W.; Ingraffea, A. R.; Cambaliza, M. O. L.; Sweeney, C.; Karion, A.; Davis, K. J.; Stirm, B. H.; Montzka, S. A.; Miller, B. R. Toward a better understanding and quantification of methane emissions from shale gas development. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 6237– 6242, DOI: 10.1073/pnas.1316546111
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Toward a better understanding and quantification of methane emissions from shale gas development
Caulton, Dana R.; Shepson, Paul B.; Santoro, Renee L.; Sparks, Jed P.; Howarth, Robert W.; Ingraffea, Anthony R.; Cambaliza, Maria O. L.; Sweeney, Colm; Karion, Anna; Davis, Kenneth J.; Stirm, Brian H.; Montzka, Stephen A.; Miller, Ben R.
Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (17), 6237-6242CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Identifying and quantifying CH4 emissions from natural gas prodn. is increasingly important due to the increase in the natural gas component of the energy sector. An instrumented aircraft platform identified large CH4 sources and quantified emission rates in southwestern Pennsylvania in June 2012. A large regional flux, 2.0-14 g CH4/s-km2, was quantified for a ∼2800 km2 area, which did not differ statistically from a bottom-up inventory, 2.3-4.6 g CH4/s-km2. Large emissions averaging 34 g CH4/s-well were obsd. at 7 well pads detd. to be in the drilling phase; 2-3 orders of magnitude greater than USEPA ests. for this operational phase. Emissions from these well pads, representing ∼1% of the total no. of wells, accounted for 4-30% of the obsd. regional flux. More work is needed to det. all CH4 emission sources from natural gas prodn., ascertain why these emissions occur, and evaluate their climate and atm. chem. impacts.
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Petron, G.; Karion, A.; Sweeney, C.; Miller, B. R.; Montzka, S. A.; Frost, G. J.; Trainer, M.; Tans, P.; Andrew, A.; Kofler, J.; Helmig, D.; Guenther, D.; Dlugokencky, E.; Lang, P.; Newberger, T.; Wolter, S.; Hall, B.; Novelli, P.; Brewer, A.; Conley, S.; Hardesty, M.; Banta, R.; White, A.; Noone, D.; Wolfe, D.; Schnell, R. A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver-Julesburg Basin: hydrocarbon emissions in oil & gas basin. J. Geophys. Res. Atmospheres 2014, 119, 6836– 6852, DOI: 10.1002/2013JD021272
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A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver-Julesburg Basin
Petron, Gabrielle; Karion, Anna; Sweeney, Colm; Miller, Benjamin R.; Montzka, Stephen A.; Frost, Gregory J.; Trainer, Michael; Tans, Pieter; Andrews, Arlyn; Kofler, Jonathan; Helmig, Detlev; Guenther, Douglas; Dlugokencky, Ed; Lang, Patricia; Newberger, Tim; Wolter, Sonja; Hall, Bradley; Novelli, Paul; Brewer, Alan; Conley, Stephen; Hardesty, Mike; Banta, Robert; White, Allen; Noone, David; Wolfe, Dan; Schnell, Russ
Journal of Geophysical Research: Atmospheres (2014), 119 (11), 6836-6852CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
Emissions of methane (CH4) from oil and natural gas (O&G) operations in the most densely drilled area of the Denver-Julesburg Basin in Weld County located in northeastern Colorado are estd. for 2 days in May 2012 using aircraft-based CH4 observations and planetary boundary layer height and ground-based wind profile measurements. Total top-down CH4 emission ests. are 25.8 ± 8.4 and 26.2 ± 10.7 t CH4/h for the 29 and 31 May flights, resp. Using inventory data, we est. the total emissions of CH4 from non-O&G gas-related sources at 7.1 ± 1.7 and 6.3 ± 1.0 t CH4/h for these 2 days. The difference in emissions is attributed to O&G sources in the study region, and their total emission is on av. 19.3 ± 6.9 t/h, close to 3 times higher than an hourly emission est. based on Environmental Protection Agency's Greenhouse Gas Reporting Program data for 2012. We derive top-down emissions ests. for propane, n-butane, i-pentane, n-pentane, and benzene from our total top-down CH4 emission est. and the relative hydrocarbon abundances in aircraft-based discrete air samples. Emissions for these five nonmethane hydrocarbons alone total 25.4 ± 8.2 t/h. Assuming that these emissions are solely originating from O&G-related activities in the study region, our results show that the state inventory for total volatile org. compds. emitted by O&G activities is at least a factor of 2 too low for May 2012. Our top-down emission est. of benzene emissions from O&G operations is 173 ± 64 kg/h, or 7 times larger than in the state inventory.
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Karion, A.; Sweeney, C.; Petron, G.; Frost, G.; Hardesty, R. M.; Kofler, J.; Miller, B. R.; Newberger, T.; Wolter, S.; Banta, R.; Brewer, A.; Dlugokencky, E.; Lang, P.; Montzka, S. A.; Schnell, R.; Tans, P.; Trainer, M.; Zamora, R.; Conley, S. Methane emissions estimate from airborne measurements over a western United States natural gas field. J. Geophys. Res. Lett. 2013, 40, 4393– 4397, DOI: 10.1002/grl.50811
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Karion, A.; Sweeney, C.; Kort, E. A.; Shepson, P. B.; Brewer, A.; Cambaliza, M.; Conley, S. A.; Davis, K.; Deng, A.; Hardesty, M.; Herndon, S. C.; Lauvaux, T.; Lavoie, T.; Lyon, D.; Newberger, T.; Petron, P.; Rella, C.; Smith, M.; Wolter, S.; Yacovitch, T. I.; Tans, P. Aircraft-based estimate of total methane emissions from the Barnett Shale region. Environ. Sci. Technol. 2015, 49, 8124– 8131, DOI: 10.1021/acs.est.5b00217
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Aircraft-Based Estimate of Total Methane Emissions from the Barnett Shale Region
Karion, Anna; Sweeney, Colm; Kort, Eric A.; Shepson, Paul B.; Brewer, Alan; Cambaliza, Maria; Conley, Stephen A.; Davis, Ken; Deng, Aijun; Hardesty, Mike; Herndon, Scott C.; Lauvaux, Thomas; Lavoie, Tegan; Lyon, David; Newberger, Tim; Petron, Gabrielle; Rella, Chris; Smith, Mackenzie; Wolter, Sonja; Yacovitch, Tara I.; Tans, Pieter
Environmental Science & Technology (2015), 49 (13), 8124-8131CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Estd. regional CH4 emissions from oil and natural gas operations in the Barnett Shale, Texas, using airborne atm. measurements are presented. Using a mass balance approach on 8 different flight days in March and Oct. 2013, total regional CH4 emissions were estd. to be 76 ± 13 × 103 kg/h (equiv. to 0.66 ± 0.11 Tg CH4/yr; 95% confidence interval [CI]). It was estd. that 60 ± 11 × 103 kg CH4/h (95% CI) are emitted by natural gas and oil operations: prodn., processing, and distribution, in the Dallas and Fort Worth urban areas. This est. agreed with the USEPA est. for nation-wide CH4 emissions from the natural gas sector when scaled by natural gas prodn.; it was higher than emissions reported by the EDGAR inventory or by industry to the EPA Greenhouse Gas Reporting Program. This work was the first to show consistency between mass balance results on so many different days and in 2 different seasons, enabling better quantification of related uncertainty. The Barnett is one of the largest prodn. basins in the US with 8% of total US natural gas prodn.; hence, these results represent a crucial step toward detg. the US natural gas prodn. greenhouse gas footprint and are a crucial step in detg. same for on-shore natural gas prodn. in US.
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Peischl, J.; Ryerson, T. B.; Aikin, K. C.; de Gouw, J. A.; Gilman, J. B.; Holloway, J. S.; Lerner, B. M.; Nadkarni, R.; Neuman, J. A.; Nowak, J. B.; Trainer, M.; Warneke, C.; Parrish, D. D. Quantifying atmospheric methane emissions from Haynesville, Fayetteville, and northeastern Marcellus shale gas production regions. J. Geophys. Res. Atmospheres 2015, 120, 2119– 2139, DOI: 10.1002/2014JD022697
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Quantifying atmospheric methane emissions from the Haynesville, Fayetteville, and northeastern Marcellus shale gas production regions
Peischl, J.; Ryerson, T. B.; Aikin, K. C.; de Gouw, J. A.; Gilman, J. B.; Holloway, J. S.; Lerner, B. M.; Nadkarni, R.; Neuman, J. A.; Nowak, J. B.; Trainer, M.; Warneke, C.; Parrish, D. D.
Journal of Geophysical Research: Atmospheres (2015), 120 (5), 2119-2139CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
We present measurements of methane (CH4) taken aboard a NOAA WP-3D research aircraft in 2013 over the Haynesville shale region in eastern Texas/northwestern Louisiana, the Fayetteville shale region in Arkansas, and the northeastern Pennsylvania portion of the Marcellus shale region, which accounted for the majority of Marcellus shale gas prodn. that year. We calc. emission rates from the horizontal CH4 flux in the planetary boundary layer downwind of each region after subtracting the CH4 flux entering the region upwind. We find 1 day CH4 emissions of (8.0 ± 2.7) × 107 g/h from the Haynesville region, (3.9 ± 1.8) × 107 g/h from the Fayetteville region, and (1.5 ± 0.6) × 107 g/h from the Marcellus region in northeastern Pennsylvania. Finally, we compare the CH4 emissions to the total vol. of natural gas extd. from each region to derive a loss rate from prodn. operations of 1.0-2.1% from the Haynesville region, 1.0-2.8% from the Fayetteville region, and 0.18-0.41% from the Marcellus region in northeastern Pennsylvania. The climate impact of CH4 loss from shale gas prodn. depends upon the total leakage from all prodn. regions. The regions investigated in this work represented over half of the U.S. shale gas prodn. in 2013, and we find generally lower loss rates than those reported in earlier studies of regions that made smaller contributions to total prodn. Hence, the national av. CH4 loss rate from shale gas prodn. may be lower than values extrapolated from the earlier studies.
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Peischl, J.; Karion, A.; Sweeney, C.; Kort, E. A.; Smith, M. L.; Brandt, A. R.; Yeskoo, T.; Aikin, K. C.; Conley, S. A.; Gvakharia, A.; Trainer, M.; Wolter, S.; Ryerson, T. B. Quantifying atmospheric methane emissions from oil and natural gas production in the Bakken shale region of North Dakota. J. Geophys. Res. Atmospheres 2016, 121, 6101– 6111, DOI: 10.1002/2015JD024631
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Lyon, D. R.; Zavala-Araiza, D.; Alvarez, R. A.; Harris, R.; Palacios, V.; Lan, X.; Talbot, R.; Lavoie, T.; Shepson, P.; Yacovitch, T. I.; Herndon, S. C.; Marchese, A. J.; Zimmerle, D.; Robinson, A. L.; Hamburg, S. P. Constructing a spatially resolved methane emission inventory for the Barnett Shale region. Environ. Sci. Technol. 2015, 49, 8147– 8157, DOI: 10.1021/es506359c
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Constructing a Spatially Resolved Methane Emission Inventory for the Barnett Shale Region
Lyon, David R.; Zavala-Araiza, Daniel; Alvarez, Ramon A.; Harriss, Robert; Palacios, Virginia; Lan, Xin; Talbot, Robert; Lavoie, Tegan; Shepson, Paul; Yacovitch, Tara I.; Herndon, Scott C.; Marchese, Anthony J.; Zimmerle, Daniel; Robinson, Allen L.; Hamburg, Steven P.
Environmental Science & Technology (2015), 49 (13), 8147-8157CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
CH4 emissions from the oil and gas industry (O&G) and other sources in the Barnett Shale region (Texas) were estd. by developing a spatially resolved emission inventory. In total, 18 source categories were estd. using multiple datasets, including empirical measurements at regional O&G sites and a national study of collecting/processing facilities. Spatially referenced activity data were compiled from federal and state databases and combined with O&G facility emission factors calcd. by Monte Carlo simulations which accounted for high emission sites representing the very upper portion, or fat-tail, of obsd. emissions distributions. Total CH4 emissions in the 25-county Barnett Shale region in Oct. 2013 were estd. to be 72,300 (63,400-82,400) kg CH4/h. O&G emissions were estd. to be 46,200 (40,000-54,100) kg CH4/h; 19% of emissions from fat-tail sites represented <2% of sites. Estd. O&G emissions in the Barnett Shale region were higher than alternative inventories based on the USEPA Greenhouse Gas Inventory, EPA Greenhouse Gas Reporting Program, and Emissions Database for Global Atm. Research by factors of 1.5, 2.7, and 4.3, resp. Collecting compressor sites, accounting for 40% of O&G emissions in this inventory, had the largest difference from emission ests. based on EPA data sources. This inventory higher O&G emissions est. was due primarily to its more comprehensive activity factors and inclusion of fat-tail sites.
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Zavala-Araiza, D.; Lyion, D. R.; Alvarez, R. A.; Davis, K. J.; Harris, R.; Herndon, S. C.; Karion, A.; Kort, E. A.; Lamb, B. K.; Lan, X.; Marchese, A. J.; Pacala, S. W.; Robinson, A. L.; Shepson, P. B.; Sweeney, C.; Talbot, R.; Townsend-Small, A.; Yacovitch, T. I.; Zimmerle, D. J.; Hamburg, S. P. Reconciling divergent estimates of oil and gas methane emissions. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 15597– 15602, DOI: 10.1073/pnas.1522126112
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Reconciling divergent estimates of oil and gas methane emissions
Zavala-Araiza, Daniel; Lyon, David R.; Alvarez, Ramon A.; Davis, Kenneth J.; Harriss, Robert; Herndon, Scott C.; Karion, Anna; Kort, Eric Adam; Lamb, Brian K.; Lan, Xin; Marchese, Anthony J.; Pacala, Stephen W.; Robinson, Allen L.; Shepson, Paul B.; Sweeney, Colm; Talbot, Robert; Townsend-Small, Amy; Yacovitch, Tara I.; Zimmerle, Daniel J.; Hamburg, Steven P.
Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (51), 15597-15602CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Published ests. of methane emissions from atm. 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 ests. 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 ests. by using repeated mass balance measurements, as well as ethane as a fingerprint for source attribution. Similarly, our bottom-up est. incorporates a more complete count of facilities than past inventories, which omitted a significant no. 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 ests. based on the US Environmental Protection Agency's Greenhouse Gas Inventory and correspond to 1.5% of natural gas prodn. This rate of methane loss increases the 20-y climate impacts of natural gas consumed in the region by roughly 50%.
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Zavala-Araiza, D.; Alvarez, R. A.; Lyon, D. R.; Allen, D. T.; Marchese, A. J.; Zimmerle, D. J.; Hamburg, S. P. Super-emitters in natural gas infrastructure are caused by abnormal process conditions. Nat. Commun. 2017, 8, 14012– 1421, DOI: 10.1038/ncomms14012
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Super-emitters in natural gas infrastructure are caused by abnormal process conditions
Zavala-Araiza, Daniel; Alvarez, Ramon A.; Lyon, David R.; Allen, David T.; Marchese, Anthony J.; Zimmerle, Daniel J.; Hamburg, Steven P.
Nature Communications (2017), 8 (), 14012CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
Effectively mitigating methane emissions from the natural gas supply chain requires addressing the disproportionate influence of high-emitting sources. Here we use a Monte Carlo simulation to aggregate methane emissions from all components on natural gas prodn. sites in the Barnett Shale prodn. region (Texas). Our total emission ests. are two-thirds of those derived from independent site-based measurements. Although some high-emitting operations occur by design (condensate flashing and liq. unloadings), they occur more than an order of magnitude less frequently than required to explain the reported frequency at which high site-based emissions are obsd. We conclude that the occurrence of abnormal process conditions (for example, malfunctions upstream of the point of emissions; equipment issues) cause addnl. emissions that explain the gap between component-based and site-based emissions. Such abnormal conditions can cause a substantial proportion of a site's gas prodn. to be emitted to the atm. and are the defining attribute of super-emitting sites.
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Zavala-Araiza, D.; Lyon, D.; Alvarez, R. A.; Palacios, V.; Harris, R.; Lan, X.; Talbot, R.; Hamburg, S. P. Toward a functional definition of methane super-emitters: application to natural gas production sites. Environ. Sci. Technol. 2015, 49, 8167– 8174, DOI: 10.1021/acs.est.5b00133
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Toward a Functional Definition of Methane Super-Emitters: Application to Natural Gas Production Sites
Zavala-Araiza, Daniel; Lyon, David; Alvarez, Ramon A.; Palacios, Virginia; Harriss, Robert; Lan, Xin; Talbot, Robert; Hamburg, Steven P.
Environmental Science & Technology (2015), 49 (13), 8167-8174CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Natural gas prodn. site emissions are characterized by skewed distributions, where a small percentage of sites, commonly labeled super-emitters, account for a majority of emissions. A better characterization of super-emitters is needed to operationalize ways to identify them and reduce emissions. This work designed a conceptual framework to functionally define super-emitting sites as those with the highest proportional loss rates (Ch4 emitted vs. CH4 produced). Using this concept, total CH4 emissions from Barnett Shale natural gas prodn. sites (Texas) were estd.; super-emitting sites functionally accounted for approx. 3/4 of total emissions. The potential to reduce emissions from these sites is discussed under the assumption that sites with high proportional loss rates have excess emissions resulting from abnormal or otherwise avoidable operating conditions, e.g., malfunctioning equipment. Since the population of functionally super-emitting sites is not expected to be static over time, continuous monitoring will be necessary to identify them and improve their operation. This work suggested that achieving and maintaining uniformly low emissions across the entire population of prodn. sites will require mitigation steps at a large fraction of sites.
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Lyon, D. R.; Alvarez, R. A.; Zavala-Araiza, D.; Brandt, A. R.; Jackson, R. B.; Hamburg, S. P. Aerial surveys of elevated hydrocarbon emissions from oil and gas production sites. Environ. Sci. Technol. 2016, 50, 4877– 4886, DOI: 10.1021/acs.est.6b00705
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Aerial Surveys of Elevated Hydrocarbon Emissions from Oil and Gas Production Sites
Lyon, David R.; Alvarez, Ramon A.; Zavala-Araiza, Daniel; Brandt, Adam R.; Jackson, Robert B.; Hamburg, Steven P.
Environmental Science & Technology (2016), 50 (9), 4877-4886CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Oil and gas (O&G) well pads with high hydrocarbon emission rates may disproportionally contribute to total CH4 and volatile org. compd. (VOC) emissions from the prodn. sector. These emissions may be missing from most bottom-up emission inventories. This work performed helicopter-based IR camera surveys of >8000 O&G well pads in 7 US basins to assess the prevalence and distribution of high-emitting hydrocarbon sources (detection threshold, ∼1-3 g/s). The proportion of sites with such high-emitting sources was 4% nationally, but ranged from 1% in the Powder River (Wyoming) to 14% in the Bakken (North Dakota) basins. Emissions were obsd. 3 times more frequently at sites in the oil-producing Bakken and oil-producing regions of mixed basins (p <0.0001, χ2 test); however, statistical models using basin and well pad characteristics explained ≤14% of obsd. emission pattern variances, indicating stochastic processes dominate the occurrence of high emissions at individual sites. More than 90% of nearly 500 detected sources were from tank vents and hatches. Although tank emissions may be partially attributable to flash gas, obsd. frequencies in most basins exceeded those expected if emissions were effectively captured and controlled, demonstrating tank emission control systems commonly under-perform. Tanks represent a key mitigation opportunity to reduce CH4 and VOC emissions.
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Brandt, A. R.; Heath, G. A.; Kort, E. A.; O’Sullivan, F.; Petron, G.; Jordaan, S. M.; Tans, P.; Wilcox, J.; Gopstein, A. M.; Arent, D.; Wofsy, S.; Brown, N. J.; Bradley, R.; Stucky, G. D.; Eardley, D.; Harris, R. Methane leaks from North American natural gas systems. Science 2014, 343, 733– 735, DOI: 10.1126/science.1247045
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Barkley, Z. R.; Lauvaux, T.; Davis, K. J.; Deng, A.; Miles, N. L.; Richardson, S. J.; Cao, Y.; Sweeney, C.; Karion, A.; Smith, M.; Kort, E. A.; Schwietzke, S.; Murphy, T.; Cervone, G.; Martins, D.; Maasakkers, J. D. Quantifying methane emissions from natural gas production in northeastern Pennsylvania. Atmos. Chem. Phys. 2017, 17, 13941– 13966, DOI: 10.5194/acp-17-13941-2017
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Smith, M. L.; Gvakharia, A.; Kort, E. A.; Sweeney, C.; Conley, S. A.; Faloona, I.; Newberger, T.; Schnell, R.; Schwietzke, S.; Wolter, S. Airborne quantification of methane emissions over the Four Corners Region. Environ. Sci. Technol. 2017, 51, 5832– 5837, DOI: 10.1021/acs.est.6b06107
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Airborne Quantification of Methane Emissions over the Four Corners Region
Smith, Mackenzie L.; Gvakharia, Alexander; Kort, Eric A.; Sweeney, Colm; Conley, Stephen A.; Faloona, Ian; Newberger, Tim; Schnell, Russell; Schwietzke, Stefan; Wolter, Sonja
Environmental Science & Technology (2017), 51 (10), 5832-5837CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Methane (CH4) is a potent greenhouse gas and the primary component of natural gas. The San Juan Basin (SJB) is one of the largest coal-bed methane producing regions in North America and, including gas prodn. from conventional and shale sources, contributed ∼2% of U.S. natural gas prodn. in 2015. In this work, we quantify the CH4 flux from the SJB using continuous atm. sampling from aircraft collected during the TOPDOWN2015 field campaign in Apr. 2015. Using five independent days of measurements and the aircraft-based mass balance method, we calc. an av. CH4 flux of 0.54±0.20 Tg yr-1 (1σ), in close agreement with the previous space-based est. made for 2003-2009. These results agree within error with the U.S. EPA gridded inventory for 2012. These flights combined with the previous satellite study suggest CH4 emissions have not changed. While there have been significant declines in natural gas prodn. between measurements, recent increases in oil prodn. in the SJB may explain why emission of CH4 has not declined. Airborne quantification of outcrops where seepage occurs are consistent with ground-based studies that indicate these geol. sources are a small fraction of the basin total (0.02-0.12 Tg yr-1) and cannot explain basinwide consistent emissions from 2003 to 2015.
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Ravikumar, A. P.; Wang, J.; Brandt, A. R. Are optical gas imaging technologies effective for methane leak detection?. Environ. Sci. Technol. 2017, 51, 718– 724, DOI: 10.1021/acs.est.6b03906
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Are Optical Gas Imaging Technologies Effective For Methane Leak Detection?
Ravikumar, Arvind P.; Wang, Jingfan; Brandt, Adam R.
Environmental Science & Technology (2017), 51 (1), 718-724CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Concerns over mitigating methane leakage from the natural gas system have become ever more prominent in recent years. Recently, the US Environmental Protection Agency proposed regulations requiring use of optical gas imaging (OGI) technologies to identify and repair leaks. In this work, we develop an open-source predictive model to accurately simulate the most common OGI technol., passive IR (IR) imaging. The model accurately reproduces IR images of controlled methane release field expts. as well as reported min. detection limits. We show that imaging distance is the most important parameter affecting IR detection effectiveness. In a simulated well-site, over 80% of emissions can be detected from an imaging distance of 10 m. Also, the presence of 'super-emitters' greatly enhance the effectiveness of IR leak detection. The min. detectable limits of this technol. can be used to selectively target 'super-emitters', thereby providing a method for approx. leak-rate quantification. In addn., model results show that imaging backdrop controls IR imaging effectiveness: land-based detection against sky or low-emissivity backgrounds have higher detection efficiency compared to aerial measurements. Finally, we show that min. IR detection thresholds can be significantly lower for gas compns. that include a significant fraction non-methane hydrocarbons.
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Methane Leaks from Natural Gas Systems Follow Extreme Distributions
Brandt, Adam R.; Heath, Garvin A.; Cooley, Daniel
Environmental Science & Technology (2016), 50 (22), 12512-12520CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
Future energy systems may rely on natural gas as a low-cost fuel to support variable renewable power. However, leaking natural gas causes climate damage because methane (CH4) has a high global warming potential. In this study, we use extreme-value theory to explore the distribution of natural gas leak sizes. By analyzing ∼15,000 measurements from 18 prior studies, we show that all available natural gas leakage data sets are statistically heavy-tailed, and that gas leaks are more extremely distributed than other natural and social phenomena. A unifying result is that the largest 5% of leaks typically contribute over 50% of the total leakage vol. While prior studies used log-normal model distributions, we show that log-normal functions poorly represent tail behavior. Our results suggest that published uncertainty ranges of CH4 emissions are too narrow, and that larger sample sizes are required in future studies to achieve targeted confidence intervals. Addnl., we find that cross-study aggregation of data sets to increase sample size is not recommended due to apparent deviation between sampled populations. Understanding the nature of leak distributions can improve emission ests., better illustrate their uncertainty, allow prioritization of source categories, and improve sampling design. Also, these data can be used for more effective design of leak detection technologies.
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Alvarez, R. A.; Zavala-Araiza, D.; Lyon, D. R.; Allen, D. T.; Barkley, A. R.; Brandt, A. R.; Davis, K. J.; Herndon, S. C.; Jacob, D. J.; Karion, A.; Kort, E. A.; Lamb, B. K.; Lauvaux, T.; Maasakkers, J. D.; Marchese, A. J.; Omara, M.; Pacala, S. W.; Peischl, J.; Robinson, A. L.; Shepson, P. B.; Sweeney, C.; Townsend-Small, A.; Wofsy, S. C.; Hamburg, S. P. Assessment of methane emissions from the U.S. oil and gas supply chain. Science 2018, 361, 186– 188, DOI: 10.1126/science.aar7204
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Assessment of methane emissions from the U.S. oil and gas supply chain
Alvarez, Ramon A.; Zavala-Araiza, Daniel; Lyon, David R.; Allen, David T.; Barkley, Zachary R.; Brandt, Adam R.; Davis, Kenneth J.; Herndon, Scott C.; Jacob, Daniel J.; Karion, Anna; Kort, Eric A.; Lamb, Brian K.; Lauvaux, Thomas; Maasakkers, Joannes D.; Marchese, Anthony J.; Omara, Mark; Pacala, Stephen W.; Peischl, Jeff; Robinson, Allen L.; Shepson, Paul B.; Sweeney, Colm; Townsend-Small, Amy; Wofsy, Steven C.; Hamburg, Steven P.
Science (Washington, DC, United States) (2018), 361 (6398), 186-188CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
Considerable amts. of the greenhouse gas methane leak from the U.S. oil and natural gas supply chain. Alvarez et al. reassessed the magnitude of this leakage and found that in 2015, supply chain emissions were ∼60% higher than the U.S. Environmental Protection Agency inventory est. They suggest that this discrepancy exists because current inventory methods miss emissions that occur during abnormal operating conditions. These data, and the methodol. used to obtain them, could improve and verify international inventories of greenhouse gases and provide a better understanding of mitigation efforts outlined by the Paris Agreement.
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Anna M. Robertson, Rachel Edie, Robert A. Field, David Lyon, Renee McVay, Mark Omara, Daniel Zavala-Araiza, Shane M. Murphy. New Mexico Permian Basin Measured Well Pad Methane Emissions Are a Factor of 5–9 Times Higher Than U.S. EPA Estimates. Environmental Science & Technology 2020, 54 (21) , 13926-13934. https://doi.org/10.1021/acs.est.0c02927
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Abstract
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Figure 1
Figure 1. Site-level measurement data synthesized in this study. Numbers in parentheses indicate the number of sites with emissions data for each basin; the citations to the original studies are indicated as superscripts after basin names. New measurements are indicated with an (a) in the Denver-Julesburg (DJB; n = 18, or 17% of all DJB sites), Marcellus (NE PA unconventional sites, n = 45, or 57% of all Marcellus data), and Uinta (n = 29, or 50% of all Uinta sites). The boxes represent the 25th and 75th percentiles, while the whiskers extend to 1.5× the interquartile range, and values outside this range are the outliers, marked with red crosses. The black horizontal line inside each box represents the median while means are shown in purple. The notches visually depict the 95% confidence interval on the median. For Eagle Ford, measurement data for the four sites are represented with an error bar indicating the minimum and maximum. For Fayetteville, the notch extends beyond the 25th percentile as a result of the sample size and the data spread in this basin. “All Abs.” and “All Norm.” represent combined data set for all absolute and production-normalized CH₄ emission rates (n = 1009), respectively.
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Figure 2
Figure 2. Comparison of CH₄ emissions distributions among basins. (a) absolute and (b) production-normalized emissions. Only basins with n ≥ 50 sites were evaluated. For the <390 Mcfd bin, comparisons were made among the Barnett (n = 245), DJB (n = 95), Pinedale (n = 50), and Uinta (n = 50) Basins. For the >390 Mcfd bin, comparisons were made among the Barnett (n = 309), Pinedale (n = 56), and the Marcellus (n = 57) Basins. The lightly colored lines indicate the 10 000 bootstrap distributions obtained by randomly sampling 50 sites, with replacement, from the empirical distributions (shown in bold solid lines). Differences in distributions were assessed using the 2-sample Kolmogorov–Smirnov test with significance established at p < 0.01.
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Figure 3
Figure 3. Relationship between site-level CH₄ emissions and NG production. (a) absolute and (b) production-normalized CH₄ emissions. Solid cyan lines show quadratic robust weighted least-squares regressions with bisquare weighting (see Materials and Methods) performed on the entire data set. Measured site-level CH₄ emissions were also binned by deciles of their site-level NG production, which are numbered sequentially on the top x axis. The notched box plots (outliers not shown) visually depict the data spread in each production decile. The black horizontal line in each notched box shows the median. The triangular purple symbols show the mean CH₄ emission rate in each production decile and the solid red lines show the polynomial fit through the mean CH₄ emission rate in each decile. These regression equations are (a) log₁₀[CH₄ (kg/h)] = 0.30 ± 0.14 × log₁₀[Prod (Mcfd)] – 0.23 ± 0.38; r_adj² = 0.72) and (b) log₁₀[%CH₄(^kg/h/_kg/h)] = −0.71 ± 0.15 × log₁₀[Prod(Mcfd)] + 2.0 ± 0.41; r_adj² = 0.93) for the absolute and production-normalized CH₄ emissions, respectively.
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Figure 4
Figure 4. Total CH₄ emissions are dominated by a small fraction of high-emitting sites. (a) Site-level absolute CH₄ emissions distribution plotted in descending rank-order. Empirically (n = 1009), the top 5% of sites contribute 57% of total absolute CH₄ emissions (solid blue line); their cumulative CH₄ emissions are equivalent to 1.6% of their total CH₄ production (solid red line). The light blue and orange bands visually depict the 95% confidence intervals on the cumulative fraction of absolute and production-normalized CH₄ emissions, respectively. The dotted green line shows the predicted CH₄ distribution for all 498 000 U.S. onshore NG production sites as obtained from the nonparametric model–the top 5% of sites account for 50% of total CH₄ and have mean site-level CH₄ emissions of 17 kg/h/site (CI: 10–25). (b) Comparison of estimated total U.S. production CH₄ emissions based on (i) nonparametric model, (ii) total CH₄ emission estimate for all production sources reported by Alvarez et al., (45) (iii) a regression model approach, and (iv) total onshore CH₄ emissions from the 2017 EPA GHGI (see Main Text). The top bubble plots visually depict the differences in production-normalized CH₄ emissions (see SI Section 2.2).
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Figure 5
Figure 5. Distribution of sites, NG production, and CH₄ emissions based on four cohorts of site-level NG production. (a) Distribution of U.S. NG production sites in 2015 (n = 498 000). (b) Distribution of their NG production (total = 83 Bcfd). (c) Distribution of their estimated CH₄ emissions (total = 830 Mg/h). (d) CH₄ emissions from the high-emitting sites (none of the 220 000 sites producing <10 Mcfd was estimated to be a high emitter). High-emitting sites are defined as the top 5% of U.S. sites (based on the cumulative fraction of site-level CH₄ emissions (n = 25 000)) and that emit >7.2 kg/h/site. Total CH₄ from high-emitting sites were estimated to be 420 Mg/h (95% CI: 260–630 Mg/h).
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Figure 6
Figure 6. Spatial distribution of CH₄ emissions, plotted on 35km × 35km grid cells. Percentages and numbers in parentheses above and below basin names indicate the basin-level production-normalized and average site-level CH₄ emission rates, respectively. Pie chart labels indicate the predicted mean total CH₄ emissions (kg/h); percentages above the labels indicate the production-normalized CH₄ emissions for the top five states, while percentages inside each pie indicate the predicted fraction of total U.S. CH₄ contributed by that state. Additional data can be found in SI Tables S7, S9, and S11 and in the provided Google Earth kmz file. The oil and gas basin boundaries are from the U.S. EIA. (46) Map data source: ArcUSA, U.S. Census, and ESRI. The map was created using ArcGIS software by ESRI (www.esri.com) and used herein under license.
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  Allen, David T.; Torres, Vincent M.; Thomas, James; Sullivan, David W.; Harrison, Matthew; Hendler, Al; Herndon, Scott C.; Kolb, Charles E.; Fraser, Matthew P.; Hill, A. Daniel; Lamb, Brian K.; Miskimins, Jennifer; Sawyer, Robert F.; Seinfeld, John H.
  Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (44), 17768-17773,S17768/1-S17768/77CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Engineering ests. of CH4 emissions from natural gas prodn. led to varied projections of national emissions. This work reports direct measurements of CH4 emissions at 190 on-shore natural gas sites in the US (150 prodn. sites, 27 well completion flow-backs, 9 well unloadings, 4 work-overs). For well completion flow-backs, which clear fractured wells of liq. to allow gas prodn., CH4 emissions were 0.01-17 Mg (mean, 1.7 Mg; 95% confidence interval bounds, 0.67-3.3 Mg) vs. an av. of 81 Mg/event in the 2011 USEPA national emission inventory (Apr. 2013). Emission factors for pneumatic pumps/controllers and equipment leaks were comparable to and higher than national inventory ests. If emission factors from this work for completion flow-backs, equipment leaks, and pneumatic pumps/controllers were assumed to be representative of national populations and were used to est. national emissions, total annual emissions from these source categories were calcd. to be 957 Gg CH4 (with sampling and measurement uncertainties estd. at ±200 Gg). The est. for comparable source categories in the USEPA national inventory is ∼1200 Gg. Addnl. measurements of unloadings and work-overs are needed to produce national emission ests. for these source categories. The 957 Gg emissions for completion flow-backs, pneumatics, and equipment leaks, in conjunction with USEPA national inventory ests. for other categories, led to an estd. 2300 Gg CH4 emissions from natural gas prodn. (0.42% of gross gas prodn.).
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13. 13
  Allen, D. T.; Sullivan, D. W.; Zavala-Araiza, D.; Pacsi, A. P.; Harrison, M.; Keen, K.; Fraser, M. P.; Hill, A. D.; Lamb, B. K.; Sawyer, R. F.; Seinfeld, J. H. Methane emissions from process equipment at natural gas production sites in the United States: pneumatic controllers. Environ. Sci. Technol. 2015, 49, 633– 640, DOI: 10.1021/es5040156
  [ACS Full Text ], [CAS], Google Scholar
  13
  Methane Emissions from Process Equipment at Natural Gas Production Sites in the United States: Pneumatic Controllers
  Allen, David T.; Pacsi, Adam P.; Sullivan, David W.; Zavala-Araiza, Daniel; Harrison, Matthew; Keen, Kindal; Fraser, Matthew P.; Daniel Hill, A.; Sawyer, Robert F.; Seinfeld, John H.
  Environmental Science & Technology (2015), 49 (1), 633-640CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Pollutant emissions from 377 gas-actuated (pneumatic) controllers were measured at natural gas prodn. sites and several oil prodn. sites throughout the US. A small subset of devices (19%), with whole gas emission rates >6 std. ft3/h (scf/h) accounted for 95% of emissions. More than half the controllers recorded emissions of ≤0.001 scf/h during a 15-min measurement. Pneumatic controllers in level control applications on separators and in compressor applications had higher emission rates than controllers in other types of applications. Regional emission differences were obsd.; lowest emissions were measured in the Rocky Mountains, highest emissions were measured at the Gulf Coast. Av. reported CH4 emissions/controller were 17% higher than av. emissions/controller in the 2012 USEPA greenhouse gas national emission inventory (2012 GHG NEI, released in 2014). The av. of 2.7 controllers/well obsd. in this work was higher than the 1.0 controllers/well reported in the 2012 GHG NEI.
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14. 14
  Lamb, B. K.; Edburg, S. L.; Ferrara, W. W.; Howard, T.; Harrison, M. R.; Kolb, C. E.; Townsend-Small, A.; Dyck, W.; Possolo, A.; Whetstone, J. R. Direct measurements show decreasing methane emissions from natural gas local distribution systems in the United States. Environ. Sci. Technol. 2015, 49, 5161– 5169, DOI: 10.1021/es505116p
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  14
  Direct Measurements Show Decreasing Methane Emissions from Natural Gas Local Distribution Systems in the United States
  Lamb, Brian K.; Edburg, Steven L.; Ferrara, Thomas W.; Howard, Touche; Harrison, Matthew R.; Kolb, Charles E.; Townsend-Small, Amy; Dyck, Wesley; Possolo, Antonio; Whetstone, James R.
  Environmental Science & Technology (2015), 49 (8), 5161-5169CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Fugitive losses from natural gas distribution systems are a significant source of anthropogenic methane. Here, we report on a national sampling program to measure methane emissions from 13 urban distribution systems across the U. S. Emission factors were derived from direct measurements at 230 underground pipeline leaks and 229 metering and regulating facilities using stratified random sampling. When these new emission factors are combined with ests. for customer meters, maintenance, and upsets, and current pipeline miles and nos. of facilities, the total est. is 393 Gg/yr with a 95% upper confidence limit of 854 Gg/yr (0.10% to 0.22% of the methane delivered nationwide). This fraction includes emissions from city gates to the customer meter, but does not include other urban sources or those downstream of customer meters. The upper confidence limit accounts for the skewed distribution of measurements, where a few large emitters accounted for most of the emissions. This emission est. is 36% to 70% less than the 2011 EPA inventory, (based largely on 1990s emission data), and reflects significant upgrades at metering and regulating stations, improvements in leak detection and maintenance activities, as well as potential effects from differences in methodologies between the two studies.
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15. 15
  Mitchell, A. L.; Tkacik, D. S.; Roscioli, J. R.; Herndon, S. C.; Yacovitch, T. I.; Martinez, D. M.; Vaughn, T. L.; Williams, L. L.; Sullivan, M. R.; Floerchinger, C.; Omara, M.; Subramanian, R.; Zimmerle, D.; Marchese, A. J.; Robinson, A. L. Measurements of methane emissions from natural gas gathering facilities and processing plants: measurement results. Environ. Sci. Technol. 2015, 49, 3219– 3227, DOI: 10.1021/es5052809
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  15
  Measurements of Methane Emissions from Natural Gas Gathering Facilities and Processing Plants: Measurement Results
  Mitchell, Austin L.; Tkacik, Daniel S.; Roscioli, Joseph R.; Herndon, Scott C.; Yacovitch, Tara I.; Martinez, David M.; Vaughn, Timothy L.; Williams, Laurie L.; Sullivan, Melissa R.; Floerchinger, Cody; Omara, Mark; Subramanian, R.; Zimmerle, Daniel; Marchese, Anthony J.; Robinson, Allen L.
  Environmental Science & Technology (2015), 49 (5), 3219-3227CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Facility-level methane emissions were measured at 114 gathering facilities and 16 processing plants in the United States natural gas system. At gathering facilities, the measured methane emission rates ranged from 0.7 to 700 kg per h (kg/h) (0.6 to 600 std. cfm (scfm)). Normalized emissions (as a % of total methane throughput) were less than 1% for 85 gathering facilities and 19 had normalized emissions less than 0.1%. The range of methane emissions rates for processing plants was 3 to 600 kg/h (3 to 524 scfm), corresponding to normalized methane emissions rates <1% in all cases. The distributions of methane emissions, particularly for gathering facilities, are skewed. For example, 30% of gathering facilities contribute 80% of the total emissions. Normalized emissions rates are neg. correlated with facility throughput. The variation in methane emissions also appears driven by differences between inlet and outlet pressure, as well as venting and leaking equipment. Substantial venting from liqs. storage tanks was obsd. at 20% of gathering facilities. Emissions rates at these facilities were, on av., around four times the rates obsd. at similar facilities without substantial venting.
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16. 16
  Subramanian, R.; Williams, L. L.; Vaughn, T. L.; Zimmerle, D.; Roscioli, J. R.; Herndon, S. C.; Yacovitch, T. I.; Floerchinger, C.; Tkacik, D. S.; Mitchell, A. L.; Sullivan, M. R.; Dallmann, T. R.; Robinson, A. L. Methane emissions from natural gas compressor stations in the transmission and storage sector: measurements and comparisons with the EPA Greenhouse Gas Reporting Program protocol. Environ. Sci. Technol. 2015, 49, 3252– 3261, DOI: 10.1021/es5060258
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  16
  Methane Emissions from Natural Gas Compressor Stations in the Transmission and Storage Sector: Measurements and Comparisons with the EPA Greenhouse Gas Reporting Program Protocol
  Subramanian, R.; Williams, Laurie L.; Vaughn, Timothy L.; Zimmerle, Daniel; Roscioli, Joseph R.; Herndon, Scott C.; Yacovitch, Tara I.; Floerchinger, Cody; Tkacik, Daniel S.; Mitchell, Austin L.; Sullivan, Melissa R.; Dallmann, Timothy R.; Robinson, Allen L.
  Environmental Science & Technology (2015), 49 (5), 3252-3261CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Equipment- and site-level methane emissions from 45 compressor stations in the transmission and storage (T&S) sector of the US natural gas system were measured, including 25 sites required to report under the EPA greenhouse gas reporting program (GHGRP). Direct measurements of fugitive and vented sources were combined with AP-42-based exhaust emission factors (for operating reciprocating engines and turbines) to produce a study onsite est. Site-level methane emissions were also concurrently measured with downwind-tracer-flux techniques. At most sites, these two independent ests. agreed within exptl. uncertainty. Site-level methane emissions varied from 2-880 SCFM. Compressor vents, leaky isolation valves, reciprocating engine exhaust, and equipment leaks were major sources, and substantial emissions were obsd. at both operating and standby compressor stations. The site-level methane emission rates were highly skewed; the highest emitting 10% of sites (including two superemitters) contributed 50% of the aggregate methane emissions, while the lowest emitting 50% of sites contributed less than 10% of the aggregate emissions. Excluding the two superemitters, study-av. methane emissions from compressor housings and noncompressor sources are comparable to or lower than the corresponding effective emission factors used in the EPA greenhouse gas inventory. If the two superemitters are included in the anal., then the av. emission factors based on this study could exceed the EPA greenhouse gas inventory emission factors, which highlights the potentially important contribution of superemitters to national emissions. However, quantification of their influence requires knowledge of the magnitude and frequency of superemitters across the entire T&S sector. Only 38% of the methane emissions measured by the comprehensive onsite measurements were reportable under the new EPA GHGRP because of a combination of inaccurate emission factors for leakers and exhaust methane, and various exclusions. The bias is even larger if one accounts for the superemitters, which were not captured by the onsite measurements. The magnitude of the bias varied from site to site by site type and operating state. Therefore, while the GHGRP is a valuable new source of emissions information, care must be taken when incorporating these data into emission inventories. The value of the GHGRP can be increased by requiring more direct measurements of emissions (as opposed to using counts and emission factors), eliminating exclusions such as rod-packing vents on pressurized reciprocating compressors in standby mode under Subpart-W, and using more appropriate emission factors for exhaust methane from reciprocating engines under Subpart-C.
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17. 17
  Marchese, A. J.; Vaughn, T. L.; Zimmerle, D. J.; Martinez, D. M.; Williams, L. L.; Robinson, A. L.; Mitchell, A. L.; Subramanian, R.; Tkacik, D. S.; Roscioli, J. R.; Herndon, S. C. Methane emissions from United States natural gas gathering and processing. Environ. Sci. Technol. 2015, 49, 3219– 3227
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18. 18
  Zimmerle, D. J.; Williams, L. L.; Vaughn, T. L.; Quinn, C.; Subramanian, R.; Duggan, G. P.; Wilson, B.; Opsomer, J. D.; Marchese, A. J.; Martinez, D. M.; Robinson, A. L. Methane emissions from the natural gas transmission and storage system in the United States. Environ. Sci. Technol. 2015, 49, 9374– 9383, DOI: 10.1021/acs.est.5b01669
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  18
  Methane Emissions from the Natural Gas Transmission and Storage System in the United States
  Zimmerle, Daniel J.; Williams, Laurie L.; Vaughn, Timothy L.; Quinn, Casey; Subramanian, R.; Duggan, Gerald P.; Willson, Bryan; Opsomer, Jean D.; Marchese, Anthony J.; Martinez, David M.; Robinson, Allen L.
  Environmental Science & Technology (2015), 49 (15), 9374-9383CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Recent growth in natural gas prodn. and utilization offers potential climate benefits which depend on CH4 (primary natural gas component and greenhouse gas) life cycle emissions. This work estd. CH4 emissions from transmission and storage (T&S) sector of the US natural gas industry using data collected in 2012, including 2292 on-site measurements, addnl. emissions data from 677 facilities, and activity data from 922 facilities. The largest emission sources were fugitive emissions from compressor-related equipment and super-emitter facilities. Total CH4 emissions estd. from the T&S sector was 1503 (1220-1950) Gg/yr (95% confidence interval) vs. the 2012 USEPA Greenhouse Gas Inventory (GHGI) est. of 2071 (1680-2690) Gg/yr. While the overlap in confidence intervals indicated the difference is not statistically significant, this is due to several significant, but offsetting, factors. Factors which reduce the study est. include: a lower estd. facility count, a shift away from engines toward lower-emitting turbine and elec. compressor drivers, and redns. in use of gas-driven pneumatic devices. Factors which increase the study est. relative to the GHGI include: updated emission rates in certain emission categories and explicit treatment of skewed emissions at component and facility levels. For T&S stations required to report to the EPA Greenhouse Gas Reporting Program (GHGRP), this study estd. total emissions to be 260% (215-330%) of reportable emissions for these stations, primarily due to inclusion of emission sources not reported under GHGRP rules, updated emission factors, and super-emitter emissions.
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19. 19
  Caulton, D. R.; Shepson, P. B.; Santoro, R. L.; Sparks, J. P.; Howarth, R. W.; Ingraffea, A. R.; Cambaliza, M. O. L.; Sweeney, C.; Karion, A.; Davis, K. J.; Stirm, B. H.; Montzka, S. A.; Miller, B. R. Toward a better understanding and quantification of methane emissions from shale gas development. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 6237– 6242, DOI: 10.1073/pnas.1316546111
  [Crossref], [PubMed], [CAS], Google Scholar
  19
  Toward a better understanding and quantification of methane emissions from shale gas development
  Caulton, Dana R.; Shepson, Paul B.; Santoro, Renee L.; Sparks, Jed P.; Howarth, Robert W.; Ingraffea, Anthony R.; Cambaliza, Maria O. L.; Sweeney, Colm; Karion, Anna; Davis, Kenneth J.; Stirm, Brian H.; Montzka, Stephen A.; Miller, Ben R.
  Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (17), 6237-6242CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Identifying and quantifying CH4 emissions from natural gas prodn. is increasingly important due to the increase in the natural gas component of the energy sector. An instrumented aircraft platform identified large CH4 sources and quantified emission rates in southwestern Pennsylvania in June 2012. A large regional flux, 2.0-14 g CH4/s-km2, was quantified for a ∼2800 km2 area, which did not differ statistically from a bottom-up inventory, 2.3-4.6 g CH4/s-km2. Large emissions averaging 34 g CH4/s-well were obsd. at 7 well pads detd. to be in the drilling phase; 2-3 orders of magnitude greater than USEPA ests. for this operational phase. Emissions from these well pads, representing ∼1% of the total no. of wells, accounted for 4-30% of the obsd. regional flux. More work is needed to det. all CH4 emission sources from natural gas prodn., ascertain why these emissions occur, and evaluate their climate and atm. chem. impacts.
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20. 20
  Petron, G.; Karion, A.; Sweeney, C.; Miller, B. R.; Montzka, S. A.; Frost, G. J.; Trainer, M.; Tans, P.; Andrew, A.; Kofler, J.; Helmig, D.; Guenther, D.; Dlugokencky, E.; Lang, P.; Newberger, T.; Wolter, S.; Hall, B.; Novelli, P.; Brewer, A.; Conley, S.; Hardesty, M.; Banta, R.; White, A.; Noone, D.; Wolfe, D.; Schnell, R. A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver-Julesburg Basin: hydrocarbon emissions in oil & gas basin. J. Geophys. Res. Atmospheres 2014, 119, 6836– 6852, DOI: 10.1002/2013JD021272
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  20
  A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver-Julesburg Basin
  Petron, Gabrielle; Karion, Anna; Sweeney, Colm; Miller, Benjamin R.; Montzka, Stephen A.; Frost, Gregory J.; Trainer, Michael; Tans, Pieter; Andrews, Arlyn; Kofler, Jonathan; Helmig, Detlev; Guenther, Douglas; Dlugokencky, Ed; Lang, Patricia; Newberger, Tim; Wolter, Sonja; Hall, Bradley; Novelli, Paul; Brewer, Alan; Conley, Stephen; Hardesty, Mike; Banta, Robert; White, Allen; Noone, David; Wolfe, Dan; Schnell, Russ
  Journal of Geophysical Research: Atmospheres (2014), 119 (11), 6836-6852CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
  Emissions of methane (CH4) from oil and natural gas (O&G) operations in the most densely drilled area of the Denver-Julesburg Basin in Weld County located in northeastern Colorado are estd. for 2 days in May 2012 using aircraft-based CH4 observations and planetary boundary layer height and ground-based wind profile measurements. Total top-down CH4 emission ests. are 25.8 ± 8.4 and 26.2 ± 10.7 t CH4/h for the 29 and 31 May flights, resp. Using inventory data, we est. the total emissions of CH4 from non-O&G gas-related sources at 7.1 ± 1.7 and 6.3 ± 1.0 t CH4/h for these 2 days. The difference in emissions is attributed to O&G sources in the study region, and their total emission is on av. 19.3 ± 6.9 t/h, close to 3 times higher than an hourly emission est. based on Environmental Protection Agency's Greenhouse Gas Reporting Program data for 2012. We derive top-down emissions ests. for propane, n-butane, i-pentane, n-pentane, and benzene from our total top-down CH4 emission est. and the relative hydrocarbon abundances in aircraft-based discrete air samples. Emissions for these five nonmethane hydrocarbons alone total 25.4 ± 8.2 t/h. Assuming that these emissions are solely originating from O&G-related activities in the study region, our results show that the state inventory for total volatile org. compds. emitted by O&G activities is at least a factor of 2 too low for May 2012. Our top-down emission est. of benzene emissions from O&G operations is 173 ± 64 kg/h, or 7 times larger than in the state inventory.
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21. 21
  Karion, A.; Sweeney, C.; Petron, G.; Frost, G.; Hardesty, R. M.; Kofler, J.; Miller, B. R.; Newberger, T.; Wolter, S.; Banta, R.; Brewer, A.; Dlugokencky, E.; Lang, P.; Montzka, S. A.; Schnell, R.; Tans, P.; Trainer, M.; Zamora, R.; Conley, S. Methane emissions estimate from airborne measurements over a western United States natural gas field. J. Geophys. Res. Lett. 2013, 40, 4393– 4397, DOI: 10.1002/grl.50811
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22. 22
  Karion, A.; Sweeney, C.; Kort, E. A.; Shepson, P. B.; Brewer, A.; Cambaliza, M.; Conley, S. A.; Davis, K.; Deng, A.; Hardesty, M.; Herndon, S. C.; Lauvaux, T.; Lavoie, T.; Lyon, D.; Newberger, T.; Petron, P.; Rella, C.; Smith, M.; Wolter, S.; Yacovitch, T. I.; Tans, P. Aircraft-based estimate of total methane emissions from the Barnett Shale region. Environ. Sci. Technol. 2015, 49, 8124– 8131, DOI: 10.1021/acs.est.5b00217
  [ACS Full Text ], [CAS], Google Scholar
  22
  Aircraft-Based Estimate of Total Methane Emissions from the Barnett Shale Region
  Karion, Anna; Sweeney, Colm; Kort, Eric A.; Shepson, Paul B.; Brewer, Alan; Cambaliza, Maria; Conley, Stephen A.; Davis, Ken; Deng, Aijun; Hardesty, Mike; Herndon, Scott C.; Lauvaux, Thomas; Lavoie, Tegan; Lyon, David; Newberger, Tim; Petron, Gabrielle; Rella, Chris; Smith, Mackenzie; Wolter, Sonja; Yacovitch, Tara I.; Tans, Pieter
  Environmental Science & Technology (2015), 49 (13), 8124-8131CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  Estd. regional CH4 emissions from oil and natural gas operations in the Barnett Shale, Texas, using airborne atm. measurements are presented. Using a mass balance approach on 8 different flight days in March and Oct. 2013, total regional CH4 emissions were estd. to be 76 ± 13 × 103 kg/h (equiv. to 0.66 ± 0.11 Tg CH4/yr; 95% confidence interval [CI]). It was estd. that 60 ± 11 × 103 kg CH4/h (95% CI) are emitted by natural gas and oil operations: prodn., processing, and distribution, in the Dallas and Fort Worth urban areas. This est. agreed with the USEPA est. for nation-wide CH4 emissions from the natural gas sector when scaled by natural gas prodn.; it was higher than emissions reported by the EDGAR inventory or by industry to the EPA Greenhouse Gas Reporting Program. This work was the first to show consistency between mass balance results on so many different days and in 2 different seasons, enabling better quantification of related uncertainty. The Barnett is one of the largest prodn. basins in the US with 8% of total US natural gas prodn.; hence, these results represent a crucial step toward detg. the US natural gas prodn. greenhouse gas footprint and are a crucial step in detg. same for on-shore natural gas prodn. in US.
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23. 23
  Peischl, J.; Ryerson, T. B.; Aikin, K. C.; de Gouw, J. A.; Gilman, J. B.; Holloway, J. S.; Lerner, B. M.; Nadkarni, R.; Neuman, J. A.; Nowak, J. B.; Trainer, M.; Warneke, C.; Parrish, D. D. Quantifying atmospheric methane emissions from Haynesville, Fayetteville, and northeastern Marcellus shale gas production regions. J. Geophys. Res. Atmospheres 2015, 120, 2119– 2139, DOI: 10.1002/2014JD022697
  [Crossref], [CAS], Google Scholar
  23
  Quantifying atmospheric methane emissions from the Haynesville, Fayetteville, and northeastern Marcellus shale gas production regions
  Peischl, J.; Ryerson, T. B.; Aikin, K. C.; de Gouw, J. A.; Gilman, J. B.; Holloway, J. S.; Lerner, B. M.; Nadkarni, R.; Neuman, J. A.; Nowak, J. B.; Trainer, M.; Warneke, C.; Parrish, D. D.
  Journal of Geophysical Research: Atmospheres (2015), 120 (5), 2119-2139CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
  We present measurements of methane (CH4) taken aboard a NOAA WP-3D research aircraft in 2013 over the Haynesville shale region in eastern Texas/northwestern Louisiana, the Fayetteville shale region in Arkansas, and the northeastern Pennsylvania portion of the Marcellus shale region, which accounted for the majority of Marcellus shale gas prodn. that year. We calc. emission rates from the horizontal CH4 flux in the planetary boundary layer downwind of each region after subtracting the CH4 flux entering the region upwind. We find 1 day CH4 emissions of (8.0 ± 2.7) × 107 g/h from the Haynesville region, (3.9 ± 1.8) × 107 g/h from the Fayetteville region, and (1.5 ± 0.6) × 107 g/h from the Marcellus region in northeastern Pennsylvania. Finally, we compare the CH4 emissions to the total vol. of natural gas extd. from each region to derive a loss rate from prodn. operations of 1.0-2.1% from the Haynesville region, 1.0-2.8% from the Fayetteville region, and 0.18-0.41% from the Marcellus region in northeastern Pennsylvania. The climate impact of CH4 loss from shale gas prodn. depends upon the total leakage from all prodn. regions. The regions investigated in this work represented over half of the U.S. shale gas prodn. in 2013, and we find generally lower loss rates than those reported in earlier studies of regions that made smaller contributions to total prodn. Hence, the national av. CH4 loss rate from shale gas prodn. may be lower than values extrapolated from the earlier studies.
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24. 24
  Peischl, J.; Karion, A.; Sweeney, C.; Kort, E. A.; Smith, M. L.; Brandt, A. R.; Yeskoo, T.; Aikin, K. C.; Conley, S. A.; Gvakharia, A.; Trainer, M.; Wolter, S.; Ryerson, T. B. Quantifying atmospheric methane emissions from oil and natural gas production in the Bakken shale region of North Dakota. J. Geophys. Res. Atmospheres 2016, 121, 6101– 6111, DOI: 10.1002/2015JD024631
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25. 25
  Lyon, D. R.; Zavala-Araiza, D.; Alvarez, R. A.; Harris, R.; Palacios, V.; Lan, X.; Talbot, R.; Lavoie, T.; Shepson, P.; Yacovitch, T. I.; Herndon, S. C.; Marchese, A. J.; Zimmerle, D.; Robinson, A. L.; Hamburg, S. P. Constructing a spatially resolved methane emission inventory for the Barnett Shale region. Environ. Sci. Technol. 2015, 49, 8147– 8157, DOI: 10.1021/es506359c
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  25
  Constructing a Spatially Resolved Methane Emission Inventory for the Barnett Shale Region
  Lyon, David R.; Zavala-Araiza, Daniel; Alvarez, Ramon A.; Harriss, Robert; Palacios, Virginia; Lan, Xin; Talbot, Robert; Lavoie, Tegan; Shepson, Paul; Yacovitch, Tara I.; Herndon, Scott C.; Marchese, Anthony J.; Zimmerle, Daniel; Robinson, Allen L.; Hamburg, Steven P.
  Environmental Science & Technology (2015), 49 (13), 8147-8157CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
  CH4 emissions from the oil and gas industry (O&G) and other sources in the Barnett Shale region (Texas) were estd. by developing a spatially resolved emission inventory. In total, 18 source categories were estd. using multiple datasets, including empirical measurements at regional O&G sites and a national study of collecting/processing facilities. Spatially referenced activity data were compiled from federal and state databases and combined with O&G facility emission factors calcd. by Monte Carlo simulations which accounted for high emission sites representing the very upper portion, or fat-tail, of obsd. emissions distributions. Total CH4 emissions in the 25-county Barnett Shale region in Oct. 2013 were estd. to be 72,300 (63,400-82,400) kg CH4/h. O&G emissions were estd. to be 46,200 (40,000-54,100) kg CH4/h; 19% of emissions from fat-tail sites represented <2% of sites. Estd. O&G emissions in the Barnett Shale region were higher than alternative inventories based on the USEPA Greenhouse Gas Inventory, EPA Greenhouse Gas Reporting Program, and Emissions Database for Global Atm. Research by factors of 1.5, 2.7, and 4.3, resp. Collecting compressor sites, accounting for 40% of O&G emissions in this inventory, had the largest difference from emission ests. based on EPA data sources. This inventory higher O&G emissions est. was due primarily to its more comprehensive activity factors and inclusion of fat-tail sites.
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Methane Emissions from Natural Gas Production Sites in the United States: Data Synthesis and National Estimate

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Abstract

Introduction

Figure 1

Materials and Methods

Overview of Site-Level Measurement Data

National Activity Data

Extrapolation of Measured Site-Level Emissions to Total Population of Sites

Two-Sample Kolmogorov–Smirnov Tests

Results and Discussion

Variability in Empirical Site-Level and Basin-Level Methane Emission Rates

Comparison of Empirical Site-Level Methane Emissions Distributions among Basins

Figure 2

Relationship between Measured Site-Level Methane Emissions and NG Production

Figure 3

Influence of High-Emitting Sites on Total Methane Emissions

Figure 4

Distribution of Methane Emissions among Natural Gas Production Sites

Figure 5

Spatial Distribution of Methane Emissions

Figure 6

Comparison with Previous Literature Estimates

Supporting Information

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Author Information

Acknowledgments

References

Cited By

Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

References

Supporting Information

Supporting Information

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