April 2019
Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2017:
Updates to Natural Gas Gathering & Boosting Pipeline Emissions
This memorandum documents the updates implemented in EPA's final 2019 Inventory of U.S. Greenhouse Gas
Emissions and Sinks (GHGI) for natural gas gathering and boosting (G&B) emissions. Specifically, an updated
emissions calculation methodology was implemented for gathering pipeline leaks and gathering pipeline
blowdowns.
Additional considerations for the G&B segment, including G&B stations, were previously discussed in memoranda
released in June 2018 (Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2017: Updates Under
Consideration for Incorporating GHGRP Data) and October 2018 (Inventory of U.S. Greenhouse Gas Emissions and
Sinks 1990-2017: Updates Under Consideration for Natural Gas Gathering & Boosting Emissions).1 During the
stakeholder process for developing the 2019 GHGI, stakeholders supported making updates to the gathering
pipeline calculation methodology using Greenhouse Gas Reporting Program (GHGRP) data. Stakeholders did not
support updates to the G&B station calculation methodology at this time, instead requesting that EPA continue to
review GHGRP data and wait for additional research findings to become available. EPA did not update the G&B
station methodology as a result, and will instead consider updates to G&B station emissions for future GHGIs. The
stakeholder feedback for G&B is summarized in Section 5.
1 Available GHGRP Data
This section summarizes the GHGRP data that EPA reviewed to develop the approach implemented in the 2019
final GHGI for gathering pipeline leaks and blowdowns.
Subpart W of the EPA's GHGRP collects annual activity and emissions data on numerous sources from onshore
natural gas and petroleum systems that meet a reporting threshold of 25,000 metric tons of C02 equivalent (mt
C02e) emissions. Facilities that meet the subpart W reporting threshold have been reporting since reporting year
(RY) 2011; however, certain sources including G&B facilities were first required to be reported in RY2016. Subpart
W activity and emissions data are currently used in the GHGI to calculate CH4 and C02 emissions for many
production, processing, and transmission and storage sources.
Subpart W specifies facility definitions specific to certain segments. Onshore production and G&B facilities in
subpart W are each defined as a unique combination of operator and basin of operation. Therefore, subpart W
does not delineate data for G&B stations versus pipelines. However, the data are reported on an emission source
level, so each source can be assigned as likely occurring at either G&B stations or pipelines. For the analyses in this
memo, certain sources were assumed to be associated with gathering pipelines. Blowdown vent stacks from the
"pipeline venting" emission source were assigned to gathering pipelines, and all other blowdown venting data
were assigned to G&B stations. For equipment leaks, data for pipelines (cast iron, plastic/composite, protected
steel, and unprotected steel gathering pipelines) were assigned to gathering pipelines, and all other equipment
leak data were assigned to G&B stations.
The GHGRP data used in the analyses discussed in this memo are those reported to the EPA as of August 19, 2018.
Stakeholders have suggested additional or alternate uses of GHGRP data, such as for certain sources using
measurement data only. Stakeholders have also suggested modifications to the reported GHGRP data for use in
the GHGI, such as through removal of stakeholder-identified outliers. In the current GHGI, EPA uses the publicly
available GHGRP data set without modification for the GHGI, to ensure transparency and reproducibility of GHGI
estimates. Prior to public release of the GHGRP data, the EPA has a multi-step data verification process for the
1 EPA memoranda for the 1990 to 2017 Inventory stakeholder process are available at < https://www.epa.gov/ghgemissions/stakeholder-
process-natural-gas-and-petroleum-systems-1990-2017-inventory>.
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data, including automatic checks during data-entry, statistical analyses on completed reports, and staff review of
the reported data. Based on the results of the verification process, the EPA follows up with facilities to resolve
identified potential issues before public release.
2	Previous GHGI Methodology for Gathering Pipelines
While EPA made updates to the G&B segment methodology to incorporate recent study data for G&B stations in
the 2016 GHGI, the methodology for gathering pipelines has been unchanged in recent years. The GHGI estimates
gathering pipeline mileage as the total producing gas wells in a given year, multiplied by a factor of pipeline miles
per well from the GRI/EPA 1996 study, plus an assumed 82,600 miles of gathering pipeline owned by transmission
companies (per GRI/EPA 1996). The pipeline leakage, pipeline blowdown, and pipeline mishap CH4 EFs are also
obtained from the 1996 GRI/EPA study. The 2018 (previous) GHGI estimated C02 emissions from gathering
pipelines using C02 EFs developed by applying a default production segment ratio of C02-to-CH4 gas content.
3	Review of GHGRP Data for Gathering Pipelines
Table 1 and Table 2 compare the reported subpart W gathering pipeline source-specific emissions and activity
(pipeline miles) to the 2018 GHGI emissions and pipeline miles, for years 2016 and 2017. Appendix A documents
the subpart W calculation methodologies for each source.
Table 1. Gathering Pipeline Leak Emissions and Mileage Data from Subpart W and National Totals
from 2018 GHGI, for Years 2016 and 2017

2016
2017
Emission Source
Total CH4
Total CO2
Pipeline
Miles
Total CH4
Total CO2
Pipeline
Miles
Emissions
Emissions
Emissions
Emissions

(mt)
(mt)
(mt)
(mt)
Subpart W Equipment Leaks
136,776
15,821
404,639
141,577
16,307
407,254
Cast iron gathering pipeline
1,266
25
307
1,723
517
533
Plastic/composite gathering pipeline
24,135
1,265
78,119
30,675
1,369
96,073
Protected steel gathering pipeline
18,438
938
280,795
17,024
959
265,611
Unprotected steel gathering pipeline
92,937
13,593
45,418
92,155
13,462
45,037
2018 GHGI Pipeline Leaks
157,798
18,820
398,554
n/a
n/a - Not applicable.
Table 2. Gathering Pipeline Blowdown Emissions from Subpart W and National Totals from 2018
(Previous) GHGI, for Years 2016 and 2017

2016
2017
Emission Source
Total CH4
Total CO2
Total CH4
Total CO2

Emissions (mt)
Emissions (mt)
Emissions (mt)
Emissions (mt)
Subpart W Blowdown Vent Stacks3
14,713
801
19,777
718
2018 GHGI Venting Sources
3,874
462


Pipeline blowdowns
2,513
300
n/a
Pipeline mishaps
1,360
162


n/a - Not applicable.
a - Includes blowdown emissions reported by G&B facilities for pipeline venting.
EPA calculated EFs from the subpart W and GHGI data by dividing the total reported emissions from Table 1 and
Table 2 by the total reported miles shown in Table 1. Table 3 and Table 4 compare the resulting EFs calculated
from the subpart W data to those used in the 2018 GHGI.
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Table 3. Gathering Pipeline Leak EFs Calculated from Subpart W and 2018 GHGI
Data Source
CH4 ef
(kg/mile)
C02 EF
(kg/mile)
Subpart W - Year 2016
338
39
Subpart W - Year 2017
348
40
2018 GHGIa
396
47
a - The 2018 GHGI uses specific EFs for each NEMS region, which are
adjusted for methane content. This table presents calculated EFs
which represent the national average.
Table 4. Gathering Pipeline Blowdown EFs Calculated from Subpart W and 2018 GHGI

CH4 EF
CO2 EF
Data Source
(kg/mile)
(kg/mile)
Subpart W - Year 2016
36
2
Subpart W - Year 2017
49
2
2018 GHGIa
10
1
a - The 2018 GHGI uses specific EFs for each NEMS region, which are
adjusted for methane content. This table presents calculated EFs
that represent the national average.
EPA also considered how to evaluate the subpart W reporting coverage in terms of activity (pipeline miles). As
seen in Table 1, the gathering pipeline miles reported to subpart W exceed the estimated national miles from the
previous GHGI for year 2016. PHMSA collects data for "regulated gathering lines," but this is a small subset of the
total (11,494 miles were reported for 20162). PHMSA does have a proposed rule, however, that would collect
gathering line data, but it is not final and data are not available.3 Year 2015 gathering pipeline miles were
estimated for the proposed rule by PHMSA (355,509 miles) and industry (399,579 miles), and so while the
estimates are based on more recent data than the previous GHGI and are of similar magnitude, the estimates are
still lower than the reported subpart W miles. EPA is maintaining an approach to estimate gathering pipeline
emissions that relies on total national miles, and based on the available data, the subpart W data provides the
most complete estimate for recent years and will be used as-is (i.e., the subpart W gathering pipeline miles will
not be scaled up and will represent the national total for 2016 and forward). National miles from PHMSA may be
available in the future, and EPA will compare that data to the subpart W data should it become available.
4	Time Series Considerations for Gathering Pipelines
The EPA also considered temporal variability, and ways to reflect emissions changes over the time series.
However, limited historical data are available for gathering pipelines. Subpart W data are only available for two
years (2016 and 2017), and the previous GHGI approach and other recent studies only examined data at a single
point in time. The previous GHGI methodology applied the same EFs for all years of the time series, and the
activity data vary with changes in gas production or gas wells (which is used to drive estimates of pipeline
mileage).
5	Updated Methodology and National Emissions Estimates for Gathering
Pipelines in the 2019 GHGI
For the gathering pipelines updates implemented in the 2019 GHGI, EPA applied the subpart W year-specific EFs
for pipeline leaks and pipeline blowdowns (as shown in Table 3 and Table 4) for year 2016 and 2017, and then
applied the year 2016 EFs to all prior years of the time series. EPA also maintained the GHGI methodology to
2	https://cms.phmsa.dot.gov/data-and-statistics/pipeline/annual-report-mileage-natural-gas-transmission-gathering-systems
3	See docket PHMSA-2011-0023 at regulations.gov.
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calculate pipeline miles for 1990-2015, and then used the reported subpart W miles to represent the national
total for 2016 and 2017. Using the reported subpart W miles as-is results in the national emissions equaling the
reported subpart W emissions for 2016 and 2017. The use of subpart W data allows the GHGI to reflect trends, as
year-specific subpart W EFs and pipeline miles will be applied for 2016 and forward.
Table 5 shows national total CH4 and C02 emissions for 2016 and 2017 based on the update described above for
gathering pipelines compared to previous GHGI estimates (also shown in Table 1 and Table 2). The subpart W-
based update implemented for the 2019 GHGI has a similar magnitude of emissions compared to the 2018 GHGI
emissions.
Table 5. Comparison of National-Level CH4 and C02 Emissions Estimates for Gathering Pipeline
Emissions, for Year 2016 and 2017
Emissions Source/Approach
2016
2017
CH4 (mt)
CO2 (mt)
CH4 (mt)
CO2 (mt)
Gathering Pipeline Leaks
2019 GHGI (Subpart W-based Update3)
136,776
15,821
141,577
16,307
2018 (Previous) GHGI
157,798
18,820
n/a
n/a
Gathering Pipeline Blowdowns
2019 GHGI (Subpart W-based Update3)
14,713
801
19,777
718
2018 (Previous) GHGI
3,874
462
n/a
n/a
n/a - Not applicable.
a - Estimates use subpart W data-as reported.
6 Requests for Stakeholder Feedback
EPA sought stakeholder feedback on the approaches under consideration discussed in the June and October 2018
memos, including the particular questions below. In response to those memos and public review draft emissions
for the 2019 GHGI, stakeholders provided specific feedback on the G&B station and gathering pipeline
approaches, and their feedback is summarized here:
•	A stakeholder supported the use of subpart W data for gathering pipelines, including showing the
emissions for each pipeline type. The stakeholder did not support using the mileage estimate from the
proposed PHMSA rule, but did recommend comparing the subpart W mileage to the PHMSA data once
PHMSA begins collecting this data.
•	Stakeholders did not support the proposed approach of using component-level data reported under
subpart W for G&B stations. Instead, stakeholders supported the current GHGI approach, which relies on
the station-level EF from Marchese et al. 2015. Stakeholders recommended the following to update the
G&B station methodology:
o Wait for results from additional research on G&B station emissions.
o Scale up component-level data from GHGRP to the Marchese et al. 2015 station-level estimates or
using the component-level data from GHGRP but adding an uncategorized source of emissions
that makes up the difference between Marchese et al. and the GHGRP.
o Apply the G&B station facility-level EF presented in an Alvarez et al. 2018 study to better account
for high emitting sites.
G&B Segment-Specific
1. What data source(s) and methodology are most appropriate to develop national G&B station and pipeline
emissions (both steady-state and episodic) in light of newly available data (GHGRP subpart W and peer-
reviewed studies)?
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a.	EPA seeks feedback on whether additional data sources or methods should be considered for
specific equipment types for gathering stations (e.g. compressors).
b.	What other new or upcoming studies might provide useful data to consider for the GHGI, to use
as a quality check against GHGRP-based estimates, and/or to supplement GHGRP data? For
example, EPA is aware of several DOE-funded field studies being conducted by researchers
including GSI Environmental, Inc., Utah State University, Colorado State University, and Houston
Advanced Research Center; focused on topics such as component-specific measurements to
develop gathering compressor emission factors4; developing nationally representative emission
factors for equipment at G&B stations5; and methane emissions rate quantification for natural gas
storage wells and fields6.
2.	For subpart W, which reported G&B activity data elements should be evaluated to assess the fraction of
national activity represented in the reporting data (for considerations toward developing appropriate
emissions factors that can be combined with available national-level activity data to develop national
emission estimates for the GHGI)?
a.	Does the fraction of national activity represented in subpart W vary by equipment type due to the
G&B facility definition (e.g., is it possible that close to 100% of G&B pipeline mileage is
represented, but equipment such as G&B compressors or G&B tanks have different coverage)?
b.	EPA seeks feedback on data sources that provide national-level totals for purposes of considering
G&B scaling approaches (e.g., while total gathering pipeline mileage is reported to GHGRP,
PHMSA only reports gathering miles for "regulated gathering lines," which is a small subset of the
total).
3.	In addition to reciprocating compressors, are there other specific G&B emission sources that EPA should
examine to assess the difference between the subpart W-based estimates and the current GHGI
estimates, for example episodic events (blowdowns)?
4.	For G&B reciprocating compressor seal and valve leakage emissions, is the GHGI EF for gas processing or
transmission reciprocating compressors more appropriate for calculating emissions from G&B in the GHGI
than the current subpart W EF? EPA seeks feedback on the considerations and approaches discussed in
Section 2.3.2.1 (of the Oct. 2018 memo) for this source (wherein the GHGI gas processing or transmission
segment EF is used as a surrogate), or other methodologies to consider.
5.	EPA is considering using the current GHGI EF for processing or transmission to calculate G&B reciprocating
compressor exhaust emissions (refer to Section 2.3.2.2 (of the Oct. 2018 memo) for additional detail). EPA
seeks feedback on the approach discussed in Section 2.3.2.2 (of the Oct. 2018 memo) wherein it is
assumed that reported emissions from reciprocating compressor drivers fueled by natural gas indicate
use of the subpart C EF. EPA acknowledges a limitation in this approach: that the subpart C EF would only
have been applied for use of pipeline quality natural gas—but believes the potential over-estimate of
activity due to this limitation is minimal, based on the similarity in the ratios of C02 to CH4 between
reported emissions and the subpart C prescribed EFs.
6.	EPA seeks feedback on how to consider regional and temporal variability for G&B stations.
a. Specifically, EPA seeks feedback on the detailed basin-level approach for scaling and estimating
emissions at the basin-level, compared with the simplified scaling approach (which involves
analyzing basin-level throughput to develop a national scaling factor) as discussed in Section 2.3.1
4	https://www.netl.doe.gov/research/oil-and-gas/project-summaries/natural-gas-midstream-projects/fe0029084-gsi
5	https://www.netl.doe.gov/research/oil-and-gas/project-summaries/natural-gas-midstream-projects/fe0029068-csu
6	https://www.netl.doe.gov/research/oil-and-gas/project-summaries/natural-gas-midstream-projects/fe0029085-gsi
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(of the Oct. 2018 memo). Note that national estimates presented in Section 5 and Appendix C (of
the Oct. 2018 memo) were developed using the detailed basin-level approach,
b. Specifically, EPA seeks feedback on an activity data element and data source that is appropriate to
estimate emissions in time series years before subpart W data are available for the G&B segment
(i.e., 1990 through 2015). EPA seeks feedback on the use of basin-level Drillinglnfo well
production data, described in Section 4 (of the Oct. 2018 memo) and used to develop time series
estimates presented in Appendix C (of the Oct. 2018 memo).
7.	EPA seeks feedback on which of Scenarios 1, 2, and 3 presented in Section 5 (of the Oct. 2018 memo) for
calculating national G&B station emissions is most appropriate for incorporation in the GHGI—or another
approach that addresses considerations discussed in Section 2 (of the Oct. 2018 memo). Note that all
three scenarios presented use the detailed basin-level approach for scaling and estimating emissions at
the basin-level, but EPA is also considering using a simplified scaling approach (refer to Question #6)
which could be combined with other elements of these three scenarios. For example, EPA might use a
simplified scaling approach to scale up reported subpart W station emissions by a factor of 1.17 combined
with surrogate EF methodology for reciprocating compressor sources.
8.	EPA seeks feedback on the most appropriate EF to use over the time series for G&B pipelines. Table 3 and
Table 4 compare the previous GHGI EFs and the subpart W EFs. Because the EFs are similar, EPA applied
the subpart W EFs to all years of the time series for the update implemented for the 2019 GHGI. However,
EPA could apply the previous GHGI EF for early time series year, apply the subpart W EF to recent time
series years, and interpolate between the two EFs for intermediate years.
9.	EPA seeks feedback on how to consider the subpart W definition of the G&B segment which includes
equipment that serves more than one well pad (e.g., tank batteries) that might generally be considered
production equipment. EPA notes that the GHGI approach for developing activity estimates for the
production segment relies on data from production segment facilities that report under subpart W, so
incorporating data from the subpart W G&B segment facilities should theoretically avoid double-counting.
10.	EPA seeks feedback on the level of detail for presenting emissions from gathering and boosting in the
GHGI. For example, emissions could be presented by equipment type (similar to how other production
segment equipment emissions are presented) or could be presented at the station-level (as in the current
GHGI) or at the basin level.
General (might impact other GHGI segments)
11.	Stakeholders have suggested that CH4 emissions from compressor exhaust in the GHGI (which are
currently based on the 1996 GRI/EPA EFs for all segments except G&B, which uses a facility-level
measurement) might be improved by developing activity data and EFs specific to rich burn versus lean
burn modes of operation and by reflecting control technologies. The current GHGI CH4 EFs for compressor
exhaust were originally developed in the 1996 GRI/EPA study from an industry survey that weighted
various operating characteristics of compressors to develop average EFs representative of the natural gas
value chain. EPA is evaluating available data (e.g., from GHGRP, AP-42, EPA's Nonpoint Oil and Gas
Emission Estimation Tool (NEI O&G Tool), background analyses for engine NSPS and NESHAP rules) to
consider developing a revised methodology that reflects the fraction of reciprocating compressors that
are rich burn versus lean burn and use of control technologies. EPA seeks stakeholder input on specific
data sources that distinguish the prevalence of rich versus lean burn and controlled versus uncontrolled
engines—for each industry segment, and across the time series.
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12. EPA seeks feedback or suggestions on the general approach for incorporating GHGRP data into recently
updated GHGI estimates, which has been:
•	Apply existing historical EFs and AFs (e.g., control category splits) for early time series years
•	Apply GHGRP-based EFs and AFs for GHGRP years
•	Develop intermediate EFs and AFs through linear interpolation
•	Apply a basin-level approach for sources with large regional variability and where national-level
emissions estimates are impacted by a basin-level versus national level approach (e.g., associated
gas venting and flaring, miscellaneous production flaring)
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Appendix A - GHGRP Measurement Methodologies
Emission Source Measurement and/or Calculation Type | # Sources
Location & Representativeness
EF Calculation Method
GHGRP Subpart W
G&B Equipment Leaks
Emissions calculated using: (1) default EFs,
by source type; (2) source type counts (rule
provides default counts e.g., valves per
wellhead) including miles of gathering
pipelines by material type; (3) estimated
time the source was operational; and (4)
concentration of C02 and CH4.
Emissions data (for 2016) are
available from 297 facilities.
Facilities in the U.S. that exceed
25,000 mt C02e reporting
threshold.
Reported emissions divided by
reported pipeline miles.
G&B - Blowdown Vent Stacks
Emissions calculated from the available
methods: (1) use blowdown volumes, the
number of blowdowns, and the ideal gas
law modified with a compressibility factor,
or (2) used a flowmeter to directly measure
emissions for each equipment type or all
equipment associated with a blowdown
event.
Emissions data (for 2016) are
available from 236 facilities.
Facilities in the U.S. that exceed
25,000 mt C02e reporting
threshold.
Reported emissions divided by
reported pipeline miles.
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