October 2022
Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2021: Updates Under
Consideration for Incorporating Additional Geographically Disaggregated Data
1 Introduction
This memo discusses updates under consideration for the 2023 Inventory of U.S. Greenhouse Gas Emissions and
Sinks (GHGI) to develop national emission estimates for certain emissions sources by quantifying emissions for
those sources at the basin level and aggregating those estimates to develop the national emission estimates.
Currently in the GHGI, EPA estimates emissions from most of the emission sources in Natural Gas and Petroleum
Systems at the national-level using emission factors (EFs) and activity data (AD) at the national-level. For
example, for liquids unloading, EPA uses Greenhouse Gas Reporting Program (GHGRP) data to develop average
national activity factors (e.g., fraction of wells conducting liquids unloading) and average national emission
factors (e.g., annual emissions per well that conducts liquids unloading with a plunger lift). These average factors
are then applied to the national well population to estimate national emissions.
Currently, EPA uses a basin-specific aggregation approach for two emission sources (i.e., associated gas venting
and flaring and miscellaneous onshore production flaring).1 For these emission sources, it was determined that
national-level EFs and activity factors (AFs) would not reflect differences in associated gas venting and flaring
among geographic regions and that over- or under-representation in GHGRP data by geographic regions where
associated gas is vented or flared more or less frequently may disproportionately contribute to national-level
factors. For associated gas venting and flaring (Petroleum Systems) and miscellaneous production flaring (both
Petroleum and Natural Gas Systems), EPA calculates basin-specific activity and emission factors for basins that,
in any year from 2011 forward, contributed at least 10 percent of total source emissions (on a C02 equivalents
basis) in the GHGRP data. For associated gas venting and flaring, EPA calculates basin-specific factors for four
basins: Williston, Permian, Gulf Coast, and Anadarko. For miscellaneous production flaring, EPA calculates basin-
specific factors for three basins: Williston, Permian, and Gulf Coast. For these emission sources, data from all
other basins are aggregated, and EPA calculates activity and emission factors for the other basins as a single
group.
In recent years, EPA has developed additional GHG Inventory products that break out emissions from the
national-level into gridded and state-level estimates.
• Gridded Inventory.2 In an effort to improve the ability to compare the national-level Inventory with
measurement results that may be at other spatial and temporal scales, a team at Harvard University
along with EPA and other coauthors developed a gridded inventory of U.S. anthropogenic methane
emissions with 0.1 degree x 0.1 degree spatial resolution, monthly temporal resolution, and detailed
scale-dependent error characterization. The gridded methane inventory is designed to be consistent
with the U.S. EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2014 estimates for the
year 2012, which presents national totals. An updated version of the gridded inventory is being
developed and will improve efforts to compare results of the GHG Inventory with atmospheric studies.
1 EPA 2018. Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2016: Revisions to C02 Emissions Estimation Methodologies.
Available online at: .
2 U.S. EPA. Gridded 2012 Methane Emissions, https://www.epa.gov/ghgemissions/gridded-2012-methane-emissions
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• State Inventory.3 In 2022, EPA released its first annual publication of state greenhouse gas (GHG) data
consistent with theGHGI, meaning state GHG totals when summed, will equal national totals in
the GHGI. For Petroleum and Natural Gas Systems, the methods used to develop state-level estimates
generally rely on relative differences in basic activity levels (e.g., petroleum production), and do not
reflect differences between states due to differences in practices, technologies, or formation types.
Both the gridded and the state versions of the GHGI generally rely on national-level average activity and emission
factors, along with location-specific information on activity drivers such as well counts or production. The update
under consideration discussed in this memo seeks to improve the ability of the gridded and state inventories to
reflect variation due to differences in formation types, technologies and practices, regulations, or voluntary
initiatives, and not only the differences in key activity levels that are reflected in the current gridded and state
inventories.
This memo discusses considerations for developing emissions estimates for the national GHGI using basin-
specific data that are currently aggregated and averaged to develop national-level estimates. In this memo, EPA
evaluates options to incorporate additional basin-level data from GHGRP subpart W in the GHGI. GHGRP subpart
W data are used in the GHGI to calculate numerous EFs and AFs for emission sources across the industry
segments in Natural Gas and Petroleum Systems.
The incorporation of these data would improve future versions of both the gridded and state-level inventories.
This would allow EPA to use the gridded inventory for improved comparisons of the GHGI with various
atmospheric observation studies (since regions will better reflect the local differences in emissions rates as
reported to GHGRP) and would allow the state-level inventory to reflect differences in state-level programs,
formation type mixes, and varying technologies and practices.
For many sources, an approach that develops estimates using geographically disaggregated data may not be
possible or preferable to a national level approach based on the currently available data. For some emission
sources in the GHGI, emission factor data come from research studies and are applied at the national level. For
example, many of the emission factors used to quantify emissions in the GHGI for the gathering and boosting,
transmission and storage, distribution, and post-meter segments are from research studies and do not have a
level of detail or total population comparable to GHGRP. For petroleum refineries, because there is no reporting
threshold for GHGRP subpart Y, facility-level data are generally available for all refineries in the U.S., and these
site-specific data are already used to develop the gridded and state-level GHG estimates.4 Even in cases where
geographically disaggregated data are available, such an approach may not always be preferable. In cases with
limited variation between areas, such an approach would have limited impact on emissions estimates regionally
or nationally. In cases with limited data in certain areas, disaggregated approaches might substantially increase
the uncertainty of estimates and basin-specific calculations would not be an improvement over use of a national
average.
For this memo, EPA focused on the onshore oil and gas production segment, where data are available from the
GHGRP that could be used to reflect distinctions in emissions levels by region, and which could impact (to varying
extents) total emissions in the GHGI.
3 U.S. EPA. State GHG Emissions and Removals, https://www.epa.eov/eheemissions/state-ehe-emissions-and-removals.
4 Some refineries have ceased reporting under GHGRP. A GHGRP facility that has reported total non-biogenic GHG emissions below
15,000 metric tons of carbon dioxide equivalent (mt C02e) for three consecutive years or below 25,000 mt C02e for five consecutive years
can discontinue reporting for all direct emitter subparts. In these cases, in the GHGI estimates for refineries, EPA has used previously
reported data for a proxy for years without reports.
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Key considerations for use of the GHGRP data for onshore production to develop subnational estimates are the
variability of emissions and activity levels between basins and GHGRP coverage of total activity for each basin.
EPA examined subpart W data at the basin-level to assess variability in the data and calculated the coverage of
subpart W data for each basin.
An assessment of data variability between basins is used to identify if for a given emission source there is a
potential impact when using basin-level activity or emissions data. To perform variability assessments, EPA
identified relevant emissions or activity data to compare across basins. Examples of emission sources where
emissions and activity differ greatly between basins are presented in this memo.
GHGRP subpart W reporting coverage (assessed as activity that is included in GHGRP for a basin versus total
activity for that basin) information is useful for assessing representativeness of reported data. Subpart W of the
EPA's GHGRP collects annual activity and emissions data on numerous sources from Natural Gas and Petroleum
Systems that meet a reporting threshold of 25,000 metric tons of C02 equivalent (C02e) emissions. Reporting
requirements under subpart W began in reporting year (RY) 2011 for onshore production. Onshore production
facilities in subpart W are defined as a unique combination of operator and basin of operation (i.e., all operator
sites within a basin). Coverage of onshore production activities varies by basin. EPA is considering a variety of
approaches to take coverage into consideration when evaluating and potentially implementing basin-specific
calculations such as including basin-specific data for all basins regardless of coverage, selecting a coverage
threshold and aggregating basin data from basins with lower coverage and developing a combined AF/EF for
those basins, combining data from neighboring basins, retaining the national level approach, or developing
another approach to rely on a larger (combined) dataset for emissions in those areas. In this memo, several
examples are provided of an approach that would use 50 percent as a coverage threshold for basin-specific
calculations. This coverage threshold was selected only for demonstration purposes. EPA is seeking stakeholder
feedback on approaches for incorporating additional basin-level data including the use of a coverage threshold,
and in case of its use, an appropriate coverage threshold level. Other approaches to address coverage could also
be considered, such as a threshold based on the number of reported wells or number of reporters in each basin.
Coverage data are also needed to determine how to scale emissions and/or activity from reported totals to basin
totals.
For this memo, EPA examined the coverage of RY2020 subpart W data by estimating the percentage of each
basin's oil and gas operations that subpart W data represents for each industry segment. The GHGRP subpart W
data used in the analyses discussed in this memo are for RY2020 reported to the EPA as of August 7, 2021. In
general, the coverage calculations equal subpart W RY2020 activity for a given basin divided by activity from a
national dataset for the year 2020 for the given basin. In most cases, the national dataset is the same data source
used in the GHGI.
GHGRP has proposed revisions to subpart W, some of which could impact sources discussed in this memo. The
general analyses and approaches discussed in this memo would likely be applicable to both current and future
GHGRP data.
The variability and coverage assessments conducted for onshore production are discussed in Section 2, with
additional detail in Appendix A. Time series considerations are discussed in Section 3. Considerations for other
segments are discussed in Section 4, with additional detail in Appendix B. Section 5 presents questions for
stakeholder feedback.
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2 Onshore Production
EPA assessed the three highest emitting sources within the onshore production segment for potential basin-
level approaches. EPA analyzed subpart W basin-level data for pneumatic controllers (45 MMT C02e in the GHGI
for 2020), associated gas flaring (15 MMT C02e in the GHGI in 2020), and well pad equipment (12 MMT C02e,
including well pad equipment leaks and chemical injection pumps).
In addition, EPA assessed basin-level coverage of subpart W onshore production segment data relative to
national data. Enverus data are currently used in the GHGI for national well counts and production volumes.
Subpart W reports contain well counts and oil and gas production volumes for operators that meet the GHGRP
reporting threshold, while Enverus contains well counts and oil and gas production volumes for all operators in
the country. Coverage was determined at the basin level as the percentage of the national dataset included in
subpart W data.
2.1 Variability
EPA reviewed pneumatic controller, associated gas flaring, and well pad equipment data to assess variability in
subpart W data between basins.
2.1.1 Pneumatic Controllers—CH4
In the current GHGI, EPA estimates pneumatic controller emissions using AFs and EFs developed from subpart
W data at the national-level. The AFs are updated annually. Two types of AFs are calculated: (1) average
controllers per well and (2) the fraction of controllers that are low-bleed, intermittent bleed, and high-bleed
controllers. These AFs were calculated separately for gas wells and for oil wells. EFs were calculated for the
different types of controllers using RY2014 data and combining oil and gas. To assess basin variability, EPA
calculated the average controllers per well for each basin separately for gas wells and oil wells, using RY2020
subpart W data. Table 1 presents examples of AFs (i.e., pneumatic controllers per well) across select basins (i.e.,
basins with high well counts) listed in order of well population in Enverus and compares to the current GHGI
estimate.
Table 1. Pneumatic Controllers Per Well for Select Basins (RY2020 Subpart W)
Basin Name
Basin
Controllers/Well
Controllers/Well
Number
(Gas)
(Oil)
Current GHGI3
1.9
1.37
Permian
430
1.35
1.18
Appalachian
160A
1.06
3.34
(Eastern Overthrust)
Appalachian
160
0.43
4.88
Anadarko
360
1.34
2.80
Gulf Coast
220
2.38
2.97
Williston
395
0.14
1.13
a. The current GHGI equals the average of all subpart W data for RY2020.
Compared to the current GHGI AF, most of the basins in Table 1 have a lower AF for gas well controllers and a
higher AF for oil well controllers. Conversely, the Gulf Coast basin AF for gas well controllers is close to or more
than double the AF for other basins while the oil well controller AFs for the two Appalachian basins are also
higher than other basins.
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EPA also calculated basin-level fractions of low-bleed, intermittent bleed, and high-bleed controllers for oil and
gas wells separately. Table 2 presents examples of controller type fractions across select basins and compares
to the current GHGI estimate.
Table 2. Pneumatic Controller Type Fractions for Select Basins (RY2020 Subpart W)
Basin Name
Basin
Number
Bleed Type Fractions (Gas)
Bleed Type Fractions (Oil)
Low
Intermittent
High
Low
Intermittent
High
Current GHGI3
0.26
0.73
0.01
0.29
0.70
0.01
Permian
430
0.13
0.87
0.01
0.37
0.62
0.01
Appalachian
(Eastern Overthrust)
160A
0.16
0.84
0
0.20
0.80
0
Appalachian
160
0.06
0.94
0
0.11
0.89
0
Anadarko
360
0.18
0.74
0.08
0.21
0.74
0.06
Gulf Coast
220
0.19
0.81
0
0.20
0.80
0
Williston
395
0.01
0.97
0.02
0.13
0.86
0.01
a. The current GHGI equals the average of all subpart W data for RY2020.
b. The bleed type fractions don't add up to 1, in some instances, due to rounding.
EPA applies separate pneumatic controller EFs in the current GHGI for low-bleed, intermittent-bleed, and high-
bleed controllers calculated using subpart W RY2014 data. The same EFs are used for both gas wells and oil wells.
As part of its analyses, EPA calculated EFs separately for controllers at gas wells and oil wells using RY2020
subpart W data. The EFs also show differences across basins and for controllers at gas wells versus oil wells. The
complete set of AFs and EFs calculated for all basins are provided in Table A.4 of Appendix A.
EPA is considering several options for the GHGI approach for pneumatic controllers:
1. National-level approaches
Option 1: Maintain current GHGI approach. Use RY2014 EFs, that combine data for controllers at oil
wells and gas wells, with year-specific AFs.
Option 2: Apply year-specific EFs, that combine data for controllers at oil wells and gas wells, at the
national level with year-specific AFs.
2. Basin-level approaches
Option 3: Apply year- and basin-specific AFs and EFs, developed separately for controllers at oil wells
and gas wells, for all basins. Apply national-level average AFs and EFs, developed separately for
controllers at oil wells and gas wells, for basins that have no GHGRP reporting.
Option 4: Apply year- and basin-specific AFs and EFs, developed separately for controllers at oil wells
and gas wells, for the 20 basins with more than 50 percent coverage (as an example threshold) of
total wells and aggregate data from all other basins together. Aggregated AFs and EFs, calculated
using data from those aggregated basins, were also used for basins that have no GHGRP reporting.
Table 3 shows the impact on the overall pneumatic controller emissions for 2020 for each option.
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Table 3. Pneumatic Controller Emissions for Different Options, Year 2020
Option
Gas Well Controller
CH4 Emissions (mt)
Oil Well Controller
CH4 Emissions (mt)
Total Well Controller
CH4 Emissions (mt)
Option 1: Current GHGI
950,718
853,546
1,804,264
Option 2: Updated National-
Level Factors
836,441
705,393
1,541,834
Option 3: Basin-Specific Factors
for All Basins
765,884
1,096,239
1,862,123
Option 4: Basin-Specific Factors
for Basins with >50% Coverage
731,994
968,184
1,700,178
Option 2, using RY2020 data to calculate separate national-level AFs and EFs for oil and gas well pneumatic
controllers, results in lower calculated national emissions for 2020 compared to the current GHGI estimate
(Option 1) calculated using RY2014 EFs. In both approaches that utilize basin-level data (i.e., Option 3 and Option
4), gas well pneumatic controller emissions decrease and oil well pneumatic controller emissions increase,
compared to both national-level approaches (i.e., Option 1 and Option 2). The basin-specific approaches lead to
a decrease in gas well controller emissions because the AFs for certain basins with high activity generally have
lower AFs than the national AFs, and the aggregated AFs are lower than the national AFs. For example, the
Permian basins and the two Appalachian basins have much lower emissions using a basin-specific approach
compared to using national average data. In addition, the Appalachian Eastern Overthrust basin (160A) does not
meet the total wells coverage example threshold of 50%, but accounts for the majority of the aggregated data
because there is much more subpart W data reported for this basin compared to other basins that are below
the threshold. Conversely, the basin-specific approaches lead to an increase in oil well controller emissions
because the AFs for certain basins with high activity generally have higher AFs than the national AFs, and the
aggregated AFs are higher than the national AFs. For example, the Gulf Coast, Anadarko, and the two
Appalachian basins have significantly higher emissions from oil well controllers using the basin-specific approach
compared to using national average data.
The difference in emissions between the two basin-specific approaches (i.e., Option 3 and Option 4) is largely
due to the two Appalachian basins, which have oil well AFs (controllers per well) that are higher than other
basins. Neither Appalachian basin meets the total wells coverage example threshold of 50% and thus basin-
specific calculations for the Appalachian basins are only in Option 3. When Appalachian AFs and EFs from subpart
W are applied to all oil wells in the Appalachian basins, it has a large impact on total calculated national emissions
from oil wells (i.e., emissions increase). The Appalachian basins have a relatively large population of oil wells
owned by facilities that do not report under subpart W. The Appalachian Eastern Overthrust basin (basin 160A)
data reported to subpart W for 2020 includes information from 1,129 oil wells. In the Appalachian basin (basin
160), data reported to subpart W for 2020 includes information from 26 wells. For comparison, national level
activity and emission factors for pneumatic controllers currently used in the GHGI draw on subpart W data from
around 500,000 wells.
2.1.2 Associated Gas Flaring—C02
EPA assessed C02 emission factor variability between basins for associated gas flaring. The current GHGI uses
basin-specific calculations for four basins (i.e., 220 Gulf Coast, 360 Anadarko, 395 Williston, and 430 Permian)
and applies an aggregated approach to the remaining basins. These four basins represent nearly 98 percent of
total oil produced in 2020. For this memo, EPA calculated EFs and AFs for all 16 basins with associated gas
emissions data reported to subpart W in RY2020. Of the 16 basins, ten basins have more than 50 percent
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coverage (example threshold) for oil production. Table 4 presents the flaring C02 EFs, percent of gas flared, and
percent of liquids production with associated gas for these ten basins (listed in ascending order by basin number,
with the four basins used in the current GHGI included first and shown in italics).
Table 4. Associated Gas Flaring C02 EFs and AFs (RY2020 Subpart W)
Basin Name
Basin Number
C02 EF
(scf/bbl)
Percent of
Associated Gas
that is Flared
Percent of
Liquids
Production with
Associated Gas
Gulf Coasf
220
430
98%
7%
Anadarko
360
159
64%
1%
Williston
395
584
100%
58%
Permian
430
203
98%
21%
Michigan
305
1,155
87%
25%
Powder River
515
438
100%
19%
Green River
535
1,432
100%
1%
Denver
540
2,521
100%
0.5%
Uinta
575
986
99%
5%
San Juan
580
22
99%
11%
All Others
1,508
21%
1%
Current GHGI All
Others
737
75%
3%
National Average in
GHGRP
All reporting
397
98%
20%
a. The current GHGI relies on basin-specific calculations for the Anadarko, Gulf Coast, Permian, and Williston
Basins and aggregates data from all other basins. Italicized rows show the input values currently used in
the GHGI.
As seen in Table 4, there is considerable variability between the basins in terms of the EF and the AF for the
percent of liquids production with associated gas. The C02 EFs vary from 22 to 2,521 scf per barrel of liquids
production, while the percent of liquids production with associated gas ranges from 0.5 percent to 58 percent
across the basins.
Next, EPA examined the impact of including additional basin-specific information on national emissions. EPA
calculated associated gas flaring C02 emissions using the following three options:
1. Applying a basin-level approach for all 16 basins with associated gas emissions data reported to subpart
W. For basins with no GHGRP reporting, aggregated EFs and AFs developed for all other basins were
used.
2. Applying a basin level approach for the 10 basins with more than 50 percent of oil production coverage
(example threshold) and that have associated gas emissions data reported to subpart W, and using
aggregated data from all other basins. For basins with no GHGRP reporting, aggregated EF and AF
developed for all other basins were used.
3. Applying a national-level approach.
Table 5 compares associated gas flaring emissions from the current GHGI to the two basin-level approaches and
the national-level approach.
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Table 5. Associated Gas Flaring C02 Emissions for Different Options, Year 2020
Option
Associated Gas Flaring C02
Emissions (mt)
Current GHGI
13,041,364
Option 1: Basin-Specific Factors for All
Basins
13,201,231
Option 2: Basin-Specific Factors for Basins
with >50% Coverage (example threshold)
13,013,920
Option 3: National-Level Factors
14,613,229
After calculating and comparing the emissions for each method of basin selection, the national emissions
estimates exhibited only a small change between the different basin-level options. Furthermore, estimating
basin-specific emissions for basins with lower oil production coverage did not lead to dramatically higher or
lower emissions at the national level. The associated gas venting and flaring emission source is unique in that a
few basins dominate the emissions. As such, it is not unexpected that adjusting the approaches for the other
basins does not significantly change the national emissions because the EPA already accounts for basin-specific
calculations for the basins that account for the majority of the emissions. As discussed in more detail in EPA
2018s, use of a national-level approach for associated gas (Option 3) results in higher calculated emissions.
2.1.3 Other Well Pad Equipment
The current GHGI uses subpart W data to calculate AFs (e.g., average number of separators per gas well) for
several emission sources in onshore production. EPA calculates the current GHGI AFs separately for equipment
on gas wells versus oil wells using RY2014 data. For gas wells, EPA calculates AFs for separators, heaters,
dehydrators, meters/piping, compressors, and chemical injection pumps. For oil wells, EPA calculates AFs for
separators, heater-treaters, headers, and chemical injection pumps.
For this memo, EPA calculated AFs for each emission source at the basin-level, using RY2020 subpart W data.
Table A.5 in Appendix A presents the results for all basins and sources. Table 6 presents the results for separators
and chemical injection pumps for select basins.
Table 6. Well Pad Equipment Leak AFs for Select Basins (RY2020 Subpart W)
Basin Name
Basin
Number
Separator Activity Factors
Chemical Injection Pump
Activity Factors
Separators/
Gas Well
Separators/
Oil Well
Pumps/
Gas Well
Pumps/
Oil Well
Current GHGI (RY2014
AFs)
0.71
0.36
0.18
0.07
Combined RY2020 AFs
0.76
0.45
0.06
0.22
Permian
430
0.90
0.43
0.09
0.01
Appalachian (Eastern
Overthrust)
160A
0.50
0.46
0.03
0.03
Appalachian
160
0.27
0.73
0.00
0.04
5 EPA 2018. Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2016: Revisions to C02 Emissions Estimation Methodologies.
https://www.epa.gov/sites/default/files/2018-04/documents/ghgemissions_co2_2018.pdf
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Basin Name
Basin
Number
Separator Activity Factors
Chemical Injection Pump
Activity Factors
Separators/
Gas Well
Separators/
Oil Well
Pumps/
Gas Well
Pumps/
Oil Well
Anadarko
360
0.57
1.17
0.13
0.40
Gulf Coast
220
0.87
0.87
0.22
0.36
Williston
395
1.08
0.59
0.00
0.03
There is variability between basins for the AFs for separators and chemical injection pumps, with values above
and below the current GHGI. For example, separators vary from 0.27 to 1.08 separators per gas well and 0.43 to
1.17 separators per oil well.
2.2 Coverage
For the pneumatic controller and other well equipment examples, EPA assessed basin-level coverage of wells in
GHGRP. Well coverage was considered in three different ways: for all well types, for oil wells only, and for gas
wells only. The GHGI uses the gas-to-oil ratio (GOR) to determine whether a well in Enverus is an "oil well" or a
"gas well." Wells with a GOR less than or equal to 100 are classified as oil wells, while wells with a GOR greater
than 100 are classified as gas wells. EPA uses the formation type to classify subpart W wells as either oil wells or
gas wells. Specifically, subpart W wells reported under the "oil" formation type are classified as oil wells and
wells reported under all other formation types are classified as gas wells. Both datasets reflect wells that
produced at some time within a year; for subpart W data that equals the number of producing wells at the end
of the calendar year plus wells permanently taken out of production.
Throughout the memo, an example threshold of 50 percent overage is applied. Of the 65 basins with oil or gas
production, 20 have more than 50 percent coverage of the national dataset in subpart W for all wells. Eleven of
the 65 basins have at least 50 percent coverage in subpart W for oil wells and 24 basins have at least 50 percent
coverage in subpart W for gas wells.
For the 13 basins with well counts exceeding 20,000 (collectively accounting for 76 percent of all wells in 2020);
eight basins have more than 50 percent coverage of the national dataset in subpart W for all wells, four basins
have more than 50 percent coverage of oil wells in subpart W, and ten basins have more than 50 percent
coverage of gas wells in subpart W.
Appendix A contains detailed tables comparing the basin-level populations of subpart W wells versus the
national dataset for total wells, oil wells, and gas wells. The well counts tables in Appendix A are organized in
descending order of national well counts.
2.3 Considerations for Production Emission Sources
EPA reviewed the onshore production segment emissions data and calculation methodologies to identify update
considerations for individual emission sources. A majority of the sources with the largest emissions for Natural
Gas and Petroleum Systems rely on subpart W data for emission factors and/or activity factors. Table 7 presents
emissions by source, for the ten highest emitting sources, and information on how subpart W data are used,
where applicable, in the calculation methodology. For emission sources that use subpart W data, EPA is
considering updating the calculation methodology to use basin-level data.
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Table 7. Current GHGI Onshore Exploration and Production Emission Source Emissions and
Methodology Information
Emission Source
Natural Gas Systems
Petroleum Systems
Subpart W Used
in GHGI?
Specific Year vs.
Range in GHGI
Subpart W Used
in GHGI?
Specific Year vs.
Range in GHGI
Pneumatic Controllers
Yes (AFand EF)
AF updated annually,
EF static value from
2014
Yes (AFand EF)
AF updated annually,
EF static value from
2014
Associated Gas Venting / Flaring
N/A
N/A
Yes (AFand EF)
updated annually,
2015 - 2020
Well Pad Equipment Leaks3
Yes (AF)
2015
Yes (AF)
2015
Tanks
Yes (AFand EF)
updated annually,
2015 - 2020
Yes (AFand EF)
updated annually,
2015 - 2020
Gas Engines
No
N/A
No
N/A
Miscellaneous Onshore Production
Flaring
Yes (AFand EF)
updated annually,
2015 - 2020
Yes (AFand EF)
updated annually,
2015 - 2020
Produced Water
No
N/A
No
N/A
Chemical Injection Pumps
Yes (AFand EF)
N/A
Yes (AFand EF)
N/A
Liquids Unloading
Yes (AFand EF)
updated annually,
2011 - 2020
N/A
N/A
Dehydrator Vents
No
N/A
N/A
N/A
a. For NG systems, GHGRP data are used for AFs for heaters, separators, dehydrators, compressors, and meters/piping.
For Petroleum systems, GHGRP data are used for Afs for separators, compressors, headers, and heater/treaters. Leak
emissions from wellheads are not calculated using GHGRP data.
3 Time Series Considerations
For most sources, reporting under GHGRP for onshore production began with emissions data for the year 2011.
Basin-specific emissions information is unavailable for previous years. EPA is considering several approaches for
developing the 1990-2021 times series for the 2023 GHGI. For example, EPA could generally apply at the basin-
level the approaches currently used across the time series at the national level. For pneumatic controllers for
example, in each basin, EPA could apply the 1992 (earliest year with available data) national average AF values
for 1990-1992, use year-specific data for 2011-2021, and interpolate AD between 1992 and 2011. A similar
approach could be taken for "other well pad equipment." Values for years prior to GHGRP are not available for
associated gas venting and flaring. In addition, GHGRP-reported production data used to calculate associated
gas venting and flaring data were first reported in RY2015. To be consistent with the current GHGI, a time series
approach for this source would likely apply the basin-level EFs developed for 2015 to all previous years of the
time series.
4 Considerations for Other Industry Segments
Considerations for implementing basin or state-level approaches vary by industry segment due to the extent to
which GHG Inventory approaches already reflect variation and available data for basin or state-level approaches.
• Exploration. The current gridded GHGI and state GHGI reflect region-specific completion and drilling
counts; however, the EFs applied are national averages (developed from GHGRP data). EPA could
consider use of basin-specific information (AD and EFs) on completion type (e.g., with and without RECs)
from GHGRP. This would likely have a small impact on total basin-level oil and gas emissions, since this
Page 10 of 26
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is a relatively small source. Basin-specific EFs for drilling are unavailable. See Appendix B for additional
information.
• Gathering and Boosting. Spatial allocation for gathering and boosting in the gridded GHGI and state GHGI
is largely based on production data. For this sector, the GHGI uses national EFs and GHGRP AD. The GHGI
scales up emissions using an assumption of 93 percent subpart W coverage for gathering and boosting
stations and 100 percent subpart W coverage for gathering pipelines. Because of these coverage
assumptions and current lack of other data to assess coverage, incorporating basin-level variation in the
GHGI would not impact national emissions. See Appendix B for additional information.
• Processing. The current GHGI uses GHGRP data for AFs and EFs and Oil and Gas Journal data to scale up
to the national level. An initial assessment of variation between states found limited variation in CH4
emissions, but larger variation in C02 emissions. See Appendix B for additional information.
• Transmission and Storage. The current GHGI uses GHGRP data for AFs and largely relies on a national-
level research study for EFs. Transmission and storage segments have lower GHGRP coverage than other
segments, which would mean that state-level EFs would rely on a small data set in many states. See
Appendix B for additional information.
• Distribution. Both the gridded GHGI and state GHGI reflect location-specific pipeline materials. Regional
EFs are unavailable in the GHGI data set for other distribution sources.
• Post-meter. The EFs used in the GHGI do not include region-specific information.
• Refineries. The gridded GHGI and state GHGI use GHGRP data directly and therefore already reflect
facility and state differences.
5 Requests for Stakeholder Feedback
EPA seeks stakeholder feedback on the update under consideration discussed in this memo and the questions
below.
1. The potential benefits and potential disadvantages of updating the GHGI to use an approach that
incorporates additional basin-level calculations.
2. Approaches for quantifying emissions for the full time series.
3. Considerations for prioritizing sources for application of a more disaggregated approach.
4. Prioritization for the production segment examples given.
5. Use of basin-specific data for all basins, or application of a coverage threshold for use of basin-specific
data for a basin versus a national or other average value.
6. Type and level of coverage threshold (e.g., percentage total activity covered by subpart W, a certain
number of wells included in the data set), and the rationale for a threshold.
7. If a coverage threshold were to be applied, approaches for basins with coverage below a threshold (e.g.,
combining data for basins below the threshold to develop EFs/AFs for all basins below the threshold or
using data from all basins to apply to basins below the threshold).
8. Data sources in addition to GHGRP that EPA should consider for disaggregating emissions data to a basin-
level.
Page 11 of 26
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9. Additional industry segments for which EPA should consider basin-/state-level approaches.
10. Underlying reasons for examples of variation noted in this memo (e.g., differences form national
averages for Gulf Coast and Appalachia for pneumatic controllers). For example, are production
conditions in certain basins likely to result in the use of more or fewer controllers per well?
Page 12 of 26
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Appendix A - Onshore Production Segment Assessment Tables
All appendix A tables (A.l - A.5) are ordered in descending order of well counts in the national dataset. Subpart
W and national well counts reflect wells that produced at some time within a year; for subpart W data that
equals the number of producing wells at the end of the calendar year plus wells permanently taken out of
production.
Table A.l. Total Well Population Coverage for Year 2020 - Subpart W Versus National Dataset (Enverus),
for Basins with Reported Wells in Either Dataset
Basin Name
Basin Number
Subpart W Wells
Total Wells
% Coverage
Permian
430
104,650
156,740
67%
Appalachian (Eastern Overthrust Area)
160A
63,403
139,955
45%
Anadarko
360
38,397
69,759
55%
Appalachian
160
31,058
68,432
45%
Gulf Coast
220
39,843
62,993
63%
San Joaquin
745
41,346
39,020
106%
Arkla
230
9,075
29,554
31%
East Texas
260
15,494
29,060
53%
Illinois
315
0
29,017
0%
San Juan
580
21,317
23,479
91%
Williston
395
19,928
22,541
88%
Denver
540
18,027
21,090
85%
Chautauqua Platform
355
501
20,968
2%
Arkoma
345
9,555
17,654
54%
Cherokee
365
0
16,826
0%
Piceance
595
13,304
14,701
90%
Fort Worth Syncline
420
7,123
14,667
49%
Central Kansas Uplift
385
0
13,895
0%
Bend Arch
425
0
13,877
0%
South Oklahoma Folded Belt
350
1,668
13,156
13%
Michigan
305
8,841
12,491
71%
Green River
535
11,290
12,484
90%
Forest City
335
0
11,001
0%
Uinta
575
9,821
10,728
92%
Powder River
515
1,932
10,571
18%
Strawn
415
8,068
7,211
112%
Sedgwick
375
189
7,167
3%
Palo Duro
435
281
6,224
5%
Black Warrior
200
0
4,460
0%
Ouachita Folded Belt
400
167
3,811
4%
Sweetgrass Arch
500
0
3,553
0%
Las Vegas-Raton
455
2,455
3,522
70%
Mid-Gulf Coast
210
1,103
3,231
34%
Los Angeles
760
446
3,160
14%
Wind River
530
567
2,801
20%
Page 13 of 26
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October 2022
Basin Name
Basin Number
Subpart W Wells
Total Wells
% Coverage
Big Horn
520
0
2,516
0%
Nemaha Anticline
370
0
2,212
0%
Arctic Coastal Plains Province
890
2,927
1,883
155%
Central Western Overthrust
507
1,482
1,723
86%
Las Animas Arch
450
278
1,711
16%
Ventura
755
0
1,328
0%
Cincinnati Arch
300
0
1,283
0%
Paradox
585
424
1,214
35%
Chadron Arch
390
0
977
0%
Coastal
740
201
894
22%
Central Montana Uplift
510
0
893
0%
Santa Maria
750
741
844
88%
Sacramento
730
683
799
85%
Sierra Grande Uplift
445
0
681
0%
Salina
380
0
297
0%
AK Cook Inlet
820
287
170
169%
North Park
545
18
133
14%
Llano Uplift
410
0
65
0%
Great Basin Province
625
0
65
0%
Overthrust & Wasatch Uplift
630
0
33
0%
Eel River
720
0
26
0%
Mojave
640
0
24
0%
Upper Mississippi Embayment
250
0
19
0%
Black Mesa
590
0
17
0%
Northern Coast Range Prov
725
0
17
0%
Florida Platform
140
0
16
0%
Western Columbia
710
0
10
0%
Kerr
405
0
3
0%
Santa Cruz
735
0
2
0%
Sierra Nevada Province
650
0
1
0%
Page 14 of 26
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October 2022
Table A.2. Oil Well Population Coverage for Year 2020 - Subpart W Versus National Dataset (Enverus), for
Basins with Reported Wells in Either Dataset
Basin Name
Basin Number
Subpart W Oil Wells
Total Oil Wells
% Coverage
Permian
430
91,547
132,457
69%
Gulf Coast
220
22,671
48,770
46%
San Joaquin
745
41,346
38,228
108%
Appalachian (Eastern Overthrust Area)
160A
1,127
32,587
3%
Anadarko
360
11,468
32,143
36%
Illinois
315
0
27,513
0%
Williston
395
17,261
19,644
88%
Appalachian
160
26
18,790
0%
Chautauqua Platform
355
143
16,312
1%
Denver
540
8,018
14,313
56%
Arkla
230
845
14,282
6%
Central Kansas Uplift
385
0
13,456
0%
Cherokee
365
0
12,719
0%
East Texas
260
810
11,957
7%
South Oklahoma Folded Belt
350
783
11,114
7%
Forest City
335
0
10,961
0%
Bend Arch
425
0
10,430
0%
Palo Duro
435
5
5,696
0%
Powder River
515
1,751
5,578
31%
Uinta
575
4,662
5,374
87%
Fort Worth Syncline
420
491
5,119
10%
Sedgwick
375
188
4,237
4%
Michigan
305
1,847
3,551
52%
Ouachita Folded Belt
400
0
3,293
0%
Los Angeles
760
446
3,102
14%
Green River
535
192
3,068
6%
San Juan
580
898
2,247
40%
Big Horn
520
0
2,182
0%
Nemaha Anticline
370
0
2,130
0%
Mid-Gulf Coast
210
612
2,108
29%
Wind River
530
10
2,032
0%
Sweetgrass Arch
500
0
1,749
0%
Arctic Coastal Plains Province
890
2,926
1,745
168%
Piceance
595
24
1,299
2%
Ventura
755
0
1,258
0%
Arkoma
345
0
1,143
0%
Cincinnati Arch
300
0
1,109
0%
Chadron Arch
390
0
977
0%
Coastal
740
201
894
22%
Las Animas Arch
450
0
856
0%
Santa Maria
750
741
768
96%
Page 15 of 26
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Basin Name
Basin Number
Subpart W Oil Wells
Total Oil Wells
% Coverage
Paradox
585
343
652
53%
Central Western Overthrust
507
26
406
6%
Salina
380
0
297
0%
Central Montana Uplift
510
0
231
0%
Black Warrior
200
0
138
0%
North Park
545
0
131
0%
Strawn
415
139
86
162%
Great Basin Province
625
0
65
0%
Llano Uplift
410
0
65
0%
AK Cook Inlet
820
56
34
165%
Overthrust & Wasatch Uplift
630
0
33
0%
Mojave
640
0
24
0%
Northern Coast Range Prov
725
0
17
0%
Florida Platform
140
0
16
0%
Black Mesa
590
0
9
0%
Sacramento
730
0
9
0%
Kerr
405
0
3
0%
Santa Cruz
735
0
2
0%
Page 16 of 26
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October 2022
Table A.3. Gas Well Population Coverage for Year 2020 - Subpart W Versus National Dataset (Enverus)
for Basins with Wells Reported in Either Dataset
Basin Name
Basin Number
Subpart W Gas Wells
Total Gas Wells
% Coverage
Appalachian (Eastern Overthrust Area)
160A
62,276
107,368
58%
Appalachian
160
31,032
49,642
63%
Anadarko
360
26,929
37,616
72%
Permian
430
13,103
24,283
54%
San Juan
580
20,419
21,232
96%
East Texas
260
14,684
17,103
86%
Arkoma
345
9,555
16,511
58%
Arkla
230
8,230
15,272
54%
Gulf Coast
220
17,172
14,223
121%
Piceance
595
13,280
13,402
99%
Fort Worth Syncline
420
6,632
9,548
69%
Green River
535
11,098
9,416
118%
Michigan
305
6,994
8,940
78%
Strawn
415
7,929
7,125
111%
Denver
540
10,009
6,777
148%
Uinta
575
5,159
5,354
96%
Powder River
515
181
4,993
4%
Chautauqua Platform
355
358
4,656
8%
Black Warrior
200
0
4,322
0%
Cherokee
365
0
4,107
0%
Las Vegas-Raton
455
2,455
3,522
70%
Bend Arch
425
0
3,447
0%
Sedgwick
375
1
2,930
0%
Williston
395
2,667
2,897
92%
South Oklahoma Folded Belt
350
885
2,042
43%
Sweetgrass Arch
500
0
1,804
0%
Illinois
315
0
1,504
0%
Central Western Overthrust
507
1,456
1,317
111%
Mid-Gulf Coast
210
491
1,123
44%
Las Animas Arch
450
278
855
33%
San Joaquin
745
0
792
0%
Sacramento
730
683
790
86%
Wind River
530
557
769
72%
Sierra Grande Uplift
445
0
681
0%
Central Montana Uplift
510
0
662
0%
Paradox
585
81
562
14%
Palo Duro
435
276
528
52%
Ouachita Folded Belt
400
167
518
32%
Central Kansas Uplift
385
0
439
0%
Big Horn
520
0
334
0%
Cincinnati Arch
300
0
174
0%
Arctic Coastal Plains Province
890
1
138
1%
Page 17 of 26
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October 2022
Basin Name
Basin Number
Subpart W Gas Wells
Total Gas Wells
% Coverage
AK Cook Inlet
820
231
136
170%
Nemaha Anticline
370
0
82
0%
Santa Maria
750
0
76
0%
Ventura
755
0
70
0%
Los Angeles
760
0
58
0%
Forest City
335
0
40
0%
Eel River
720
0
26
0%
Upper Mississippi Embayment
250
0
19
0%
Western Columbia
710
0
10
0%
Black Mesa
590
0
8
0%
North Park
545
18
2
900%
Sierra Nevada Province
650
0
1
0%
Page 18 of 26
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October 2022
Table A.4. RY2020 Subpart W Basin-Level Pneumatic Controllers AFs and EFs (for Basins Reporting Pneumatic Controllers in 2020)
Basin
Number
Controllers/Well
Fraction
Low-Bleed EF
Fraction
Intermittent
Bleed EF
(scfd/device)
Fraction High-
High Bleed EF
Basin Name
AF
Low-Bleed
(scfd/device)
Intermittent-Bleed
Bleed
(scfd/device)
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Permian
430
1.3
1.2
0.1
0.4
21.0
22.2
0.9
0.6
89.8
153.0
0.01
0.01
488.7
545.3
Appalachian (Eastern)
160A
1.1
3.3
0.2
0.2
28.5
25.8
0.8
0.8
196.1
223.4
0.002
-
599.7
-
Anadarko
360
1.3
2.8
0.2
0.2
25.4
23.1
0.7
0.7
214.8
223.8
0.08
0.06
620.5
716.7
Appalachian
160
0.4
4.9
0.1
0.1
25.7
27.7
0.9
0.9
229.8
261.8
-
-
-
-
Gulf Coast
220
2.4
3.0
0.2
0.2
21.3
21.9
0.8
0.8
169.7
206.2
0.005
0.002
670.7
629.9
San Joaquin
745
-
0.0
-
-
-
-
-
1.0
-
220.7
-
-
-
-
Arkla
230
2.1
1.2
0.1
0.0
29.9
23.4
0.8
0.9
256.7
229.6
0.01
0.12
682.9
593.0
East Texas
260
1.5
1.2
0.2
0.1
31.9
28.1
0.7
0.9
261.1
254.7
0.03
0.01
795.7
545.9
San Juan
580
5.1
2.4
0.5
0.7
23.2
21.5
0.5
0.3
238.9
279.4
0.004
0.01
751.5
781.2
Williston
395
0.1
1.1
0.0
0.1
12.8
11.9
1.0
0.9
306.7
156.9
0.02
0.01
868.2
466.3
Denver
540
3.2
2.7
0.1
0.1
13.7
14.1
0.9
0.9
138.4
125.3
-
-
-
-
Chautauqua Platform
355
1.0
0.5
0.1
1.0
23.6
17.8
0.9
-
247.4
-
-
-
-
-
Arkoma
345
1.7
-
0.0
-
30.1
-
0.9
-
86.6
-
0.07
-
793.7
-
Piceance
595
3.4
2.7
0.2
-
28.4
-
0.8
1.0
163.5
272.1
-
-
-
-
Fort Worth Syncline
420
1.5
0.7
0.5
0.3
27.7
18.5
0.5
0.7
232.1
236.0
0.01
0.01
739.5
686.5
South Oklahoma
Folded Belt
350
1.9
2.1
0.2
0.1
21.9
21.7
0.8
0.9
213.0
209.2
-
-
-
-
Michigan
305
0.5
0.6
0.1
0.2
25.7
24.9
0.9
0.7
236.3
230.2
0.01
0.08
663.6
666.9
Green River
535
4.2
3.2
0.2
0.6
27.2
20.5
0.7
0.4
127.9
196.7
0.001
0.002
704.4
996.6
Uinta
575
3.5
6.0
0.6
0.5
27.9
19.2
0.4
0.5
174.5
162.2
0.0001
-
809.4
-
Powder River
515
0.7
1.4
0.9
0.2
16.4
18.5
0.1
0.8
198.2
195.3
-
0.0004
-
358.1
Strawn
415
2.6
1.4
0.1
0.8
28.8
30.5
0.9
0.2
241.4
296.9
0.01
-
771.1
-
Sedgwick
375
4.0
2.6
-
-
-
-
1.0
1.0
264.3
264.5
-
-
-
-
Palo Duro
435
0.9
0.8
-
-
-
-
-
-
-
-
1.00
1.00
688.7
739.2
Ouachita Folded Belt
400
4.1
-
-
-
-
-
0.7
-
243.6
-
0.25
-
664.9
-
Las Vegas-Raton
455
0.1
-
-
-
-
-
1.0
-
319.9
-
-
-
-
-
Mid-Gulf Coast
210
1.0
0.9
0.0
0.5
32.3
3.2
1.0
0.5
299.7
267.5
0.01
0.02
793.9
895.8
Wind River
530
4.2
3.6
0.6
0.5
26.6
27.6
0.3
0.2
249.2
306.0
0.12
0.31
787.9
812.8
Central Western
Overthrust
507
5.4
4.7
0.3
0.5
23.7
15.5
0.7
0.5
194.8
151.7
0.01
0.01
711.9
490.9
Las Animas Arch
450
0.4
-
-
-
-
-
1.0
-
215.3
-
-
-
-
-
Page 19 of 26
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Basin Name
Basin
Number
Controllers/Well
AF
Fraction
Low-Bleed
Low-Bleed EF
(scfd/device)
Fraction
Intermittent-Bleed
Intermittent
Bleed EF
(scfd/device)
Fraction High-
Bleed
High Bleed EF
(scfd/device)
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Gas
Oil
Paradox
585
3.1
0.0
-
-
-
-
1.0
1.0
257.2
265.6
-
-
-
-
Santa Maria
750
-
0.0
-
-
-
-
-
1.0
-
292.3
-
-
-
-
Sacramento
730
0.0
-
-
-
-
-
1.0
-
316.3
-
-
-
-
-
AK Cook Inlet
820
1.0
0.8
0.1
0.0
28.9
36.8
0.9
0.9
268.6
265.2
0.02
0.02
487.6
476.8
North Park
545
1.3
1.3
-
-
-
-
1.0
1.0
231.8
232.7
-
-
-
-
- Indicates no subpart W data were reported for this basin.
Page 20 of 26
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October 2022
Table A.5. RY2020 Subpart W Basin-Level Well Pad Equipment AFs (for Basins Reporting Well Pad Equipment in 2020)
Basin Name
Basin
Number
Separators/
Well
Chemical Injection
Pump/ Well
Heaters/
Well
Dehydrators/
Well
Meters/Piping /
Well
Compressors/
Well
Heater-Treaters/
Well
Headers/
Well
Gas
Oil
Gas
Oil
Gas
Gas
Gas
Gas
Oil
Oil
Permian
430
0.90
0.43
0.09
0.01
0.04
0.00
1.40
0.20
0.19
0.18
Appalachian
(Eastern)
160A
0.50
0.46
0.03
0.03
0.10
0.00
0.96
0.01
0.01
0.01
Anadarko
360
0.57
1.17
0.13
0.40
0.11
0.01
1.01
0.13
0.71
0.22
Appalachian
160
0.27
0.73
0.00
0.04
0.01
-
0.92
0.00
-
-
Gulf Coast
220
0.87
0.87
0.22
0.36
0.05
0.02
1.27
0.08
0.28
0.37
San Joaquin
745
-
0.02
-
-
-
-
-
-
0.00
0.07
Arkla
230
1.02
-
0.28
-
0.02
0.03
0.92
0.03
-
-
East Texas
260
0.63
1.06
0.29
0.55
0.06
0.01
1.04
0.04
0.17
-
San Juan
580
0.97
1.04
0.02
0.01
0.97
0.00
1.06
0.20
0.00
0.21
Williston
395
1.08
0.59
0.00
0.03
0.04
0.00
1.71
0.00
0.83
-
Denver
540
0.56
0.54
0.02
0.03
0.22
0.00
0.69
0.09
0.12
0.17
Chautauqua
Platform
355
0.95
1.05
0.27
-
0.01
0.00
0.52
0.08
0.96
-
Arkoma
345
0.91
-
0.15
-
0.00
0.01
1.18
0.07
-
-
Piceance
595
0.54
1.00
0.07
-
0.16
0.00
0.39
0.00
-
1.00
Fort Worth
Syncline
420
1.04
1.16
0.31
0.02
0.00
0.00
0.97
0.16
0.03
0.29
South Oklahoma
Folded Belt
350
1.49
0.70
0.28
0.39
0.05
-
0.82
0.21
0.23
0.33
Michigan
305
0.04
0.26
0.02
0.01
0.00
0.02
0.88
0.06
0.30
0.10
Green River
535
0.81
-
1.26
0.96
0.22
0.22
0.77
0.02
0.47
0.01
Forest City
335
-
-
-
-
-
-
-
-
-
0.08
Uinta
575
0.99
0.57
2.27
0.06
0.52
0.02
0.82
0.00
0.39
0.70
Powder River
515
1.33
0.96
19.00
0.15
1.67
-
40.33
-
1.13
0.40
Strawn
415
1.05
0.88
1.49
0.94
-
0.00
0.96
0.24
-
0.34
Sedgwick
375
-
1.02
-
-
-
-
-
-
0.67
0.29
Palo Duro
435
0.21
0.20
-
-
-
-
1.00
0.11
-
0.20
Ouachita Folded
Belt
400
1.07
-
1.80
-
-
-
1.00
0.25
-
-
Page 21 of 26
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October 2022
Basin Name
Basin
Number
Separators/
Well
Chemical Injection
Pump/ Well
Heaters/
Well
Dehydrators/
Well
Meters/Piping /
Well
Compressors/
Well
Heater-Treaters/
Well
Headers/
Well
Gas
Oil
Gas
Oil
Gas
Gas
Gas
Gas
Oil
Oil
Las Vegas-Raton
455
1.00
-
-
-
-
-
1.00
0.22
-
-
Mid-Gulf Coast
210
0.95
-
0.30
-
0.12
0.03
0.78
0.04
-
-
Wind River
530
1.05
-
0.98
-
0.77
0.02
0.89
0.05
-
-
Arctic Coastal
Plains Province
890
0.03
0.05
-
-
-
0.03
0.21
0.08
0.00
0.08
Central Western
Overthrust
507
0.97
-
0.84
0.93
0.75
0.62
1.01
0.01
-
-
Las Animas Arch
450
0.01
-
-
-
-
-
1.00
-
-
-
Paradox
585
1.40
0.25
0.15
-
0.14
0.02
3.68
0.20
0.05
0.10
Coastal
740
-
0.01
-
-
-
-
-
-
0.01
0.34
Salina
380
-
-
-
-
-
-
-
-
-
0.52
AK Cook Inlet
820
0.53
0.59
0.22
0.07
0.53
0.23
0.44
0.21
-
0.91
North Park
545
0.20
0.26
-
-
-
-
1.00
-
-
1.00
Upper Mississippi
Embayment
250
-
-
-
-
-
-
-
-
-
0.28
- Indicates no subpart W data were reported for this basin.
Page 22 of 26
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October 2022
Appendix B - Preliminary Example Analyses and Considerations for Other
Industry Segments
B.l Exploration
EPA assessed hydraulically fractured (HF) completion C02 emissions for RY2019. HF completions are the largest
emission source within exploration (on a C02 equivalent basis). RY2019 data were evaluated because the number
of HF completion events in RY2020 was almost half of the number of events in RY2019. EPA calculated the
average HF completion C02 emissions per completion event, separately for oil well and gas well completions, for
each basin that reported HF completion data to subpart W. Table B.l shows the average emissions per event for
the nine basins with the highest number of completions (collectively accounting for 92 percent of HF completions
and 96 percent of HF completion C02 emissions). The basins listed in the table below are in descending order of
total HF completion events for RY2019.
Table B.l. Well Completion C02 Emissions Per Completion Event, by Basin (Subpart W RY2019)
Basin
Oil Well Completion C02
Emissions (mt/event)
Gas Well Completion C02
Emissions (mt/event)
Permian
117
434
Gulf Coast
43
97
Williston
670
N/A
Appalachian (Eastern
Overthrust Area)
N/A
6
Anadarko
33
3
Denver
61
5
Arkla
N/A
1
Green River
N/A
10
Piceance
N/A
1
All Other Basins
111
23
There is significant variability in HF completion C02 emissions across the basins, as seen in Table B.l. This
variability reflects differences in completion practices such as reduced emission completions. EPA is considering
calculating basin-level activity factors for exploration sources such as HF completions and well testing.
B.2 Gathering and Boosting
To quantify national level emission from gathering and boosting (G&B) stations in the current GHGI, EPA uses
national emissions factors and GHGRP activity data. EPA scales up GHGRP activity data using a factor calculated
by the Zimmerle et al. 2020 study (i.e., subpart W accounts for 93 percent of total G&B stations).6 For G&B
pipeline emissions, EPA assumes GHGRP covers 100 percent of pipelines.
6 Zimmerle et al. 2020. Methane Emissions from Gathering Compressor Stations in the U.S. Environ. Sci. Technol. 2020, 54, 12, 7552-
7561. Available at: https://doi.org/10.1021/acs.est.0c00516
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October 2022
EPA assessed AD within the G&B segment to determine potential sources of variability. Three of the largest G&B
sources that utilize subpart W data are compressors, tanks, and pneumatic controllers, with intermittent bleed
controllers representing the bulk of pneumatic controller emissions. Table shows information on intermittent
bleed pneumatic controllers, tanks, and compressors in the 10 basins with the largest amount of pipeline
mileage, totaling 83 percent of the national mileage total. To show variability for compressors and tanks, each
is normalized against gathering pipeline miles in a basin.
Table B.2. Percent Intermittent Bleed Controllers and Compressors and Tanks per Mile of G&B
Pipeline (RY2020 Subpart W)
Basin Name
Basin
Number
Percent of All Pneumatic
Controllers that are
Intermittent Bleed
Compressors
per Mile
Tanks per
Mile
Appalachian
160
94%
0.02
0.02
Appalachian- Eastern
Overthrust
160A
82%
0.07
0.03
Gulf Coast
220
66%
0.06
0.15
East Texas
260
81%
0.01
0.06
Arkoma
345
82%
0.07
0.01
Anadarko
360
56%
0.03
0.03
Williston
395
87%
0.03
0.31
Fort Worth Syncline
420
86%
0.03
0.01
Permian
430
61%
0.05
0.17
San Juan
580
45%
0.04
0.03
Examining the activity factors for intermittent bleed pneumatic controllers, compressors, and tanks indicates
that the basins do have variability in their G&B operations. For example, the Gulf Coast basin, which in addition
to having the highest count of equipment for the three emission sources evaluated, has higher activity factors
than most other basins.
All G&B station emission sources in the current GHGI rely on subpart W data for national activity estimates.
However, based on the current GHGI approach where a single national scaling factor is used, basin-level
calculations would not result in any differences to the national GHGI emissions (assuming each basin has the
same scaling applied). Emission factor data are not available to develop regional disaggregation.
The current GHGI G&B station scaling factor of 93 percent could be applied at the basin-level; this approach
would not account for potentially different coverages of subpart W data among basins nor would it account for
variability across basins. However, the basin-level data could be more directly incorporated in the approach used
to develop the state-level GHGI without impacting the national GHGI.
B.3 Processing
In the current GHGI, the AFs and EFs for natural gas processing plants are from GHGRP and the values are scaled
to the national level using plant counts from the Oil & Gas Journal (O&GJ). Year 2015 processing plant counts
are the most recent and are applied for each year from 2015 forward.
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October 2022
EPA calculated average emissions per processing plant for each state by summing the total reported subpart W
emissions from all emission sources in a state and dividing by the number of plants in that state. Table 2 shows
CH4 and C02 emissions per processing plant for select states.
Table B.3. Natural Gas Processing Emissions per Plant by State (RY2020 Subpart W)
State
Total Number of Plants
Average CH4 Emissions
per Plant (mt)
Average C02 Emissions
per Plant (mt)
Colorado
24
236
70,209
Louisiana
50
209
38,428
New Mexico
25
341
76,800
North Dakota
18
210
13,305
Oklahoma
50
290
7,907
Texas
214
194
35,904
West Virginia
10
220
7,300
Wyoming
20
225
114,301
All Others
51
206
28,159
Subpart W
462
220
38,182
The average CH4 emissions per plant have minimal differences amongst the states. As shown in Table B.3, the
average CH4 emissions per plant range from 194 mt/plant to 341 mt/plant. With the exception of a few states,
most states have average plant CH4 emissions that are similar to the subpart W average of 220 mt CH4 per plant
(considering all data together). Conversely, average C02 emissions per state vary widely, from 7,300 mt/plant to
114,301 mt/plant. These differences in average C02 emissions per plant indicate there is significant variability in
flaring practices and in acid gas removal emissions (the key contributors to C02 emissions) between the states.
The current GHGI relies on subpart W to calculate EFs and AFs for a majority of natural gas processing emission
sources. Therefore, based on the observed variability in emissions shown above, updating the GHGI
methodology for C02 from processing plants to use state-level EFs and AFs calculated from subpart W could
improve state-level estimates. Preliminary assessment of coverage at the state-level with Oil and Gas Journal
data indicates that many states have coverage lower than 50 percent on a processing plant count basis.
B.4 Transmission
For this memo, EPA used subpart W data on average compressors per station, by state, for a preliminary
assessment of the variability of subpart W transmission segment data across states. These results are shown in
Table B.4 for the states that collectively represent 70 percent of the total number of transmission stations in
subpart W. As seen in the table below, Kentucky has the highest number of compressors per station at 9
compressors followed by Kansas, Michigan, and Mississippi with 7 compressors per station for each state. All
the other states (12 out of 16 states) have compressors per station close to the subpart W average of 5
compressors per station (i.e., between 4 and 6 compressors per transmission station).
Table B.4. Compressors Per Station by State (RY2020 Subpart W)
State
Compressors per Station
Alabama
6
Arizona
4
Florida
5
Illinois
6
Page 25 of 26
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October 2022
State
Compressors per Station
Kansas
7
Kentucky
9
Louisiana
6
Michigan
7
Mississippi
7
New Mexico
4
New York
4
Ohio
5
Oklahoma
4
Pennsylvania
5
Texas
5
West Virginia
5
Subpart W Average
5
The variability analysis for compressor station shows some variability across states. However, most states have
similar compressors per station as the subpart W average. The current GHGI approach to develop national level
activity data involves applying a single national scale up factor. Preliminary assessment of coverage at the state-
level with Pipeline and Hazardous Materials Safety Administration (PHMSA) data indicates that many states have
coverage lower than 50 percent for transmission.
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