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EPA’s Oil and Gas Emissions Estimation Tool Improvements for 2014
Mike Pring and Regi Oommen
Eastern Research Group, Inc., 1600 Perimeter Park Drive, Morrisville, NC 27560
Jennifer Snyder
U.S. EPA, 109 T.W. Alexander Drive, Mail Code C339-02, Research Triangle Park, NC 27709
ABSTRACT
Quantification of nonpoint source emissions from the oil and gas exploration and production sector has
gained interest in recent years in the United States. Drilling technology has allowed development of
unconventional oil and gas plays (such as shale or tight sands) in areas where there was previously no
activity, or where activity had subsided after depletion of the conventional reserves. For example, the
areas in and around the Marcellus Shale in Ohio, Pennsylvania, and West Virginia; the Barnett and
Eagle Ford Shales and Permian Basin in Texas; and the Bakken Shale/Williston Basin in North Dakota
have all experienced a rapid expansion in activity over the last ten years.
While the major emissions sources associated with oil and gas collection, processing, and distribution
have traditionally been included in the National Emissions Inventory (NEI) as point sources (e.g. gas
processing plants, pipeline compressor stations, and refineries), the activities occurring “upstream” of
these types of facilities were not previously as well inventoried. Upstream activities refer to emission
units and processes associated with the exploration and drilling of oil and gas wells, and the equipment
used at the well pad (e.g. separators, dehydrators, storage tanks, and compressor engines) to then extract
the product from the well and deliver it “downstream” to a central collection point or processing facility.
EPA addressed the deficiency in the upstream inventory in the NEI starting in 2011 with development of
the Nonpoint Oil and Gas Emission Estimation Tool.
The NEI nonpoint oil and gas emissions inventory is primarily developed using data supplied to the
U.S. Environmental Protection Agency (EPA) by state air agencies. Where state data are not
supplied, the EPA populates the NEI with the best available data. In the case of nonpoint oil and
gas emissions estimates, the EPA uses the Nonpoint Oil and Gas Emission Estimation Tool
described in this paper to estimate emissions from this category.
This paper presents a summary of the improvements made to the tool to develop the 2014 inventory
through collaboration with other EPA offices, states, local air quality agencies, and non-governmental
organizations (NGO). These improvements include enhancing the architecture of the Tool to include a
Dashboard View and improve the user interface, the addition of a changes log, incorporation of process
characterization data from Subpart W of the Greenhouse Gas Reporting Program (GHGRP),
disaggregation of activity between conventional and unconventional formations, incorporation of
updated gas composition data, emission factor updates, and various basin factor updates as
provided by state and local tool users. Planned improvements to the tool in preparation for the 2017
inventory are also discussed.
INTRODUCTION
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The EPA Emission Inventory and Analysis Group (EIAG) produces the National Emission Inventory
(NEI) for criteria air pollutants and their precursors (CAPs) and hazardous air pollutants (HAPs)
every three years.1 The NEI is a comprehensive and detailed estimate of air emissions of both
criteria and HAP from all air emissions sources, including both stationary (e.g. power plants and
petroleum refineries) and mobile (e.g. automobiles and aircraft) sources. The NEI is prepared by
the EPA based primarily upon emission estimates and emission model inputs provided by State,
Local, and Tribal air agencies for sources in their jurisdictions, and supplemented by data
developed by the EPA. These data are needed for a variety of reasons, including modeling
demonstrations, regulatory analyses, international reporting, and to produce the National Air
Pollutant Emission Trends report.
While the major emissions sources associated with oil and gas collection, processing, and distribution
have traditionally been included in the NEI as point sources (e.g. gas processing plants, pipeline
compressor stations, and refineries), the activities occurring “upstream” of these types of facilities have
not been as well characterized. Upstream activities refer to emission units and processes associated with
the exploration and drilling of oil and gas wells, and the equipment used at the well pad (e.g. storage
tanks, separators, and compressor engines) to extract the product from the well and deliver it
“downstream” to a central collection point or processing facility.
Until 2011, where State data were not supplied to the EPA, the NEI did not contain any estimates
for this increasingly important source category. Therefore, the EPA developed the Nonpoint Oil
and Gas Emission Estimation Tool (the “Tool”) described in this paper to develop such estimates
for the 2011 and 2014 NEI, and to assist State and Local air agencies in developing their own
nonpoint emissions inventories for the upstream oil and gas sector. Where states did not provide an
inventory, the EPA utilized the estimates contained within the Tool to fully populate the NEI such
that the 2014 NEI contains a comprehensive, national nonpoint oil and gas emissions inventory for
CAPs and HAPs.
Concurrent with the recent spike in oil and gas exploration and production in the United States, the
EPA has undertaken several related regulatory and inventory development efforts. Notably, New
Source Performance Standards (NSPS) Subpart OOOO (Standards of Performance for Crude Oil and
Natural Gas Production, Transmission and Distribution for which Construction, Modification or
Reconstruction Commenced after August 23, 2011, and on or before September 18, 2015)2 and Subpart
OOOOa (Standards of Performance for Crude Oil and Natural Gas Facilities for which Construction,
Modification or Reconstruction Commenced After September 18, 2015) 3 cover additional oil and gas
processes, and Subpart W (Petroleum and Natural Gas Systems)4 of the GHGRP requires facility-
level reporting of emissions of greenhouse gases to the EPA. The EPA also develops a national
Greenhouse Gas Emissions Inventory (GHG EI) that includes natural gas and petroleum systems.5
Given these recent and ongoing regulatory and inventory initiatives, updated data continues to
become available to help inform the inventory. This paper discusses the updates made to the Tool
to improve the inventory for the 2014 NEI, as well as planned updates for the 2017 NEI.
TOOL DEVELOPMENT HISTORY
Several state, local, and regional air quality organizations were involved in initial development of
the 2011 version of the Tool. These organizations include the Central States Air Resources
Agencies (CenSARA), Western Regional Air Partnership (WRAP), the Mid-Atlantic Regional Air
Management Association (MARAMA), and staff from numerous states including Colorado,
Kansas, Oklahoma, Pennsylvania, Texas, West Virginia, and Wyoming. The Tool was initially
developed based on work that was ongoing at the time by various States, inter-governmental agencies,
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and EPA. In particular, CenSARA was developing a Microsoft® Excel-based emission estimation tool
to improve the nonpoint oil and gas emissions inventories for the seven CenSARA states (Texas,
Louisiana, Oklahoma, Arkansas, Kansas, Missouri and Nebraska). These different groups have been
actively involved in development of the Tool since its inception.
The basic methodology used to develop the Tool consists of the following steps:
• Compile activity data - Oil and gas activity data was obtained to include, but not limited to, the
number of active wells by well type, gas production, oil production, spud counts, feet drilled, and
water production. The activity data for the Tool was primarily obtained from DrillingInfo’s
HPDI database,8 a commercial database that processes state-level oil and gas commission data
into a comprehensive database of production statistics.
• Compile process characterization data - To initially populate the Tool, basin-level data on
equipment, process parameters, and emissions factors from the CenSARA study were used for
the counties in the CenSARA states, and an average of the data for the CenSARA basins were
used for the remainder of the country.
• Incorporate updated process characterization data - CenSARA averaged data was then replaced
with state or county-specific data as it became available. EPA data7 were used to develop
updated default characterization data for several categories, including condensate tanks, liquids
unloading, pneumatic devices, and well completion emission algorithms. Additionally, several
State, local, and regional air quality agencies provided updates to replace the default CenSARA
and EPA process characterization data.
• Develop a tool to house the inventory - A Microsoft® Access tool that estimates 2011 nonpoint
oil and gas emissions by county and emissions process was then developed using the compiled
activity and process characterization data. The flexible Tool design allows for user-specified
inputs such that users may update activity and emissions rate data at the basin and/or county
level.
A first draft of the Tool was distributed to the National Oil and Gas Committee in the fall of 2012,
with the version used in the 2011 NEI version 2 finalized in November of 2014. During the two-
year period between the draft and final versions of the tool, numerous changes were made to both
the structure and functionality of the tool, as well as to the process characterization data. Structural
and functional changes made to the tool include:
• Resolve data input tables at the county level - The initial spatial resolution of the process
characterization data used was at the basin level, consistent with the spatial resolution used in the
existing nonpoint emission inventory efforts at the time (such as the tool developed for
CenSARA). The Tool has since been revised to allow for county-level process characterization
data.
• Separation of coal-bed methane wells - Initially, coal-bed methane activity was aggregated with
natural gas well activity. Subsequently, coal-bed methane activity was separated such that
distinct well counts and production data are now included for coal-bed methane wells.
• Updated EPA Basin Factors – Ongoing EPA data collection and oil and gas emissions inventory
and regulatory development efforts were utilized to improve data in the Tool. In particular, data
reported under Subpart W of the GHGRP, data and methodologies used in the US GHG
inventory, and gas composition data used in rulemaking were employed in the Tool as
applicable.
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Process characterization data were also updated extensively over the course of the Tool
development effort. Updated data were obtained from the WRAP, individual States, and the EPA,
and included the following:
• WRAP process characterization data - Over the last seven years, there has been an active
emissions inventory effort conducted in the Rocky Mountain States to assess nonpoint oil and
gas emissions.9 As a result of these projects, more specific data were included in the Tool for
numerous States, including New Mexico, Utah, Colorado, Wyoming, Montana, and North
Dakota.
• State process characterization data - Individual States that provided updates to one or more
parameters used to estimate emissions in the Tool include Kansas, Michigan, Nevada, New
Mexico, New York, North Dakota, Ohio, West Virginia, Texas, and Utah.
2014 TOOL IMPROVEMENTS
Upon completion of the Tool for use in the 2011 NEI, additional enhancements were implemented for
the 2014 version of the Tool. These improvements included:
• Tool structural updates
• Updated activity analysis
• State activity/factor updates
• Gas composition updates
• Emission factor updates
• GHGRP Subpart W updates
Tool structural updates - Based on feedback from Tool users, the graphical interface used in the Tool
was revised from an Access-based interface to a more graphical “button” based in interface. Figures 1
and 2 below show the interface from the 2011 Tool and from the 2014 Tool, respectively. In addition,
and Excel-based import/export routine was added to facilitate input of updated basin factor and activity
data. Other updates include inclusion of a change log, inclusion of 2011 legacy data, and resolving
emission factors at the state/basin/attainment category level.
Updated activity analysis – County-level oil and gas exploration and production activity was reviewed
and was revised to resolve oil, condensate, and gas production between conventional and
unconventional resources. At this time, the emission estimation methodologies have not been
distinguished between resource type but this may be a future improvement. In addition, a 100
(MCF/BBL) gas-to-oil ratio has been used to distinguish between oil and gas wells.
State activity/factor updates – As with the 2011 Tool, numerous state and local agencies have provided
updated activity and basin factor updates for the 2014 Tool. State agencies supplying updated data
include Alaska, Kansas, Michigan, New Mexico, Ohio, Tennessee, Texas, and West Virginia.
Gas composition updates – Gas composition data is used within the Tool to speciate VOC and HAP
emissions from numerous sources types. For the 2014 Tool, updated gas composition data was provided
by Alaska, Kansas, WRAP, and from EPA’s SPECIATE database.
Emission factor updates – Based on new AP-42 emissions factors and other recent research, updated
emission factors were input in the Tool for heaters (based on a California Air Resources Board report),
flares (AP-42), and unconventional oil well completions (EPA NSPS subpart OOOOa analysis). In
addition, emission factors for carbon tetrachloride for compressor engines and artificial lift engines used
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in the 2011 Tool were removed from the Tool. EPA determined that the current emission factor was
based solely on “non-detect” values and was not appropriate for use in the Tool.
GHGRP Subpart W updates – EPA’s GHGRP has continually evolved since inception. As a result of the
changes to the GHGRP reporting requirements year-over-year in 2014 and 2015, additional data has
become available which allow for estimation of updated Tool input factors for the 2014 inventory.
Updated basin factors derived from GHGRP Subpart W data used in the 2014 Tool include:
• Associated Gas Venting and Flaring fraction flared
• Condensate Tanks fraction flared
• Crude Oil Tanks fraction flared
• Heaters (number of heaters per well)
• Pneumatic Devices (number of low, intermittent, and high-bleed devices per well)
• Fraction of gas wells requiring compression
• Wellhead compressor engine sizes
2014 TOOL RESULTS
As a result of an ongoing, collaborative, multi-year effort, the EPA has been able to develop and
continually refine a Tool that provides State, local, and regional air quality agencies with a
standardized approach that may be used to develop county-level emissions inventories for nonpoint
oil and gas operations. While the Tool was initially populated using default data from oil and gas
activity occurring in the CenSARA region, it has since been updated to reflect national data
generated by the EPA for other inventory and regulatory efforts, as well as data compiled by
numerous State and regional agencies. This has resulted in a more complete inventory, as well as a
more refined and accurate inventory with each subsequent inventory cycle.
The 2014 NEI is now complete with respect to inclusion of nonpoint oil and gas emission estimates
for all the oil and gas producing counties in the country. The data in the NEI comes from a
combination of State-supplied data (e.g. California, Colorado, and Wyoming) and data derived
from the Tool (e.g. Montana, New Mexico, and North Dakota). In some cases, the State-supplied
data was generated in part by the Tool (e.g. West Virginia and Pennsylvania). Likewise, much of
the data in the NEI that came from the Tool utilizes inputs provided to EPA by State and regional
agencies involved in development of the Tool over the last few years (e.g. Kansas).
Figures 3 and 4 below show 2011 and 2014 national estimates for NOx and VOC, respectively, as
calculated by the Tool. As shown in Figure 3, the most significant NOx sources include compressor
engines, heaters, artificial lift engines, and drilling and hydraulic fracturing engines. As shown in
Figure 4, the most significant VOC sources include oil tanks, pneumatic devices, fugitive
components, and condensate tanks.
The 2008 NEI contained nonpoint oil and gas emissions inventories for only a few states, as shown
below in Figure 5. Figures 6 and 7 show the expansions of the nonpoint oil and gas inventory in the
2011 and 2014 NEI, respectively. Figures 6 and 7 show both the coverage of nonpoint oil and gas
emissions in the 2011 NEI Version 2 and the 2014 NEI Version 2, respectively, as well as the
source of the NEI data for each State (“EPA Data” refers to data derived by the Tool). As stated
above, the NEI contains data submitted directly by States where available, so the emission estimates in
the NEI do not necessarily match those output by the Tool.
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IMPROVEMENTS PLANNED FOR 2017
EPA is currently planning for structural improvement to the Tool for 2017 as well as updating the
factors used to estimate emissions for processes currently addressed in the Tool. Structural
improvements include planned expansion of the SCC codes to provide greater resolution of emission
estimates (particularly to handle flaring emissions), disaggregation of activity data for conventional and
unconventional resources, and additional analysis of HPDI data to better allocate liquids production
between crude oil and condensate. Updates to existing processes include accommodating the use of
electric-powered compressor engines, and incorporating vapor recovery units to control emissions from
storage tanks (and other processes as appropriate).
Other planned updates include analysis of 2017 GHGRP (Subpart W) data that will include new data
elements reported by industry, including well counts and well IDs, equipment counts, and emissions
data which can be used as inputs for the Tool. The 2017 GHGRP data is due to the EPA in April of
2018, so the EPA anticipates the use of these data in the 2017 Tool.
Additional subcategories that were not originally included in the Tool continue to be examined by the
EPA and the National Oil and Gas Emissions Committee. New subcategories such as coalbed methane
dewatering pumps may also be added to the scope of the Tool. Finally, DrillingInfo 2017 production
and activity data will be pulled in late summer or early fall 2018, which will form the basis of activity
data in both the NEI and the GHG EI.
The EPA anticipates releasing Version 1 of the 2017 NEI Tool in the Spring of 2018. As additional
improvements are made and new data incorporated into the Tool, EPA will continue to issue Tool
updates for use by state and local agencies to compile and update the 2017 inventory.
REFERENCES
1. U.S. EPA, 2017. 2014 National Emissions Inventory (NEI). U.S. Environmental Protection Agency.
Internet address: https://www.epa.gov/air-emissions-inventories/2014-national-emissions-inventory-
nei-data
2. U.S. EPA, 2016. Subpart OOOO— Standards of Performance for Crude Oil and Natural Gas
Production, Transmission and Distribution for which Construction, Modification or Reconstruction
Commenced after August 23, 2011, and on or before September 18, 2015. Internet address:
https://www.ecfr.gov/cgi-
bin/retrieveECFR?gp=&SID=c72684771b97c0b89a2cdedd14993386&mc=true&n=pt40.8.60&r=PA
RT&ty=HTML#sp40.8.60.oooo
3. U.S. EPA, 2016. Subpart OOOOa— Standards of Performance for Crude Oil and Natural Gas
Facilities for which Construction, Modification or Reconstruction Commenced After September 18,
2015. Internet address: https://www.ecfr.gov/cgi-
bin/retrieveECFR?gp=&SID=c72684771b97c0b89a2cdedd14993386&mc=true&n=pt40.8.60&r=PA
RT&ty=HTML#sp40.8.60.oooo_0a
4. U.S. EPA, 2017. Subpart W – Petroleum and Natural Gas Systems. Internet address:
http://www.epa.gov/ghgreporting/reporters/subpart/w.html
5. U.S. EPA, 2017. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 - 2015. Internet
address: https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks-
1990-2015
6. Drillinginfo, Inc. 2016. “DI Desktop Database powered by HPDI.” Internet address:
http://www.didesktop.com/
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7. U.S. EPA, 2013a. “Updates to default factors in the EPA OAQPS Nonpoint National Oil and Gas
Emissions Calculator Tool”, Memorandum from Mike Pring and Regi Oommen to Roy Huntley and
Jennifer Snyder. September 12, 2013.
KEY WORDS
Criteria Pollutants
Emissions Inventory
Hazardous Air Pollutants
Oil and Gas
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Figure 1. 2011 Tool Interface
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Figure 2. 2014 Tool Interface
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Figure 3. 2014 and 2011 Tool-estimated NOx emissions
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Figure 4. 2014 and 2011 Tool-estimated VOC Emissions
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Figure 5. 2008 NEI Nonpoint Oil and Gas Inventory Coverage
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Figure 6. 2011 NEI Version 2 Nonpoint Oil and Gas Inventory Coverage
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Figure 7. 2014 NEI Version 2 Nonpoint Oil and Gas Inventory Coverage
