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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460 

OFFICE OF 

 AIR AND RADIATION 

Petroleum and Natural Gas Systems (Subpart W) of the Greenhouse Gas Reporting 

Rule 

Docket: EPA-HQ-OAR-2009-0923 

Title: Portable Combustion Emissions 

Summary: This memo discusses how EPA evaluated data on portable GHG emissions to 

determine what portion of the GHG emissions from the onshore production petroleum 

and natural gas industry segment are coming from portable equipment. 
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Portable Combustion Emissions 

EPA received numerous comments from the April 2010 proposal for 40 CFR Part 98 – 

Subpart W Petroleum and Natural Gas Systems of the Greenhouse Gas Reporting Rule 

on whether or not portable greenhouse gas (GHG) emissions should be included as an 

emissions source for the onshore production segment. In response to those comments 

received, EPA evaluated data on portable GHG emissions to determine what portion of 

the GHG emissions from the onshore production petroleum and natural gas industry 

segment are coming from portable equipment. 

EPA determined that 45 percent of the CO2e emissions from the onshore production 

segment originate from portable equipment combustion and process emission sources. 

EPA based this finding on the process and combustion emissions from onshore petroleum 

and natural gas sources from the U.S. Inventory1

 with corrections for under-estimated 

sources (see Section 4.C and Appendix B of the Technical Support Document to final 

subpart W in docket EPA-HQ-OAR-2009-0923). EPA then made assumptions on 

stationary and portable equipment based on the Natural Gas STAR Program data2

, further 

collaboration with industrial partners in the Natural Gas STAR Program and on expert 

opinion. With these analyses, subpart W covers 85% of the emissions (including 

stationary combustion and process emissions) from the onshore petroleum and natural 

gas production industry segment , from 3% of the onshore production owner-operator 

companies only by including the portable equipment emissions in the rule. Excluding 

portable equipment emissions would reduce the coverage well below 55% of emissions 

because many facilities (basins) do not have significant well liquid unloading emissions 

(34% of total onshore production). 

Portable Equipment Emission Sources 

EPA reviewed emission sources in the 2006 U.S. Inventory and those estimated in the 

Technical Support Document (TSD) to determine what emissions originate from 

equipment that fits the definition of “portable equipment.” EPA determined that portable 

equipment in the onshore production segment certainly includes all drilling rigs and 

workover rigs. Therefore, all combustion from these equipment (17% of total) as well as 

all process emissions associated with drilling and workover operations, primarily venting 

gas hydraulic fracturing completions (8% of total) would be excluded without portable 

equipment included. Furthermore, EPA has determined that most wellhead equipment 

such as compressors, dehydrators and heater-treaters are skid mounted and thereby could 

be considered portable. Indeed, EPA has learned that many owner-operators move 

equipment such as glycol dehydrators that move from well to well when the production 

rate from one well declines below production rate to operate effectively. Therefore, 

considering that this type of wellhead equipment is portable, the sum from drilling rigs 

and workovers, plus production gas fired compressor engines (17%), production gas 

fueled dehydrators and heater-treaters (3%), and the total portable equipment contribution 

 

1

 EPA. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2008. April 2010, U.S. EPA # 

430-R-10-006. <http://epa.gov/climatechange/emissions/usinventoryreport.html>. 2

 Onshore Production Basin Threshold Analysis EPA-HQ-OAR-2009-0923 
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to onshore production emissions is 45%. The wellhead equipment considered to be 

portable by subpart W includes: 

1) Petroleum Systems Compressors and Engines 

2) Natural Gas Systems Compressors and Engines 

3) Petroleum Heater-Treaters 

4) Petroleum Drilling Rigs 

5) Natural Gas Drilling and Workover Rigs and Unconventional Well Completion Vents 

6) Dehydrator Reboilers 

Appendix B in the TSD (docket EPA-HQ-OAR-2009-0923) discusses how the emissions 

from these combustion sources were estimated. Table 1 below shows the total and 

reported process emissions from the onshore production sector broken into U.S. 

Inventory Natural Gas Systems emissions, U.S. Inventory Petroleum Systems emissions, 

unconventional well completion and workover emissions, conventional well completions 

and workover emissions, and well liquids unloading emissions. Table 2 shows the total 

and reported combustion emissions from the onshore production industry segment broken 

into Natural Gas Systems engines, Petroleum Systems engines, petroleum heater-treaters 

, natural gas drilling rigs, petroleum drilling rigs, and dehydrator reboilers, and other 

natural gas and petroleum combustion emissions. 

Portable Equipment Emissions 

EPA summed the emissions from the sources selected above and determined the 

emissions from portable equipment to be about 120 million tonnes CO2-equivalent or 45 

percent of the 265 million tonnes CO2-equivalent for the entire onshore production 

segment. 
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Table 1. Onshore Production Process Emissions by Source Nationwide and Reported (tonnes CO2-

U.S. Nationwide 

Emissions 

% of U.S. 

Process 

Emissions 

% of Total 

Emissions 

Reported 

Emissions 

% of 

Pr

Em

U.S. Inventory Petroleum Systems 13,577,120 8% 5% 9,945,225 U.S. Inventory Natural Gas Systems 41,441,133 25% 16% 38,083,392 Unconventional Well Workovers and Completions 21,008,000 13% 8% 20,492,624 Conventional Well Workovers and Completions 122,816 0.1% 0.0% 113,042 Well Unloading 90,092,000 54% 34% 83,672,205 Total Process 166,300,327 100% 63% 152,353,196 Total Emissions 265,115,434 100% 226,010,270 

Table 2. Onshore Production Combustion Emissions by Source Nationwide and Reported (tonnes CO

Nationwide 

Emissions 

% of U.S. 

Combustion 

Emissions 

% of Total 

Emissions 

Reported 

Emissions 

% of 

Com

Em

Petroleum Systems Engines 10,768,282 11% 4% 7,887,755 Natural Gas Systems Engines 33,618,693 34% 13% 30,894,760 Natural Gas Combustion Emissions 7,203,340 7% 3% 6,619,694 Petroleum Combustion Emissions 1,455,124 1% 1% 1,065,877 Petroleum Heater-Treater Combustion Emissions 378,632 0.4% 0.1% 277,348 Petroleum Drilling Emissions 12,636,856 13% 5% 7,462,623 Natural Gas Drilling Emissions 32,348,607 33% 12% 19,103,285 

3 Onshore Production Basin Threshold Analysis EPA-HQ-OAR-2009-0923 4 Onshore Production Basin Threshold Analysis EPA-HQ-OAR-2009-0923 
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Dehydrator Combustion Emissions 93,324 0.1% 0.04% 85,763 Total Combustion Emissions 98,815,107 100% 33% 73,657,074 Total Onshore Production Emissions 265,115,434 100% 226,010,270