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Oil and Gas Production Emissions and NSPS Baseline Estimates

Background: Methane Emission from the 2009 U.S. Greenhouse Gas Inventory

This section provides background information on methane emissions from the oil and gas sector 

from the 1990 - 2009 U.S. Greenhouse Gas Inventory (published in 2011) with a focus on oil and 

gas production. As shown in Figure 1, the 1990-2009 U.S. GHG Inventory estimates 2009 

methane emissions from oil and natural gas systems (not including petroleum refineries) to be 

624 Bcf (251.55 MMtCO2e). Of that total, oil and gas production is estimated to be 64%1. 

Figure 1: 2009 U.S. Oil and Gas Industry Methane Emissions 

Figure 2 shows the breakdown of production related methane emissions by source. For onshore 

oil and gas production, well venting and flaring (detailed below) is the largest source followed by 

pneumatic devices, dehydrators and pumps, compressor emissions sources, and storage tanks. 

 

1 EPA. (2011) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2009, U.S. EPA. April, 2011. 

Available online at <http://epa.gov/climatechange/emissions/usinventoryreport.html>
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Figure 2. U.S. Inventory Emissions by Source Categories

The following table shows a detailed breakdown of the specific emission sources related to well 

venting and flaring. As shown below, well clean ups by liquids unloading is the largest source of 

well related emissions followed by hydraulically fractured completions and workovers.

However liquid loading is not typically an issue for new wells and only becomes a serious 

problem when the well has reached a mature age (which will vary by well but may be 10 years or 

greater). 

Well Venting and Flaring 2009 Methane Emissions 

(Bcf)

2

Well Clean Ups (LP Gas Wells) 187.3

Unconventional Gas Wells Workovers 14.6 

Unconventional Gas Well Completions 12.1 

Power River CBM Venting 2.4 

Black Warrior CBM Venting 0.48 

Well Drilling 0.05 

Conventional Gas Wells Workovers 0.04 

Oil Well Completions 0.007 

Oil Well Flaring 0.004 

Oil Well Workovers 0.003 

Conventional Gas Well Completions 0.0004 

Total 217

 

2 The estimates already account for existing controls first in the form of existing regulations and then in the form of 

voluntary reductions via Natural Gas STAR. The U.S. Inventory subtracts Natural Gas STAR reductions at the 

sector (e.g. Production or Natural Gas Processing) level. For the purpose of this analysis, EPA apportioned 

reductions to each source category. These emissions pie charts are posted on the Gas STAR Program web site 

annually - http://www.epa.gov/gasstar/basic-information/index.html
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Baseline Well Emission Factors used in NSPS Impacts Analysis: 

In the development of this proposed rule, for emissions from well completions and 

workovers, EPA analyzed emissions from conventional gas well completions, unconventional 

gas well completions (e.g., “tight” formations such as shale, tight sands, and CBM), and oil well 

completions from the 2009 U.S. Inventory and the Mandatory Reporting Rule, Subpart W 

Technical Support Document (TSD). Since hydraulic fracturing is most common in and closely 

associated with unconventional formations, this analysis assumes unconventional wells are 

hydraulically fractured and conventional wells are not. The 2009 U.S. Inventory and the Subpart 

W TSD estimate that on average 9,175 thousand cubic feet of natural gas (or 158.3 and 44 short 

tons of methane and VOC respectively) is emitted per gas well completion with hydraulic 

fracturing. 

3 As illustrated below, the largest methane and VOC emissions are from hydraulically 

fractured wells. Conventional wells and oil wells are relatively small source of methane and 

VOC emissions. 

For the purposes of the NSPS estimates, EPA used 0.0208 short tons per Mcf of methane 

density from EPA Method 18 to convert volume of methane emissions. Volatile organic 

compound (VOC) emissions are estimated assuming a 0.278 to 1 VOC to methane (by mass) 

ratio. Hazardous air pollutant (HAP) emissions are estimated assuming a 0.0105 to 1 to HAP to 

methane (by mass) ratio. The resulting methane, VOC, and HAP emission factors for gas well 

completions with hydraulic fracturing, without hydraulic fracturing, and for oil well completions 

are summarized in the table below.

 

3 EPA (2010). Greenhouse Gas Emission Reporting from the Petroleum and Natural Gas Industry: Background 

Technical Support Document. Appendix B page 87. Available Online at: 

http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD.pdf (note: the Subpart W TSD

published the whole gas factor and the inventory publishes the methane portion, determined by NEMS region). 

Emissions from Tight Sands: It is important to note that the emissions estimates for well 

completions and recompletions (workovers) in the 1990 – 2009 GHG Inventory exclude a 

significant number of wells completed in tight sand plays due to limited data availability 

when the 2009 Inventory was developed. The estimate in the NSPS proposal includes an 

adjustment for tight sand plays when estimating new sources (which is being considered as a 

planned improvement in development of next year’s Inventory for not just new sources but 

all sources). This adjustment is estimated to increase the 2009 Inventory estimate by 

approximately 76.7 MMtCO2e (or at 0.02082 tons methane/1000 scf, about 175 Bcf).
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NSPS Baseline Emission Factors by Species in Short Tons per Completion

Emission Species Unconventional Gas 

Well Completions with 

Hydraulic Fracturing 

(short 

tons/completion)

Conventional Gas Well 

Completions without 

Hydraulic Fracturing 

(short 

tons/completion)

Oil Well Completions 

(short 

tons/completion)

Methane 158.3 0.76 0.02

VOC 44.0 0.21 0.004

HAP 1.662 0.008 0.0002