1

Emission Factor for Hydraulically Fractured Gas Well 

Completions: Data Assessment and Calculation Methodology

This memo details the development of the emission factor used in the NSPS regulation impacts analysis, 

including a discussion of the methodology and the data sources, and then provides detail on the analysis 

of data presented to EPA through the public comment period of the NSPS. 

EMISSION FACTOR DEVELOPMENT

Previous estimates of CH4 from natural gas production

EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks1 methane emissions estimates for natural 

gas systems depend heavily on emissions factors from a 1996 EPA/GRI study.2 Hydraulically fractured 

gas well completions and workovers are a significant proportion of completions and workovers today, 

but were not as common during the development of the 1996 EPA/GRI study. The study calculated an 

emission factor for gas well completions by making several key assumptions. These assumptions were 

based on the data and knowledge of industry practices available at the time but which are not 

appropriate for hydraulically fractured wells. These assumptions included:

 Emissions that occur during the well completion are equal to one day of the average gas 

production rate per gas well in 1992 based on the American Gas Association’s Gas Facts.

 All completion emissions are flared (reducing the methane emissions from each completion by 

approximately 98 percent). 

Since the publication of the 1996 EPA/GRI study, the number of hydraulically fractured gas well 

completions has increased significantly to 9,000 hydraulically fractured wells completed annually

according to the National Energy Modeling System’s (NEMS) models. Through extensive interactions 

with oil and gas companies and industry experts and through a review of best available data, EPA 

became aware that the assumptions made in the 1996 EPA/GRI study did not accurately characterize 

methane emissions from hydraulically fractured gas wells. In particular:

 The 16.97 million cubic feet per year average gas production rate in 1992, which GRI used as a 

surrogate for the average completion flow rate, includes a large number of marginal wells. 

Marginal low pressure wells have significantly less gas production than newly completed 

hydraulically fractured wells. 

 Significant quantities of gas are produced during the completion process for a hydraulically 

fractured gas well, in particular during the flowback period. In 2004, companies began sharing 

information with EPA on voluntary activities to reduce emissions from hydraulically fractured 

gas well completions, referred to as Reduced Emission Completions. Companies reported that 

the extended flowback time (as compared to conventional completions) and increased volume 

of natural gas during the flowback period, made it cost-effective to bring portable equipment 

on-site to capture the increased natural gas for market. EPA currently assumes an average 

flowback period of 3 to 10 days for hydraulically fractured wells.

 

1

http://epa.gov/climatechange/emissions/usinventoryreport.html

2

EPA/GRI. Methane Emissions from the Natural Gas Industry. June 1996. Available at 

http://www.epa.gov/gasstar/tools/related.html
2

 Finally, through interactions with companies at EPA and industry events as well as public data 

and experiences shared by the industry, EPA is aware that not all companies are flaring the 

flowback gas, contrary to the assumption in the 1996 EPA/GRI study. In fact, many companies 

are either capturing through reduced emissions completions (RECs) or venting the gas that 

occurs during the completion of hydraulically fractured gas wells.

Based on this information, and review of available data on emissions per completions from hydraulic 

fracturing, EPA has concluded that the emission factor for gas well completions from the 1996 EPA/GRI

study no longer represents the diversity of gas well completion practices in use today, and the emission 

rates associated with these practices. On the basis of the available data reported by companies carrying 

out hydraulic fracturing and RECs, EPA developed an emission factor specifically for gas well completions 

with hydraulic fracturing, as described in the section below. 

Emission factor for completions with hydraulic fracturing

In 2010, on the basis of information reported by companies carrying out hydraulic fracturing and RECs, 

EPA developed an emission factor specifically for hydraulically fractured gas well completions. This 

factor was first used in the Technical Support Document for Subpart W of Part 98, GHG reporting rule, 

and also in the 1990-2009 Inventory (published in April 2011). 

The emission factor for gas well completions with hydraulic fracturing was developed using four data 

sources, together representing data from over 1,000 well completions with hydraulic fracturing. These 

data were provided to EPA or its technical contractors by industry for presentation at Natural Gas STAR 

technology transfer workshops from 2004-2007. For each data source, EPA calculated the average gas 

release per gas well completion. The data from these wells collectively indicate that the true average gas 

release rate for a hydraulically fractured well completion is substantially higher (greater than two orders 

of magnitude) than the 1996 GRI/EPA emission factor that is applicable to conventional well 

completions. These data also indicate that there is a high degree of variability in gas release rates across 

hydraulically fractured well completions due to geology, technology and operating conditions. The four 

calculated averages were each then rounded to the nearest single significant digit to reflect the 

precision of these averages. The resulting emission factors from each of the four data sources were 

arithmetically averaged to determine the final emission factor for gas well completions with hydraulic 

fracturing. 

As the variability was already reflected through the rounding of the rate developed from each data set, 

EPA elected not to round the final average calculated from the four rates. Had EPA rounded the final 

number, it would be 9,000 Mcf per completion or 2% lower. If EPA had not rounded the results of the 

initial studies, the final average well emission factor would be 11,057 Mcf per completion, or 21% 

higher. 

A detailed breakdown of each source and the calculation method to determine the emission factor is 

shown in Table 1 below. The calculation is as follows:
3

completion

completion Mcf

Mcf

completion

Mcf

completion

Mcf

completion

Mcf

9,175

4

6,000 10,000 700 20,000



































































Table 1: Summary of Data Sources Used for the Original EF (9,175 Mcf per completion)

Data Source Year(s) # of 

Wells

Total Gas 

Emissions 

(Mcf)

Average Gas 

Emissions Per 

Completion 

(Mcf)

Formation 

Type

Industry Data

Set #1 - 106 616,887 5,820 Tight Sands

Williams

2002 

to 

2006

1,064 26,014,000 24,449 Tight Sands

Devon 2004 30 357,000 11,900 Shale

Weatherford 2004 3 2,000 667 CBM

Industry Data Set #1

In 2004, an EPA technical contractor received a data set from a natural gas production company. The 

data set included information on volumes of gas sold and flared from 106 completions at wells with 

hydraulic fracturing (see Appendix A). On the basis of this information, the contractor calculated a

national-level estimate of recoverable gas from completions3. In the original work to develop the factor, 

EPA used the national estimate of recoverable gas from completions and information on national well 

counts to calculate average potential emissions per well.4 The calculated results of average value of 

potential emissions per well, 5,815 Mcf per completion, was rounded to the nearest 1,000 Mcf per 

completion (one significant figure), equaling 6,000 Mcf per completion.

The gas producer has since provided EPA with the detailed data set, after removing identifying 

information such as the geographic area and company name. EPA used this data set on number of

completions, gas flared, and sold for each completion to calculate average potential emissions (5,820 

Mcf per completion, which also rounds to 6,000 Mcf per completion). The detailed data set is shown in 

Appendix 1. 

Williams Data

 

3

Green Completions. Natural Gas STAR Producer’s Technology Transfer Workshop. September 21, 2004. 

<http://epa.gov/gasstar/workshops/techtransfer/2004/houston-02.html> Slide 4.

4

Appendix B of the Background Technical Support Document for Subpart W of the Greenhouse Gas Reporting Rule, 

available at http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD.pdf
4

Williams Inc. reported generating 26,014 MMcf of natural gas from 1,064 completions5. Therefore,

[26,014 MMcf]*[1000 Mcf/MMcf] / [1064 completions] = 24,449 Mcf per completion

The 24,449 Mcf per completion was rounded to the nearest 10,000 Mcf per completion, equaling 20,000 

Mcf per completion.

Devon Data

Devon Energy provided EPA6 with data from 30 wells with an average rate of 11,900 Mcf natural gas 

recovered and sold rather than vented. To calculate a potential emissions rate from this data, EPA 

assumed that 90% of the natural gas vented was recovered. Therefore,

[11,900 Mcf/completion] / [90%] = 13,222 Mcf per completion

The 13,222 Mcf per completion was rounded to the nearest 10,000 Mcf per completion, equaling 10,000 

Mcf per completion.

Weatherford Data

Weatherford provided data from 3 well completions7. Those completions together captured 2 MMcf of 

gas. Again to calculate a potential emissions factor, EPA assumed that 90% of the natural gas released

was recovered. Therefore,

[2 MMcf/completion]*[1000 Mcf/MMcf] / ([90%]*[3 completions]) = 740 Mcf per completion

The 740 Mcf per completion was rounded to the nearest 100 Mcf per completion, equaling 700 Mcf per 

completion.

Use of the factor in estimating emissions from completions with hydraulic fracturing

The 9,175 Mcf per completion emission factor represents natural gas released from the completion of a 

hydraulically fractured natural gas well (i.e., potential gas emissions from the completion process in the 

absence of controls to capture or flare the released gas). It is very important to note that in calculating 

total national emissions from completions with hydraulic fracturing for the Inventory of U.S. Greenhouse 

Gas Emissions and Sinks, the EPA adjusts calculated methane release for methane that is actually not 

emitted (i.e., that is instead flared or controlled with certain technologies and practices) due to both 

voluntary action and State regulations. EPA uses this approach across the national inventory for many 

source categories, and it is consistent with internationally agreed guidance from the IPCC.8 

Specifically, the development of the national emissions estimates for unconventional natural gas wells 

requiring hydraulic fracture involves the following key steps:

 

5

EPA. Reducing Methane Emissions During Completion Operations. Natural Gas STAR Producer’s Technology 

Transfer Workshop. September 11, 2007. <http://www.epa.gov/gasstar/documents/workshops/glenwood2007/04_recs.pdf> Slide 14.

6

Green Completions. Natural Gas STAR Producer’s Technology Transfer Workshop. September 21, 2004. 

<http://epa.gov/gasstar/workshops/techtransfer/2004/houston-02.html> Slide 13.

7

Green Completions. Natural Gas STAR Producer’s Technology Transfer Workshop. September 21, 2004. 

<http://epa.gov/gasstar/workshops/techtransfer/2004/houston-02.html> Slide 14.

8

IPCC 2006, 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Prepared by the National

Greenhouse Gas Inventories Programme, Eggleston H.S., Buendia L., Miwa K., Ngara T. and Tanabe K. (eds).

Published: IGES, Japan.
5

 Start with the unmitigated national emission factor (9,175 Mcf of gas per completion).

 Adjust this factor by the average methane content of gas (on a regional basis). 

 Multiply by the number of well completions and recompletions that involve hydraulic fracturing

in each region, and then sum the regional totals to calculate total unmitigated methane 

emissions from the completions and workovers with hydraulic fracturing in the U.S.

 Calculate the amount of the methane that is not emitted, using data on voluntary action and 

State regulations in order to develop a more accurate picture of actual emissions from this 

source category and sector.

 Deduct the calculated amount of methane that is not emitted from the calculated total 

unmitigated methane emissions value to develop the net emissions estimate that is included in 

national emissions totals. The EPA notes that there has been a great deal of confusion about 

the Inventory methods for calculating net emissions. In the 1990-2010 Inventory of U.S. 

Greenhouse Gas Emissions and Sinks (public review draft), EPA has provided improved 

information on this methodology. 

As noted above, EPA acknowledges that some producers conduct well completions using REC techniques 

voluntarily based upon economic and environmental objectives. The EPA normally does not account for 

voluntary actions in baseline emissions estimates when we analyze potential impacts of our regulatory 

actions, since there is no assurance that such voluntary emission reduction measures will continue in the 

future. Additionally, the EPA does not have comprehensive, national-level information on the number 

of RECs currently being performed for reasons other than regulatory compliance. As such, emissions 

reductions obtained from RECs implemented voluntarily are not reflected in our baseline emissions 

estimates in the analysis of potential impacts of this regulatory action. To the extent that RECs would be 

implemented voluntarily absent the NSPS, the compliance costs and emissions reductions obtained from 

the NSPS may be somewhat overstated.

DATA SUBMITTED THROUGH NSPS COMMENTS

This section describes well completions emissions information received through the NSPS public 

comments. Table 2 summarizes this information.

Table 2: Summary of Data Presented to EPA through NSPS Comment Period

Data Source Year(s) # of 

Wells

Total Gas 

Emissions 

(Mcf)

Average Gas 

Emissions 

Per 

Completion 

(Mcf)

Formation 

Type

Noble Energy Unkno

wn

5 16,800 3,360 Tight Sands

ConocoPhillips 2010 101 1,276,290 12,637 Tight Sands, 

Shale, CBM

URS Data 2011 98 71,948 734 Tight Sands, 

Shale

BP 1 2007 50 30,000 600 CBM
6

BP 2 2007 18 16,938 941 CBM

La Plata CEAP Unkno

wn

Unkno

wn

Unknown 1,875 CBM

Southern Ute 

Tribe 

Unkno

wn

Unkno

wn

Unknown 5,000 CBM

Noble Energy Data

In a comment submitted by Noble Energy for the NSPS9, Noble provided average emissions per 

completion for 2 vertical and 3 horizontal wells. These wells were hydraulically fractured and produced 

from tight formations. The average from all 5 wells was 3,360 Mcf per completion. The emissions 

estimates were determined using a calibrated roots meter.

ConocoPhillips Data

In a comment submitted by Earthworks10, Earthworks provided a reference to a presentation given by 

ConocoPhillips for a Natural Gas STAR Workshop in 2009. The presentation includes 101 wells in three 

different formation types. The average from all wells is 12,637 Mcf per completion.

URS Data

A comment submitted by Devon Energy11 contains a study conducted by URS for ANGA. In the study, 7 

companies submitted information relating to hydraulically fractured well completions at nearly 1200 

wells. Of the 7 companies, 4 submitted emissions estimates for 98 well non-green completions (i.e., 

completions without RECs). Emissions estimates were not included for wells with RECs, which 

comprised 92% of the URS data set. For each of the 98 wells without REC completions, URS provided 

information on pressure, flowback duration, and choke size and used Equation W-11B from Subpart W 

of EPA’s Greenhouse Gas Reporting Rule to calculate the emissions from completions. Using this 

equation, and a number of simplifying assumptions, URS calculated an average emissions rate of 765 

Mcf per completion. 

The 765 Mcf per completion is an emissions average per completion for wells in each of 5 basins. The

765 Mcf per completion factor represents the simple average of the average emissions calculated from 

the 5 basins. For the calculations shown below, we have recalculated the rate per completion so that it 

is not weighted by basin, as the other data sets evaluated do not allow for separate estimates by basin. 

Instead, using the data from the 98 wells with emissions estimates, EPA calculated the average 

emissions rate of the data set to be 734 Mcf per completion. 

In reviewing the data and URS’ use of the Subpart W equation, it became apparent that the resulting 

flowrate from Equation W-11B was misinterpreted to result in a flow rate at standard conditions when it 

was in fact a flowrate at actual conditions. Converting from actual volume (actual conditions) to 

standard volume (standard conditions), which is the input intended to estimate emissions from gas well 

venting during completions and workovers following hydraulic fracturing in Equation W-10A, EPA recalculated an average value greater than 50,000 Mcf per completion. This value is far higher than the 

value presented by URS (765 Mcf per completion) and values given in other data sets. EPA’s evaluation 

shows that the URS study is significantly underestimating emissions by using the equation at actual 

 

9

http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OAR-2010-0505-4228

10 http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OAR-2010-0505-4275

11 http://www.epa.gov/quality/informationguidelines/documents/12003-attB.pdf
7

conditions, however, due to highly conservative assumptions made by URS (e.g., 100% gas in flowback 

instead of a mixture of gas and fluids, and maximum choke size and casing pressure), the recalculated 

emissions average of over 50,000 Mcf per completion may also be an inaccurate depiction of emissions. 

Given this uncertainty, EPA performed the analyses presented below both with (using the URScalculated values per completion, averaged to 734 Mcf as noted above) and without the URS emissions 

estimates, for completeness and transparency. 

BP

In a comment submitted by Earthworks9, the commenter referenced two BP presentations. The two 

data sets corresponded to well completions carried out in CBM formations. The first BP data set 

contained 50 well completions and had average emissions of 600 Mcf per completion. The second BP 

data set contained 18 well completions and had average emissions of 941 Mcf per completion.

La Plata CEAP

In a comment submitted by Earthworks9, the commenter referenced a La Plata Climate and Energy 

Action Plan process in which two companies (BP and Red Cedar, the Southern Ute Indian Tribe’s gas 

company) estimated that green completions could recover 1,875 Mcf per completion in the San Juan 

Basin based on industry completion data.

Southern Ute Tribe

In a comment submitted by Earthworks9, the commenter referenced a figure from a Programmatic 

Environmental Assessment (PEA) for 80‐acre Infill Oil & Gas Development on the Southern Ute Indian 

Reservation developed by Environ. The emissions estimate was 5,000 Mcf per completion.

EVALUATION OF ALTERNATIVE EF CALCULATIONS

In this section, EPA presents an analysis to determine whether or not the new information submitted 

through NSPS comments indicates a need to revise the emission factor used in the proposal, and to 

present and evaluate alternative emission factors. 

To determine how the factor might change by including data received through the public comment 

period, EPA used the data submitted through NSPS comments (Table 2 above) in combination with the 

data used for the original emission factor calculation (Table 1 above) to calculate a number of 

alternative emission factors for consideration. 

First, EPA developed two simple averages of the data: alternative factor 1, which is an average by well 

completion, and alternative factor 2, which is an average by data source. 

The data used to calculate alternative factors 1, and 2, which includes the data used in the original 

emission factor and the data submitted through NSPS comments is presented in Table 3 below. 
8

Table 3: Data Used for Alternative Factors 1 and 2

Data Source Year(s) # of Wells

Total Gas 

Emissions 

(Mcf)

Average Gas 

Emissions Per 

Completion 

(Mcf)

Formation 

Type

Industry Data

Set #1*

Unknown 106 616,887 5,820 Tight Sands

Williams*

2002 to 

2006 1,064 26,014,000 24,449 Tight Sands

Noble Energy Unknown 5 16,800 3,360 Tight Sands

ConocoPhillips 2010 101 1,276,290 12,637 Tight Sands, 

Shale, CBM

URS Data 2011 98 71,948 734 Tight Sands, 

Shale

Devon* 2004 30 357,000 11,900 Shale

Weatherford* 2004 3 2,000 667 CBM

BP 1 2007 50 30,000 600 CBM

BP 2 2007 18 16,938 941 CBM

La Plata CEAP Unknown Unknown Unknown 1,875 CBM

Southern Ute 

Tribe Unknown Unknown Unknown 5,000 CBM

* Denotes sources used for the original emission factor calculation.

Alternative Factor 1: Average by Well Completion

The first alternative emission factor assessed was calculated by summing the total emissions from all 

data sets and dividing by the total number of completions. The La Plata CEAP and Southern Ute Tribe 

data points cannot be included when calculating emission factors using this method because these 

sources did not provide total emissions and total number of well completions. The resulting emission 

factor was about 20,574 Mcf per completion. Including the URS data, the calculated value is 19,256 

Mcf per completion. 

This method results in an emission factor heavily affected by the number of well completions included in 

a given data set resulting in bias for one formation type. Tight sands are over represented, as 86% of 

the well completions in the data set were in tight sands, while nationally 26.5 percent of annual 

completions are tight sands (NEMS 2010). Further, one data source (Williams) accounts for 1,064 of the 
9

1,377 well completions, which may bias the result towards the Williams data, which has an average 

emissions rate of 24,449 Mcf per completion.

Alternative Factor 2: Average by Data Source

The second alternative emission factor for assessment was estimated by performing a simple 

unweighted average of the average emissions per completion reported for each data source. This 

method calculated an average emission factor for each data source individually and then calculated a 

simple average of the 10 resulting emission factors. The average emission factors ranged from 600 Mcf 

per completion to 24,449 Mcf per completion. Of the 10 data sources, 3 corresponded to tight sands, 1 

to shale, 5 to CBM, and 1 was a mix of all three formations. The resulting emission factor was about 

6,725 Mcf per completion. Including the URS data, the calculated factor is 6,180 Mcf per completion. 

This method has the advantage of having no over-representation due to number of well completions 

included in a given data set resulting in bias for one formation type. However, it does result in overrepresentation of CBM completions due to the number of data sets including those types of completions 

(6 out of 10 data sources). In the U.S., 26.9 percent of annual completions are CBM (NEMS 2010). It 

also results in underrepresentation of shale completions, as only 2 of the data sources included shale 

completions. In the U.S. 46.5% of annual well completions are in shale (NEMS 2010). 

Alternative Factors 3 through 5

Next, a number of alternatives that weigh the factor by formation type were developed. Based on the 

data EPA has evaluated, formation type can greatly influence the volume of gas released per 

completion. Based on observations of the available data from the U.S. oil and natural gas industry, of 

the three formation types included here, coal bed methane typically has the lowest volume of gas 

released per completion, and shale typically has the highest. As the national average gas release per 

completion will therefore be influenced by the mix of completions nationally, here we examine factors 

that take this mix into account. To develop these weighted factors, we disaggregated data sets that 

included more than one formation type. The Conoco Phillips data set included three formation types, 

and was split into three data sets, and the URS data included two formation types and was likewise split 

into two data sets. Table 5 below presents this disaggregation. 

Table 5: Data Used for Alternative Factors 3, 4, and 5

Data Source Year(s) # of 

Wells

Total Gas 

Emissions 

(Mcf)

Average Gas 

Emissions Per 

Completion 

(Mcf)

Formation 

Type

Industry Data

Set #1*

Unkno

wn

106 616,887 5,820 Tight 

Sands

Williams*

2002 

to 

2006

1,064 26,014,000 24,449 Tight 

Sands

Noble Energy Unkno

wn

5 16,800 3,360 Tight 

Sands

ConocoPhillips 2010 10 60,222 6,022 Tight 

Sands
10

URS Data 2011 41 16282 397 Tight 

Sands

ConocoPhillips 2010 83 1,200,000 14,458 Shale

Devon* 2004 30 357,000 11,900 Shale

URS Data 2011 57 55,666 977 Shale

Weatherford* 2004 3 2,000 667 CBM

BP 1 2007 50 30,000 600 CBM

BP 2 2007 18 16,938 941 CBM

La Plata CEAP Unkno

wn

Unkno

wn

Unknown 1,875 CBM

Southern Ute 

Tribe

Unkno

wn

Unkno

wn

Unknown 5,000 CBM

ConocoPhillips 2010 8 16,068 2,009 CBM

* Denotes sources used for the original emission factor calculation.

Alternative Factor 3: Weighting by 3 formation types based on production per formation type

Using all of the data for which we had information on formation type, an emission factor for tight sands

(9,913 Mcf per completion), an emission factor for shale (13,179 Mcf per completion), and an emission 

factor for CBM (1,849 Mcf per completion) were generated.12 A final emission factor was then 

determined using a weighted average based on ratios of production from each formation to the total 

production from CBM, shale, and tight sands (see Table 6 below). The resulting emission factor was 

about 9,984 Mcf per completion. Including the URS data, the calculated factor is 7,554 Mcf per 

completion. 

This alternative factor has the advantage of having no over-representation of a given formation type due 

to greater number of data sets or wells. This method recognizes within the calculation methodology 

that formation can be a significant variable affecting the total emissions per completion. However, a 

ratio of gas production may not be a good surrogate for a ratio of completion flowback volumes. In 

addition, a ratio of gas production might not be representative of the current completions being carried 

out in each formation.

 

12 Including the URS, the calculated emissions rate for tight sands is 8,010 Mcf/completion and for shale is 9,111 

Mcf/completion. The URS data set did not include data on CBM, so that rate is unaffected. 
11

Table 6: Summary of Data Used for Weighting of Alternative Factor 3

U.S. Gas Production 

(Mcf/year)

Fraction of average 

production flowrate

Tight Sands 6,287,544,084 0.49

Shale 4,656,526,998 0.37

CBM 1,773,088,295 0.14

U.S. Total 

Production

(CBM, Shale, Tight 

Sands)

12,717,159,377 1.0

Alternative Factor 4: Weighting by 3 formation types, based on the number of completions for each 

formation type

The same calculated emissions rates for each of the three formation types used to calculate Alternative 

Factor 3 were used to calculate Alternative Factor 4. A final emission factor was then determined using 

a weighted average based on the number of well completions carried out in each formation according to 

the NEMS models (see Table 7 below). The resulting emission factor was about 9,263 Mcf per 

completion. Including the URS data, the value is 6,864 Mcf per completion.

Table 7: Summary of Data Used for Weighting of Alternative Factors 4 and 5

U.S. Completion 

Counts

% of Total

Tight Sands 2,393 26.5%

Shale 4,196 46.5%

CBM 2,426 26.9%

U.S. Total (All 

formation types) 9,015 100%
12

This alternative factor has the advantages of having no over-representation of a given formation type 

due to greater number of data sets, and that it represents current trends in the natural gas industry 

based on the formations being developed. This method recognizes within the calculation methodology 

that formation can be a significant variable affecting the total emissions per completion. This method 

also weighs by the number of completions by formation type, which is more closely corresponds to total 

completion emissions per formation, than production by formation. 

Alternative Factor 5: Weighing by 2 formation categories (CBM and tight sands/shale), based on 

number of completions in formation categories

As data for tight sands and shale showed similar levels of emissions, EPA evaluated an option that 

treated the two formation types as one. To develop the fifth alternative emission factor for analysis, the 

data was first separated into one of two groups according to formation: tight/shale and CBM. An 

emission factor for tight sands/shale (11,025 Mcf per completion)13 and an emission factor for CBM 

(1,849 Mcf per completion) were generated. 

A final emission factor was then determined using a weighted average based on the number of well 

completions carried out in each formation according to the National Energy Modeling System (NEMS) 

models. The resulting emission factor was about 8,538 Mcf per completion. Including the URS data, the 

factor is 7,466 Mcf per completion. 

This alternative factor has the advantages of having no over-representation of a given formation type 

due to greater number of data sets, and that it represents current trends in the natural gas industry 

based on the formations being developed. This method recognizes within the calculation methodology 

that formation can be a significant variable affecting the total emissions per completion, but that 

emissions from tight sands and shale formations may not be significantly different. This method also 

weighs by the number of completions by formation type, which is more closely corresponds to total 

completion emissions per formation, than production by formation. 

FINAL EMISSION FACTOR

The EPA evaluated all of the data (both data used to develop the original factor and data from the 

commenters) in several ways to determine whether an improved emission factor could be developed. 

Every calculation methodology considered had positive and negative aspects with respect to arriving at 

a single average emissions factor to represent U.S. flowback emissions following hydraulic fracture of a 

gas well. Excluding the URS data, 3 of the 5 analyses produced factors higher than the 9,175 factor, and 

2 produced lower results. Including the URS data, 1 of the analyses produced a factor higher than 9,175, 

and 4 analyses produced factors lower than 9,175. 

 

13 Including the URS data, this value is 9,969 Mcf/completion. 
13

Table 8: Summary of Results

Alternative 

Factor 

Number Description

Factor Value

(Mcf per 

completion)

1 Average by Well Completion 20,574

2 Average by Data Source 6,725

3 Weighting by 3 formation types based on production per formation 

type

9,984

4 Weighting by 3 formation types, based on the number of 

completions for each formation type

9,263

5 Weighing by 2 formation categories (CBM and tight sands/shale), 

based on number of completions in formation categories

8,538

The alternative factors weighted by formation type have the advantage of having no overrepresentation of a given formation type due to greater number of data sets or wells. These methods 

recognize within the calculation methodology that formation can be a significant variable affecting the 

total emissions per completion. The factors weighted by formation type were very similar to EPA’s 

emission factor of 9,175. 

The five different calculation methodologies incorporating new data submitted through NSPS comments 

result in emission factors that fall within the expected interval estimate calculated for EPA’s original 

factor (See Appendix B for a statistical analysis of the original factor). Additionally, EPA’s original factor 

is closest to the results produced when formation type is taken into account, which is a key factor in 

influencing the emission rate. As a result of this analysis EPA concludes that, although it does have 

uncertainty, the original EPA emission factor provides a valid central estimate of emissions from this 

source in the U.S. Therefore, EPA has decided to retain the data set and the methodology used to 

develop the original emission factor of 9,175 thousand cubic feet (Mcf) per completion. 

EPA acknowledges that it did not apply a consistent approach to rounding of significant figures in each 

individual steps of the original calculation. To address this inconsistency, EPA is making a minor revision 

by rounding the end result to 1 significant figure. The final emission factor is 9,000 Mcf per 

completion.
14

APPENDIX A: Industry Data Set #1

This appendix contains Industry Data Set #1. This data set includes vertical gas wells drilled before 2004, 

that were in tight sand and hydraulically fractured and were used to assist in development of the 

Reduced Emissions Completions Lessons Learned study. This data set was used in developing the first 

depiction of national methane emissions potential from hydraulic fracturing completion, presented in a 

Gas STAR workshop September 21, 2004.

The data set is provided in the below table. Each row contains one well completion at a well with 

hydraulic fracture. The columns report the total gas flared during the completion event, the total gas 

sold (recovered by reduced emissions completion methods into the flow line), and the sum of the flared 

and sold volumes which represents the total gas produced by the well during the completion event. 

At the bottom of the table are the sums for the total completions, total gas flared, total gas sold, and 

total gas produced. The sums are then used to calculate an average emissions factor.

Well Completion 

Number

Flared Gas 

(Mcf)

Gas Sold (Mcf)

TOTAL Mcf

PRODUCED PER 

COMPLETION

1 878 6,919 7,797

2 3,293 8,638 11,931

3 3,348 2,386 5,734

4 1,670 2,339 4,009

5 7,095 800 7,895

6 2,884 11,549 14,433

7 1,553 4,503 6,056

8 4,005 169 4,174

9 6,614 363 6,977

10 2,536 3,377 5,913

11 2,043 5,203 7,246

12 1,010 3,792 4,802

13 3,586 149 3,735

14 12,659 4,692 17,351

15 4,307 970 5,277

16 4,053 1,105 5,158

17 3,987 12,058 16,045

18 1,320 964 2,284

19 1,576 2,133 3,709

20 2,752 3,903 6,655

21 4,063 2,629 6,692

22 5,591 12,652 18,243

23 2,550 3,696 6,246

24 1,057 7,429 8,486

25 2,206 9,085 11,291

26 1,274 6,832 8,106

27 4,333 6,558 10,891
15

28 4,626 941 5,567

29 1,091 23 1,114

30 861 278 1,139

31 3247 331

32 774 1,197 1,971

33 1,503 4,489 5,992

34 6,525 7,398 13,923

35 1,125 2,027 3,152

36 1,894 1,723 3,617

37 1,398 4,876 6,274

38 2,104 11,331 13,435

39 1,149 3,641 4,790

40 803 420 1,223

41 2,619 445 3,064

42 889 2,532 3,421

43 1530 153

44 1,749 3,986 5,735

45 1,450 1,248 2,698

46 807 277 1,084

47 1,8330 1,833

48 1,577 3,508 5,085

49 1,219 1,552 2,771

50 1,557 4,444 6,001

51 1,880 5,795 7,675

52 2,501 4,096 6,597

53 5,698 211 5,909

54 2,355 1,922 4,277

55 971 2,034 3,005

56 6,213 159 6,372

57 2,599 2,592 5,191

58 1,502 8,212 9,714

59 2,219 931 3,150

60 6,114 1,925 8,039

61 1,020 740 1,760

62 594 2,727 3,321

63 2,189 5,840 8,029

64 1,350 1,528 2,878

65000

66 2,674 3,872 6,546

67 2,424 3,352 5,776

68 941 3,639 4,580

69 1,126 4,683 5,809

70 1,264 2,117 3,381

71 1,833 1,953 3,786

72 1,513 4,940 6,453

73 9,157 9,568 18,725

74 3,026 7,770 10,796

75 3,026 501 3,527
16

76 16,849 5,802 22,651

77 1,768 2,137 3,905

78 9,234 1,027 10,261

79 2,191 4,017 6,208

80 0 0 0

81 5,300 1,260 6,560

82 1,694 2,022 3,716

83 673 1,234 1,907

84 313 2,651 2,964

85 0 0 0

86 3,883 5,211 9,094

87 1,800 4,296 6,096

88 1,468 3,237 4,705

89 2,946 8,984 11,930

90 1,059 1,715 2,774

91 2,381 5,754 8,135

92 882 658 1,540

93 255 1,933 2,188

94 742 2,356 3,098

95 889 0 889

96 844 3,606 4,450

97 1,052 2,863 3,915

98 1,006 581 1,587

99 502 723 1,225

100 3,470 3,079 6,549

101 2,529 5,688 8,217

102 4,725 3,283 8,008

103 523 1,203 1,726

104 2,469 5,548 8,017

105 602 1,571 2,173

106 812 4,782 5,594

Well Completions Flared Gas (Mcf) Gas Sold (Mcf)

Total Produced Gas

(Mcf)

TOTAL 106 267,323 349,564 616,887

EF Calculation: 

completion

Mcf

completions

Mcf 5,820

106

616,887



APPENDIX B: Statistical Analysis Memo
17

Development of a Bayesian Posterior Interval for the 

Emission Factor for Hydraulically Fractured Well 

Completions

OVERVIEW

EPA conducted a statistical analysis of the data and developed a Bayesian posterior interval to assess the 

validity of the emission factor for hydraulically fractured completions. This memo describes that 

analysis.

Data set for analysis

As described in the main text of this memo, the development of the emission factor of 9,175 Mcf per 

completion used data from four data sets, together representing data from over 1,000 

well completions with hydraulic fracturing. To calculate a robust interval estimate for the 

emission factor EPA used data on emissions reductions reported by industry to EPA’s Natural Gas STAR 

program. The data set includes several years of data, and contains 56 data entries on reductions from 

over 9,000 completions at wells with hydraulic fracturing. This larger data set was able to provide a 

robust interval estimate for the emission factor. 

As will be shown in this memo, the distributional characteristic of the Gas STAR data appeared to be 

similar in nature to the data used to calculate the emission factor. Based on these statistical similarities 

it is justifiable to use the Gas STAR data to construct prior distributions for purposes of improving the 

estimate of variance and interval estimates centered on the emission factor. Specifically, the Gas STAR 

data is used to estimate the variance of the emission factor. 

Gas content 

The emission factor of 9,175 Mcf per completion represents total potential gas emissions. The Gas STAR 

data represents potential methane emissions (i.e., a subset of the total potential gas emissions). To 

develop a meaningful comparison for the purposes of this analysis, we’ve converted the 9,175 Mcf 

whole gas emission factor to a methane emission factor of 8,900 Mcf CH4 per completion, using a 

methane content value of 83.24% (ECR 2011) and the same averaging and rounding methodologies that 

were used in the calculation of the 9,175 factor. More detail is presented below. 

Within Data Set Variation 

Due to the nature of the data—data entries in Gas STAR are aggregate, not at the well-level--a within 

data set estimate of variance was unobtainable. All estimates of variance explored here are due to 

between data set variability. For this reason, it is possible that the overall variance presented in this 

memo is an underestimate of the true amount of variability. However, if the between source variability 

is significantly greater than the within source variability then the bias in the variance is likely to be small. 

Bayesian Model 
18

A relatively simple model was chosen for purposes of analysis14. The four observations of the emission 

factors data are assumed to be normally distributed with a mean of and a known variance of 

 

set 

equal to the sample variance. From prior knowledge, it is reasonable to assume that the mean is 

normally distributed with known mean and variance 

 

. The mean of the prior distribution will be set 

to the mean of the sample, which is the current emission factor, converted to Mcf CH4 per completion, 

of 8,900. As a result, the posterior distribution will be normal with a mean equal to the emission factor 

(8,900) and variance that is a sum of inverse of the prior variance, 

 

, and the inverse of the sample 

variance, 

 ⁄ . The sample size for the emission factor data set is 4. Therefore the posterior variance 

of the emission factor is:

 

 

 

(

 

 

 

 

 

 

)

Note that the sample size is 4. The interval estimate will be based on the posterior distribution of the 

mean. 

DATASETS

Emission Factor Data

Information from the 4 data sets used to calculate the emission factor is shown in Table 1 below. The 

data sets together represent emissions calculated for over 1,000 well completions; however, not all of 

the data sets presented well-level information. As such, we were unable to analyze the within data set 

variance. The values presented below are the average emissions for each data set. The column labeled 

‘Whole Gas’ shows the value used to develop the Mcf gas per completion value of 9,175. The column 

labeled ‘Modified’ is the Whole Gas column multiplied by the methane content correction factor of 

0.8324. The final column labeled ‘Rounded’ is the values that are used to calculate the emission factor. 

Table 1 Data Used to Calculate the Emission Factor

Data Source

Whole 

Gas Modified Rounded

Weatherford

667 555 600

Industry Data Set 

#1

5820 4844 5000

Devon

11900 9905 10000

 

14 Gelman, A., Carlin, J. B., Stern, H. S., & Rubin, D. B. (1995). Bayesian Data Analysis. Boca

Raton, FL: Chapman & Hall/CRC.
19

William

24449 20351 20000

The average of the four rounded values is 8900 and the standard error is 4168. Due to the small number 

of data sets (4) the confidence interval around the emission factor that is very wide. Using the standard 

distributional assumptions, the 95% confidence interval based on the 4 data sets is:

 ̅ ̅ 

where is a value from the T-distribution with 3 degrees of freedom. The values for ̅ and ̅ 

are given in Table 2. Using these values the 95% CI for the current emission factor is (-8,510, 26,100). In 

the sections below the goal will be to obtain a more useful interval estimate for the emission factor.

Table 2 Descriptive Statistics for Emission Factor Data

Observations 4

Minimum 555

Maximum 20,351

Mean 8,900

Variance 6.95+07

Standard Error 4168.5

Gas STAR Data

A key assumption for employing Bayesian analysis in this situation is that the data used to calculate the 

emission factor should have a similar distribution to the Gas STAR data. If this assumption appears to be 

reasonable then the Gas STAR data can be used to create a prior distribution for the emission factor. In 

order to provide justification for this assumption the means, variances and ranges of the Gas STAR data 

are compared to the emission factor data. The Gas STAR data set reviewed contained 55 data entries 

with information on completion type. The Williams observation, which was included in both the 

emission factor and in the Gas STAR data set, was removed from the Gas STAR data for purposes of 

calculating theses statistics which are shown in Table 3. Inclusion of this observation would imply that 

the statistics calculated using the Gas STAR data are correlated with statistics from the emission factor 

data. This type of correlation would make the analysis needlessly complicated. 

Table 3 Descriptive Statistics for all Gas STAR Data

Observations 55

Minimum 76

Maximum 65,322

Mean 9,174

Variance 1.25E+08

Mean

The mean of the Gas STAR data is 9,174 Mcf CH4 per completion, which compares favorably to the 

converted methane emission factor of 8,900. Given the amount of variability in the data, these values 

are statistically equivalent. 

Range
20

The range of observations for the emission factor data are (555, 20,351) and for Gas STAR the range is 

(76, 65,322). It should be noted that the second largest value in the Gas STAR data is 27,305. While the 

range of the Gas STAR data appears to be greater than the emission factor data, it appears to be a 

function of one large observation. If that observation is removed then the ranges of the two datasets 

are nearly identical. The minimum for the emission factor data is 555. Of the 55 Gas STAR observations

only 6 have a value less than 555. This implies that the value of 555 is not an expected minimum when 

only four observations are in the sample. 

Variance of the Mean

Assuming that the observations in the Gas STAR dataset are independent and identically distributed, the 

variance of the mean is calculated by dividing the variance shown above by the sample size. Thus:

 ̂

 

 

 

 or 2.27E6

The standard error of the mean is the square root of this value which is 1,506. 

Corrected Standard Error for the Emission Factor

The estimate of the standard error for the emission factor ̂ can now be determined using the Gas 

STAR information and the emission factor data. This will be done by using Bayesian analysis. The Gas 

STAR data will be used to create a normal prior distribution for the mean emission factor value. This 

prior will has a mean of 8900 and variance 

 

as defined above. The new variance of emission factor the 

will be determined using the following formula: 

 ̂ 

 

 

(

 

 ̂

 

 

 

 

)

where 

 

is the variance of four emission factor observations (6.95E7). This gives:

 ̂ 

 

and

 ̂ 

The posterior distribution for the emission factor is then N(8,900, 2,006,067). The 95% prediction 

interval is 

 

This gives bounds of (6,123, 11,676).

SUMMARY OF RESULTS
21

The 95% posterior interval for the converted methane emission factor of 8,900 Mcf methane per 

completion was determined to be (6,123, 11,676). As this is a Bayesian interval estimate the 

interpretation is there is a probability of 0.95 that the true emission factor is between 6,123 and 11,676 

given that the posterior distribution for the emission factor is N(8,900, 2,006,067). 

The posterior interval shown above is calculated for the methane emission factor of 8,900. It is likely 

that the emission factor of 9,175 Mcf whole gas per completion, and the rounded factor of 9,000 Mcf

whole gas per completion, would have a similar interval estimate as that of the methane emission 

factor. The center of the two interval estimates would clearly be similar (8,900 versus 9,000 or 9,175). 

However, it is likely that the whole gas interval would be wider as the variance would likely be larger. As 

a result, it is reasonable to assume that probability that the true emission factor for whole gas being 

contained in the interval 6,123 and 11,676 is similar to 0.95.