Characterizing Pivotal 

Sources of Methane 

Emissions from Natural 

Gas Production

Summary and Analysis of API and ANGA Survey 

Responses

Terri Shires and Miriam Lev-On

URS Corporation and The LEVON Group

FINAL REPORT (Updated September, 2012)

September 21, 2012
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) i

Table of Contents

1. Overview..........................................................................................................................1

1.1 Context......................................................................................................................1

1.2 Introduction to the API/ANGA Survey ......................................................................3

2. Well Data .........................................................................................................................4

2.1 National Gas Well Counts .........................................................................................4

2.2 Gas Well Completions...............................................................................................6

2.3 Data Limitations Concerning Wells...........................................................................9

3. Gas Well Liquids Unloading...........................................................................................11

4. Hydraulic Fracturing and Re-fracturing (Workovers)......................................................17

4.1 API/ANGA Survey..................................................................................................17

4.2 WRAP Survey.........................................................................................................19

4.3 Impact of Completions and Re-fracture Rate Assumptions ......................................21

4.4 Completion and Re-fracture Emission Factor...........................................................23

4.5 Data Limitations for Completion and Re-fracture Emissions....................................23

5. Other Surveyed Information ...........................................................................................25

5.1 Centrifugal Compressors .........................................................................................25

5.2 Pneumatic Controllers .............................................................................................26

6. Conclusions....................................................................................................................28

7. References......................................................................................................................30

Appendix A. API/ANGA Survey Forms .....................................................................................32

Appendix B. ANGA/API Well Survey Information.....................................................................37

Appendix C. Emission Estimates for Gas Well Liquids Unloading............................................41
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) ii

Tables

ES-1. Emission Comparison between EPA and Industry Data ....................................... iv

1. API/ANGA Survey – Summary of Gas Well Counts by Type and NEMS Region ..... 5

2. API/ANGA Survey – Additional Details on Gas Well Counts.................................. 7

3. API/ANGA Survey – Summary of Liquids Unloading Data ..................................... 8

4. API/ANGA Survey –Liquids Unloading Emissions Comparison.............................. 8

5. API/ANGA Survey – Summary of Gas Well Workovers with Hydraulic Fracturing in 

2010 and First Half of 2011 by NEMS Region And Well Type (First Survey Data 

Request Phase) .....................................................................................................12

6. API/ANGA Survey – Summary of 2010 Gas Well Workovers on Unconventional

Wells by AAPG Basin and NEMS Region (Second Survey Data Request Phase).....13

7. WRAP Survey – Summary of Gas Well Workovers by AAPG Basin for the Rocky 

Mountain Region, 2006 Data.................................................................................16

8. API/ANGA Survey –Gas Well Workover Emissions Comparison ...........................18

9. WRAP Survey – Summary of Completion Emissions for the Rocky Mountain 

Region, 2006 Data ................................................................................................19

10. ANGA Survey – Summary of Completion Emissions.............................................21

11. API/ANGA Survey –Pneumatic Controller Counts.................................................22

12. Pneumatic Controller Emission Comparison – Production Operations.....................24

13. Pneumatic Controller Emission Comparison – Production Operations.....................25

14. Emission Comparison between EPA and Industry Data ..........................................26

B-1. ANGA/API Survey – Summary of Gas Well Counts by Type and NEMS Region ..36

B-2. ANGA/API Survey – Additional Details on Gas Well Counts...............................37

C-1. Liquids Unloading for Conventional Gas Wells without Plunger Lifts ..................40

C-2. Liquids Unloading for Conventional Gas Wells with Plunger Lifts .......................41

C-3. Liquids Unloading for Unconventional Gas Wells without Plunger Lifts...............42

C-4. Liquids Unloading for Unconventional Gas Wells with Plunger Lifts....................45

Figures

1. Comparison of EPA to API/ANGA Gas Well Count Data by AAPG Basin .............29

A.1 Survey Instructions ...............................................................................................30

A.2 Gas Well Survey Data ...........................................................................................31

A.3 Gas Well Workover Survey Data ...........................................................................32

A.4 Gas Well Liquids Unloading Survey Data..............................................................33

A.5 Other Survey Data ................................................................................................34
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) iii

Executive Summary

This document presents the updated results from a collaborative effort among members 

of the American Petroleum Institute (API) and America’s Natural Gas Alliance (ANGA) to

gather data on key natural gas production activities and equipment emission sources that are 

essential to developing estimates of methane emissions from upstream natural gas production. 

API and ANGA members undertook this effort as part of an overall priority to develop

new and better data about natural gas production and make this information available to the 

public. This information acquired added importance in 2011, when the EPA released an 

inventory of U.S. greenhouse gases (GHG) emissions that substantially increased estimates of 

methane emissions from Petroleum and Natural Gas Systems. Public comments submitted by 

both trade associations reflected a number of concerns – most notably that EPA’s estimates were 

based on a small set of data submitted by a limited number of companies in a different context 

(i.e., data not developed for the purpose of estimating nationwide emissions). 

The API/ANGA data set (also referred to as ANGA/API) provides data on 91,000 wells 

distributed over a broad geographic area and operated by over 20 companies. This represents 

nearly one-fifth (18.8%) of the estimated number of total wells used in EPA’s 2010 emissions 

inventory.1 The ANGA/API data set is also more than 10 times larger than the set of wells in 

one of EPA’s key data sources taken from an older Natural Gas Star sample that was never 

intended for developing nationwide emissions estimates. Although more and better data efforts 

will still be needed, API/ANGA members believe this current collaborative effort is the most 

comprehensive data set compiled for natural gas operations. 

As Table ES-1 demonstrates, survey results in two source categories – liquids 

unloading and unconventional gas well re-fracture rates - substantially lower EPA’s estimated 

emissions from natural gas production. The right-hand column of this table shows the impact of 

ANGA/API data on the estimated emissions for each source category. Gas well liquids 

unloading and the rate at which unconventional gas wells are re-fractured are key contributors to 

the overall GHG emissions estimated by EPA in the national emissions inventory. For example, 

methane emissions from liquids unloading and unconventional well re-fracturing accounted for 

59% of EPA’s estimate for overall natural gas production sector methane emissions. Overall, 

API/ANGA activity data for these two source categories indicate that EPA estimates of potential 

emissions from the production sector of “Natural Gas Systems” would be 53% lower if EPA 

were to use ANGA/API’s larger and more recent survey results. 

 

1 EPA’s 2010 national inventory indicates a total of 484,795 gas wells (EPA, 2012).
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) iv

TABLE ES-1. EMISSION COMPARISON BETWEEN EPA AND INDUSTRY DATA

Source 

Category EPA API/ANGA

Impact on Source 

Category 

Emissions

Metric tons of CH4 % of EPA 

Emissions 

Total

Metric tons of CH4 % of 

Revised 

Emissions 

Total

% Difference in 

Emissions

Gas Wells Liquids 

Unloading

4,501,465 * 51% 319,664 8% -93%

Unconventional 

Well Re-fracture 

Rates

712,605 * 8% 197,311 5% -72%

Other Production 

Sector Emissions**

3,585,600 41% 3,585,600 87%

Total Production 

Sector Emissions

8,799,670 4,102,575 -53%

* EPA’s estimates are adjusted to industry standard conditions of 60 degrees F and 14.7 psia for comparison 

to the ANGA/API emission estimates.

**The “Other Production Sector Emissions” are comprised of over 30 different source categories detailed in 

Table A-129 in the Annex of the EPA’s 2012 national inventory. The “Other Production Sector 

Emissions” are the same values for this comparison between the EPA national inventory and the 

API/ANGA survey to focus the comparison on quantified differences in emission estimates for gas well 

liquids unloading and unconventional well re-fracture rates.

As mentioned above, the differences between EPA and ANGA/API estimates hinge on 

the following key differences in activity data and thus considerably impact overall emissions 

from Natural Gas Systems:

 Liquids unloading and venting. API/ANGA data showed lower average vent times, a 

significantly lower percentage of wells venting for liquids unloading, and a higher 

number of vents per vented well than EPA assumed in their inventory calculation. The 

API/ANGA survey also found a slightly larger percentage of wells with plunger lifts than 

EPA assumed, but a significant percentage of wells with artificial lift which EPA had not 

considered. These findings are particularly significant because liquids unloading 

accounted for 51% of EPA’s total “Natural Gas Systems” methane emissions in the 2010

inventory. Applying emission factors based on ANGA/API data reduces the calculated

emissions for this source by 93% (from 4,501,465 metric tons of CH4 to 319,664 metric 

tons of CH4 when compared on an equivalent basis) from EPA’s 2010 national GHG 

inventory.

 Re-fracture rates for unconventional wells. API/ANGA members collected data on refracture rates for unconventional wells in two phases. The first phase collected data for 

all well types (conventional and unconventional), while the second phase targeted 

unconventional gas wells. Both phases of the survey data show significantly lower rates 

of well re-fracturing than the 10% assumption used by EPA. As discussed in detail in 

API & ANGA - EPA

EPA
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) v

this report, the re-fracture rate varied from 0.7% to 2.3%. The second phase of the survey 

gathered data from only unconventional well activity and using the re-fracture rate data 

from this second phase of the ANGA/API survey reduces the national emission estimate 

for this source category by 72%, - from 712,605 metric tons of CH4 to 197,311 metric 

tons of CH4 when compared on an equivalent basis.

This report also discusses an important related concern that the government lacks a single 

coordinated and cohesive estimate of well completions and well counts. Although the 2010 

national GHG inventory appears to under-represent the number of well completions according to 

the numbers reported through both the API/ANGA data and IHS CERA, differences in national 

well data reporting systems make it difficult to accurately investigate well completion 

differences with any certainty. The EPA inventory, which uses data from the Energy 

Information Administration, various state governments and privately sourced data, such as 

HPDI, does not consistently distinguish between conventional and unconventional wells. 

The concept of conventional and unconventional wells is used differently by 

different stakeholders, is not particularly helpful from an emissions standpoint, and should 

be abandoned in favor of classifications more relevant to the emission source categories 

being evaluated. For instance, the scale of emissions from well completions is primarily 

associated with whether the wells are hydraulically fracture stimulated, the size/stages of the 

hydraulic fracture stimulation, and practices for handling the well clean-up/flow-back post 

fracture stimulation. 

Without a consistent measure for the quantity and type of wells, it is difficult to be 

confident of the accuracy of the number of wells that are completed annually, let alone the 

amount of emissions from them. Natural gas producers strongly believe that the effects of any 

possible under-representation of well completions will be offset by a more realistic emission 

factor for the rate of emissions per well. Analysis of the data reported under Subpart W of the 

GHG reporting rule should be used to inform a more realistic emission factor for well 

completions.

This survey also collected data on centrifugal compressors and pneumatic controllers. 

While the sample sizes are too small to make strong conclusions, the results discussed in the 

body of the report indicate that further research is necessary to accurately account for the 

different types of equipment in this area (e.g., wet vs. dry seal centrifugal compressors and “high 

bleed,” “low bleed,” and “intermittent bleed” pneumatic controllers). 

As government and industry move forward in addressing emissions from unconventional 

gas operations, three key points are worth noting:

 In addition to the voluntary measures undertaken by industry, more data will become 

available in the future. Emission reporting requirements under Subpart W of the 

national Greenhouse Gas Reporting Program (GHGRP) went into effect January 1, 2011 

with the first reporting due in the fall of 2012. As implementation of the GHGRP 

progresses from year to year, the natural gas industry will report more complete and more 

accurate data. If EPA makes use of the data submitted and transparently communicates 

their analyses, ANGA/API members believe this will increase public confidence in the 

emissions estimated for key emission source categories of the Natural Gas Systems 

sector. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) vi

 Industry has a continuous commitment to improvement. It is clear that companies are 

not waiting for regulatory mandates or incentives to upgrade equipment, or to alter

practices like venting and flaring in favor of capturing methane where practical. Instead, 

operators are seizing opportunities to reduce the potential environmental impacts of their 

operations. Industry is therefore confident that additional, systematic collection of 

production sector activity data will not only help target areas for future reductions but 

also demonstrate significant voluntary progress toward continually ‘greener’ operations. 

 Members of industry participating in this survey are committed to providing 

information about the new and fast-changing area of unconventional oil and gas 

operations. API and ANGA members look forward to working with the EPA to revise 

current assessment methodologies as well as promote the accurate and defensible uses 

of existing data sources. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 1

1.Overview

The accuracy of GHG emission estimates from natural gas production has become a 

matter of increasing public debate due in part to limited data, variability in the complex 

calculation methodologies, and assumptions used to approximate emissions where measurements 

in large part are sparse to date. Virtually all operators have comprehensive methane mitigation 

strategies; however, beyond the requirements of the Environmental Protection Agency’s (EPA) 

Mandatory Reporting Rule or incentives of programs like the EPA’s Natural Gas Star program, 

data is often not gathered in a unified way that facilitates comparison among companies.

In an attempt to provide additional data and identify uncertainty in existing data sets, the 

American Petroleum Institute (API) and America’s Natural Gas Alliance (ANGA) began a joint 

study on methane (CH4) emissions from natural gas production operations in July 2011. The 

first part of this section offers context to the decision to conduct this survey, while the second 

offers a brief introduction to the survey itself.

1.1 Context

Onshore gas resources, including shale, will undoubtedly play a key role in America’s 

energy future, and additional information must be collected to better quantify the methane 

emissions from natural gas production. Meaningful, publicly available data is a priority, 

especially in light of EPA’s 2011 revision of its calculation methodology for Natural Gas 

Systems in the 2009 national inventory (EPA, 2011c). (EPA added two new sources for 

unconventional gas well completions and workovers, and also significantly revised its estimates 

for liquids unloading and made adjustments to other source categories.) These changes

substantially increased EPA’s estimated GHG emissions for the production sector of the Natural 

Gas Systems by 204%. Equally problematic is the methodology used by EPA to credit voluntary 

and regulatory reductions from the emissions reported in Table A-128 in the 2010 inventory. 

These reductions are taken as a “lump sum” reduction at the segment (e.g. production) level with 

no transparency of what source categories the reductions are applied towards. Presentation of 

the pre-reduction values in Table A-129 (over a 300% increase) coupled with lack of source 

specific transparency has led many stakeholders to use the emissions in Table A-129 directly in 

their various advocacy actions. API recommends that the EPA change their methodology of 

crediting reductions to be transparent down to the source type/category and that the postreduction emissions be reported in the future analogues to Table A-129. 

Industry was alarmed by the upward adjustment, especially since previous EPA estimates 

had been based on a 1996 report prepared by the EPA and GRI – and did not take into account 

the considerable improvements in equipment and industry practice that have occurred in the

fifteen years between 1996 and 2011 (GRI, 1996).

An EPA technical note to the 2009 inventory attributed the changes to adjustments in 

calculation methods for existing sources, including gas well liquids unloading, condensate 

storage tanks, and centrifugal compressor seals. EPA also added two new sources not previously 

included in its inventories, namely unconventional gas well completions and workovers (recompletions) (EPA, 2011f). 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 2

Industry did not have an adequate opportunity to examine EPA’s rationale for the new 

emissions factor prior to its initial release due to the structure of the inventory process and the 

lack of formal opportunity for meaningful input. Unlike changes in regulatory requirements, 

EPA is not required to initiate a formal comment process for changes in methodologies like 

emission factors and calculations methods in the national GHG inventory. As such, EPA is not 

compelled to incorporate or consider input provided by stakeholders and experts. Indeed, 

changes to methodologies are often made without the benefit of dialogue or expert review. 

Although EPA further acknowledged in the 2010 inventory (released in 2012), that their natural 

gas calculations needed work, their practice is to continue using the same numbers until adjusted 

estimates have been made. It is important to note that EPA has indicated a willingness to engage 

and discuss this matter with some members of industry. API and ANGA look forward to 

working collaboratively with EPA to improve the national GHG inventory. 

Under the best of circumstances, EPA had remarkably little information to draw on in 

determining their new emission estimates. Input from industry on this topic was not directly 

solicited, specific guidance or information did not exist on the international level, nor was it 

available from other national regulators. A review of the Intergovernmental Panel on Climate 

Change (IPCC) and other inventories submitted to the United Nations Framework Convention on 

Climate Change (UNFCCC) indicate that the U.S. is currently the only country to date to 

differentiate between conventional and unconventional natural gas production. As discussed 

above, the distinction between conventional and unconventional natural gas production is not 

particularly useful from an emission estimation standpoint and should be abandoned. 

Regulators, academics, and environmentalists around the world therefore considered the new 

estimated emission factor as an unprecedented development in a controversial issue. 

Widespread criticism of the figures and analysis of the assumptions and methodologies

revealed problematic methodology and less justification for the underlying numbers than 

originally anticipated. In a paper entitled Mismeasuring Methane, the well-respected energy 

consultancy IHS CERA succinctly detailed several concerns about the revisions – most notably 

that EPA’s new estimate was based on only four (4) data points that natural gas well operators 

had submitted voluntarily under the Natural Gas Star Program, which highlights emissions 

reductions. Together, the four data points cover approximately 8,880 wells – or roughly 2% of 

those wells covered in the EPA’s national greenhouse gas inventory. Those numbers, which 

were submitted in the context of showcasing achieved emissions reductions and not to estimate 

emissions, were then extrapolated to over 488,000 wells in the 2009 emissions inventory (IHS 

CERA, 2011). 

With an emerging topic like shale energy development, however, the impact of EPA’s 

revised estimates was, and continues to be, enormous. Emission estimates from production using 

EPA’s figures were used to question the overall environmental benefits of natural gas and have 

led to speculation of the role natural gas can play in a clean energy future.. They were cited 

widely by unconventional gas opponents - many of whom used the new figures selectively, 

inappropriately using the pre-reduction (voluntary and regulatory) figures, and without caveats 

like “estimated” to argue against further development of shale energy resources. For example, 

an article published by ProPublica cited the revised EPA emission factors as “new research” 

which “casts doubt” on whether natural gas contributes lower GHG emissions than other fossil 

fuels (Lustgarten, 2011). Many of these studies – e.g., the work of Howarth et al. were widely 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 3

reported in the popular press (Zellers, 2011) with little attention to the quality of analysis behind 

their conclusions.

Notably, other authors using more robust and defensible scientific methodologies argued 

that - even with undoubtedly high emissions estimates - natural gas still possessed a lifecycle 

advantage when its comparative efficiency in electricity generation was taken into account. For 

example, a study by Argonne National Laboratory utilizing the same EPA data sources 

concluded that taking into account power plant efficiencies, electricity from natural gas shows 

significant life-cycle GHG benefits over coal power plants (Burnham, 2011). Unfortunately, the 

complex technical arguments in these studies generated considerably less media and public

attention.

It is important to understand that the ongoing debate about the accuracy of EPA’s 

adjusted emission factor as contained in the 2009 inventory did not keep these numbers from 

being used in a series of rules that have wide ranging ramifications on national natural gas 

policies both in the United States and globally. Many countries considering shale energy 

development remain bound by the emissions reduction targets in the Kyoto Protocol and their 

regulatory discussions reflect greenhouse gas concerns. In addition to the very real risk that 

other countries could adopt the emission factor before the EPA can refine its calculations, the 

possibility of higher emissions (even if only on paper) might deter other nations from developing 

their own unconventional energy resources.

By the summer of 2011, it was clear to ANGA/API members (also referred to as 

API/ANGA members) that gathering additional data about actual emissions and points of 

uncertainty during unconventional gas production was essential to improve GHG life cycle 

analysis (LCA) of natural gas for the following reasons: 1) to focus the discussion of emissions 

from natural gas production around real data; 2) to promote future measurement and mitigation 

of emissions from natural gas production; and 3) to contribute to improving the emission 

estimation methods used by EPA for the natural gas sector in their annual national GHG 

inventory and its use globally. 

1.2 Introduction to the API/ANGA Survey

API and ANGA members uniformly believed that EPA’s current GHG emissions 

estimates for the natural gas production sector were overstated due to erroneous activity data in 

several key areas - including liquids unloading, well re-fracturing, centrifugal compressors, and

pneumatic controllers. Members worked cooperatively to gather information through two data 

requests tailored to focus on these areas and collect reasonably accessible information about 

industry activities and practices. Specifically, information was requested on gas well types, gas 

well venting/flaring from completions, workovers, and liquids unloading, and the use of

centrifugal compressor and pneumatic controllers. 

The actual data requests sent to members can be found in Appendix A, and Appendix B 

provides more detailed data from the ANGA/API well survey information. 

Survey results and summaries of observations, including comparisons to EPA’s emission 

estimation methods, are provided in the following sections.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 4

2.Well Data

This section examines well data gathered by API and ANGA members. Overall, 

ANGA/API’s survey effort gathered activity data from over 20 companies covering nearly 

91,000 wells and 19 of the 21 American Association of Petroleum Geologists (AAPG) basins2

containing over 1% of the total well count in EPA’s database of gas wells. Members believe that 

the API/ANGA survey represents the most comprehensive data set compiled for natural gas 

operations and, as such, provides a much more accurate picture of operations and emissions than 

the information EPA has relied on for its emission estimates.

Information to characterize natural gas producing wells was collected by survey in two 

parts: 

 The first part of the survey requested high-level information on the total number 

of operating gas wells, the number of gas well completions, and the number of gas 

well workovers with hydraulic fracturing. Data on over 91,000 wells was 

collected primarily for 2010, with some information provided for the first half of 

2011. 

 The second part of the survey requested more detailed well information about key 

activities. The well information collected through the two surveys is provided in 

Appendix B.

Section 2.1 looks at overall natural gas well counts, Section 2.2 examines completion 

data from ANGA/API members, and Section 2.3 briefly identifies several unresolved issues 

concerning well counts and classifications that could benefit from future analysis for 

examination. For the purposes of this report, unconventional wells are considered to be shale gas 

wells, coal bed wells, and tight sand wells which must be fractured to produce economically.

2.1 National Gas Well Counts

To provide context for the information collected by API and ANGA, comparisons were 

made to information about national gas wells from EPA and the U.S. Energy Information 

Administration (EIA). Unfortunately, the government lacks a single coordinated and cohesive 

set of estimates for gas wells. 

Industry grew concerned when it became apparent that significant discrepancies existed 

among different sources of national gas well data. The EPA inventory, the EIA, and IHS data 

EPA published in conjunction with the GHGRP all reported different well counts that do not 

consistently distinguish between key areas used by the inventory like conventional and 

unconventional wells. Furthermore, there does not appear to be a single technical description for 

classifying wells that is widely accepted. Without consistent measures and definitions for the 

 

2 Basins are defined by the American Association of Petroleum Geologists (AAPG) AAPG–CSD Geologic 

Provinces Code Map: AAPG Bulletin, Prepared by Richard F. Meyer, Laure G. Wallace, and Fred J. Wagner, Jr., 

Volume 75, Number 10 (October 1991) and the Alaska Geological Province Boundary Map, Compiled by the 

American Association of Petroleum Geologists Committee on Statistics of Drilling in Cooperation with the USGS, 

1978.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 5

quantity and type of wells, it is difficult to reach agreement on the number of hydraulically 

fractured and unfractured natural gas wells completed annually - let alone their emissions.

Both the EIA data and the EPA data accompanying the national GHG inventory lack 

sufficient detail for well classifications to provide a basis for helpful comparison with the survey 

data reported here. Instead, national well data developed as part of mandatory emissions 

reporting is used for comparison because it has the most appropriate level of detail in well 

categories (EPA, 2011e). 

In EPA’s database gas well count (EPA, 2011e), 21 of the AAPG basins each have more 

than 1% of the total well count. The API/ANGA survey has wells from 19 of those 21 basins. In 

terms of wells represented by these basins, 92% of the total EPA database well count is 

accounted for by wells in those 21 basins, while 95% of the ANGA/API surveyed gas wells are 

accounted for by those 21 basins. These results are summarized in Table 1 and illustrated in 

Figure 1. This indicates that the API/ANGA survey results have good representation for the 

basins with the largest numbers of wells nationally.

TABLE 1. COMPARISON OF GAS WELL COUNT DATA BY AAPG BASIN: SUMMARY 

STATISTICS

EPA Database 

Gas Well 

Count*

API/ANGA Survey 

Data

ANGA/API as a 

% of EPA 

Total number of U.S. gas wells 355,082 gas wells 91,028 gas wells 26%

Number of significant AAPG 

basins**

21 basins Data on wells in 19 of 

those 21 basins

90%

Number of wells in significant AAPG 

basins 

325,338 wells 86,759 wells 27%

% of total wells in significant AAPG

basins

92% 95%

* EPA’s database gas well count (EPA, 2011e) differs from the well count provided in EPA’s 2010 national 

inventory, but provides more detail on the types of wells. Additional details are provided in Appendix B.

** Significant basins are defined as basins with more than 1% of the total national gas wells.

As shown in Figure 1, the API/ANGA survey results more heavily represent gas wells in 

specific AAPG basins when compared to EPA’s basin-level well counts (EPA, 2011d). Unlike 

the EPA data, the ANGA/API data is more heavily influenced by AAPG 160 and 160A. AAPG 

basins 360, 230, and 580 are important for both data sets.

The smaller data set provided by EPA (2011e) may not include all of the Marcellus shale

wells (particularly in Pennsylvania), and the well classification system used in this smaller data 

set could probably be made more rigorous. Although this comparison may not show a perfect 

distributional match for the basin by basin distribution of the API/ANGA survey data presented 

here, it does not change the fundamental conclusion that the ANGA/API survey information is 

the largest and most representative data set yet collected for onshore U.S. gas production, since 

this data set does cover 90% of the basins and 27% of the national gas well count for the 

significant basins as reported by EPA (EPA, 2011e). The data discussed in this report provides 

substantial new information for understanding the emissions from Natural Gas Systems and 

offers a compelling justification for re-examining the current emission estimates for 

unconventional gas wells. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 6

Appendix B contains more detail about the industry well data sample compared to the 

overall data maintained by the government. Unless otherwise noted, further statistical 

comparisons of well data throughout this paper are done with reference to the EPA data

furnished with the Subpart W sub-basin categorization because it was the only one which 

effectively parsed the data by well type (EPA, 2011e).

FIGURE 1. COMPARISON OF EPA TO API/ANGA GAS WELL COUNT DATA BY AAPG

BASIN

2.2 Gas Well Completions

Acknowledging the somewhat different time periods covered, the API/ANGA survey 

data represents 57.5% of the national data for tight gas well completions and 44.5% of shale gas 

well completions, but only 7.5% of the national conventional well completions and 1.5% of coalbed methane well completions. About one-third of the surveyed well completions (2,205) were 

not classified into the well types requested (i.e., tight, shale, or coal-bed methane) by the 

respondents. The survey results for well completions are provided in Table 2 and compared to 

national data provided to ANGA by IHS.3 

 

3 Data provided in e-mail from Mary Barcella (IHS) to Sara Banaszak (ANGA) on August 29,2011. Data were 

pulled from current IHS well database and represent calendar year 2010.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 7

EPA's 2010 inventory showed 4,169 gas well completions with hydraulic fracturing 

(EPA, 2012, Table A-122); however, EPA does not provide a breakout of completions by well 

type (shale gas, tight gas or coal-bed methane). In comparing the EPA 2010 count of gas well 

completions with hydraulic fracturing (4,169 completions) to both the survey results and data 

provided by IHS, it seems that EPA’s national GHG inventory underestimates the number of 

well completions. Even accounting for the difference in time periods (2010 for EPA compared 

to 2010/2011 data from the ANGA/API survey), the national inventory appears to underrepresent the number of well completions. 

TABLE 2. API/ANGA SURVEY – SUMMARY OF GAS WELL COMPLETIONS BY NEMS

REGION AND WELL TYPE*

(FIRST SURVEY DATA REQUEST PHASE)

NEMS Region

Conventional 

Wells Shale

Coal-bed 

Methane Tight Unspecified

Regional 

Total

API/ANGA Survey Data Gas Well Completions

Northeast 2 291 3 67 126 489

Gulf Coast 81 588 - 763 374 1,806

Mid-Continent 22 734 - 375 270 1,401

Southwest 425 442 - 346 310 1,523

Rocky Mountain 10 30 977 1,017

Unspecified - - - - 1,125 1,125

Survey TOTAL 540 2,055 33 2,528 2,205 7,361

% of Survey Total 7.3% 27.9% 0.4% 34.3% 30.0%

2010 IHS Gas Well Completions IHS Total

2010 National 

Well Completions

(from IHS)1

7,178 4,620 2,254 4,400 18,452

38.9% 25.0% 12.2% 23.8%

API/ANGA as % of 

IHS National Well 

Counts

7.5% 44.5% 1.5% 57.5%

* ANGA/API survey data represents well counts current for calendar year 2010 or the first 

half of 2011. 

** EPA’s national GHG inventory does not designate gas wells by classifications of “shale”, 

“coal bed methane” or “tight”.

As shown in Table 3, the ANGA/API survey noted 7,361 gas well completions for 2010 

and the first half of 2011. This is equivalent to approximately 40% of the gas well completions 

reported by IHS for 2010. Although EPA’s 2010 national GHG inventory appears to underrepresent the number of gas well completions according to the numbers reported through both 

the API/ANGA data and the IHS, differences in national well data reporting systems make it 

difficult to accurately investigate well completion differences with certainty. Data used in the 

EPA inventory, which includes information from the Energy Information Administration, 

various state governments, and privately sourced data, such as HPDI, does not consistently 

distinguish between conventional and unconventional wells. Without a consistent measure for 

the quantity and type of wells, it is difficult to be confident of the accuracy of how many wells 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 8

are completed annually, let alone to estimate their emissions. Industry strongly believes that the 

effects of any current under-representation of well completions will be offset by a more realistic 

emission factor for the rate of emissions per well.

TABLE 3. SUMMARY OF GAS WELL COMPLETIONS DATA 

(FIRST SURVEY DATA REQUEST PHASE)

# Completions 

for Gas Wells 

without 

hydraulic 

fracturing

# Completions 

for Gas Wells 

with hydraulic 

fracturing

Total 

Completions

2010 National Well Completions 

(from EPA; EPA 2012) 702 4,169 4,871

% of National Total 14% 86%

API/ANGA Survey Well Completions 540 6,821 7,361

% of Survey Total 7% 93%

Well Completions from IHS 7,178 11,274 18,452

% of National Total 39% 61%

Table 4 provides detailed data for well completions from the ANGA/API survey. From 

the survey, 94% of gas well completions in 2010 and the first half of 2011, were conducted on 

wells with hydraulic fracturing. About one-half of all gas well completions for this time period 

were for tight wells, and about one-half of all gas well completions were for vertical wells with 

hydraulic fracturing. Any differences in totals between Tables 2, 3 and 4 are because these 

tables were derived from the two different data requests sent to member companies as described 

previously in the introduction to Section 2.

TABLE 4. API/ANGA SURVEY – ADDITIONAL DETAILS ON GAS WELL COMPLETIONS

(SECOND SURVEY DATA REQUEST PHASE)

# Completions for Gas Wells with hydraulic 

fracturing (HF)

Gas Wells without 

hydraulic fracturing

Total 

Completions

# Vertical 

wells 

completions

# Horizontal 

well 

completions

Total 

Wells 

with HF

% of 

Wells 

with 

HF

# 

Completions

% of 

Wells 

without 

HF

TOTAL 

Conventional 315 57 372 69% 164 31% 536

TOTAL Shale 317 1,863 2,180 99% 30 1% 2,210

TOTAL Tight 2,054 368 2,422 96% 106 4% 2,528

TOTAL Coal Bed 

Methane 27 3 30 91% 3 9% 33

TOTAL OVERALL 2,713 2,291 5,004 94% 303 6% 5,307
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 9

The following points summarize survey information provided in Tables 2, 3 and 4. These 

tables represent a snapshot of well activity data during this time.

 Overall, the survey showed 94% of the 5,307 wells reported in the API/ANGA data set as 

completed in 2010 and the first half of 2011 used hydraulic fracturing.

 536 conventional gas wells were completed in 2010 and the first half 2011. 

◦ 59% were vertical wells with hydraulic fracturing,

◦ 11% were horizontal wells with hydraulic fracturing, and

◦ 31% were wells without hydraulic fracturing.

 2,210 shale gas wells were completed in 2010 and the first half 2011. 

◦ 14% were vertical wells with hydraulic fracturing,

◦ 84% were horizontal wells with hydraulic fracturing, and

◦ 1% were wells without hydraulic fracturing.

 2,528 tight gas wells were completed in 2010 and the first half 2011.

◦ 81% were vertical wells with hydraulic fracturing,

◦ 15% were horizontal wells with hydraulic fracturing, and

◦ 4% were wells without hydraulic fracturing.

 33 coal-bed methane wells were completed in 2010 and the first half 2011. 

◦ 82% were vertical wells with hydraulic fracturing, 

◦ 9% were horizontal wells with hydraulic fracturing, and 

◦ 9% were wells without hydraulic fracturing.

2.3 Data Limitations Concerning Wells

In response to follow-up questions on well data, EPA indicated that they classified gas 

well formations into four types (conventional, tight, shale, and coal-bed) (EPA, 2011e). When 

developing the gas well classifications, EPA applied their judgment where data were not 

available in the database. ANGA and API are interested in using the well database compiled by 

IHS or a similar database, to more completely classify gas wells at some point in the future. The 

API/ANGA survey did not specifically define conventional wells for collecting the well data 

presented in this section, leaving the respondents to determine the classification of wells based 

on their knowledge of the well characteristics or state classifications. As such, this well 

classification may vary somewhat according to the respondent’s classification of wells.

It should be noted that there is not a generally accepted definition for “gas wells.” 

Producers might be producing from several zones in the same formation, and different states 

define “gas” or “oil” wells differently due to the historical structure of royalties and revenues. 

There is also no commonly used definition of “conventional” gas wells. Thus, different 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 10

definitions of these terms may have produced inconsistency in the classification of wells between 

gas and oil, and conventional and unconventional for the surveyed results, as well as for the EPA 

and EIA national data. For the purposes of this report, unconventional wells are considered to be 

shale gas wells, coal bed wells, and tight sand wells which must be fractured to produce 

economically. Given the counts of wells in the 2010 inventory versus the API/ANGA survey it 

is clear that the definition of conventional versus unconventional wells is not uniform and that 

the definition used by the individual companies responding to the survey likely differs from 

whatever distinction EPA uses.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 11

3.Gas Well Liquids Unloading 

Gas well clean ups, also known as liquids unloading, account for 51% of total CH4

emissions from the natural gas production sector in EPA’s national GHG inventory (EPA, 

2012).4 Methane emissions from well venting for liquids unloading in the 2010 inventory 

represent a dramatic increase from the 6% of CH4 emissions that liquids unloading represented in 

the 2008 inventory. The magnitude of the increase, the accuracy of the underlying assumptions,

and the methodology used in estimating emissions from well venting for liquids unloading 

became major concerns to API/ANGA members. 

As the name indicates, ‘Venting Wells for Liquids Unloading’ is a technique to remove 

water and other liquids from wellbores to improve the flow of natural gas in gas wells. 

In EPA’s national inventory, emissions from gas well liquids unloading are based on the 

following assumptions (EPA, 2011a and Hanle, 2011):

 41.3% of conventional wells require liquids unloading.

 150,000 plunger lifts are in service, which equates to 31% of the gas wells in the national 

inventory.

 The average gas well is blown down to the atmosphere 38.73 times per year.

 The average casing diameter is 5 inches.

 A gas well is vented to the atmosphere for 3 hours.

Due to the dramatic increase in EPA’s estimated emissions from this source and 

ANGA/API’s concern, a survey of member companies was conducted to gather data on current 

operating practices from a large and broad cross-section of the industry regarding well venting 

for liquids unloading. The survey was structured to gather activity information along with data 

required for estimating emissions using the methodologies from the GHGRP Subpart W. Sixtyone data sets with information relevant to liquids unloading covering 59,880 wells and 18 AAPG 

basins were received. 

The following information was requested:

 Geographic area represented by the information provided;

 Time period – data were annualized to 12 months if the information was provided for a 

partial year;

 Number of operated gas wells represented by the information provided;

 Number of gas wells with plunger lift installed;

 Number of gas wells with other artificial lift (beam pump; ESP; etc.);

 Total number of gas well vents;

 Number of wells with and without plunger lifts that vent to the atmosphere;

 

4

See EPA Table A-129, of Annex 3 of the 2010 inventory report. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 12

 Total count of gas well vents for time period with and without plunger lifts;

 Average venting time for wells with and without plunger lifts;

 Average daily production of venting gas wells (Mcf/day);

 Average depth of venting wells (feet);

 Average casing diameter of venting gas wells (inches);

 Average tubing diameter of venting gas wells with plunger lift (inches); and

 Average surface pressure of venting gas wells (psig).

Not all respondents reported information for each parameter requested in the survey. 

Hence, the subsequent analysis determining the sample size and other factors used the well 

counts and other information from only those data sets which included information for the 

parameter being determined or analyzed. As a result, the analysis of the survey data reflects 

three levels of information:

1. High-level data were used to relate the survey information to national well counts. This 

survey data set provided the broadest representation of wells, totaling 59,880 total gas 

wells in the liquids unloading data sets.

2. Mid-level data consisted of survey information used to determine the fraction of both 

plunger equipped and non-plunger equipped gas wells that vent gas due to liquids 

unloading. The mid-level survey data represented a total of 49,124 wells.

3. Detailed survey data were used to calculate emissions using the methodologies in the 

GHGRP Subpart W and to compare the survey data to assumptions EPA used in deriving 

the emissions for the 2010 national inventory. Detailed survey information was provided 

for a total of 42,681 wells. 

Although the survey was split into “conventional” and “unconventional” categories, this 

specific distinction was not carried forward into the liquids unloading analysis. Liquid loading 

of well-bores is a function of the physics of flow up the well-bore and the fluids’ properties; the 

type of producing formation is not relevant when the conditions for liquid loading occur. 

Table 5 summarizes the high-level results from the API/ANGA survey and characterizes the 

national well population in EPA’s 2010 inventory using the survey derived information. Based 

on the survey results, 36% of gas wells are equipped with plunger lift. Applying this percentage 

to the national gas well count results in 174,743 wells nationally with plunger lift. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 13

TABLE 5. LIQUIDS UNLOADING HIGH LEVEL DATA SUMMARY

High Level Survey Data

Gas wells with plunger lifts (59,648 in sample) 21,500 wells (36.0%)

Gas wells with artificial lift (54,660 in sample) 7,329 wells (13.4%)

Gas Wells Vented to the atmosphere for Liquids Unloading 

(49,124 in sample) 6,462 wells (13.2%)

# Vents per vented well 145.1 vents/vented well

Total gas well vents represented by the data sets 937,663 vents

National Well Characterization

National # of wells from 2010 Inventory 484,795 wells

Calculated national # wells with plunger lift 174,743 wells

Calculated national # wells without plunger lift 310,052 wells

Calculated national # wells with artificial lift 

(subset of wells without plunger lift 65,003 wells

Methane mole percentage from EPA 2010 national inventory 78.8%

The survey also collected information on gas wells with artificial lift. A key distinction is 

that wells with artificial lift do not build up liquid columns in the well bore and hence are not 

vented for liquids unloading. The ANGA/API survey results indicated that 13.4% of wells use 

artificial lift which yields approximately 65,000 wells with artificial lift when extrapolated on a 

national basis. EPA has acknowledged that their current emission estimation method for liquids 

unloading does not account for activities used to reduce CH4 emissions by many different 

artificial lift methods used in industry. 

Table 6 summarizes the mid-level API/ANGA survey results, which characterizes the 

national venting well population, and applies the survey derived annual emissions per venting 

well to calculate national emissions. Based on the mid-level survey data, 21.1% of plunger 

equipped wells vent gas for liquids unloading, and 9.3% of wells without plunger lift vent gas for 

liquids unloading. These percentages were applied to the calculated national gas well counts 

with and without plunger lift (shown in Table 5) and are used to estimate the number of wells 

with and without plunger lift that vent for liquids unloading (36,806 and 28,863 wells, 

respectively, shown in Table 6). 

Emissions were calculated for the survey data by applying Equation W-8 or W-9 from the 

EPA GHG reporting rule in 40 CFR 98 Subpart W, where Equation W-8 applies to gas wells 

without plunger lifts, and Equation W-9 applies to gas wells with plunger lifts. Appendix C 

summarizes the survey data used to estimate the emissions shown in Table 6. The total volume 

of gas emitted based on the survey data was converted to methane emissions by applying the 

methane content used in the national inventory (78.8%). When the total volume is divided by the 

number of venting wells represented by the survey data, it results in an emission factor of 

approximately 254 Mscfy CH4/well.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 14

TABLE 6. LIQUIDS UNLOADING EMISSION ESTIMATION BASED ON SURVEY DATA

Mid-Level Survey Data

Total number of wells with plunger lift (42,681 in sample) 11,518 wells

Total number of wells without plunger lift (42,681 in sample) 31,163 wells

Number of plunger equipped wells that vent (42,681 in sample) 2,426 wells (21.1%)

Number of non-plunger equipped wells that vent (42,681 in sample) 2,901 wells (9.3%)

Total annual volume gas vented for venting wells 1,719,843,596 scf gas/year

Calculated volume vented gas per venting well 322,854 scfy gas/well

Calculated methane volume vented per venting well 254,409 scfy ch4/well

Calculated National Well Data

Calculated national # wells with plunger lift that vent for 

unloading 36,806 wells

Calculated national # wells without plunger lift that vent for 

unloading 28,863 wells

National Emission Calculations

Total gas venting for liquids unloading volume 

(scaled for national wells) 21,201,410,618 scf gas/yr

Total methane venting for liquids unloading 

(scaled for national wells) 16,706,711,567 scf CH4/yr

Total liquid unloading vented methane 

(scaled for national wells) 319,664 metric tons CH4/yr

Comparison to 2010 National Inventory Liquids 

Unloading

2010 National Inventory CH4 emissions from Liquids Unloading 4,501,465 metric tons CH4/yr

% difference between survey and national data -92.9%

For comparison to EPA’s estimated emissions from liquids unloading, the survey 

calculated emission factor was applied to the calculated national number of gas wells with and 

without plunger lift. When extrapolated to a national basis, as shown above, the survey data 

estimates that 319,664 tonnes of CH4 are emitted from liquids unloading, compared to EPA’s 

national emissions of 4,501,465 tonnes CH4 (based on industry standard conditions of 60 °F and 

14.7 psia) – for the same activities. Hence, the results of the ANGA/API survey would predict 

emissions that are a factor of 14 times lower than the emissions reported in the 2010 national 

GHG inventory for liquids unloading.

Table 7 summarizes the detailed API/ANGA survey results for relevant individual 

parameters used in the emission calculation. These are compared to EPA’s assumptions to 

demonstrate why the survey results produce a much lower emissions than EPA’s assumptions. It 

should be noted, the ANGA/API parameters from each data set were weighted by the number of 

vents in that particular data set, where appropriate. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 15

TABLE 7. DETAILED SURVEY DATA COMPARED TO EPA ASSUMPTIONS

Detailed Survey Data Survey Data EPA Assumptions

Total national number of wells that vent for liquids unloading 65,669 wells 179,391 wells

Average number of vents per venting plunger well 343.72 vents/well38.7 vents/well

per venting non-plunger well 32.57 vents/well

Average time per vent plunger equipped wells 0.11 hour3 Hours

non-plunger equipped wells 1.90 hours

Average depth plunger equipped wells 4,370 feet6,000 Feet

non-plunger equipped wells 5,433 feet

Average tubing diameter - plunger equipped wells 2.15 inches 2 inches

Average casing diameter - non-plunger equipped wells 4.57 inches 5 inches

Average production rate plunger equipped wells 104.3 Mcfd gas

non-plunger equipped wells 45.9 Mcfd gas

Average pressure plunger equipped wells 91.0 psig 100 psig sales line 

pressure (200-1000 psig 

non-plunger equipped wells 110.8 psig shut-in pressure)

Calculated methane volume venting per venting wells 254,409 scf CH4/well 1,316,750 scf CH4/well

As noted previously, the data sets used to derive the liquids unloading emission estimates 

are summarized in Appendix C. When examining Appendix C, it is important to note the 

presence of several data responses that can be viewed as outliers. Two data responses for 

operations with conventional wells reported very high frequencies of vents to the atmosphere. 

These data sets represent 174 gas wells with plunger lifts that vent to the atmosphere and are 

located in the Mid-Continent region (out of a total 1,140 conventional gas wells represented by 

the two data sets). The wells represented by these data points have plunger lifts that vent to the 

atmosphere for each plunger cycle. The information was confirmed by the two data respondents 

and is an artifact of the plunger control for these wells which results in very short venting 

durations (between 4 and 5 minutes) for each plunger cycle, which is not the operating practice 

assumed by the EPA in the equations that are used to estimate emissions associated with liquids 

unloading. Accounting for the high frequency of plunger lift cycles for these wells results in a 

high average vent frequency, but still leads to the derivation of a lower emission factor than the 

one derived by EPA in accordance with their assumptions about liquids unloading operations. 

The survey derived factor is likely somewhat high due to the calculation assumption that the 

entire tubing string is de-pressured in each venting event which is not likely for the two “outlier” 

data sets discussed above. 

These variances among operators in the ANGA/API data demonstrate the challenge of 

applying national emissions estimates to conditions in which there can be considerable variation 

in wells and operating techniques, among and even within various regions. As member 

companies have noted in various comments to regulators, oil and natural gas production 

operations vary considerably according to factors such as local geology, hydrology, and state 

law.

As production companies continue to collect and report information for EPA’s mandatory 

GHG reporting program, better information on liquids unloading frequency and emissions will 

be available. One area that would benefit from additional information is an investigation of 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 16

regional differences, or plunger lift control practices, in view of the high frequency of vents 

observed for two data sets containing conventional gas wells with plunger lifts in the MidContinent region.

Key findings of the API/ANGA survey on liquids unloading are:

 Overall, the change in emission factors based on data collected from the ANGA/API 

survey reduces estimated methane emissions for this source by 93% from the methane 

emissions reported in EPA’s 2010 national GHG inventory. This is a factor of 14 times 

lower than EPA’s reported methane emissions.

 When compared to EPA’s assumptions used to derive the national GHG emission 

estimates for liquids unloading, the API/ANGA survey data indicated a lower percent of 

gas wells that vent for liquids unloading and a much shorter vent duration. The 

difference in these two parameters from EPA’s assumptions more than offset the higher 

number of vents observed from the survey data.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 17

4.Hydraulic Fracturing and Re-fracturing (Workovers)

A well workover refers to remedial operations on producing natural gas wells to try to 

increase production. Starting with the 2009 inventory, EPA split the estimation of emissions 

from producing gas wells into conventional (i.e., without hydraulic fracturing) and 

unconventional (i.e., with hydraulic fracturing). For workovers of wells without hydraulic 

fracturing, the 2009 and 2010 national inventories used emission factors of the same order of 

magnitude as the 2008 inventory (2,454 scf of CH4/workover). In contrast, the unconventional 

(with hydraulic fracturing) well workover emission factor increased by a factor of three thousand

(3,000). 

EPA did acknowledge that the new emission factor for well workovers was based on 

limited information (EPA, 2011b). Moreover, several publications including Mismeasuring 

Methane by IHS CERA underscored the perils of extrapolating estimates using only four (4) data 

points representing approximately two percent (2%) of wells – particularly when the data was 

submitted in the context of the Natural Gas Star program, which was designed to highlight 

emissions reduction options (IHS CERA, 2011). Unfortunately, even if the EPA’s workover 

factor is high, it must be used in estimated emissions calculations until it is officially changed. 

EPA’s new emission factor is 9.175 MMscf of natural gas per re-fracture (equivalent to 

7.623 MMscf CH4/re-fracture). Additionally, EPA used this new emission factor in conjunction 

with an assumed re-fracture rate of 10% for unconventional gas well workovers each year to 

arrive at their GHG emission estimate for this particular category. 

4.1 API/ANGA Survey

The ANGA/API survey requested counts for gas well workovers or re-fractures in two 

separate phases of the survey, covering 91,028 total gas wells (Table 8 covering 2010 and first 

half of 2011 data) and 69,034 unconventional gas wells (Table 9, 2010 data only), respectively. 

The first phase of the survey was part of the general well data request. Counts of 

workovers by well type (conventional, tight, shale, and coal bed methane) and by AAPG basin 

were requested. The frequency of workovers was calculated by dividing the reported workover 

rates by the reported total number of each type of gas well. These results are summarized in 

Table 8, which includes a comparison to national workover data from EPA’s annual GHG 

inventory. The high number of workovers in the Rocky Mountain region is discussed further 

below.

Table 8 indicates that even for the high workover rates associated with unconventional 

tight gas wells in the Rocky Mountain region, the workover rate is much less than EPA’s 

assumed 10% of gas wells re-fractured each year. Based on this first phase of the survey,

 The overall workover rate involving hydraulic fracturing was 1.6%. 

 However, many of these workovers were in a single area, AAPG-540, where workovers 

are known to be conducted more routinely than in the rest of the country (as described in 

more detail below Table 9). Excluding AAPG 540, the overall workover rate involving 

hydraulic fracturing was 0.7% which is a more likely range for a national re-fracture rate.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 18

 For shale, coal-bed methane, and tight formation wells in Table 8, the overall workover 

rate involving hydraulic fracturing was 2.2%. Excluding AAPG 540, the overall 

workover rate involving hydraulic fracturing was 0.5%.

TABLE 8. API/ANGA SURVEY – SUMMARY OF GAS WELL WORKOVERS WITH HYDRAULIC 

FRACTURING IN 2010 AND FIRST HALF OF 2011 BY NEMS REGION AND WELL TYPE

(FIRST PHASE DATA SURVEY)

NEMS Region

Conventional 

Wells

Unconventional Wells

Shale

Coal-bed 

Methane Tight Unspecified

Northeast - - - - -

Gulf Coast - 5 - 38 73

Mid-Continent 8 1 - 73 33

Southwest 60 25 - 8 7

Rocky Mountain 4 - 25 901 -

West Coast - - - - -

Unspecified - - - - 200

Survey TOTAL 72 31 25 1,020 313

1,076

% of national 0.3% 21.3%

Overall Survey Total

% of national

1,461

5.6%

National Workover Counts 

(from EPA’s 2010 national 

inventory)

Conventional 

Wells Unconventional Wells

21,088 5,044

80.7% 19.3%

26,132

Conventional 

Wells

Unconventional Wells

Shale

Coal-bed 

Methane Tight Unspecified

% Workover Rate with 

Hydraulic Fracturing 

(from ANGA/API Survey) 0.3% 0.3% 0.5% 3.0% 2.4%

Tight w/out AAPG 540 0.5%

Unconventional Wells 2.2%

 W/out AAPG 540 0.5%

All Wells 1.6%

All Wells w/out AAPG

540 0.7%
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 19

A second phase of the survey was conducted which targeted collecting gas well refracture information for 2010 to provide a comparator for EPA's assumption that 10% of wells 

are re-fractured each year. This portion of the ANGA/API survey requested information just for 

“unconventional” gas wells (i.e., those located on shale, coal-bed methane, and tight formation

reservoirs), where the formations require fracture stimulation to economically produce gas. A 

re-fracture or workover was defined for this second phase of the survey as a re-completion to a 

different zone in an existing well or a re-stimulation of the same zone in an existing well. These 

results are summarized in Table 9.

While there likely is significant overlap of unconventional well data reported in the first 

and second phases of the survey (which covered over 62,500 unconventional wells and 69,000 

unconventional wells respectively), combined these data indicate an unconventional well refracture rate of 1.6% to 2.3% including AAPG 540 and 0.7% to 1.15% excluding AAPG 540.

AAPG Basin 540 (i.e. DJ Basin) which is part of the Rocky Mountain Region stands out 

in Tables 8 and 9. After four (4) to eight (8) years of normal production decline, the gas wells in 

this basin can be re-fractured in the same formation and returned to near original production. 

Success of the re-fracture program in the DJ Basin is uniquely related to the geology of the 

formation, fracture reorientation, fracture extension and the ability to increase fracture 

complexity. Also, most DJ Basin gas wells are vertical or directional, which facilitates the 

ability to execute re-fracture operations successfully and economically. These characteristics 

result in a high re-fracture or workover rate specific to this basin/formation.

ANGA and API believe the high re-fracture rate observed in the DJ Basin is unique and 

not replicated in other parts of the country. This was a limited program that was occurring 

during the data survey activities and has currently stopped. There may be a few other formations 

in the world that have similar performance, but the successful re-fracture rate in the DJ Basin is 

not going to be applicable to every asset/formation and there is no evidence of the high refracture rate in any of the other 22 AAPGs covered in the API/ANGA survey. It is highly 

dependent on the type of rock, depositional systems, permeability, etc. For these reasons, refracture rates for tight gas wells and all gas wells with and without AAPG Basin 540 are 

summarized in Tables 8 and 9.

4.2 WRAP Survey

Other information on re-fracture rates is available in a survey conducted by the Western 

Regional Air Partnership (WRAP). WRAP conducted a survey of production operators in the 

Rocky Mountain Region (Henderer, 2011) as part of the initiative to develop GHG reporting 

guidelines for a regional GHG cap and trade program. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 20

TABLE 9. API/ANGA SURVEY – SUMMARY OF 2010 GAS WELL WORKOVERS ON 

UNCONVENTIONAL WELLS BY AAPG BASIN AND NEMS REGION

(SECOND PHASE SURVEY DATA)

NEMS 

Region

AAPG

Basin

Number of 

Unconventional 

Operating Gas 

Wells

Number of 

Hydraulic 

Fracture 

Workovers on 

Previously 

Fracture 

Stimulated Wells 

% Wells refractured 

per year

Regional % 

Wells refractured 

per year

Northeast 160 1,976 0 0.00% 0%

160A 760 0 0.00%

Gulf Coast

200 2 0 0.00%

0.91%

220 649 2 0.31%

222 629 3 0.48%

230 820 4 0.49%

250 13 0 0.00%

260 2,830 36 1.27%

MidContinent

345 3,296 11 0.33%

0.95%

350 213 3 1.41%

355 282 8 2.84%

360 7,870 89 1.13%

375 12 0 0.00%

385 1 0 0.00%

400 64 0 0.00%

Southwest

415 1,834 0 0.00%

1.04% 420 838 8 0.95%

430 1,548 36 2.33%

435 2 0 0.00%

Rocky 

Mountain

515 1 0 0.00%

4.7% 540 5,950 866 14.55%

580 8,197 8 0.10%

595 5,222 32 0.61%

Not specified 26,025 487 1.87% 1.87%

Unconventional TOTAL 

(all wells) 69,034 1,593 2.31%

Unconventional Median 790 3

Rocky Mountain Region 

Unconventional Total 19,370 906 4.68%

Unconventional TOTAL 

(Without AAPG 540) 63,084 727 1.15%
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 21

Within each basin in this region, the top oil and gas producers were identified and invited 

to participate in the survey. The goal was to have operator participation that represented 80% of 

the production for the region. The spreadsheet survey requested information on the completions, 

workovers, and emissions associated with these activities. An emission factor and frequency of 

re-fracturing was developed for each basin as a weighted average of the operator responses. 

The re-fracture rates from the WRAP survey are shown in Table 10 (Henderer, 2011). 

TABLE 10. WRAP SURVEY – SUMMARY OF GAS WELL WORKOVERS BY AAPG BASIN FOR 

THE ROCKY MOUNTAIN REGION, 2006 DATA

AAPG Basin

# Wells 

represented 

by survey

# Wells 

Recompleted

% 

Recompleted

515 4,484 121 2.70%

530 731 5 0.68%

535 4,982 201 4.03%

540 8,247 636 7.71%

580 3,475 14 0.40%

595 4,733 275 5.81%

Total 26,652 1,252

Weighted average 4.70%

AAPG Basin 540 results in the highest re-fracture rate for this data set, consistent with 

the ANGA/API survey as noted above. It is noteworthy that, while there are differences among 

individual AAPG Basin results, the weighted average re-fracture rate from the WRAP survey in 

2006 is the same as the Rocky Mountain regional 4.7% re-fracture rate from the API/ANGA 

survey shown in Table 9.

4.3 Impact of Completions and Re-fracture Rate Assumptions

Table 11 compares the considerable reduction in the national GHG inventory that would 

result from applying a lower re-fracture rate. 

EPA indicated that the national inventory assumes 10% of unconventional gas wells are 

re-fractured each year. Table 11 replaces this value with results from the ANGA/API survey. A 

re-fracture rate of 1.15% is applied to unconventional gas wells in the Mid-Continent and 

Southwest regions (No unconventional gas wells were assigned to the Northeast and Gulf Coast 

regions. The West Coast region is not shown since the API/ANGA survey did not include any 

responses for gas well operations in this region.) A re-fracture rate of 4.7% is applied to 

unconventional gas wells in the Rocky Mountain region. 

With these adjustments to the re-fracture rate for unconventional gas wells, the 

national emission estimate is reduced by 72% for this emission source category, from 712,605

metric tons of CH4 to 197,311 metric tons of CH4 when compared on a consistent basis.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 22

TABLE 11. API/ANGA SURVEY –GAS WELL WORKOVER EMISSIONS COMPARISON

NEMS Region Well type

2010 EPA 

National 

Inventory 

# 

workover

Adjusted # 

workovers

(based on 

API/ANGA 

survey)

2010 EPA National 

Inventory

Revised 

Emissions, 

tonnes 

CH4

(based on 

ANGA/API 

survey)

Emission 

Factor, scf 

CH4/workover

Estimated 

Emissions, 

tonnes 

CH4* % Difference

Northeast

Wells without Hydraulic 

Fracturing 8,208 8,208 2,607 409 409

Wells with Hydraulic 

Fracturing 0 0 7,694,435 0 0

Mid Continent

Wells without Hydraulic 

Fracturing 3,888 3,888 2,574 191 191

Wells with Hydraulic 

Fracturing 1,328 153 7,672,247 194,950 22,462** -89%

Rocky

Mountain

Wells without Hydraulic 

Fracturing 3,822 3,822 2,373 174 174

Wells with Hydraulic 

Fracturing 2,342 1,100 7,194,624 322,402 151,432** -53%

Southwest

Wells without Hydraulic 

Fracturing 1,803 1,803 2,508 87 87

Wells with Hydraulic 

Fracturing 1,374 158 7,387,499 194,217 22,382** -89%

Gulf Coast

Wells without Hydraulic 

Fracturing 3,300 3,300 2,755 174 174

Wells with Hydraulic 

Fracturing 0 0 8,127,942 0 0

TOTAL 712,605 197,311 -72%

* EPA Estimated emissions = 2010 # Workovers x EPA 2010 Emission Factor, converted to mass emissions based on 60°F and 14.7 psia.

** Revised emissions = Adjusted # Workovers x Emission Factor, converted to mass emissions based on 60°F and 14.7 psia.

API & ANGA - EPA

EPA
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 23

4.4 Completion and Re-fracture Emission Factor

In the 2009 GHG national inventory, EPA applies an emission factor of 2,454 scf 

CH4/event for conventional gas well workovers, while the emission factor for unconventional 

gas well completions and workovers was increased to 7,623,000 scf CH4/event (EPA, 2011c). 

Similarly, for the 2010 national GHG inventory, EPA maintained the emission factor of 2,454 

scf CH4/event for gas well workovers without hydraulic fracturing, but applied an average 

emission factor of 7,372,914 to gas well workovers with hydraulic fracturing (EPA, 2012). 

(EPA applies slightly different emission factors for each NEMS region based on differing gas 

compositions.)

The ANGA/API survey focused on activity data and did not collect data to revise the 

emission factor for unconventional gas well completions and workovers. 

Emissions Data from WRAP Study

The WRAP study discussed in Section 4.2 also gathered data on emissions from 

completions. This information supports a revised emission factor but was reported by sources 

outside the ANGA/API data survey. The results are summarized in Table 12. The WRAP 

emission factor is 78% lower than EPA’s emission factor (9.175 MMscf gas/event). The WRAP 

survey did not provide a methodology for determining emissions data.

TABLE 12. WRAP SURVEY – SUMMARY OF COMPLETION EMISSIONS FOR THE ROCKY 

MOUNTAIN REGION, 2006 DATA

AAPG Basin

Weighted average gas 

emissions from 

completion, Mcf 

gas/well

# 

completions 

represented

515 167 207

530 268 54

535 76 642

540 59 608

580 6,559 283

595 4,053 819

Total 2,613

Weighted average 2,032 Mcf/well

4.5 Data Limitations for Completion and Re-fracture Emissions

Although the data sets are limited, it appears that EPA’s assumed re-fracture rate of 10% 

is a significant overestimate. Information from the API/ANGA survey indicates that even 

including what appears to be unique activity in AAPG-540, the re-fracture rate is much less 

frequent, ranging from 1.6% to 2.3% based on two sets of survey information (Tables 8 and 9, 

respectively). The re-fracture rate for AAPG Basin 540 appears to be higher than other areas in 

the U.S. due to unique geologic characteristics in that region (4.7% based on a weighted average 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 24

of data reported for that region). Without AAPG Basin 540, the national rate of re-fracturing is 

between 0.7% and 1.15% of all gas wells annually. 

Additionally, limited information on the emissions from completions and workovers with 

hydraulic fracturing indicate that EPA’s GHG emission factor for these activities is significantly 

overestimated. It is expected that better emissions data will develop as companies begin to 

collect information for EPA’s mandatory GHG reporting program (EPA, 2011d).
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 25

5.Other Surveyed Information

EPA had indicated that activity data for centrifugal compressor wet seals and pneumatic 

devices used in the national inventory is lacking. Note that the need for better equipment data 

persists throughout the majority of the U.S. inventory and is not unique to the oil and natural gas 

industry. The ANGA/API survey requested the following information related to centrifugal 

compressors and pneumatic devices:

 The number of centrifugal compressors, reported separately for production/gathering 

versus processing;

 The number of centrifugal compressors with wet versus dry seals, reported separately for 

production/gathering versus processing;

 The number of pneumatic controllers, classified as “high-bleed,” “low-bleed,” and 

“intermittent,” reported separately for well sites, gathering/compressor sites, and gas 

processing plants; and

 The corresponding number of well sites, gathering/compressor sites, and gas processing 

plants, associated with the pneumatic controller count.

5.1 Centrifugal Compressors

Processing Facilities

The API/ANGA survey collected the equivalent of 5% of the national centrifugal 

compressor count for gas processing operations (38 centrifugal compressors from the survey, 

compared to 811 from EPA’s 2010 national GHG inventory). For the gas processing centrifugal 

compressors reported through the survey, 79% were dry seal compressors and 21% were wet 

seals. EPA’s 2010 national inventory reported 20% of centrifugal compressors at gas processing 

plants were dry seal, and 80% were wet seal. EPA’s emission factor for wet seals (51,370 scfd 

CH4/compressor) is higher than the emission factor for dry seals (25,189 scfd CH4/compressor).5 

Based on the ANGA/API survey, EPA appears to be overestimating emissions from 

centrifugal compressors. If the small sample size from the API/ANGA survey is representative, 

non-combustion emissions from centrifugal compressors would be 173,887 metric tons of 

methane compared to 261,334 metric tons of methane from the 2010 national inventory (when 

applying industry standard conditions of 60 °F and 14.7 psia to convert volumetric emissions to 

mass emissions). Although based on very limited data, if the ANGA/API survey results reflect 

the population of wet seal versus dry seal centrifugal compressors, the emissions from this 

source would be reduced by 34% from EPA’s emission estimate in the national inventory. Better 

data on the number of centrifugal compressors and seal types will be available from companies 

reporting to EPA under the mandatory GHG reporting program.

 

5 EPA Table A-123, of Annex 3 of the 2010 inventory report. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 26

Production and Gathering Facilities

Very few of the data sets reported through the API/ANGA survey indicate counts of 

centrifugal compressors associated with production/gathering operations - only 550 centrifugal 

compressors from 21 participating companies. EPA’s 2010 GHG inventory did not include 

centrifugal compressors in production/gathering operations. On a well basis, the survey 

responses equate to 0.07 centrifugal compressors per gas well, with 81% dry seal centrifugal 

compressors and the remaining wet seal compressors. Information reported through EPA’s 

mandatory GHG reporting program will provide additional information to account for GHG 

emissions from centrifugal compressors in production operations.

5.2 Pneumatic Controllers

Table 13 summarizes the survey responses for pneumatic controllers. For each type of 

location – gas well sites, gathering compressor sites, and gas processing plants – the count of the 

number of sites represented by the survey data is shown. Table 13 also shows the percent of 

each pneumatic controller type for each type of location.

TABLE 13. ANGA/API SURVEY –PNEUMATIC CONTROLLER COUNTS

Gas Well Sites

Gathering/ 

Compressor 

Sites

Gas 

Processing 

Plants

# wells, sites or plants 48,046 wells 1,988 sites 21 plants

# controllers/well, site or 

plant 0.99 per well 8.6 per site 7.8 per plant

# Low Bleed Controllers 12,850 27% 5,596 33% 117 71%

# High Bleed Controllers 11,188 24% 1,183 7% 47 29%

# Intermittent Controllers 23,501 49% 10,368 60% 0 0%

The survey requested that the responses designate pneumatic controllers as either “high 

bleed”, “low bleed”, or “intermittent” following the approach each company is using for Subpart 

W reporting. For example, Subpart W defines high-bleed pneumatic devices as automated, 

continuous bleed flow control devices powered by pressurized natural gas where part of the gas 

power stream that is regulated by the process condition flows to a valve actuator controller where 

it vents continuously (bleeds) to the atmosphere at a rate in excess of 6 standard cubic feet per 

hour (EPA, 2011d). 

EPA does not currently track pneumatic controllers by controller type in the national 

inventory. This information will be collected under 40 CFR 98 Subpart W starting in September 

2012. From the API/ANGA survey, intermittent bleed controllers are the more prevalent type at 

gas well sites and gathering/compressor sites, while gas plants predominately use low-bleed 

controllers. No intermittent controllers were reported for gas plants by the survey respondents.

Table 14 compares emission results based on applying the emission factors from the 

EPA’s GHG reporting rule to emissions presented in the 2010 national GHG inventory, using the 

counts of pneumatic controller from the ANGA/API survey for production operations. 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 27

For production, the EPA national inventory combines pneumatic controller counts 

associated with large compressor stations with pneumatic controllers in production. An emission 

factor for each NEMS region is applied to the count of total controllers in each NEMS region. 

For this comparison, a weighted average emission factor of 359 scfd CH4/device was applied to 

the count of pneumatic controllers located at well sites and gathering/compressor sites. 

Under the EPA mandatory reporting rule (40 CFR 98 Subpart W), separate emission 

factors are applied to pneumatic controllers based on the controller type and whether the 

controller is located in the Eastern or Western region of the United States, as specified in the rule 

(EPA, 2011d). For this comparison, an average of the eastern and western emission factors is 

applied to each device type in computing the emission estimates resulting from the EPA GHG 

reporting rule.

TABLE 14. PNEUMATIC CONTROLLER EMISSION COMPARISON – PRODUCTION 

OPERATIONS

API/ANGA Survey 

Count of Controllers

EPA GHG Reporting 

Rule (Subpart W)

2010 National GHG 

Inventory

Gas 

Well 

Sites

Gathering/ 

Compressor 

Sites Total

Emission 

Factor,*

scfh 

CH4/device

Emissions, 

tonnes 

CH4/yr

Emission 

Factor, 

scfd 

CH4/device

Emissions, 

tonnes 

CH4/yr

# Low Bleed 

Controllers 12,850 5,596 18,446 1.58 4,885

359

46,286

# High Bleed 

Controllers 11,188 1,183 12,371 42.35 87,814 31,042

# 

Intermittent 

Controllers

23,501 10,368 33,869 15.3 86,856 84,987

Total 64,686 179,556 162,315

* Emission factors shown are the average of the eastern and western emission factors from Table W1A (EPA, 2011d).

Based on the types of pneumatic controllers reported in the ANGA/API survey, EPA’s 

mandatory GHG reporting rule could increase CH4 emissions 11% over the pneumatic controller 

portion of the 2010 national GHG inventory. To put this in context, in EPA’s inventory report 

for 2010, emissions from pneumatic controllers accounted for approximately 13% of CH4

emissions from the natural gas field production stage. Any increase from that initially reported 

data, however, will likely represent a worst case scenario. It is important to remember that 

pneumatic controllers operate only intermittently, so variability such as the frequency and 

duration of the activations will be important information to consider when defining an accurate 

and effective reporting regime for these sources. 

EPA’s mandatory GHG reporting rule does not require reporting emissions from 

pneumatic controllers at gas processing plants, so no emission factors are specified. The GHG 

national inventory applies an emission factor of 164,721 scfy CH4 per gas plant for pneumatic 

controllers. For the national inventory, this results in 1,856 tonnes CH4 emissions - a very small 

contribution to CH4 emissions from onshore oil and gas operations.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 28

6.Conclusions

API and ANGA members believe this to be the most comprehensive set of natural gas 

data to date and are pleased to share these results with both regulators and the public. 

Based on the information gathered from member companies during this project, it 

appears that EPA has overstated several aspects of GHG emissions from unconventional natural 

gas production. As summarized in Table 15, the ANGA/API survey data results in significantly 

lower emission estimates for liquids unloading and unconventional gas well refracturing when 

compared to EPA’s emission estimates in the national inventory. Using the combined emission 

estimates from the survey for these two key emission sources would indicate a 53% reduction in

calculated natural gas production sector emissions compared to EPA’s estimates. 

TABLE 15. EMISSION COMPARISON BETWEEN EPA AND INDUSTRY DATA

Source 

Category EPA National Inventory API/ANGA Survey

Impact on Source 

Category 

Emissions

Metric tons of CH4

% of EPA 

Production 

Total

Metric tons of CH4

% of 

Revised 

Production 

Total

% Difference in 

Emissions

Liquids Unloading 4,501,465 * 51% 319,664 8% -93%

Unconventional 

Well Re-fracture 

Rates

712,605 * 8% 197,311 5% -72%

Other Production 

Sector Emissions** 3,585,600 41% 3,585,600 87%

Total Production 

Sector Emissions 8,799,670 4,102,575 -53%

* EPA’s estimates are adjusted to industry standard conditions of 60 degrees F and 14.7 psia for comparison 

to the ANGA/API emission estimates.

**The “Other Production Sector Emissions” are comprised of over 30 different source categories detailed in 

Table A-129 in the Annex of the EPA’s 2012 national inventory. The “Other Production Sector 

Emissions” are the same values for this comparison between the EPA national inventory and the 

API/ANGA survey to focus the comparison on quantified differences in emission estimates for gas well

liquids unloading and unconventional well re-fracture rates.

This project was directed toward gathering more robust information on workovers, 

completions, liquids unloading, centrifugal compressors, and pneumatic controllers with the 

intent of supporting revisions to the activity factors used in EPA’s national inventory and cited 

by many media publications. Although limited information was collected on centrifugal 

compressors and pneumatic controllers, the survey results indicated potential additional 

differences, which are not included in the Table 15 comparison, when comparing total emissions 

API & ANGA - EPA

EPA
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 29

from all sources to the national inventory. Additional future data collection efforts, including 

more detailed reporting under Subpart W of the GHGRP will likely resolve these differences and 

continue to inform the overall natural gas emissions data. 

While API and ANGA recognize that the data collected for this report represents a 

sample of the universe of natural gas wells operating in the U.S., we believe that the conclusions 

drawn from the data analysis are relevant and representative of natural gas production as whole. 

In EPA’s gas well count, 21 of the AAPG basins each have more that 1% of the total well count. 

The ANGA/API survey has wells from 19 of those 21 basins. In terms of wells represented by 

these basins, 92% of the total EPA well count is accounted for by wells in those 21 basins, while 

95% of the API/ANGA surveyed gas wells are accounted for by those 21 basins. This indicates 

that the ANGA/API survey results have good representation for the basins with the largest 

numbers of wells nationally. 

Moreover, the API/ANGA survey results are based on a large number of wells - at least 

an order of magnitude or more - higher than the number of wells used by EPA to develop their 

revised emission factors that purport to be representative of U.S. industry operations nationwide. 

Such a richer data set allows for improved granularity of emission characteristics for various 

operations. A case in point is the information on liquids unloading where the ANGA/API survey 

results were obtained from over 59,000 gas wells and which indicate that 21% of wells equipped 

with plunger lift and 9.3% of wells without plunger lift vent for liquids unloading. In 

comparison, EPA’s approach is based on the assumption that the survey of 25 well sites 

conducted by GRI (1996) for the base year 1992 continues to provide representative data for the 

fraction of conventional wells requiring unloading, which EPA set at 41.3%. Industry also 

believes that the systematic approach in which the API/ANGA data were collected and vetted by 

natural gas experts is an improvement over the ad hoc way in which EPA collected some of their 

data. This study indicates that EPA should reconsider their inventory methodologies for natural 

gas production particularly in light of more comprehensive and emerging data from the industry. 

ANGA and API members look forward to working with the agency to continue to educate and 

evaluate the latest data as it develops about the new and fast-changing area of unconventional 

well operations.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 30

7.References

Energy Information Administration (EIA). "Number of Producing Gas Wells”, U.S. and State 

level data, annual, 2010 Data, Released February 29, 2012. 

http://www.eia.gov/dnav/ng/ng_prod_wells_s1_a.htm

Hanle, Lisa (U.S. National GHG Inventory, Petroleum and Natural Gas Systems Chapter 

Coordinator, U.S. EPA). “Background Information to Support 1990-2009 GHG Inventory 

Estimates for Petroleum and Natural Gas Systems”, Memorandum to Karin Ritter (Manager, 

Regulatory and Scientific Affairs, American Petroleum Institute), April 15, 2011.

Henderer, Douglas. Personal communications on the survey instrument utilized for the WRAP 

III inventory. KleinFelder, Littleton, Colorado, dhenderer@kleinfelder.com, October, 2011.

http://www.wrapair2.org/PhaseIII.aspx

IHS CERA, “Mismeasuring Methane,” 2011. http://www.ihs.com/info/en/a/mis-measuringmethane-report.aspx

United States Environmental Protection Agency, Natural Gas STAR Lessons Learned: Installing 

Plunger Lift Systems In Gas Wells, 2006. 

http://www.epa.gov/gasstar/documents/ll_plungerlift.pdf

United States Environmental Protection Agency (EPA). “Greenhouse Gas Emissions Reporting 

from the Petroleum and Natural Gas Industry, Background Technical Support Document,” U.S. 

Environmental Protection Agency, Climate Change Division, Washington DC, November, 2010.

United States Environmental Protection Agency (EPA). “EPA U.S. Oil and Gas GHG 

Inventory, July 14, 2011 Webcast, Review Inventory Method, Emission Reductions of Gas Well 

Liquids Unloading by Plunger Lifts,” Distributed for discussion purposes only. Final Draft, July 

15, 2011(a).

United States Environmental Protection Agency (EPA). “EPA U.S. Oil and Gas GHG 

Inventory, July 14, 2011 Webcast, Review Inventory Method, Potential Inventory Improvement 

on Well Completions and Workovers,” Distributed for discussion purposes only. Final Draft, 

July 15, 2011(b).

United States Environmental Protection Agency (EPA). Inventory of Greenhouse Gas Emissions 

and Sinks: 1990-2009, Washington DC, April, 2011(b) 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 31

http://www.epa.gov/climatechange/emissions/downloads11/US-GHG-Inventory-2011-

Complete_Report.pdf

United States Environmental Protection Agency (EPA). Inventory of Greenhouse Gas Emissions 

and Sinks: 1990-2010, Washington DC, April, 2012. 

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

United States Environmental Protection Agency (EPA). “Mandatory Reporting of Greenhouse 

Gases: Technical Revisions to the Petroleum and Natural Gas Systems Category of the 

Greenhouse Gas Reporting Rule”, Final Rule, Federal Register, Vol. 76, No. 247, December 23, 

2011(d). 

United States Environmental Protection Agency (EPA). “Supplement to Appendix D of the 

Revisions to Subpart I and Subpart W Technical Support Document – Listing of Well Count by 

Group Type 2010: Well Counts by Group.pdf”. Supporting information provided by EPA with 

the pre-Federal Register version of amendments to Subpart W. August 22, 2011(e). 

http://www.epa.gov/ghgreporting/documents/pdf/2011/documents/Well-Counts-by-Group.pdf

United States Environmental Protection Agency, Technical Note on the 1990 to 2009 Inventory 

Estimates for Natural Gas Systems, Washington DC, 2011(f). 

http://www.epa.gov/outreach/downloads/TechNote_Natural%20gas_4-15-11.pdf

Zeller, Tom Jr. “Studies Say Natural Gas Has Its Own Environmental Problems,” New York 

Times, April 11, 2011.
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 32

Appendix A. API/ANGA Survey Forms

The following provides the survey forms used to gather data presented in this report.

FIGURE A-1. SURVEY INSTRUCTIONS
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 33

FIGURE A-2. GAS WELL SURVEY DATA
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 34

FIGURE A-3. GAS WELL WORKOVER SURVEY DATA
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 35

FIGURE A-4. GAS WELL LIQUIDS UNLOADING SURVEY DATA
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 36

FIGURE A-5. OTHER SURVEY DATA
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 37

Appendix B. ANGA/API Well Survey Information

Responses from the second part of the API/ANGA survey, which focused on more detailed well 

information, covered more than 60,000 wells and provided data on:

 # of gas wells without hydraulic fracturing (anytime in their history)

 # of gas wells with hydraulic fracturing (any time in their history);

◦ # of vertical gas wells with hydraulic fracturing (anytime in their history);

◦ # of horizontal gas wells with hydraulic fracturing (anytime in their history);

 # of completions for vertical gas wells with hydraulic fracturing;

 # of completions for horizontal gas wells with hydraulic fracturing;

 # of completions for gas wells without hydraulic fracturing;

 # of workovers for vertical wells with hydraulic fracturing;

 # of workovers for horizontal wells with hydraulic fracturing; and

 # of workovers for wells without hydraulic fracturing.

Table B-1 summarizes the well data collected by the ANGA/API survey and presents its 

distribution by formation type and region. The regional distribution follows the National Energy 

Modeling System (NEMS) regions defined by the EIA. The data are compared to EPA’s 

national well counts classified by type as provided in the August 2011 database file (EPA, 

2011d). 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 38

TABLE B-1. API/ANGA SURVEY – SUMMARY OF GAS WELL COUNTS BY TYPE AND 

NEMS REGION*

NEMS Region

Conventional 

Wells Shale

Coal-bed 

Methane Tight Unspecified

Northeast 12,144 3,541 9 3,874 2,563

Gulf Coast 2,870 1,990 - 7,968 1,521

Mid-Continent 9,081 2,333 - 3,747 5,579

Southwest 646 1,208 - 726 2,326

Rocky Mountain 3,707 366 5,458 18,053 11

West Coast - - - - -

Unspecified 1,307

Survey TOTAL 28,448 9,438 5,467 34,368 13,307

% of EPA 2010 Well 

Counts (from 

database file) 14.2% 30.1% 11.5% 45.6%

Overall Survey Total 91,028

EPA Well Counts

(2010, from 

database file)

200,921 31,381 47,371 75,409

56.6% 8.8% 13.3% 21.2%

355,082

EPA National 

Inventory (2010) 484,795

EIA National Well 

Count (2010) 487,627

* ANGA/API survey data represents well counts current for calendar year 2010 or the first 

half of 2011. 

As shown in Table B-1, data from the API/ANGA survey represent approximately 26% 

of the national gas wells reported by EPA’s database (or 18.7% of the EIA well count data). 

This includes almost 46% of all tight gas wells and 30% of shale gas wells. This may indicate 

that the ANGA/API information has an uneven representation of unconventional gas wells, and 

in particular shale and tight gas wells, but it also appears that EPA’s data may mis-categorize 

these types of wells. For example, the EPA/HPDI data set contains few wells from Pennsylvania 

and West Virginia while the API/ANGA survey includes 9,422 wells from that area (AAPG 

160A). 
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 39

Table B-2 summarizes additional details on the natural gas wells information collected through 

the second data collection effort by the ANGA/API survey which covered 60,710 wells.

TABLE B-2. ANGA/API SURVEY – ADDITIONAL DETAILS ON GAS WELL COUNTS*

# Wells w/out 

hydraulic 

fracturing 

(anytime in 

their history)

# Wells with hydraulic fracturing 

(any time in their history)

Total # Vertical wells

# Horizontal 

wells

TOTAL Conventional 1,498 16,678 14,844 1,834

TOTAL Coal Bed 

Methane 42 3,475 3,424 42

TOTAL Shale 1,931 9,084 2,012 7,072

TOTAL Tight 122 27,880 24,045 3,835

TOTAL OVERALL 3,593 57,117 44,325 12,783

* API/ANGA survey data represents well counts current for calendar year 2010 or the first half of 2011. 

Additional information on natural gas wells with and without hydraulic fracturing was 

provided for approximately two-thirds (60,710 natural gas wells) of the total well data collected 

by the ANGA/API survey. For this subset of the well data, 94% of the gas wells have been 

hydraulically fractured at some point in their operating history, including almost 92% of the 

conventional wells. EPA’s 2010 national inventory reported 50,434 gas wells with hydraulic 

fracturing. This is very similar to the number of unconventional gas wells that EPA reported in 

the 2009 national inventory. Based on the API/ANGA survey results, it appears that EPA has 

underestimated the number of gas wells with hydraulic fracturing.

Of the ANGA/API survey responses for wells that have been hydraulically fractured, 

most (77.6%) are vertical wells. Vertical wells are predominately conventional gas wells, coalbed methane and tight gas wells; while the majority of shale gas wells are horizontal. EPA does 

not currently distinguish between vertical and horizontal gas wells.

A Short Note About EPA and EIA’s Well Counts

There is a discrepancy of over 132,000 natural gas wells between the EPA database 

information (EPA, 2011d) and the EIA national gas well counts (EIA, 2012), and a difference of 

almost 130,000 gas wells between the two EPA data sources (EPA, 2011d and EPA, 2012). This 

difference needs to be understood since ultimately both the IHS (EIA) and HPDI (EPA) data 

originate from the same state-level sources of information. 

The EIA provides a gas well count of 487,627 for 2010 based on Form EIA-895A6, the 

Bureau of Ocean Energy Management, Regulation and Enforcement (formerly the Minerals 

 

6

Form EIA-895, Annual Quantity And Value Of Natural Gas Production Report; 

http://www.eia.gov/survey/form/eia_895/form.pdf
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 40

Management Service) data, and World Oil Magazine (EIA, 2010). However, the EIA does not 

classify gas wells by conventional and unconventional, or by formation types, precluding more 

detailed comparison against the EIA data. For some parameters the classifications were based on 

qualitative descriptions of the formations’ physical properties (e.g. permeability) rather than on 

actual measurements (i.e. permeability data in millidarcy readings).7 

EPA provides a similar well count in the 2010 national inventory: 434,361 nonassociated gas wells + 50,434 gas wells with hydraulic fracturing, resulting in a total of 484,795 

gas wells (EPA, 2012). Further classification of gas wells or description on what constitutes a 

“non-associated” gas well versus a “gas well with hydraulic fracturing” is not provided in EPA’s 

national inventory. 

Small differences in the HPDI and IHS original data may arise from definitional 

differences as HPDI and IHS compile the raw data. In addition, each state may have a different 

interpretation of well definitions of gas versus oil wells that introduces differences among states 

for the wells reported. EPA had indicated in discussions with the API/ANGA group that their 

database well count information may not include all of the wells in the Marcellus basin. EIA 

indicates 44,500 gas wells in Pennsylvania in 2010. However, even in accounting for these 

wells, there is still a large difference (almost 88,000 wells) between EPA’s total gas well number 

from their database source and EIA’s well data.

Nevertheless, these discrepancies among the well counts need to be understood since 

these data all originate from the same state-level sources of information. Differences could arise, 

for example, from different interpretations of well definitions. 

Since the EIA data is the de facto benchmark in the energy industry, the difference 

between the EIA and EPA well count data needs to be understood before any meaningful 

conclusions can be made from the EPA data.

Since EPA’s well count from HPDI was much lower than the EIA, this report does not 

attempt to come up with a national gas well count but chose to use the 355,082 number from the 

EPA HPDI database because it was the only available database which parsed the wells into 

conventional and unconventional categories (EPA, 2011d).

 

7

Information provided by Don Robinson of ICF (EPA’s contractor).
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 41

Appendix C. Emission Estimates for Gas Well Liquids Unloading

Tables C-1 through C-4 summarize the liquids unloading emissions data collected through the API/ANGA survey and the resulting 

emission estimates. The emission factors reported in Table 4 are based on a regional weighted average of the conventional and 

unconventional gas wells, with and without plunger lifts. This provided a consistent comparison against the EPA emission factors 

which are reported only on a regional basis and do not differentiate between conventional and unconventional wells or wells with and 

without plunger lifts.

TABLE C-1. LIQUIDS UNLOADING FOR CONVENTIONAL GAS WELLS WITHOUT PLUNGER LIFTS

NEMS Region Northeast Gulf Coast Mid-Continent Southwest

# venting gas wells 190 916 12 6 1 38 220

Total # gas well vents 4,335 39,668 144 60 1 2,444 880

Average casing diameter, inches 5 4.5 5.5 3.65 4.83 4 5.5

Average well depth, feet 3,375 3,448 10,000 19,334 7,033 4,269 8,000

Average surface pressure, psig (for 

venting wells) 85 50

(Weighted 

average 

applied) 65.2

224 25.5 60.8 100

Average venting time, hours 1 2 1 2.5 .25 4.95 1

Average daily production rate, Mscfd 12.83 7.21 300 664 58.43 84 100

Total emissions, scf gas/yr 13,492,728 78,217,933 1,287,782 3,854,938 2,440 38,451,629 9,037,809

Emissions per well, scfy gas/well 71,014 85,391 107,315 642,490 2,440 1,011,885 41,081
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 42

TABLE C-2. LIQUIDS UNLOADING FOR CONVENTIONAL GAS WELLS WITH PLUNGER LIFTS

NEMS Region Northeast Mid-Continent

# venting gas wells 33 109 164 2 10

Total # gas well vents 1,272 4,217 489,912 23 7,300

Average tubing diameter, inches 2 2.375 1.995 2 2.375

Average well depth, feet 3,375 3,448 4,269 7,033 9,500

Average surface pressure, psig (for 

venting wells) 85 50 60.8 25.5 500

Average venting time, hours 1 0.3 0.067 0.75 0.08

Average daily production rate, Mscfd 12.83 7.21 84 58.43 30

Total emissions, scf gas/yr 973,442 1,963,379 232,529,848 23,623 74,495,422

Emissions per well, scfy gas/well 29,498 18,013 1,417,865 11,811 7,449,542
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 43

TABLE C-3. LIQUIDS UNLOADING FOR UNCONVENTIONAL GAS WELLS WITHOUT PLUNGER LIFTS

NEMS Region Northeast Gulf Coast

# venting gas wells 337 6 8 27 11 15

Total # gas well vents 27,720 6 104 207 572 15

Average casing diameter, 

inches

4.5 5.5 5.5 4.5 5.5 10.75

Average well depth, feet 4,845 6,000 11,000 9,000 13,752 16,000

Average surface pressure, psig 

(for venting wells)

121.6 400 200 50 450 540

Average venting time, hours 1.36 3 1 5.3 2 2

Average daily production rate, 

Mscfd 26 200 25 130 353 8,500

Total emissions, scf gas/yr 148,079,273 267,095 2,749,066

5,736,344

49,326,199 11,004,804

Emissions per well, scfy 

gas/well

439,404 44,516 343,633 212,457 4,484,200 733,654
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 44

TABLE C-3. LIQUIDS UNLOADING FOR UNCONVENTIONAL GAS WELLS WITHOUT PLUNGER LIFTS, CONTINUED

NEMS Region Gulf Coast Mid-Continent

# venting gas wells 146 2 10 40 177 3 136 215

Total # gas well vents 146 12 120 40 400 7.2 391.2 2,580

Average casing diameter, 

inches

4.5 5.5 5.5 8.625 5.5 4.92 5.02 5.5

Average well depth, feet 8,500 11,647 11,000 12,500 3,911 10,293 7,888 11,000

Average surface pressure, 

psig (for venting wells)

15 25 94 530 80 90.04 98.75 200

Average venting time, hours 0.6875 1.5 4 1 2.5 1.58 1.925 0.5

Average daily production

rate, Mscfd

99 83 92 6,500 250 727 875 100

Total emissions, scf gas/yr 276,156 82,853 2,985,945 7,496,306 7,908,154 196,019

16,457,032

68,197,984

Emissions per well, scfy 

gas/well

1,891 41,427 298,594 187,408 44,679 65,340 121,008 317,200
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 45

TABLE C-3. LIQUIDS UNLOADING FOR UNCONVENTIONAL GAS WELLS WITHOUT PLUNGER LIFTS, CONTINUED

NEMS Region Southwest Rocky Mountain

# venting gas wells 228 6 3 5 113 2 28

Total # gas well vents 221 6 1 1800 2,004 4 10,584

Average casing diameter, 

inches

9.625 5.5 5 2.375 4.038 4.7 4.5

Average well depth, feet 8,725 8,000 15,000 11,597 11,149 11,056 10,844

Average surface pressure, psig 

(for venting wells) 516 50 200 476 250 250 198

Average venting time, hours 1 0.5 6.67 0.77 1.616 0.75 3.18

Average daily production rate, 

Mscfd

1,500 12 150 41.54 127 433 83

Total emissions, scf gas/yr 35,075,995 34,759 65,227 76,746,923 42,212,132 95,677 262,702,911

Emissions per well, scfy 

gas/well

153,842 5,793 21,742 15,349,385 373,559 47,839 9,382,247
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 46

TABLE C-4. LIQUIDS UNLOADING FOR UNCONVENTIONAL GAS WELLS WITH PLUNGER LIFTS

NEMS Region Northeast Gulf Coast

# venting gas wells 308 103 5 3 2 22 59 5

Total # gas well vents 63,840 75,190 194 156 2 22 354 5

Average tubing diameter, 

inches

2.375 2.375 2.375 2.375 2.375 2.375 2.375 2.375

Average well depth, feet 4,845 2,500 7,000 13,752 16,000 8,500 11,647 12,500

Average surface pressure, 

psig (for venting wells) 121.6 200 130 450 540 15 25 530

Average venting time, 

hours

0.221 0.05 0.1 2 1 0.875 0.3 0.5

Average daily production

rate, Mscfd 26 15 628 353 8,500 99 83 6,500

Total emissions, scf gas/yr 87,985,573 84,228,892 410,107 5,522,367 391,212 45,622 341,615 71,050

Emissions per well, scfy 

gas/well

285,667 817,756 82,021

1,840,789

195,606 2,074 5,790 14,210
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 47

TABLE C-4. LIQUIDS UNLOADING FOR UNCONVENTIONAL GAS WELLS WITH PLUNGER LIFTS, CONTINUED

NEMS Region Mid-Continent Southwest

# venting gas wells 48 4 64 29 18 60

Total # gas well vents 155,742 9.6 170.4 348 25 60

Average tubing diameter, inches 2.375 3.88 4.11 2.4 1.995 2.375

Average well depth, feet 3,911 10,293 7,888 (Average Applied)

7,888 8,725 6,800

Average surface pressure, psig (for 

venting wells)

80 90.04 98.75 74.69 516 110

Average venting time, hours 0.0833 2.99 2.6 0.5425 0.5 0.2

Average daily production rate, 

Mscfd 250 727 875 (Weighted average 

Applied) 371 1500 25

Total emissions, scf gas/yr

120,385,033

781,741

13,999,323

751,651 170,468 106,183

Emissions per well, scfy gas/well 2,508,022 195,435 218,739 25,919 9,470 1,770
Summary and Analysis of API and ANGA Survey Responses (Updated September, 2012) 48

TABLE C-4. LIQUIDS UNLOADING FOR UNCONVENTIONAL GAS WELLS WITH PLUNGER LIFTS, CONTINUED

NEMS Region Rocky Mountain

# venting gas wells 247 23 296 19 793

Total # gas well vents 1,476 51.43 2,080 21,888 9,516

Average tubing diameter, inches 1.997 1.92 2.375 2.375 2.375

Average well depth, feet 11,149 11,164 11,056 10,844 7,400

Average surface pressure, psig (for 

venting wells)

250 290 250 198 150

Average venting time, hours 0.407 1.12 2.1 0.455 0.67

Average daily production rate, Mscfd 127 454 433 83 46

Total emissions, scf gas/yr 6,427,381 841,788 72,746,777 105,363,947 27,245,035

Emissions per well, scfy gas/well 26,022 36,599 245,766 5,545,471 34,357
Summary and Analysis of API and ANGA Survey Responses 49

The calculated emissions shown in Tables C-1 through C-4 are based on applying Equation W-8 

from 40 CFR 98 Subpart W to gas well liquid unloading without plunger lifts and Equation W-9 

to gas well liquid unloading with plunger lifts. The equations and the terms are provided below.

98.233(f)(2) Calculation Methodology 2. Calculate the total emissions for well venting for liquids 

unloading using Equation W–8 of this section.

Where:

Es,n= Annual natural gas emissions at standard conditions, in cubic feet/year.

W = Total number of wells with well venting for liquids unloading for each sub-basin.

0.37×10−3= {3.14 (pi)/4}/{14.7*144} (psia converted to pounds per square feet).

CDp= Casing internal diameter for each well, p, in inches.

WDp= Well depth from either the top of the well or the lowest packer to the bottom of the 

well, for each well, p, in feet.

SPp= Shut-in pressure or surface pressure for wells with tubing production and no packers 

or casing pressure for each well, p, in pounds per square inch absolute (psia) or 

casing-to-tubing pressure of one well from the same sub-basin multiplied by the 

tubing pressure of each well, p, in the sub-basin, in pounds per square inch absolute 

(psia).

Vp= Number of vents per year per well, p.

SFRp= Average flow-line rate of gas for well, p, at standard conditions in cubic feet per hour. 

Use Equation W–33 to calculate the average flow-line rate at standard conditions.

HRp,q= Hours that each well, p, was left open to the atmosphere during unloading, q.

1.0 = Hours for average well to blowdown casing volume at shut-in pressure.

Zp,q= If HRp,q is less than 1.0 then Zp,q is equal to 0. If HRp,q is greater than or equal to 1.0 

then Zp,q is equal to 1.

98.233(f)(3) Calculation Methodology 3. Calculate emissions from each well venting to the

atmosphere for liquids unloading with plunger lift assist using Equation W–9 of this section.

Where:

Es,n= Annual natural gas emissions at standard conditions, in cubic feet/year.

W = Total number of wells with well venting for liquids unloading for each sub-basin.

0.37×10−3= {3.14 (pi)/4}/{14.7*144} (psia converted to pounds per square feet).

TDp= Tubing internal diameter for each well, p, in inches.

WDp= Tubing depth to plunger bumper for each well, p, in feet.

SPp= Flow-line pressure for each well, p, in pounds per square inch absolute (psia), using 

engineering estimate based on best available data.

Vp= Number of vents per year for each well, p.

SFRp= Average flow-line rate of gas for well, p, at standard conditions in cubic feet per hour. 

Use Equation W–33 to calculate the average flow-line rate at standard conditions.

HRp,q= Hours that each well, p, was left open to the atmosphere during each unloading, q.

0.5 = Hours for average well to blowdown tubing volume at flow-line pressure.
Summary and Analysis of API and ANGA Survey Responses 50

Zp,q= If HRp,q is less than 0.5 then Zp,q is equal to 0. If HRp,q is greater than or equal to 0.5 

then Zp,q is equal to 1.