Quantifying Fugitive Emission Factors from
Unconventional Natural Gas Production
Using IPCC Methodologies

IPCC TFI Technical Support Unit Inventory InternshipFebruary ‐ August 2013
Ryan Glancy
Under the Supervision of:
Mr. Kiyoto Tanabe, Head, Technical Support Unit
IPCC Task Force on National Greenhouse Gas Inventories

This document should be referenced as:
Glancy, R.P. (2013)
Quantifying Fugitive Emission Factors from Unconventional Natural Gas Production Using IPCC
Methodologies
IGES, Hayama, JapanExecutive Summary
Continued advances in horizontal drilling techniques, when combined with a process known as‘hydraulic fracturing’, are providing access to previously uneconomical natural gas resources. It is
anticipated that these resources will play an increasing role in the global primary energy mix, withprojections that the uptake will be exponential, growing from 145 billion cubic meters in 2010 to 975
billion cubic meters by 2035.
This study reviews available literature and data sources related to the fugitive emissions from theproduction of unconventional gas sources; Shale gas, Tight sands gas and Coalbed methane. Tier 1(and Tier 2 for USA and Canada) emission factors are developed using IPCC good practicemethodologies. Factors are developed for methane, carbon dioxide, non‐methane volatile organic
compounds and nitrous oxide for both developed and developing country scenarios. Using Monte
Carlo analyses, expected values and uncertainty ranges for each emission factor are derived fromthe data retrieved through the literature review.
Emission factors and uncertainty ranges for the developed country scenario can be seen in Table E‐1
below.

Table E‐1: Proposed emission factors for production of Shale gas, Tight sands gas and Coalbed methane in developed
countries with associated 95% confidence interval upper and lower limits (those for conventional natural gas production
taken from Table 4.2.4 in Chapter 4, Volume 2 of the 2006 IPCC Guidelines are also included for comparison)

The results show that fugitive emissions arising from hydraulic fracturing activities are substantial
when compared with typical conventional gas fugitive emissions. Mean life‐cycle values for fugitive
emissions from Shale gas, Tight sands gas and Coalbed methane are 133%, 100% and 36% higher
respectively than those of conventional gas in the developed countries scenario as shown in Figure
E‐1. Developing countries show a similar scale of difference.

iFigure E‐1: Well‐to‐meter fugitive emissions for natural gas by source type in developed countries

In general the following conclusions can be drawn:
1) Fugitive emissions from completions and workovers of unconventional gas sources including
Shale gas, Tight sands gas and Coalbed methane, are significant and the relevant emission
factors should be added to the EFDB.
2) While the emission factors derived in this study are as accurate as possible, there is a deficitof measured and verified emissions data. Third party measurements to quantify fugitive
emissions from unconventional (and conventional) gas sources would vastly improve theaccuracy of the emission factors.
3) The scale of the difference in emissions between conventional and unconventional gas
sources may be indicative of other types of unconventional fuel. In its current form the 2006
IPCC Guidelines do not address unconventional fuel types sufficiently and consideration
should be given to amending the guidelines accordingly.

iiTable of Contents
Executive Summary ................................................................................................................................................. i
1

2

3

4

Introduction ................................................................................................................................................... 1
1.1

Subject of Concern ................................................................................................................................ 1

1.2

Unconventional Gas .............................................................................................................................. 1

1.3

USA and Canadian Unconventional Gas Industries ............................................................................... 3

Background and Scope................................................................................................................................... 4
2.1

Unconventional Gas and the IPCC Emissions Factor Database ............................................................. 4

2.2

Tier 1, 2 and 3 Emission Factors ............................................................................................................ 4

2.3

Scope of Study....................................................................................................................................... 5

Methodology ................................................................................................................................................. 7
3.1

Literature Review .................................................................................................................................. 7

3.2

Screening for Bias .................................................................................................................................. 7

3.3

The Model ............................................................................................................................................. 7

3.4

Assumptions .......................................................................................................................................... 9

3.5

Statistical Analysis ............................................................................................................................... 11

3.6Time Series Considerations ................................................................................................................. 11

3.7

Uncertainty ......................................................................................................................................... 12

3.8

Quality Control Procedures and Limitations ....................................................................................... 13

Results.......................................................................................................................................................... 14
4.1
Tier 1 Emission Factors for Unconventional Gas Sources in Developed Countries (Tier 2 for USA and
Canada) ............................................................................................................................................................ 14
4.2
Tier 1 Emission Factors for Unconventional Gas Sources in Developing Countries and Countries with
Economies in Transition ................................................................................................................................... 14

5

6Discussion .................................................................................................................................................... 19
5.1

Life‐cycle Assessment of Fugitive Emissions from Conventional and Unconventional Gas Sources .. 19

5.2

Uncertainty Analysis............................................................................................................................ 19

5.3

Precision .............................................................................................................................................. 21

5.4

Emerging Research and Potential Consequences ............................................................................... 21

5.5

Recommendations for Further Work .................................................................................................. 22

Conclusion.................................................................................................................................................... 23

Appendix 1: Primary Information Sources............................................................................................................ 24
Appendix 2: Datasets and Expert Judgement Sources Used ................................................................................ 25
Appendix 3: Input Variable Probability Density Functions ................................................................................... 26
Appendix 4: Dealing with Different Causes of Uncertainty .................................................................................. 34
Appendix 5: Sensitivity Analyses .......................................................................................................................... 35
References ............................................................................................................................................................ 39

Equations
Equation 1: Basis for Emissions from Unconventional Gas Sources...................................................................... 5
Equation 2: Net Fugitive Emissions from Natural Gas Operations ........................................................................ 7
Equation 3: Emissions from Venting...................................................................................................................... 8Equation 4: Emissions from Flaring ....................................................................................................................... 8Equation 5: Workover Potential Emissions Estimation ....................................................................................... 10

TablesTable 1:

Estimated global unconventional gas reserves .................................................................................. 1

Table 2:

USA natural gas reserves by type ....................................................................................................... 3

Table 3:

List of itemised and common variables for the development of Shale gas, Tightsands gas and Coalbed methane fugitive emissions .......................................................................... 9

Table 4:

Time series consistency considerations for variables affecting fugitive emissions
from unconventional gas ................................................................................................................. 12

Table 5:

Proposed emission factors for production of Shale gas, Tight sands gas andCoalbed methane in developed countries with associated 95% confidence interval
upper and lower limits ..................................................................................................................... 15

Table 6:

Proposed emission factors for production of Shale gas, Tight sands gas andCoalbed methane in in developing countries and countries with economies intransition with associated 95% confidence interval upper and lower limits ................................... 15

FiguresFigure 1:

A map of known global Shale gas reserves ........................................................................................ 2

Figure 2:

Fugitive emission factors for CH4, CO2, NMVOCs and N2O by IPCC category forconventional gas, Shale gas, Tight sands gas and Coalbed methane in developed
countries .......................................................................................................................................... 17

Figure 3:

Fugitive emission factors for CH4, CO2, NMVOCs and N2O by IPCC category forconventional gas, Shale gas, Tight sands gas and Coalbed methane for developing
countries and countries with economies in transition..................................................................... 18

Figure 4:

Life‐cycle well‐to‐meter fugitive emissions for natural gas by source type indeveloped countries......................................................................................................................... 20

Figure 5:

Sensitivity charts for total CH4 fugitive emissions arising from Shale gas, Tightsands gas and Coalbed methane with input variables and datasets ranked by effecton output mean ............................................................................................................................... 20

Figure 6:

A comparison of fugitive emission factors for Shale gas from completion andworkover activities ........................................................................................................................... 21

List of Acronyms
API

American Petroleum Institute

ANGA

America’s Natural gas Alliance

CoV

Coefficient of Variation

EFDB

Emission Factors Database

EIA

Energy Information Administration (USA)

EPA

United States Environmental Protection Agency

EUR

Estimated Ultimate Recovery

GHG

Greenhouse Gases

GHGRP

Greenhouse Gas Reporting Program (EPA)

GRI

Gas Research Institute

IEA

International Energy Agency

IPCC

Intergovernmental Panel on Climate Change

LCA

Life‐cycle assessment

MJ

Mega‐joules (106 joules)

NGGIP

National Greenhouse Gas Inventories Program

PDF

Probability Density Function

QA

Quality Assurance

QC

Quality Control

REC

Reduced Emission Completion

TFI

Task Force on National Greenhouse Gas Inventories (IPCC)

tcm

Trillion Cubic Metres

TCO2eTonnes of Carbon Dioxide Equivalent1 Introduction
1.1 Subject of Concern
Continued advances in horizontal drilling techniques, when combined with a process known as‘hydraulic fracturing’, are providing access to previously uneconomical natural gas resources. Large
reserves of this ‘unconventional’ natural gas (unconventional gas) have been identified and are
distributed globally. It is projected that these sources will play an increasing role in the global
primary energy mix, with projections that the uptake will be exponential.
From the limited amount of study that has been conducted on the subject, there is a generalconsensus that fugitive methane emissions from unconventional gas exceed those of conventional
onshore sources (conventional gas), but there is some disagreement to the scale of the difference.
Considering the increasing rate of extraction and the labelling of the fuel source as a ‘transition fuel’between coal (and other high‐emission fuels) and renewable sources of energy, the potential impactof unconventional gas on global greenhouse gas (GHG) emissions is significant. It is therefore apriority to reach a higher level of certainty with regard to the scale of these impacts so that wellinformed policy decisions can be agreed and implemented.

1.2 Unconventional Gas
Unconventional gas is defined by the International Energy Agency (IEA) as “sources of gas trapped
deep underground by impermeable rocks, such as coal, sandstone and shale. The three main types of‘unconventional’ gas are: Shale gas (found in shale deposits); coal bed methane, or CBM (extracted
from coal beds) and Tight sands gas (which is trapped underground in impermeable rock formations)”(IEA, Glossary of Terms). These sources are mined using the hydraulic fracturing process whereby
large volumes of water mixed with sand and chemicals are injected at high pressure into theformations causing fractures. This releases the trapped pockets of gas which can then flow into thewell bore for collection. The process can be applied to either horizontally‐drilled or vertically‐drilledwells.
The IEA’s World Energy Outlook 2012 (IEA 2012a) estimates that Shale gas reserves alone are 200
trillion cubic meters globally (see Table 1 below), which accounts for nearly a quarter of total
estimated natural gas reserves. Extraction was approximately 145 billion cubic meters in 2010 with
anticipated increase in production to 975 billion cubic meters by 2035 (IEA, 2012b), resulting in theshare of unconventional gas in total natural gas output rising from 14% to 32%.
SourceUnconventional
(tcm)

Gas

Reserves% of Total Estimated Natural Gas
Reserves

Shale gas

200

25%

Coal Bed Methane

47

6%

Tight sands gas

81

10%

Table 1: Estimated Global Unconventional gas reserves (IEA, 2012a)

To date, only the USA has a well‐established unconventional gas industry, which has seen
unprecedented growth in recent years. Production grew 48% between 2006 and 2010 (IEA, 2011)1and is projected to grow by a further 172% between 2010 and 2035 when it will make up 49% ofnatural gas production (IEA 2012a). Canada has also recently started commercial production.
Other major resources have been identified in a number of developed and developing countries
around the world. The full extent and locations of unconventional gas sources is not completely
understood. Figure 1 below shows the locations of confirmed Shale gas plays (gas fields). Outside of
the USA and Canada, unconventional gas industries are predominantly still in the research anddevelopment stage and there is limited information available.

Figure 1: A map of known global Shale gas reserves (EIA, 2012)

The environmental impact of the extraction of unconventional gas sources has come under scrutiny
and there remains significant uncertainty as to the scale of the effect. In terms of the potential
impact on GHG emissions there is considerable on‐going debate. One of the key issues is the level offugitive emissions arising from the well development process. There are elements of unconventional
gas extraction that do not pertain to the typical conventional process. Specifically, GHG emissions
arising from the hydraulic fracturing process have significant uncertainties and are currently a sourceof considerable controversy.
A typical well will undergo multiple hydraulic fractures during the well development stage. During
this stage, so‐called ‘completion’ (also known as ‘flow‐back’) of an unconventional gas well occurs.
This is the stage at which, after the well has been hydraulically fractured, pressure causes an amountof the fracturing liquid (anywhere from 20 – 80%) and dissolved gases to return to the surface at thewell head. The initial pressure and liquid content of this flow‐back makes this fluid difficult toprocess so that it is not always economically or technically viable to recover the natural gas
contained within the fluid. This may result in the venting or flaring of the gas which is predominantly
composed of methane, a powerful GHG. Workovers (also known as re‐fractures) involve principally
the same process as hydraulic fracture and completion, but are carried out on existing wells where
productivity has diminished. Any given well will thus undergo one completion and may or may notundergo multiple workovers, depending on its productivity and the availability of further gas
resources in the rock formation. With the exception of completions and workovers, unconventional
2

gas production, processing and transportation processes are considered consistent with those of
onshore conventional gas.
Modern technology can allow for the capture of the gases contained within the flowback fluids
during completions and workovers. This process is known as ‘Reduced Emission Completions’ (RECs)or ‘Green Completions’. Deployment of these technologies is largely dependent on the financial
incentive to do so and is not always of economic benefit to the mining operations.

1.3 USA and Canadian Unconventional Gas Industries
Unconventional gas was first mined on an economically viable scale in the USA in 1998. Since thattime the scale of proven resources has grown significantly and now exceeds that of conventional gas(see Table 2 below). There has been rapid growth in the industry and the majority of the identified
plays are now in production. The Canadian industry is now starting to follow in the USA’s footsteps
in exploiting unconventional gas sources, especially in the west of the country.
Conventional

Unconventional

OnshoreAssociatedOffshore

Tight

Shale

Tight Sands

25%

13%

7%

31%

16%

9%

44%

56%

Table 2: USA natural gas reserves by type (NETL, 2011)

During this period of growth the USA Environmental Protection Agency (EPA) has been working toestablish appropriate means of quantifying the associated GHG emissions. Based on the lack ofverified data and information available, both the activity data and the relative emission factors havebeen subject to a large degree of uncertainty and debate. The most comprehensive study to date
was conducted in partnership between the EPA and the Gas Research Institute (GRI) in 1992. The
resulting report (EPA/GRI 1996) provided fugitive emission factors for the natural gas industry
including estimates for conventional, unconventional, upstream and downstream sources. Thesefigures have been in use until recently when some of the underlying assumptions were called into
question. Subsequently the EPA has revised some of the figures including those for completions andworkovers (EPA 2013).
In addition to the default emission factors used in USA national GHG reporting (EPA 2013), the EPA
has started collecting industry‐reported data through its legislated Greenhouse Gas Reporting
Program (GHGRP). Under the mandate of Subpart W for ‘Petroleum and Natural Gas Systems’, whichstarted in 2011, industry operatives with emissions over 25,000 TCO2e are obliged to estimate andreport their emissions on an annual basis. The initial results of this exercise have recently been madeavailable to the public (EPA GHGRP 2013).
There are various other scientific initiatives underway throughout the USA to try and better estimate
fugitive emission sources from the natural gas industry. These methods include technologies such as
air‐sampling and infrared imagery to measure the emissions. Initial results have indicated thatcurrent inventory techniques are significantly underestimating fugitive emissions. This may haveserious consequences not only for the natural gas industry, but for the fossil fuel sector at large.

3

2 Background and Scope
2.1 Unconventional Gas and the IPCC Emissions Factor Database
The Intergovernmental Panel on Climate Change (IPCC) Task Force on National Greenhouse Gas
Inventories (TFI) maintains an Emission Factors Database (EFDB) as part of the National Greenhouse
Gas Inventories Programme (NGGIP). This database serves as an open‐access repository of emission
factors for use during GHG inventory compiling by users globally. The database is organizedaccording to IPCC sectors and subsectors, categorised by sources of potential emissions. Within the
Energy sector, there are subsectors relating to Fuel Combustion Activities (1.A), Fugitive Emissions
from Fuels (1.B) and Carbon Dioxide Transport and Storage (1.C). Default emission factors exist for
fugitive emissions from the natural gas industry, however these factors pertain to conventional gas
production and as yet, no emission factors have been included for fugitive emissions fromunconventional gas. Similarly, there is no reference to unconventional gas in the 2006 IPCC
Guidelines for National Greenhouse Gas Inventories (2006 IPCC Guidelines).
The IPCC mechanism for prioritising various sources of GHG emissions and their related uncertainties
is referred to as ‘Key Category Analysis’. This quantitative and qualitative analysis considers the level,
trend and uncertainty of any given GHG source or sink in order to qualify its importance in thecontext of any given inventory. By categorising these elements it allows inventory compilers to focus
resources on the most relevant subcategories. When considering unconventional gas in this context,
the emission source could be defined as a key category on a global scale given the existing level ofproduction, the trend and the level of uncertainty of the related fugitive emissions. It is thus
considered timely that fugitive emission factors for the production of unconventional gas be
developed for inclusion in the EFDB.

2.2 Tier 1, 2 and 3 Emission Factors
The IPCC approach to developing GHG inventories is based on a three‐tier system. Tiers are allocatedto emission factors and activity data to categorize the level of rigour adopted in their development.
The tiers are ordered in ascending complexity with the concept that that the higher the tier, themore accurate the reported activity data or emission factors are likely to be. In terms of emission
factors; in general Tier 1 refers to a global default (least accurate), Tier 2 refers to a country‐specific(moderate accuracy) and Tier 3 refers to a rigorous bottom‐up analysis of localised factors andparameters (most accurate).
The natural gas industry qualifies as a ‘Key Category’ within a number of producing, exporting andimporting states around the world. A significant amount of research and analysis has been
undertaken to evaluate the level of fugitive emissions from conventional gas sources to allow for
accurate inventorying. Guidance on the application of these emission factors is provided in the 2006
IPCC Guidelines1. Default Tier 1 emission factors for fugitive emissions from the conventional natural
gas industry are included in both the 2006 IPCC Guidelines2 and the EFDB, while there are also someTier 2 factors for certain countries (including the USA and Canada).

12Volume 2, Chapter 4
Volume 2, Chapter 4, Tables 4.2.4 and 4.2.5

4

2.3 Scope of Study
This study reviews available literature and data sources relative to fugitive emissions from theproduction of unconventional gas. Emission factors are developed consistent with IPCC goodpractice methodologies to arrive at Tier 1 and Tier 2 (USA and Canada) emission factors for Shale gas,
Tight sands gas and Coalbed methane in both developed and developing country scenarios. The
analysis also discusses the potential steps towards developing Tier 3 methodologies.
The IPCC defines fugitive emissions as “intentional or unintentional releases of gases fromanthropogenic activities. In particular, they may arise from the production, processing, transmission,storage and use of fuels, and include emissions from combustion only where it does not support a
productive activity (e.g., flaring of natural gases at oil and gas production facilities)” (IPCC 1996).
When applying this definition to the production of natural gas, fugitive emissions are inclusive ofunintentional leaks, intentional venting and flaring. Within the scope of this study venting and flaringof completion and workover gases are quantified and attributed to IPCC (IPCC 2006) source
subsector 1.B.2.b (categorised by the following path: Energy → Fugitive Emissions from Fuels → Oiland Natural Gas → Natural Gas).
As discussed in Section 1.2, it is considered that the typical production, processing, storage,transport and distribution processes of conventional and unconventional gas sources are equivalent
with the exception of completions and workovers of unconventional wells. Owing to this consistency,
the emission factors for completions and workovers are compatible with existing emission factors
for conventional gas such that they can be compiled as follows:
Equation 1: Basis for Emissions from Unconventional Gas Sources:.

.

/

Where:
Eunconv.

= Fugitive emissions from unconventional gas sources

Econv.

= Fugitive emissions from conventional gas sources

Ecomps/works= Fugitive emissions from completions and workovers

Based on this assumption, this study quantifies emissions from completions and workovers andapplies them to emission factors already developed for fugitive emissions from conventional gas
which can be found in the 2006 IPCC Guidelines3.
Many literature sources identify a process known as ‘liquids unloading’ as an additionalinconsistency between unconventional and conventional extraction processes as an activity (NETL
2011 and Burnham 2012) while other sources claim that it can occur in all well types (EPA 2012 andHowarth 2011). Considering the lack of clarity on this issue, it conservatively assumed that liquids
unloading does occur in unconventional gas wells. It should be noted that fugitive emissions fromliquids unloading has not received much attention and is poorly understood. This is consideredfurther in Section 5.5 below.

3

Volume 2, Chapter 4, Tables 4.2.4 and 4.2.5

5

The USA is the only country to have established a significant level of unconventional gas production
and as such, is the only country about which literature and data specific to unconventional gas has
been amassed. Therefore, this study reviews predominantly USA information and derives Tier 2(country‐specific) emission factors applicable to the USA. It is considered that in Canada the similarparameters, geological formations and technologies mean that the Tier 2 emission factors developed
here are applicable. The assumption is made that these can also serve as Tier 1 emission factors for‘Developed Countries’ on the basis that similar geological conditions, technology and resources canbe expected. This is consistent with the 2006 IPCC Guidelines4 for other natural gas systems. In the‘Developing Countries and Countries with Economies in Transition’ scenario, it is assumed thatsimilar geological conditions can be expected, but different assumptions need to be made regarding
technology and resources.
There are significant differences in the factors that affect emissions from Shale gas, Tight sands gas
and Coalbed methane source types. These arise from the differences in reservoir pressures which
effect the potential emissions at completion and workover stages as well as the estimated ultimate
recovery (lifetime productivity) of the wells. Consequently this study develops emission factors foreach source type. It should be noted that considerably more research has been carried out on Shalegas compared to Tight sands gas or Coalbed methane which has an impact on data availability.
Consistent with the 2006 IPCC Guidelines, emission factors are developed for methane (CH4), carbon
dioxide (CO2), non‐methane volatile organic compounds (NMVOCs) and nitrous oxide (N2O) for the
production of each source type and in both developed and developing country scenarios.

4

Volume 2, Chapter 4, Table 4.2.4

6

3 Methodology
3.1 Literature Review
Information and data used in this study is taken from the most recent sources available
predominantly from USA literature (as previously discussed there are limited sources of informationfrom other countries). Primary data is taken from reports and papers dating from 2010 to present(see Appendix 1). These sources were selected based on the quality of the information and toprovide a balanced contribution from the scientific community, industry representatives and theenvironmental administration (EPA).
Unconventional sources of natural gas are still a relatively new fuel source and consequently there
remains a shortage of verified data on the factors that affect fugitive emission levels. In order toovercome this issue a number of the assumptions used by scientists and industry representatives
have been used as data points in this study. In these scenarios the data can be considered as ‘Expert
Judgement’ as defined in the 2006 IPCC Guidelines5. Instances in which data from Expert Judgement
is used are listed in Appendix 2.

3.2 Screening for Bias
Following a detailed review of the relevant literature, the available information was scrutinised todetermine applicable datasets and to try to eliminate sources of bias. It was identified that a numberof papers utilised the same EPA data source for emissions from completions before applying various
assumptions for other variables affecting the level of emissions. In order to utilise all of this data, thedata and assumptions identified in the literature were disaggregated from their sources andcategorised according to the variables (detailed in Section 3.3 below). This approach allows for
maximum use of available data while minimising potential bias from potentially polarised
assumptions in any given study. It also ensures double‐counting of EPA data is avoided.
A significant amount of the data available in published literature is derived from industry reported
figures. For the most part this data is deemed permissible, especially in light of the lack of third party
measurements. One exception for the use of industry reported data was made for estimatedultimate recovery (EUR ‐ the lifetime productivity of a well), which was not included on the basis thatsuch data can be subject to inflation by industry for financial reasons and is thus considered apotential source of bias.

3.3 The Model
When considering fugitive emissions from any natural gas sources, the most effective means ofcalculating net emissions for establishing emission factors involves the following general approach:
Equation 2: Net Fugitive Emissions from Natural Gas Operations:.

.

Where:
Enet.

5= Net fugitive emissions

Volume 1, Chapter 2

7

Epot..

= Total potential fugitive emissions6

Emit.

= Mitigated fugitive emissions (i.e. gas captured through abatement
technologies)

This concept is expanded to apply its principals specifically to unconventional gas production. In
order to maintain consistency with the 2006 IPCC Guidelines, emission factors for venting and flaring
are derived separately. The emissions of a GHG ‘i’ are calculated using the following equations:
Equation 3: Emissions from Venting:
∙
,

,

∙ 1

∙

,

,

∙

∙ 1

∙

,

Equation 4: Emissions from Flaring:
,

,

∙ 1

∙

,

,

∙

∙ 1

∙

,

∙ 1

∙

∙

Where:
Ei,venting= Venting emissions factor for gas i as a result of completion and workover
activities for unconventional gas wells (g/MJ or Gg/106m3)

Ei,flaring= Flaring emissions factor for gas i as a result of completion and workover
activities for unconventional gas wells (g/MJ or Gg/106m3)

Mpot.comp.

= Total potential natural gas emissions from completion activities (g or Gg)

Mpot.work.

= Total potential natural gas emissions from workover activities (g or Gg)

Nwork.

= Number of well workovers during well lifespan

RECcomp= Ratio of natural gas captured by REC technology during completions

RECwork= Ratio of natural gas captured by REC technology during workovers

Xcomp,flare= Ratio of natural gas not captured by REC technology that is flared during
completions (i.e. Xcomp,flare = 1 ‐ Xcomp,vent)

Xwork,flare= Ratio of natural gas not captured by REC technology that is flared during
workovers (i.e. Xwork,flare = 1 – Xwork,vent)

Xcomp,vent= Ratio of natural gas not captured by REC technology that is vented during
completions (i.e. Xcomp,vent = 1 ‐ Xcomp,flare)

6

One of the most common sources of data used to estimate potential fugitive emissions from completions in the various studies reviewed
is from the amounts of gas reported as captured when using REC technology. The volume of recovered gas is assumed to be that which
would have escaped in the absence of abatement technology. Some industry representatives have questioned the legitimacy of using thismeasurement technique to estimate emissions and described it as counter‐intuitive (based on the fact that the gas in question was
captured and not vented). In this regard it must be emphasised that in using this data, the emissions are strictly defined as ‘potential
emissions’ and irrespective of abatement technologies that can be applied to mitigate the net (actual) emissions.

8

Xwork,vent= Ratio of natural gas not captured by REC technology that is vented during
workovers (i.e. Xwork,vent = 1 – Xwork,flare)FE= Flaring destruction efficiency (i.e. fraction of the gas that leaves the flarepartially or fully burned). Typically, a value of 0.995 is assumed for flares atrefineries and a value 0.98 is assumed for those used at production andprocessing facilities.

yi= Mass fraction of the associated gas that is composed of substance i (i.e.,
CH4, CO2 or NMVOC).

CEFi7= Combustion emission factor for gas i from flaring (Gg of gas i/Gg of
associated natural gas flared). Refer to the IPCC emission factor database(EFDB), manufacturer’s data or other appropriate sources for the value of
this factor.

EUR= Estimated ultimate recovery from the gas well (MJ or 106m3)

The data extracted from the literature reviews for each source type were converted to common
units and applied to these equations using the assumptions and statistical approach outlined in
Sections 3.4 and 3.5 below.

3.4 Assumptions
Emissions from the three source types of unconventional gas are derived through a combination ofitemised and common variable inputs as listed in Table 3. Information relating to the geologicalconditions in Shale gas, Tight sands gas and Coalbed methane (i.e. data regarding potential
emissions and EUR) wells is itemised. No disaggregated data is available for the level of mitigation bysource type. There is no information to suggest that the prevalence of these measures differ
between the source types so the inputs derived are considered common between them.
Itemised Variables

Common Variables

Completion potential emissions

Completions REC rate

Workover potential emissions

Completions flaring rate

Number of workovers

Workovers REC rate

Estimated Ultimate Recovery

Workovers flaring rate

Table 3: List of itemised and common variables for the development of Shale gas, Tight sands gas and Coalbed
methane fugitive emissions

When applying Equations 3 and 4 to a population of wells it should be noted that for REC rates(RECcomp and RECwork), the input should be the product of expected capture rates with the technology
in place and the prevalence of RECs in industry (i.e. the average capture rate from a gas well
population including wells where REC technology is not utilized).

7

CEF is usually used to stand for “Carbon Emission Factor”, but it stands for “Combustion Emission Factor” inthis report.

9

The results derived from USA literature provide Tier 2 emission factors for the USA (and Canada).
The assumption is made that these factors are appropriate for application as Tier 1 emission factors
for other developed countries, consistent with the 2006 IPCC Guidelines8 approach to fugitive
emissions.
In order to estimate Tier 1 emission factors for developing countries, different assumptions
regarding the level of mitigation (i.e. the common variables listed in Table 3 above) are made toaccount for the differing levels of technology, resource and knowledge. In these cases it is assumed
that no REC technology is used and that all completion and workover potential emissions are either
vented or flared. The upper limit of the range for flaring has been set to equal the mean value
derived for developed countries, with the lower limit set to 0%. This approximation is made in theabsence of available industry data from developing countries, deemed consistent with the 2006 IPCC
Guidelines9 approach to fugitive emissions in developing countries. Using the assumption thatsimilar geological conditions for each source type can be expected regardless of the location,
variables contributing towards the level of potential emissions (Mpot.comp. and Mpot.work.) were kept
consistent with those developed from USA data.
No data was identified for potential emissions from workovers (Mpot.work.) by source type. To fill thisdata gap, surrogate data is used (consistent with the 2006 IPCC Guidelines10). Workover potential
emissions from uncategorized sources is available (EPA GHGRP 2013, US EPA 2013), which is applied
to the appropriate ‘pressure group’ ratio derived from potential emissions from completions asfollows:
Equation 5: Workover Potential Emissions Estimation:

,

,

,
,

,
,

,

,

Where:
Mpot.work,source= Total potential natural gas emissions from workover activities for a given
source type (e.g. Shale gas)

Mpot.comp,source= Total potential natural gas emissions from completion activities for a given
source type (e.g. Shale gas)

Mpot.comp,all sources = Total potential natural gas emissions from completion activities for all
unconventional source types
Mpot.work,all sources = Total potential natural gas emissions from workover activities for all
unconventional source types
In this way emissions from workovers for a particular source type are proportional to the emissions
from completions for that source type. The level of emissions is also pro‐rated using data from all
sources so that the relative difference in emissions between completions and workovers is taken
into consideration.
89Volume 2, Chapter 4, Table 4.2.4
The approach taken in the development of emission factors listed in Volume 2, Chapter 4, Table 4.2.5
Volume 1, Chapter 5, Section 5.3

10

10

3.5 Statistical Analysis
Probability density functions (PDFs) were assigned to each of the data sets before a Monte Carloanalysis11 was applied to combine each contributing dataset and derive an expected value anduncertainty range for each variable (input). These input variable PDFs were subsequently put
through a further Monte Carlo cycle using Equations 3 and 4 to derive the emission factors andrelated uncertainties. The input PDFs can be found in Appendix 3. IPCC adopts a 95% confidence
interval limit in defining uncertainties and this is applied in this study.
For datasets with sufficient sample sizes, normal or lognormal PDFs were assigned using a best‐fit
algorithm available in the software package. For smaller sample data sets, PDFs were selected using
best judgement. These were assigned as follows:
1. Normal distributions were used for symmetrical samples
2. Lognormal distributions were assigned for positively skewed samples
3. Triangular distributions were applied in instances where only expected values, maximums
and minimums were provided
For the majority of variables, sufficient data is available to derive acceptable PDFs for the variables
listed. The exception was information related to workovers for which there is limited data (asdescribed in Section 3.4 above). In order to develop uncertainty ranges for this variable (Mpot.work.), a
PDF was developed using emissions from all sources (EPA GHGRP 2013, US EPA 2013) and thenapplied to the expected value for each source type, as calculated using Equation 5.

3.6 Time Series Considerations
The 2006 IPCC Guidelines12 identify the issue of time series consistency as a key consideration whendeveloping emission factors or emission inventories. In order to determine the applicability of theemission factors developed in this study to be used over time series from this point forward and to
consider the frequency at which factors should be reviewed, an analysis of the time variability of thevariables that contribute to fugitive emissions from unconventional gas is detailed in Table 4 below.
This lists the variables and potential considerations for their applicability to a time series consistentemission factor. It should be noted that for all of the variables there should be an opportunity to
reduce the level of uncertainty over time as more survey information and better technologies
become available.
Taking the considerations from Table 4 into account, as the various unconventional gas industries
evolve, it will improve accuracy if a Tier 3 approach to fugitive emissions is developed. This will allow
for updated emission factors which will better reflect improvements in technology.

11

Monte Carlo analysis uses pseudo‐random model input samples generated in reference to a probability density function. After 100
simulations of 10,000 iterations each the model output, mean, standard deviation, percentiles etc. are statistically inferred.
Volume 1, Chapter 5

12

11

Variable

Time Series Considerations

Potential fugitive emissions

Given the definition of potential emissions used in this study, there are
no changes anticipated. There is however significant opportunity to
improve the accuracy of this variable.

(Mpot.comp. and Mpot.work.)
Reductions in emissions throughapplication of reduced emission
completions/workovers (RECcomp.

and RECwork.)

In the USA and Canada, it is considered that the prevalence of RECs will
increase over time. The EPA has placed a legal requirement on the useof REC technology for the majority of unconventional gas wells(excluding some low pressure formations) from 2015 onwards throughthe New Source Performance Standards (NSPS).
It is also considered that the technology used for RECs will improveover time increasing the level of captured gas.

Reductions in emissions from flaring(Xflare.comp. and Xflare.work.)

It is anticipated that as environmental considerations develop into
policy, there will be an increase in the prevalence of flaring in thenatural gas and associated industries.

Estimated ultimate recovery

As hydraulic fracturing technology improves, it is possible that higher
EUR levels may be realised.

Combustion emission factors for
CO2, CH4 and N2O (CEFi)

No significant changes anticipated.

Natural gas composition (yi)No significant changes anticipated.

Table 4: Time series consistency considerations for variables affecting fugitive emissions fromunconventional gas

3.7 Uncertainty
The 2006 IPCC Guidelines13 suggest two approaches to quantifying uncertainty in the estimation ofemissions/removals of greenhouse gases.
Approach 1 is an error propagation methodology which uses simple standard equations to establish
the uncertainty in multiplication and addition/subtraction derived quantities. This approach isacceptable in a situation where the coefficient of variation (CoV) of a PDF is less than 0.3 and where
it is symmetrical. In the case of fugitive emissions from unconventional gas this approach wasdeemed insufficient based on the facts that:
1) A number of variables had CoVs in excess of 0.3,
2) The majority of the PDFs were asymmetric and3) The emission source is likely to be a ‘Key Category’ and warrants a more detailed
investigation and more accurate uncertainties.
On this basis Approach 1 was not applied.
Approach 2 is a more rigorous methodology which involves a Monte Carlo simulation. This applies acategory‐by‐category assessment of uncertainty through the use of defined PDFs by variable todetermine non‐normal uncertainty ranges. This approach has been applied to the emission factors
developed in this study using the @Risk software.

13

Volume 1, Chapter 3, Section 3.2

12

A number of considerations were taken to reduce the uncertainty as far as possible, consistent withthe 2006 IPCC Guidelines14:
1. Improving conceptualisation: key variables affecting the emission factors were identified in
the literature reviews to create a common framework that best utilises available data.
2. Improving models: the model was created consistent with the identified variables and thedata extracted from the literature has been parameterised accordingly. This disaggregation
approach reduces the potential for bias from literature and allows for better identification of‘outlying’ data.
3. Improving representativeness: disaggregation of the data also increased the sample data
population to allow for better representation of measured and Expert Judgement data
points. PDFs were applied to the various data populations individually using best judgement
in order to most accurately represent expected values and related uncertainties.
Further details of the strategies used to deal with different causes of uncertainty are listed inAppendix 4.

3.8 Quality Control Procedures and Limitations
The 2006 IPCC Guidelines15 set out a detailed approach to quality assurance (QA) and quality control(QC). The process is designed to minimise uncertainty and maintain integrity in the emission factors
and activity data associated with GHG inventories such that the outcomes:i.
ii.

contain neither over‐ nor under‐estimates so far as can be judged, andreduce uncertainties as far as practicable

In this study a rigorous QC process was adopted to ensure data sources, assumptions andcalculations were examined carefully.
The potential emissions data (Mpot.comp. and Mpot.work.) is based on secondary information and deemed
not to have gone through sufficient QA/QC procedures. The literature containing the data has
however been subject to peer review (with the exception of the EPA data) and so to that extent has
been verified.
In the unconventional gas scenario there are no other IPCC process‐derived emission factors forcomparison. Comparisons are drawn between emission factors derived in this study and thoseestimated in other scientific papers and publications as part of a precision test detailed in Section 5.3.

14
15

Volume 1, Chapter 3, Section 3.1.6
Volume 1, Chapter 613

4 Results
4.1 Tier 1 Emission Factors for Unconventional Gas Sources in Developed
Countries (Tier 2 for USA and Canada)
Emission factors for Shale gas, Tight sands gas and Coalbed methane for developed countries arelisted in Table 5 below, together with those for unconventional natural gas production taken from
Table 4.2.4 of Chapter 4, Volume 2 of the 2006 IPCC Guidelines. Associated 95% confidence intervals
are included in the ‘Uncertainty’ column16.
As discussed in previous sections of this report these emissions contribute solely to the Production
category (as defined by the IPCC) and are considered as appropriate Tier 1 emission factors fordeveloped countries and Tier 2 emission factors for USA and Canada.
The results show a significant contribution of fugitive emissions from completion and workover
activities when compared to conventional sources of natural gas. As would be expected given the
different pressure conditions, Shale gas shows the highest level of emissions followed by Tight sands
gas and Coalbed methane. Uncertainty ranges are positively‐skewed consistent with the lognormal
input PDFs.
Figure 2 gives a life‐cycle (including production, processing, transport and storage and distribution)comparison of conventional gas default emission factors with those of Shale gas, Tight sands gas andCoalbed methane by GHG.

4.2 Tier 1 Emission Factors for Unconventional Gas Sources in Developing
Countries and Countries with Economies in Transition
Emission factors for Shale gas, Tight sands gas and Coalbed methane for developing countries andcountries with economies in transition are listed in Table 6 below, together with those forunconventional natural gas production taken from Table 4.2.5 of Chapter 4, Volume 2 of the 2006
IPCC Guidelines. Associated 95% confidence intervals are included in the ‘Uncertainty’ column (aswith Table 5).
In the developing countries scenario there is a significant contribution of emissions from venting
gases (CH4 and NMVOCs) during completions and workovers, but the contribution from flaring
products (CO2 and N2O) is less substantial. This result is expected due to the lower level of flaring (i.e.
a higher proportion is vented).
Figure 3 gives a comparison in the same format as Figure 2 but for the developing countries scenario.
The relative effect of completions and workover emissions on total emissions is less pronounced
than in the developed countries scenario due to the higher baseline emissions (i.e. emissions fromconventional gas production).

16

Rounded up/down (to ± 10, 20, 40, 50, 75, 100, 200, 250, 500, 800 or 1,000%) depending on the magnitude of the value, consistent withthe approach in the 2006 IPCC Guidelines; Volume 2, Chapter 4, Table 4.2.4

14

Conventional Fugitives

1.B.2.b.iii.2

3.8E‐04 to
2.3E‐03
7.60E‐07

Shale Gas

Tight Sands
Gas
Production

Coalbed
Methane

Flaring

1.B.2.b.ii

Fugitives

1.B.2.b.iii.2

Venting

1.B.2.b.i

Flaring

1.B.2.b.ii

Fugitives

1.B.2.b.iii.2

Venting

1.B.2.b.i

Flaring

1.B.2.b.ii

Fugitives

1.B.2.b.iii.2

Venting

1.B.2.b.i

Flaring

1.B.2.b.ii

±100%
±25%

3.8E‐04 to ±100%
2.3E‐03
5.8E‐03‐100 to+500%
2.0E‐04‐100 to+500%
3.8E‐04 to ±100%
2.3E‐03
4.7E‐03‐100 to+500%
1.0E‐04‐100 to+550%
3.8E‐04 to ±100%
2.3E‐03
1.6E‐03‐100 to+500%
4.7E‐05‐100 to
+500%

CO2
Value

1.4E‐05 to
8.2E‐05
1.20E‐03

±100%
±25%

1.4E‐05 to ±100%
8.2E‐05
2.5E‐04‐100 to+500%
2.8E‐02‐100 to+500%
1.4E‐05 to ±100%
8.2E‐05
1.5E‐04‐100 to+500%
1.5E‐02‐100 to+500%
1.4E‐05 to ±100%
8.2E‐05
6.2E‐05‐100 to+500%
8.2E‐03‐100 to
+500%

NMVOC
Value

9.1E‐05 to
5.5E‐04
6.20E‐07

N2O
Value

Units of measure
Uncertainty(% of Value)

CH4
Value

Uncertainty(% of Value)

IPCC CodeUncertainty(% of Value)

Sub‐category Emission
sourceUncertainty(% of Value)

Category

±100%

N/A

N/A

±25%

2.1E‐08
N/A‐10 to+1000%
N/A

N/A

N/A

9.0E‐08
N/A‐100 to
+500%
N/A

N/A

N/A

6.0E‐08
N/A‐75 to
+800%
N/A

N/A

N/A

3.9E‐08‐50 to+800%

9.1E‐05 to ±100%
5.5E‐04
7.8E‐04‐100 to+500%
2.4E‐05‐100 to+500%
9.1E‐05 to ±100%
5.5E‐04
5.8E‐04‐100 to+500%
1.3E‐05‐100 to+500%
9.1E‐05 to ±100%
5.5E‐04
2.0E‐04‐100 to+250%
6.7E‐06‐100 to+500%

Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production

Table 5: Proposed emission factors for production of Shale gas, Tight sands gas and Coalbed methane in developed countries with associated 95% confidence interval
upper and lower limits (those for conventional natural gas production taken from Table 4.2.4 in Chapter 4, Volume 2 of the 2006 IPCC Guidelines are also included forcomparison)

15

CH4
Value

Conventional

Fugitives

1.B.2.b.iii.2

Flaring

1.B.2.b.ii

Fugitives
Venting

1.B.2.b.iii.2
1.B.2.b.i

3.8E‐04 to
2.4E‐02
7.6E‐07 to
1.0E‐06
3.8E‐04 to
1.8E‐02

Flaring

1.B.2.b.ii

Fugitives

1.B.2.b.iii.2

Venting

1.B.2.b.i

Flaring

1.B.2.b.ii

Fugitives

1.B.2.b.iii.2

Venting

1.B.2.b.i

Flaring

1.B.2.b.ii

Shale Gas

Tight Sands
Gas Production

Coalbed Methane‐40 to+250%±75%‐40 to‐75 to+500%
1.7E‐04‐200 to+500%
3.8E‐04 to ‐40 to
2.4E‐02+250%
7.8E‐03‐75 to+500%
9.0E‐05‐200 to+500%
3.8E‐04 to ‐40 to
2.4E‐02+250%
4.8E‐03‐75 to+300%
5.0E‐05‐200 to
+500%

CO2
Value

1.4E‐05 to
1.8E‐04
1.2E‐03 to
1.6E‐03
1.4E‐05 to
6.9E‐04‐40 to+250%±75%‐40 to‐75 to+500%
3.0E‐02‐100 to+500%
1.4E‐05 to ‐40 to
1.8E‐04+250%
9.3E‐05‐75 to+500%
2.1E‐02‐100 to+500%
1.4E‐05 to ‐40 to
1.8E‐04+250%
1.9E‐04‐50 to+500%
1.5E‐02‐100 to
+250%

NMVOC
Value

6.2E‐07 to
8.5E‐072.0E‐03
2.5E‐05

2.0E‐03
1.5E‐05

6.5E‐04
1.0E‐05‐40 to+250%±75%‐40 to‐75 to+500%‐150 to+500%‐40 to+250%‐75 to+500%‐100 to+500%‐40 to+250%‐50 to+500%‐100 to+500%

Units of measure

N2O
Value

NA

Uncertainty(% of Value)

IPCC CodeUncertainty(% of Value)

Emission source

Uncertainty(% of Value)

Sub‐category

Uncertainty(% of Value)

Category

N/A

2.1E‐08 to ‐10 to2.9E‐08+1000%
NA
N/A
N/A
N/A
2.2E‐07NA‐40 to+1000%
N/A

N/A

N/A

1.9E‐07NA‐40 to+1000%
N/A

N/A

N/A

1.7E‐07‐20 to+1000%

Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production
Gg per 106 m3 gas
production

Table 6: Proposed emission factors for production of Shale gas, Tight sands gas and Coalbed methane in in developing countries and countries with economies intransition with associated 95% confidence interval upper and lower limits (those for conventional natural gas production taken from Table 4.2.5 in Chapter 4, Volume 2of the 2006 IPCC Guidelines are also included for comparison)

16

Figure 2: Fugitive emission factors for CH4, CO2, NMVOCs and N2O by IPCC category for conventional gas, Shale gas, Tight sands gas and Coalbed methane indeveloped countries

17Figure 3: Fugitive emission factors for CH4, CO2, NMVOCs and N2O by IPCC category for conventional gas, Shale gas, Tight sands gas and Coalbed methane fordeveloping countries and countries with economies in transition18

5 Discussion
5.1 Life‐cycle Assessment of Fugitive Emissions from Conventional andUnconventional Gas Sources
By adding IPCC defaults for conventional gas processing, transport and storage and distribution(from the 2006 IPCC Guidance17) to the emission factors derived for production, a ‘well‐to‐meter’
life‐cycle analysis of fugitive emissions has been estimated. These estimates with associateduncertainty ranges are shown in Figure 4 below. The comparison indicates that fugitive emissions
from venting and flaring of natural gas during completion and workover events is significantly higher
than that of conventional sources. The mean values for Shale gas, Tight Sands gas and Coalbed
methane are 133%, 100% and 36% higher respectively than those of conventional gas in developed
countries.

Figure 4: Life‐cycle well‐to‐meter fugitive emissions18 for natural gas by source type in developed countries

5.2 Uncertainty Analysis
The main contributing factors to the level of uncertainty are the ranges for potential emissions(Mpot.comp. and Mpot.work.) and EUR as can be seen in the sensitivity analysis in Figure 5 below. Reducing
the uncertainty associated with these parameters would have a significant effect on the overall
uncertainty of the derived emission factors.

17

Volume 2, Chapter 4, Table 4.2.4
Using 100‐year global warming potentials (GWPs) of 25 for CH4, 1 for CO2, 3.4 for NMVOCs (weighted average) and 298 for N2O
consistent with IPCC Fourth Assessment Report (AR4) and supplementary data
18

19

Shale Gas Completions / CH4
Inputs Ranked By Effect on Output Mean
Jiang Shale CPE

0.0042183

Howarth Shale CPE

0.0020212

EIA Shale EUR

0.0021219

0.014414

Jiang CFRUSGS Shale EUR
Jiang REC Rate

0.020724
0.016708

0.0045491

0.016432

0.0024443

0.013878

0.0040241

Sullivan Input 1

0.012332

0.0049610

0.012736

Sullivan Input 2

0.0046458

O'Sullivan REC Rate

0.0044733

0.011702

Sullivan Input 5

0.0046987

0.011918

0.012063

Baseline = 0.006371

Shale Gas Completions / CH4

Tight Sands Completions / CH4
Inputs Ranked By Effect on Output Mean
EPA 2012 TS CPEEIA TS EUR

0.00048894

0.0084580

0.00080284

0.0053758

USGS TS EUR

0.0013154

Jiang REC Rate

0.0020767

0.0042316
0.0032248

O'Sullivan REC Rate

0.0022362

Density NG / PDF

0.0023485

Jiang CFR

0.0023374

CH4 y

0.0022492

CH4 CEF

0.0024130

Burnham CFR

0.0024449

0.0031689
0.0030657
0.0029450
0.0028047
0.0029361
0.0027728

Baseline = 0.0026292

Tight Sands Completions / CH4

CBM Completions / CH4
Inputs Ranked By Effect on Output Mean
EPA 2012 CBM CPE

0.00029240

EIA CBM EUR

0.00034162

USGS CBM EUR

0.0042393
0.0027539

0.00023588

0.0026422

Jiang REC Rate

0.0012086

0.0016004

Jiang CFR

0.0011730

0.0015545

O'Sullivan REC Rate
Burnham CFR

0.0012376
0.0013319

0.0015892
0.0016190

Density NG / PDF

0.0012688

CH4 y0.0013168

0.0015193

CH4 CEF

0.0013182

0.0014600

0.0015261

Baseline = 0.0014151

CBM Completions / CH4

Figure 5: Sensitivity charts for total CH4 fugitive emissions arising from Shale gas, Tight sands gas andCoalbed methane with input variables and datasets ranked by effect on output mean

Figure 5 also indicates the significance of the lack of completion potential emissions data for Tightsands gas and Coalbed methane. The data available (from EPA) is a limited sample size which
subsequently increases the uncertainty range (resulting in the large effects demonstrated from ‘EPA
20

2012 TS CPE’ and ‘EPA 2012 CBM CPE’). By increasing the sample size for these variables (i.e.
retrieving more data), uncertainty could be reduced.
Further sensitivity charts can be found in Appendix 5 in which similar opportunities to reduce
uncertainty can be identified.

5.3 Precision
A comparison of the proposed emission factors developed within this study with those that havebeen developed in other studies can provide an insight into the level of precision among factors. The
2006 IPCC Guidelines19 defines precision as “Agreement among repeated measurements of the same
variable. Better precision means less random error.” Figure 6 compares the emission factors
developed in this study with those found in other peer‐reviewed literature (Weber et al. 2012). This
serves as a quality control procedure as well as providing a sense of precision among studies. Withdisagreement among the level of emissions from the various studies of a factor of more than 9, it isclear that the emissions estimates developed are collectively imprecise. This is an indication that thelack of third party measured and verified emissions data from natural gas operations is a significant
issue.

Figure 6: A comparison of fugitive emission factors for Shale gas from completion and workover activities

5.4 Emerging Research and Potential Consequences
There is still a large degree of uncertainty surrounding the issue of fugitive emissions from natural
gas production, processing, transport and distribution, both conventional and unconventional. The
estimations still rely largely upon the EPA/GRI data published in 1996 which has been supplemented
with some updates as the industry has matured. The most significant concern is that the data isbased on engineering calculations and/or approximately measured volumes but as yet there has notbeen a comprehensive study to measure and verify emissions from a wide variety of wells.

19

Volume 1, Chapter 3, Page 3.7

21Technology has recently been developed whereby air sampling and infrared photography can now
offer a more accurate means of measuring and verifying these fugitive emissions. A study
undertaken by National Oceanic and Atmospheric Administration (NOAA) measured atmosphericmethane concentrations in the Denver‐Julesberg basin in Colorado (Petron et al. 2012). The findingsof this analysis indicated that methane emissions were four orders of magnitude higher than that
estimated through the inventorying process. The Authors acknowledge it is difficult to assign theexact source of the methane based on the varied fossil fuel activity in that basin, however, thechemical signature indicated that “the ambient molar ratios are lower than what could be expectedfrom condensate tank flashing emissions alone, indicating that most of the CH4 emissions observedcame from the venting of raw natural gas.” This result has raised great concern (Tollefson 2012), notonly to estimated emissions of fugitive methane but to the inventorying process at large.
The University of Texas along with NOAA and various other partners are currently undertaking adetailed study of fugitive methane emissions from the natural gas industry which will be the first of
its kind as is scheduled for publication in 2013.

5.5 Recommendations for Further Work
With the rapidly evolving global unconventional gas sector, it is considered that further research isnecessary to allow for more accurate GHG reporting. The following is a list of issues that with further
research would improve the accuracy of fugitive emission factors from unconventional gas sources.
1)

2)

3)

4)

5)

Liquids unloading ‐ emissions from the liquids unloading process in both conventional andunconventional gas wells are poorly understood (as described in Section 2.3). The number andtype of wells that require liquids unloading is not clear. The prevalence of mitigation
technologies (such as plunger lifts) is also subject to significant uncertainty. Further research
and measurement/verification of emissions is required to better understand this emission
source.
Measurement (sampling) analysis ‐ measured and verified emissions data collected by third‐
party organisations (using technology as described in Section 5.4) are needed to improve theaccuracy and confidence of fugitive emission factors for natural gas.
Update IPCC default EFs for NG ‐ existing IPCC fugitive emission factors for the natural gas
industry are still largely based on the 1996 EPA/GRI study. These emission factors may be
outdated and consideration should be given to updating the IPCC default factors, especiallyconsidering the significant increase in natural gas consumption that has been projected.
Reporting by pressure of play ‐ in order to improve the emission factors developed and movetowards a Tier 3 approach, consideration should be given to categorising the pressure of theunconventional gas ‘plays’. This will allow for improved accuracy and reduced uncertainty forpotential emissions (Mpot.comp. and Mpot.work.) which can have a significant impact on reducing theuncertainty ranges of the emission factors developed in this study, as indicated by thesensitivity analysis in Section 5.2.
Reporting by EUR of play – in order to improve the emission factors developed and movetowards a Tier 3 approach, consideration should be given to categorising the expected EUR byunconventional gas ‘plays’. Improving EUR accuracy can have a significant impact on reducing
the uncertainty of the emission factors developed in this study, as indicated by the sensitivity
analysis in Section 5.2.

22

6 Conclusion
In general the following conclusions can be drawn:
1) Fugitive emissions from completions and workovers of unconventional gas sources including
Shale gas, Tight sands gas and Coalbed methane, are significant and the relevant emission
factors should be added to the EFDB.
2) While the emission factors derived in this study are as accurate as possible, there is a deficitof measured and verified emissions data. Third party measurements to quantify fugitive
emissions from unconventional (and conventional) gas sources would vastly improve theaccuracy of the emission factors.
3) The scale of the difference in emissions between conventional and unconventional gas
sources may be indicative of other types of unconventional fuel. In its current form the 2006
IPCC Guidelines do not address unconventional fuel types sufficiently and consideration
should be given to amending the guidelines accordingly.

23

Appendix 1
Primary Information Sources
1.

API/ANGA 2012: Characterizing Pivotal Sources of Methane Emissions from Natural gas Production
An industry survey conducted using various data sources questioning the EPA approach to GHG emission
inventorying techniques, particularly from liquids unloading and re‐fractures rates.

2.

Burnham et al 2012: Life‐cycle greenhouse gas emissions of Shale gas, natural gas, coal, andpetroleum
LCAs of Shale and conventional gas GHG emissions using EPA default data with various assumptions
applied.

3.

Howarth et al 2011: Methane and the greenhouse gas footprint of natural gas from shale formations
LCA assessment of GHG emissions from Shale gas compared with conventional gas and coal using datareported from industry.

4.

Hultman et al 2011: The greenhouse impact of unconventional gas for electricity generation
LCAs of shale and conventional gas GHG emissions using EPA default data with various assumptions
applied.

5.

Jiang et al 2011: Life cycle greenhouse gas emissions of Marcellus Shale gas
LCA of Shale gas emissions in the Marcellus play using data provided by Pennsylvania Department ofEnvironmental Protection and New York State Department of Environmental Conservation Data

6.

NETL 2011: Life Cycle Greenhouse Gas Inventory of Natural Gas Extraction, Delivery and Electricity
Production
Top‐down LCA of natural gas sources using EPA default emission factors and other industry figures andassumptions.

7.

O’Sullivan et al 2012: Shale gas production: potential versus actual greenhouse gas emissions
Analysis of fugitive emissions from Shale gas well completion using data reported from 4,000 wells.

8.

Stephenson et al 2011: Modeling the Relative GHG Emissions of Conventional and Shale gas
Production
LCA of unconventional gas sources using EPA default data with various assumptions applied.

9.

US EIA 2012: Assumptions to the Annual Energy Outlook 2012
Assessment of EUR forecasting based on statistical analysis of existing well resource depletion.

10. EPA GHGRP 2013: Reported Subpart W Data
Emissions data reported through EPA’s GHGRP.
11. EPA 2013: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990‐2011
Emission factors and inventory assumptions used in the development of the 2013 National Inventory
Report for the period 1990 to 2011.
12. USGS 2010: Assembling Probabilistic Performance Parameters of Shale‐Gas Wells
Assessment of EUR forecasting based on statistical analysis of existing well resource depletion.
13. Wrap Phase III 2008: Development of Baseline 2006 Emissions from Oil and Gas Activity
An industry survey carried out by WRAP to estimate emissions from Oil and Gas activities in the western
U.S. focussing on NOx, CO, VOCs, PM and SOx.

24

Appendix 2
Datasets and Expert Judgement Sources Used
Variable
Shale gas Completion Potential
Emissions

Tight sands gas Completion
Potential Emissions
Coalbed methane Completion
Potential Emissions
Completion REC Net Capture Rate
(net of prevalence and capture rate)
Completions flaring rate

Workovers per lifespan

Workover potential emissions
Workover REC Net Capture Rate
(net of prevalence and capture rate)
Workovers flaring rate
Estimated ultimate recovery

Well lifespanCombustion emission factors
Natural gas composition

Data Sources
1) EPA 2013
2) Jiang3) O’Sullivan4) Howarth
1) EPA 2013

Data type
Industry reported
Industry reported
Industry reported
Industry reported
Industry reported

Published
2013
2011
2012
2011
2013

1) EPA 2013

Industry reported

20131) EPA 2013
2) Jiang3) O’Sullivan

1) EPA 2013
2) Jiang3) O’Sullivan
4) Burnham
5) EPA S/P W
1) API/ANGA
2) EPA 20133) NETL
4) Hultman5) API/ANGA
1) EPA 2013
2) EPA S/P W
1) EPA S/P W

Industry reported
Expert judgement
Expert judgement
Industry reported
Expert judgement
Expert judgement
Expert judgement
Industry reported
SurveyIndustry reported
Expert judgement
Expert judgement
SurveyIndustry reported
Industry reported
Industry reported

2013
2011
2012
2013
2011
2012
2012
2013
2012
2013
2011
2011
2008
2013
2013
2013

1) EPA S/P W
1) O’Sullivan
2) USGS
3) EIA
1) O’Sullivan2) Hultman3) Burnham
1) IPCC 2006
1) IPCC 2005

Industry reported
Expert judgement
Detailed analysis
Detailed analysis
Expert judgement
Expert judgement
Expert judgement
Detailed analysis
Detailed analysis

2013
2012
2010
2012
2012
2011
2012
2006
2005

25

Appendix 3
Input Variable Probability Density Functions
Variable
Shale gas
Completions
Potential
Emissions(Mpot,comp)Tight sandsgasCompletions
Potential
Emissions(Mpot,comp)

PDF

Input TS CPE
42

983
95.0%

0.00302.5%

0.00250.0020
Input TS CPE
Minimum
7.22
Maximum 4,554.03
Mean
280.64
Std Dev
266.96
Values10000

0.00150.00100.00050.0000

CoalbedmethaneCompletions
Potential
Emissions(Mpot,comp)Input CBM CPE
9
0.018

176
95.0%

2.5%

0.016
0.014
0.012
Input CBM CPE

0.010
0.008

Minimum
Maximum
Mean
Std Dev
Values2.23
719.12
52.39
46.72
10000

0.006
0.004
0.002
0.000

26

Reduced
Emissions
Completions
CaptureRate(RECcomp)

Input REC Rate
31.60%

57.60%

2.5%

6

95.0%

2.5%

5

4
Input REC Rate
Minimum
Maximum
Mean
Std Dev
Values3222.303%63.817%44.980%6.698%10000

10Developed
Countries
Completions
Flaring Rate
(Xcomp,flare)

Input CFR (DC)57.25%

66.98%

2.5%

16

95.0%

2.5%

14
12
10

Input CFR (DC)Minimum
Maximum
Mean
Std Dev
Values8
654.947%69.882%62.000%2.494%
10000

4
2
0

80%

70%

60%

50%

40%

30%

20%

10%

0%

-10%

-20%

Non‐
Developed
Countries
Completions
Flaring Rate
(Xcomp,flare)27

Shale gas
Workovers
per Lifespan(Nwork)Tight sandsgas
Workovers
per Lifespan(Nwork)CoalbedmethaneWorkovers
per Lifespan(Nwork)28

Shale gas
WorkoverPotential
Emissions(Mpot,work)

Output Shale Gas WPE
14 1,360
1.2

2.5%

2.5%

1.0

0.8

Output Shale Gas WPE
Minimum
Maximum
Mean
Std Dev
Values

0.6

0.4

1.18
399,328.94
359.86
5,024.90
10000

0.2

0.0

Tight sandsgas
WorkoverPotential
Emissions(Mpot,work)

Output Tight Sands WPE
4 573
0.00122.5%

2.5%0.00100.0008Output Tight Sands WPE
Minimum
Maximum
Mean
Std Dev
Values0.00060.00040.127
36,215.15
129.82718.41
10000

0.00020.0000

CoalbedmethaneWorkoverPotential
Emissions(Mpot,work)

Output CBM WPE
1 100
0.00252.5%

2.5%

0.00200.00150.0010

Output CBM WPE
Minimum
Maximum
Mean
Std Dev
Values

0.0203
18,911.65
24.93
215.23
10000

0.00050.000029

Workover
REC Capture
Rate(RECwork)

Developed
Countries
Workover
Flaring Rate
(Xwork,flare)

Non‐
Developed
Countries
Workover
Flaring Rate
(Xwork,flare)30

Shale gas
Estimated
Ultimate
Recovery(EUR)

Input Shale EUR
7.3
0.035

56.3

2.5%

95.0%

2.5%

0.030

0.025
Input Shale EUR

0.020

Minimum
Maximum
Mean
Std Dev
Values0.015

3.14
135.19
27.37
13.59
10000

0.010

0.005

0.000

Tight sandsgas
Estimated
Ultimate
Recovery(EUR)

Input TS EUR
6.2

66.7
95.0%

0.040

2.5%

0.035
0.030
0.025

Input TS EUR
Minimum
Maximum
Mean
Std Dev
Values

0.020
0.015

2.03
294.85
23.86
16.53
10000

0.010
0.005
0.000

CoalbedmethaneEstimated
Ultimate
Recovery(EUR)

Input CBM EUR
2
0.050

55
95.0%

2.5%

0.045
0.040
0.035
0.030
0.025
0.020

Input CBM EUR
Minimum
Maximum
Mean
Std Dev
Values0.805
646.84
12.84
20.12
10000

0.015
0.010
0.005
0.000

31

Combustion
Emission
Factor for
CarbonDioxide(CEFCO2)

Combustion
Emission
Factor for
Methane(CEFCH4)

Combustion
Emission
Factor for
NitrousOxide(CEFN2O)

32

VolumeFraction for
Methane(yCH4)

VolumeFraction for
CarbonDioxide(yCO2)

VolumeFraction for
Non‐
MethaneVolatile
Organic
Compound(yNMVOC)

33

Appendix 4
Dealing with Different Causes of Uncertainty
Causes of Uncertainty

ActionLack of completenessModel (bias and random errors)







Lack of dataLack of representativeness of data

Statistical random sampling errorMeasurement errorMissing dataAll key components of the system have been considered
Errors in the data collected through literature review are non‐quantifiable but have for the most part been verified throughpeer‐review
The model is complete as far as available data permits
Uncertainty ranges in the model predictions are rigorously
derived through Monte Carlo analysis
The model is considered accurate as available data permits
Model predictions and uncertainty ranges fall within ranges
resulting from other peer‐reviewed studies
The level of precision is not confirmable based on the level ofdisagreement between studies to date
Model predictions lie within the ranges of expert judgement
Where data was lacking, values provided by expert judgement
have been used
Where only small sample data populations were available thiswas taken into account in the statistical analysis andsubsequent uncertainty ranges
The data used is taken from the most recent and reliable
sources available
Serious doubts have arisen on the accuracy of this data basedon recent research but at this time the data used still
represents the most representative
Monte Carlo analysis was used in the derivation of model
predictions and associated uncertainty ranges to account for
statistical random sampling errors
Measurement error has not been accounted for in this study
The majority of data used has been through peer review
which is the only extent to which measurements can beverified
Monte Carlo analysis was applied to ensure missing data was
accounted for with the uncertainty ranges developed34

Appendix 5
Sensitivity Analyses
Sensitivity Tornado
Shale Gas Completions / CH4
Inputs Ranked By Effect on Output Mean
Jiang Shale CPE

0.0042183

Howarth Shale CPE

0.0020212

EIA Shale EUR

0.0021219

0.014414

Jiang CFRUSGS Shale EUR
Jiang REC Rate
Sullivan Input 1

0.020724
0.016708

0.0045491
0.013878

0.0040241

0.012332

0.0049610

Sullivan Input 2

0.0046458

O'Sullivan REC Rate

0.0044733

Sullivan Input 5

0.016432

0.0024443

0.012736
0.012063
0.011702

0.0046987

0.011918

Baseline = 0.006371

Shale Gas Completions / CH4

Tight Sands Completions / CH4
Inputs Ranked By Effect on Output Mean
EPA 2012 TS CPEEIA TS EUR

0.00048894

0.0084580

0.00080284

0.0053758

USGS TS EUR

0.0013154

Jiang REC Rate

0.0020767

O'Sullivan REC Rate

0.0042316
0.0032248

0.0022362

Density NG / PDF

0.0023485

Jiang CFR

0.0023374

CH4 y

0.0022492

CH4 CEF

0.0024130

Burnham CFR

0.0024449

0.0031689
0.0030657
0.0029450
0.0028047
0.0029361
0.0027728

Baseline = 0.0026292

Tight Sands Completions / CH4

CBM Completions / CH4
Inputs Ranked By Effect on Output Mean
EPA 2012 CBM CPE

0.00029240

EIA CBM EUR

0.00034162

USGS CBM EUR

0.0042393
0.0027539

0.00023588

0.0026422

Jiang REC Rate

0.0012086

0.0016004

Jiang CFR

0.0011730

0.0015545

O'Sullivan REC Rate
Burnham CFR

0.0012376
0.0013319

0.0015892
0.0016190

Density NG / PDF

0.0012688

0.0015261

CH4 y0.0013168

0.0015193

CH4 CEF

0.0013182

0.0014600

Baseline = 0.0014151

CBM Completions / CH435

Shale Gas Workovers / CH4
Inputs Ranked By Effect on Output Mean
EPA SP W WPE
Input Shale Works/EUR
Howarth Shale CPE
EIA Shale EUR

0.00019267

Jiang Unc. CPE

USGS Shale EUR

0.0014181

0.00018316

0.0013322

0.00026755

0.0014069

0.00022813

0.0011063

0.00013157

Input Works REC Rate
Howarth Unc. CPE

0.0018236

0.00013179

Sullivan Input 4
Jiang Shale CPE

0.0022093

8.997E-005

0.00093461

0.00028856
8.677E-005

0.0010897
0.00083195

0.00013945

0.00085660

Baseline = 0.0004475

Shale Gas Workovers / CH4

Tight Sands Workovers / CH4
Inputs Ranked By Effect on Output Mean
EPA SP W WPE

0.0010096

EPA 2012 TS CPE

0.00033635

Howarth Unc. CPE

0.00042526

EIA TS EUR

0.00048450

USGS TS EUR

0.00088141

Input Works REC Rate
Jiang Unc. CPE

0.0093768
0.0063039
0.0036353
0.0034464
0.0033288

0.0012840
0.00088398

0.0034799
0.0030657

EPA SP W Unc. CPE

0.0026583

Input TS Works/EUR

0.0013284

0.0029629

Sullivan Input 3

0.0014036

0.0029601

Baseline = 0.0019713

Tight Sands Workovers / CH4

CBM Workover / CH4
Inputs Ranked By Effect on Output Mean
EPA SP W WPE
EPA 2012 CBM CPE

8.285E-005

0.0010712

3.960E-005

0.00062252

EIA CBM EUR

3.374E-005

0.00049659

USGS CBM EUR

2.707E-005

0.00048934

Howarth Unc. CPE

3.670E-005

0.00048577

Input CBM Works/EUR

6.958E-005

Jiang Unc. CPE

7.037E-005

Input Works REC Rate

0.00012074

EPA 2012 WPE

0.00051841
0.00044437
0.00043205
0.00035574

CH4 y

0.00036303
Baseline = 0.0001978

CBM Workover / CH4

36

Shale Gas Completions / CO2
Inputs Ranked By Effect on Output Mean
Jiang Shale CPE

0.020338

Howarth Shale CPE

0.0098058

EIA Shale EUR

0.010304

USGS Shale EUR

0.10295
0.0811060.070103

0.011860

0.071626

Jiang CFR

0.020749

Jiang REC Rate

0.019602

CO2 y

0.078729
0.063425

0.022469

Sullivan Input 1

0.065738

0.023862

0.064854

CO2 CEF

0.023479

0.063261

Sullivan Input 2

0.022738

0.062437

Baseline = 0.031047

Shale Gas Completions / CO2

Tight Sands Completions / CO2
Inputs Ranked By Effect on Output Mean
EPA 2012 TS CPEEIA TS EUR

0.0023829

0.041060

0.0038852

0.026164

USGS TS EUR

0.0062923

Jiang REC Rate

0.010099

0.020720
0.015573

O'Sullivan REC Rate

0.010853

0.015211

Density NG / PDF

0.011354

0.014775

CO2 CEF

0.011823

0.014170

Burnham CFR

0.011314

0.013648

Jiang CFR

0.011770

0.014092

CO2 y

0.011760

0.013655

Baseline = 0.012729

Tight Sands Completions / CO2

CBM Completions / CO2
Inputs Ranked By Effect on Output Mean
EPA 2012 CBM CPE
USGS CBM EUR
EIA CBM EUR

0.0014112

0.020499

0.0011428

0.012881

0.0016592

0.013148

Jiang REC Rate

0.0059038

O'Sullivan REC Rate

0.0059591

0.0076465

Density NG / PDF

0.0061328

0.0074342

Burnham CFR

0.0063655

0.0076049

CO2 y

0.0063465

0.0073350

0.0077300

CO2 CEF

0.0064570

0.0071863

Jiang CFR

0.0063909

0.0070711

Baseline = 0.006837

CBM Completions / CO2

37

Shale Gas Workovers / CO2
Inputs Ranked By Effect on Output Mean
EPA SP W WPE
Input Shale Works/EUR
Howarth Shale CPE

0.00016995

Jiang Shale CPE
USGS Shale EUR
Howarth Unc. CPE

0.0011785

0.00023801

0.0012333

0.00015913

0.0011163

0.00020352

0.00095179

0.00012550

0.00081632

7.669E-005

Input Works REC Rate
Jiang Unc. CPE

0.0015949

0.00011482

Sullivan Input 4
EIA Shale EUR

0.0019138

8.016E-005

0.00075128

0.00025338

0.00092482

0.00011232

0.00075703

Baseline = 0.0003941

Shale Gas Workovers / CO2

Tight Sands Workovers / CO2
Inputs Ranked By Effect on Output Mean
EPA SP W WPE

0.00089884

EPA 2012 TS CPE

0.00029663

Howarth Unc. CPE

0.00038268

EIA TS EUR

0.00043464

USGS TS EUR

0.00077188

Input Works REC Rate
Jiang Unc. CPE

0.0011290

0.0081949
0.0055383
0.0031118
0.0029472
0.0029469
0.0030622

0.00070923 0.0025852

EPA SP W Unc. CPE

0.0024227

Input TS Works/EUR

0.0011994

0.0026454

Sullivan Input 5

0.0012434

0.0026579

Baseline = 0.0017391

Tight Sands Workovers / CO2

CBM Workover / CO2
Inputs Ranked By Effect on Output Mean
EPA SP W WPE

7.344E-005

0.00090123

EPA 2012 CBM CPE

3.490E-005

USGS CBM EUR

2.382E-005

EIA CBM EUR

2.957E-005

0.00039704

Howarth Unc. CPE

3.283E-005

0.00039022

Input CBM Works/EUR

5.956E-005

Jiang Unc. CPE

5.931E-005

Input Works REC Rate

0.00010645

0.00051204
0.00043718

0.00041610
0.00035071
0.00035154

Sullivan Input 5

0.00030945

EPA SP W Unc. CPE

0.00029037
Baseline = 0.00017067

CBM Workover / CO238

References
1.
2.
3.
4.

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39