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1Normal Operations: . Emissions from normal operations can be divided into two
main source categories: (1) venting and flaring and (2) discharges from process
vents, chronic leaks, etc.
Venting and Flaring - Venting and flaring refers to the disposal of gas that cannot
be contained or otherwise handled. Such venting and flaring activities are associatedwith combined oil and gas production and take place in production areas where gas
pipeline infrastructure is incomplete and the natural gas is not injected into
reservoirs (emissions from process vents are not included here - see below).
Venting activities release methane because the vented gas typically has a highmethane content. If the excess gas is burned in flares the emissions of methane will
depend on how efficient the burning processes are. Generally the combustion
efficiency for flare sources are assumed to be between 95 and 100 per cent.
However a new study based upon measurements carried out by Norwegian Oil
Industry Association - OLF (1993) indicates very small amounts of unburned
methane from flares, less than 0.1 per cent of the gas flared. To estimate themethane emissions from venting and flaring activities satisfactorily it is required toknow the flare efficiency rates and the division between the quantity of gas vented
and gas flared.
The combined quantity of gas vented and flared is reported by countries that
produce oil and gas (Barns and Edmonds, 1990). A few countries also are able toreport separately gas vented and gas flared. The reliability of the data isquestionable in many cases because vented and flared amounts are not normallymetered but inferred from the difference between total production and disposals.
Discharges from Process Vents, Chronic Leaks etc. - These include:
•
•

•

•
•

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Emissions from pneumatic devices (gas-operated controls such as valves andactuators) depend on the size, type, age of the devices, the frequency of theiroperation and the quality of their maintenance.
Leaks from system components are unintentional and usually continuous
releases resulting from the failure of a seal or the development of a flaw, crack
or hole in a component designed to contain or convey oil or gas. Connections,valves, flanges, instruments, and compressor shafts can develop leaks fromflawed or worn seals, while pipelines and storage tanks can develop leaks fromcracks or from corrosion.
Emissions from process vents, such as vents on glycol dehydrators and vents on
crude oil tankers and storage tanks resulting from normal operation of the
facilities. However such process vents are minor methane sources in most gas
production facilities.
Emissions from starting and stopping reciprocating engines and turbines.
Emissions during drilling activities, e.g., gas migration from reservoirs throughwells.

2

Maintenance: Includes regular and periodic activities performed in the operation of
the facility. These activities may be conducted frequently, such as launching andreceiving scrapers (“pigs”) in a pipeline, or infrequently, such as evacuation of pipes("blowdown") for periodic testing. In each case, the required procedures releasegas from the affected equipment. Releases also occur during maintenance of wells("well workovers") and during replacement or maintenance of fittings.

3

System Upsets and Accidents: System upsets are unplanned events in thesystem, the most common of which is a sudden pressure surge resulting from thefailure of a pressure regulator. The potential for unplanned pressure surges is

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systems to protect the equipment from damage due to the increased pressure.
Relief systems vary in design. In some cases, gases released through relief valves may
be collected and transported to a flare for combustion or re-compressed andre-injected into the system. In these cases, methane emissions associated withpressure relief events will be small. In older facilities, relief systems may vent gases
directly into the atmosphere or may send gases to flare systems where complete
combustion may not be achieved.
The frequency of system upsets varies with the facility design and operating
practices. In particular, facilities operating well below capacity are less likely toexperience system upsets and resulting emissions. Emissions associated withaccidents are also included under the category of upsets. Occasionally, gastransmission and distribution pipelines are accidentally ruptured by construction
equipment or other activities.
Table 1-56 lists the most important sources of emissions within each segment of theoil and gas industry. Based on available information, the sources listed as "major"
account for the majority of emissions from each segment. Because data are limited
and, because there is considerable diversity among oil and gas systems throughout
the world, other possible sources are listed which may, in some cases, be important
contributors to emissions.

TABLE 1-56
METHANE EMISSIONS FROM OIL AND NATURAL GAS ACTIVITIES
Segment
Oil and Gas Production
Oil and Gas Wells
Gathering lines
Treatment facilities
Crude oil transportation and
Refining
Pipelines
Tankers

Major Emission Sources
Venting
Normaloperations:fugitiveemissions; deliberate releases frompneumatic devices and process
vents

Other Possible Emission Sources
Flaring, maintenance, system upsets
and accidents

Normaloperations:fugitiveemissions; deliberate releases fromprocess vents at refineries, duringloading and unloading of tankers
and storage tanks

Flaring, maintenance, system upsets
and accidents

Normaloperations:fugitiveemissions; deliberate releases frompneumatic devices and process
vents

Flaring, maintenance, system upsets
and accidents

Storage tanks
Refineries
Natural Gas Processing,Transportation, andDistribution
Gas Plants
Underground storage
reservoirs
Transmission Pipelines
Distribution Pipelines

1.8.4 Available Emissions Data
Only very limited data are available that describe methane emissions from natural gas andoil activities. Estimating the types of emissions defined above is complicated by the fact

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that emissions rates from similar activities in various regions and countries are influenced
by differences in the industry's supporting infrastructure, operating and maintenancepractices, and level of technology used. Because natural gas and oil activities are complex,
it is not easy to define simple relationships between emissions and gross descriptors of
the activities undertaken.
The available published data were reviewed to identify emissions estimates that include: adetailed consideration of the physical attributes of oil and gas systems; the operation andmaintenance characteristics of key facilities; and country- or region-specific factors that
may influence emission rates. The following data were identified:•Surveys: Several studies have surveyed system operators to estimate emissions asa fraction of production or throughput. These studies include Alphatania (1989),
AGA (1989), and INGAA (1989). While these studies provide a basis for identifying
the activities that operators believe are likely to be major sources of emissions, they
are not based on detailed assessments of emission rates and therefore do notprovide a quantitative basis for making estimates of methane emissions from oil andnatural gas activities.
•

Estimates Based on “Reported Unaccounted For Gas”: Several studies, such
as Hitchcock and Wechsler (1972), Abrahamson (1989) and Cicerone andOremland (1988), have assumed that emissions can be approximated by reported
amounts of "unaccounted for" gas. Unaccounted for gas is defined as the difference
between gas production and gas consumption on an annual basis. Like estimates ofventing and flaring, unaccounted for gas often is used as an accounting convenience
to balance company or national production and consumption estimates.
The use of unaccounted for gas estimates as estimates of emissions is questionablebecause factors other than emissions account for the majority of the gas listed as
unaccounted for. These include: meter inaccuracies, use of gas within the systemitself, theft of gas (PG&E, 1990), variations in temperature and pressure anddifferences in billing cycles and accounting procedures between companies receiving
and delivering the gas (INGAA, 1989). Furthermore, because known releases of gas
are not reflected in unaccounted for gas estimates, such as emissions fromcompressor exhaust, the unaccounted for gas estimates cannot unambiguously beconsidered an upper or lower bound on emissions.

•

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Engineering Studies and Measurements: A small number of studies are basedon detailed engineering and/or field measurement analyses. Several engineering
analyses have considered the manner in which actual or model facilities are built andoperated, and extrapolate facility emissions to a system-wide basis. Several
measurement studies have measured emissions from operating facilities or identified
actual leaks and extrapolated these measurements to estimate system-wide
emissions. In general, data from engineering studies and measurements are thepreferred basis for emission estimates. However, only a few of these types ofstudies have been performed, thereby limiting the ability to estimate emissions
nationally, regionally and globally from oil and gas systems. Table 1-57 lists thestudies identified and the information they contain. The methane emissions
estimates from the studies in the table have been converted to common units ofkilograms of emissions per petajoule of energy (kg/PJ). A total of five studies arelisted, with emissions estimates for parts of North America (US EPA, 1992), Eastern
Europe (Rabchuk et al., 1991), and Western Europe (Schneider-Fresenius et al.,
1989, Norwegian SPCA, 1992b and Norwegian Oil Industry Association OLF, 1993).
Additionally, Barns and Edmonds (1990) present estimates based on a global
assessment. Further studies are needed to improve the basis for making emissions
estimates.

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SUMMARY OF METHANE EMISSION FACTORS
Data source

Study methodology

US EPA (1992)

Compilation of estimates from:All emissions have been scaled
down
to
1988
energyconsumption or productionlevels••detailed engineering analyses
field measurement studies

Emission factors

Applicability
Oil and Gas Production:

290 - 4 670 kg/PJ of oil
produced

Emissionsfrom
non-gasproducing oil wells including
fugitiveemissionsandmaintenance emissions in theUSA

39 590 - 104 220 kg/PJ of gas
produced

Emissions from gas production,
includingfugitiveemissions,dehydrator venting, bleedingfrompneumatic
devices,maintenance, and systems upsetsin the USA

2 870 - 13 920 kg/PJ of total oil
and gas produced

Venting and flaring emissions
from oil and gas production andfugitive emissions from gasproducing oil wells in the USA

Crude Oil Transportation and Refining:
110 - 1 666 kg/PJ of oil refined

Emissions from oil refining andrelated oil storage tanks in the
USA

Natural Gas Processing, Transmission and Distribution:
59 660 - 116 610 kg/PJ of gas
consumed

Rabchuk et al. (1991)

Compilation of estimates from:•previous measurement
studies•

official data for 1989

Emissions from gas processing,transmission and distribution
includingfugitiveemissions,dehydrator venting, bleedingfrompneumatic
devices,maintenance, and system upsetsin the USA

Oil and Gas Production:
218 000 - 567 600 kg/PJ of gas
produced

Emissions from leakages at gas
wells
includingroutineequipment venting in the former
USSR

Natural Gas Processing, Transmission and Distribution:
340 000 - 715 800 kg/PJ of gas
consumed

Schneider-Fresenius et al.
(1989)

Compilation of results from:•

Batelle study's 1988
literature survey

Emissions from leakages atunderground storage facilities,
compressor stations, linear partof main pipelines and distribution
networks in the former USSR

Oil and Gas Production:
14 800 - 270 00 kg/PJ of gas
produced

Emissions from gas production
and treatment facilities inGermany

Natural Gas Processing, Transmission and Distribution:
58 000 - 111 000 kg/PJ of gas
consumed

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TABLE 1-57 (CONTINUED)
SUMMARY OF METHANE EMISSION FACTORS
Data source
Barns and Edmonds (1990)

Study Methodology
Compilation of:official reports and projections
on international emissions

Norwegian SPCA (1992b)

Summary of:

Emission Factors

Applicability

Oil and Gas Production:
96 000 kg/PJ of natural gas
production

Emissions from gas production andseparation facilities in the world

6 300 - 1 019 000 kg/PJ of gas
production

Emissions from venting and flaring
activity by region of the world

Oil and Gas Production:

emissions estimates for 1989
based on:

12 800 kg/PJ of gas produced

Emissions from cold vents andfugitive emissions

information and measurements
collected from oil companies

3 200 kg/PJ of gas produced

Flare and gas turbines

and industry associations

200 kg/PJ of gas produced

Pre-production
testing)emissions(well

Crude oil transportation:
2 500 kg/PJ oil tankered

Emissions from offshore loading of
crude oil

Natural gas processing:
1 800 kg/PJ of gas processed
Norwegian Oil Industry
Association (OLF), 1993

Summary of emission estimates
based on:information and measurements
collected from oil associations

Emissions from one Norwegian gas
processing terminal

Oil and Gas Production:
3 000 - 7 500 kg/PJ of gas
produced

Emissions from cold vents andfugitive emissions

100 - 400 kg/PJ of gas
produced

Pre-production emissions

1.8.5 Recent Revisions to Emission Factors
The above methodology and emission factors are based on the report of an expert groupconvened to advise the IPCC/OECD/IEA programme on methods and data in this specificarea (Ebert, et al., 1993). Since that group delivered its report in mid-1993, a more
recent analysis (US EPA, 1994b) has provided a somewhat different interpretation ofsome emission factors. While this very detailed analysis endorses the basic tiered
methodology included in this Manual, the emission factors it derives differ from thosepresented here. This evaluation was based on essentially the same set of measurement
data as cited in this Manual, but draws different results from the limited available data.
The results of the recent US EPA analysis are summarised in Table 1-58. The mostsignificant differences are in natural gas processing, transportation and distribution, wherea more detailed set of emission factor ranges are recommended for non-OECD
countries, some which are based on production of natural gas and some which are basedon consumption of natural gas (which is the case for all of the factors provided above).
Where emission factors are provided for more than one subcategory, they are intended
to be additive, and would result in a higher total emissions estimates. Other differencesin this US EPA analysis are that venting and flaring emissions for Western Europe arebased on oil rather than gas production, and there are minor revisions to some factors
for fugitive and other emissions from gas production.
These differences are significant, even given the overall uncertainty in this category, andshould be considered carefully by national experts in regions where emissions from this

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ENERGYsource category are significant. It is hoped that the differences can be resolved or
explained in more detail in a subsequent version of these Guidelines.

TABLE 1-58
REVISED REGIONAL EMISSION FACTORS FOR METHANE FROM OIL AND GAS ACTIVITIES (kg/PJ)
Source Type

Basis

Western Europe

US & Canada

Other Oil Exporting
Former USSR,
Central & EasternCountries
EuropeRest of the
World

OIL & GAS PRODUCTION
Fugitive and Other
Maintenance Emissions
from Oil Production

Oil Produced

300 - 5 000

300 - 5 000

300 - 5 000

300 - 5 000

300 - 5 000

Fugitive and Other
Maintenance Emissions
from Gas Production

Gas Produced

15 000 - 27 000

46 000 - 84 000

140 000 - 314 000

46 000 - 96 000

46 000 - 96 000

Venting & Flaring from
Oil and Gas Production

Oil & Gas
Produced(a)-3 000 - 14 000

-

-

-

Oil Produced

1 000 - 3 000

-

-

-

-

Gas Produced--

6 000 - 30 000

758 000 - 1 046 000

175 000 209 000

CRUDE OIL TRANSPORTATION, STORAGE AND REFINING
Transportation

Oil Tankered

745

745

745

745

745

Refining

Oil Refined

90 - 1 400

90 - 1 400

90 - 1 400

90 - 1 400

90 - 1 400

Storage Tanks

Oil Refined

20 - 250

20 - 250

20 - 250

20 - 250

20 - 250

NATURAL GAS PROCESSING, TRANSPORT AND DISTRIBUTION
Emissions from
Processing, Distribution
and Transmission

Gas Produced--

288 000 - 628 000

288 000 (high)(b)288 000 (high)(b)

Gas Consumed

72 000 - 133 000

57 000 - 118 000-

118 000 (low)(c)

118 000 (low)(c)

Leakage at industrial
plants and power
stations

Non-residential Gas
Consumed(d)--175 000 - 384 000

0 - 175 000

0 - 175 000

Leakage in theresidential andcommercial sectors

Residential Gas
Consumed(e)--87 000 - 192 000

0 - 87 000

0 - 87 000(a) In the US and Canada, the emissions are based on total production of both oil and gas produced.
(b) The emission factor of 288 000 kg/PJ of gas produced is used only for the high emissions estimate.
(c) The emission factor of 118 000 kg/PJ of gas consumed is used only for the low emissions estimate.
(d) Gas consumption by utilities and industries.
(e) Gas consumption by the residential and commercial sectors.
Source: US EPA (1994b).

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1.8.6 Methodologies
EmissionsFor

Estimating

MethaneA three-tiered approach is presented for estimating methane emissions from oil and gas
activities. The specific tiers are listed below in the order of increasing sophistication, datarequirements, and accuracy:•••

Tier 1 - Production-Based Average Emission Factors Approach,
Tier 2 - Mass Balance Approach, andTier 3 - Rigorous Source-Specific Approach.

Countries should select the approach or combination of approaches that is most suited
to their circumstances. Some important considerations may include the relative
contribution of oil and natural gas to total methane emissions for the country, theavailable information and resources, and the complexity of the local oil and gas industry.
Regardless of the method that is used, the results must be aggregated back to a Tier 1
format to provide a consistent basis for comparison.
Note that methane emissions from combustion plant and incomplete combustion inflaring processes in the non-oil and gas industry are excluded. They are accounted forseparately in the section on methane emissions from combustion and industry.
Tier 1 - Production-Based Average Emission Factors ApproachThis is the simplest approach for estimating CH4 emission from oil and gas activities, andis the only one that does not require any direct interaction with the oil and gas industry
and associated regulatory agencies. Accordingly, it is the least reliable of the methods.
The required activity data may be easily referenced from published documents of the IEA
or the United Nations Statistical Division, and the necessary emission factors areprovided in this document. The Tier 1 Approach can be used as a starting point for any
country, and may be all that is needed where the emissions from a country's oil and gas
industry are comparatively small and/or where data or resources are not available topursue a more rigorous approach.
Production Base: To estimate emissions, the following steps are recommended as adefault estimation procedure:
1 Global oil and gas systems have been divided into regions with relatively
homogeneous oil and gas system characteristics. Each country should decide which
system characterisation best fits its own oil and gas system(s).
2 For each region, representative emission factors for each activity within eachsegment have been selected with the objective of taking into account the various
system designs and operating practices found in each region.
3 For each country, country-specific activity levels must be obtained and multiplied by
the appropriate emission factor. Emission factors for countries should be selectedfrom those corresponding to the appropriate region.
As more data become available for oil and gas producing activities within different
countries, the default methodology described above (including activity data and emission
factors) should be refined. Each step is discussed below in more detail.
Regional Definitions: Regions have been defined considering the limitations in data on
emission factors and activity levels, but also recognising the key differences in oil and gas
systems that are found globally. The following five regions are recommended at this time:

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USA and Canada: The United States is a large producer and importer of oil andis a large producer of gas. Detailed emissions estimates are available for the UnitedStates.

•Former USSR and Eastern Europe: Indications are that emission rates fromthis region are much higher than emission rates from other regions, in particular forthe gas system. This region includes the former USSR (which is by far the largest oil
and gas producer in the region), Albania, Bulgaria, Czech & Slovak Republics,
Hungary, Poland, Romania, and the former Yugoslavia.

•

Western Europe: This region is a net importer of oil and gas, and mainly
produces oil and gas off shore. This region includes: Austria, Belgium, Denmark,Faroe Islands, Finland, France, Germany, Gibraltar, Greece, Iceland, Ireland, Italy,
Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, andthe United Kingdom.

•

Other Oil Exporting Countries: This region includes the world's other major
oil producing countries: the 11 OPEC members (Algeria, Libya, Nigeria, Venezuela,
Indonesia, Iran, Iraq, Kuwait, Qatar, Saudi Arabia and the United Arab Emirates),
Gabon, Ecuador and Mexico. Generally, these countries produce large quantities ofoil and have limited markets for gas.

•

Rest of the World: This region includes the remaining countries of Asia, Africa,
Middle East, Oceania and Latin America.

In defining these regions, countries with relatively similar oil and gas systems were
aggregated. Additional investigation would likely improve the definition of the regions.
Emission Factors: As discussed above, the basis for selecting emission factors is weakbecause very few detailed studies of emissions have been performed. Using theinformation summarised in Table 1-57, emission factors should be selected by industry
segment and emission type for each of the regions. In some cases data from the UnitedStates were used when region-specific information was not available.
Tables 1-60 to 1-64 offer emission factors for each region. Emission factors from US
EPA (1992) were used for the United States. Key emission factors for Eastern Europe
and the Former USSR were taken from Rabchuk et al. (1990) and Barns and Edmonds(1990). Estimates were used for emission factors for venting and flaring for the several
regions, including Eastern Europe.
Studies by Schneider-Fresenius et al. (1989) and Norwegian SPCA (1992b) were adopted
as representative of emission factors for Western European gas production, venting andflaring. No region-specific data were available for the Other Oil Exporting countries andthe Rest of the World. Emission factors in these regions are expected to fall between
the relatively low rates found in North America and Western Europe and the relatively
high rates found in Eastern Europe. Consequently, a range of emission factors issuggested for these regions unless more information can be obtained.
Activity Levels: Data on the quantity of oil and gas produced, refined, and consumed
can be obtained from the IEA or the UN Statistical Division. Sources are described in
Section 1.2.1 Data Sources. Data on oil refining capacity can be used to approximate oil
refined. Data on oil tankered were not available by region. It is important for national
experts to ensure that production figures used in calculation of apparent consumption forCO2 emissions estimates (described in the Tier 1 method for CO2) are consistent withthose used in this section.

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Tier 2 - Mass Balance Approach
The Mass Balance Approach is only recommended for oil system releases of CH4, andshould not be used for releases from natural gas activities. No Tier 2 method exists fornatural gas activities. This is a priority for future work. The Mass Balance Approach
employs standard, generally easy-to-obtain, oil and gas data (i.e., production volumes, gasto-oil ratios (GORs), and gas compositions) to estimate the maximum amount ofmethane that could be emitted to the atmosphere by different sectors of the oil and gas
industry. These amounts are then scaled to reflect actual emissions by applying
appropriate emission factors. The minimum emission factors account for the amount ofgas that is disposed of by control devices, consumed by combustion equipment,
conserved, or reinjected. Leak emission factors account for the amounts lost throughleaks from these control/utilisation systems.
The basic procedures for performing the mass balance calculations for each oil and gas
activity are set out below. Total CH4 emissions is the sum of emissions for each of these
activities. Default data and factors are provided where possible.
Oil Production: Emissions from oil production may be estimated using the relation,

EOil-P = QOil − P × GOR × YCH4 × K Oil − P × DGOR × 10 −12
where
EOil-P= methane emissions from oil production (Tg/year)

QOil-P= oil produced (m3/year)

GOR= gas to oil ratio; defines the amount of gas produced (in volume) per unit ofoil produced (in volume). (m3/m3, or dimensionless).

YCH= methane Fraction; the volume of gas dissolved in the oil that is methane,on a volume basis, for example, 0.1 m3 of methane per 1.0 m3 of gas.
(may be considered dimensionless)4

KOil-P= emission factor for oil production, see below (dimensionless)

DGOR= density of methane at the temperature and pressure at which the GOR is
estimated, (g/m3). The relevant temperature and pressure may vary,depending on the values used to calculate the GOR. For example, at 0°C
and a pressure of 1 atmosphere, the density of methane is 715.4 g/m3, andat 20°C, and a pressure of 1 atmosphere, the density of methane is666.6 g/m3.

The first two terms (QOil-P and GOR) estimate the total amount of gas that is withdrawn
from the ground while the oil is produced. The term YCH converts the total gas quantity
4
into the quantity of methane (still on a volume basis). The emission factor (KOil-P) is thefraction of the total gas withdrawn that is emitted. At most, KOil-P = 1. To put theemissions estimate on a mass basis, DGOR is used to estimate grams, and the conversion
to teragrams follows.

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