Natural Gas and Climate Change

Eric D. Larson, PhD
Natural Gas
& Climate Change

Natural Gas
& Climate Change
Eric D. Larson, PhD
Climate Central
research on climate change and informs the public of
report on climate science, energy, sea level rise,
Climate Central
research on climate science; energy; impacts such
investigate and synthesize weather and climate data
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Senior Scientist
Central while also being part of the research faculty
research interests include engineering, economic, and
Report
Author

Contents
Key Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Report in Brief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
. . . . 21
2.1 Gas Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2 Gas Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3 GasTransmission and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4 Gas Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
. . 28
3.1 Leakage During Gas Production, Processing, andTransmission.. . ......... 29
3.2 Leakage from Gas Distribution Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Natural Gas and Climate Change l 1
Key Findings
Knowing how much methane is leaking from the natural gas system is essential to determining the potential climate
three factors:
The methane leak rate from the natural gas system;
gas is 1
2 2
over 1
Climate Central has developed an interactive graphic incorporating all three factors
power plants, for over 1
17 percent
1
pollution than continued coal use after 1

of coal plants are converted to natural gas, a new wave of highly potent methane leaks into the atmosphere and then
1
1
the natural gas supply system, from less than 1
level,studies have reported methane leak rates as high as 1 12 annual greenhouse gas emissions
1
to 1
Determining methane leakage is complicated by various uncertainties:
-80%
-60%
-40%
-20%
no coal
replacement
20% switch to natural
gas a disadvantage
switch to natural
gas an advantage
2010 4030 6050 8070 10090
It will be Decades Before Switching to Natural Gas From
Coal Power Brings a 50 Percent Reduction in Emissions
2050
coal-to-gas
conversion rate:
2.5% per year
5.0% per year
8% methane leakage
5% methane leakage
2% methane leakage
EmissionsReductionFrom
SwitchtoNaturalGas
Years From Today

have been relatively few measurements of vented gas volumes, and estimating an average amount of
There are orders of magnitude in variability of estimates of how much gas will ultimately be recovered
well, which introduces uncertainty in estimating the percentage of gas leaked over the life of an
The leak integrity of the large and diverse gas distribution infrastructure:
thousands of metering and regulating stations operating under varying gas pressures and other

Report in Brief
1
19 percent of all electricity
1
2
, and any
methane leakage in perspective:Depending on the rate of methane leakage,how much more climate friendly is natural
1 1
1
Production Processing Transmission Distribution
Figure 1. The four stages of the U.S. natural gas supply system.

The large uncertainties in leakage estimates arise from the sheer size and diversity of the gas supply system and a
Gas Production
2
1 1
11
1
The average methane leakage rate for gas production from hydraulically fractured shale wells estimated in different

Gas Processing
2
and other contaminants at unacceptably
1996 reported measured
leak rates from more than 1
compressors and engines at gas processing plants, on the basis of which representative daily leakage rates were
2
that originated in the natural gas is separated from the gas during processing and vented to the
1
among the four stages in the natural gas supply system, so uncertainties in gas processing estimates are of less
GasTransmission
of varying types, more than 1
Because the number of compressors and engines in the transmission system are relatively well documented and
1
seasonal movements of gas in and out of storage reservoirs was not considered when measurements were made, and
Gas Distribution
1
1996 study mentioned earlier, and nearly

1
Natural Gas System Leakage inTotal and Implications for Electricity Generation
generation, leakage from the production, processing, and transmission stages are important to consider, since nearly
1
1
percent for conventional gas and from 1
1
the global warming impact of methane leaks and the related fact that the potency of methane as a greenhouse gas is
2
1
2
1
2
, we estimated
1
1

methane leakage associated with each new increment of gas electricity has a warming potency that is initially very high
17 or 41
11
coal use would be only 1
2
capture and storage to provide
Figure 2. Impact on global warming of shifting existing coal generated electricity to natural gas over time relative to maintaining existing coal
generation at current level.The impacts are calculated for two different annual coal-to-gas substitution rates and for three assumed methane
leakage rates.
-80%
-60%
-40%
-20%
no coal
replacement
20% switch to natural
gas a disadvantage
switch to natural
gas an advantage
2010 4030 6050 8070 10090
It will be Decades Before Switching to Natural Gas From
Coal Power Brings a 50 Percent Reduction in Emissions
2050
coal-to-gas
conversion rate:
2.5% per year
5.0% per year
8% methane leakage
5% methane leakage
2% methane leakage
EmissionsReductionFrom
SwitchtoNaturalGas
Years From Today

1. Introduction
Natural gas is the second most abundant fossil
The estimates of the total amount of natural
the past decade with the discovery of new forms of
unconventional gas,which refers broadly to gas residing
carbonate, and coal formations can all trap natural
4
The production of shale gas,
the most recently discovered unconventional gas, is
a
in the decades ahead, along with total gas production
growth in shale gas and this has dramatically increased
6
generation is widely thought to be an important way to
2
emitted into
the atmosphere, because combustion of natural gas by
2
than the combustion
into electricity than coal, resulting in an even larger
natural gas has a clear greenhouse gas emissions
also be considered to get an accurate picture of the full
greenhouse emissions impact of natural gas compared
considered together are often called the lifecycle
a
Horizontal drilling and hydraulic fracturing are also applied to produce gas from some tight sandstone and tight carbonate formations.A key distinction
between the term tight gas and shale gas is that the latter is gas that formed and is stored in the shale formation, whereas the former formed external
to the formation and migrated into it over time (millions of years).4
Table 1. Number of years that estimated recoverable resources of natural gas, petroleum, and coal would last
if each are used at the rate that they were consumed in 2011.*
Years left at 2011 rate of use
Conventional Natural Gas
Unconventional Natural Gas
Petroleum
Coal
116
1021
171
2475
42
49
36
140
* Calculated as the average of estimated reserves plus resources from Rogner,et al1
, divided by total global use of
gas,petroleum,or coal in 2011 from BP.2
The consumption rates in 2011 were 122 exajoules for gas,170 exajoules
for oil, and 156 exajoules for coal. One exajoule is 1018
joules, or approximately 1 quadrillion BTU (one quad).
** Including Alaska. Calculated from resource estimates and consumption data of EIA. 3

Figure 3. Number of gas wells drilled per month in the U.S. 5
Figure 4. Past and projected U.S. natural gas production (in trillion cubic feet per year).A trillion cubic feet of natural gas
contains about one quadrillion BTU (quad), or equivalently about 1 exajoule (EJ) of energy. Source: EIA.7

Figure 5. Unlike other sectors, natural gas for electricity generation has been growing since around 1990 and is now the single largest user of
natural gas.This graph shows gas use (in million cubic feet per year) by different sectors. Lease, plant, and pipeline fuel refers to natural gas
consumed by equipment used to produce and deliver gas to users, such as natural gas engines that drive pipeline compressors. Source: EIA.
Burning Natural Gas Produces Much Less CO2
Than Burning Coal
Figure 6. Average emissions by fuel type from combustion of fossil fuels in the U.S. in 2011:7
average emissions per million BTU (higher
heating value) of fuel consumed (left) and average emissions per kWh of electricity generated (right).

The recent and dramatic appearance of shale
whether or not lifecycle greenhouse gas emissions for
natural gas are as favorable as suggested by the simple
4
2
,so small leaks
from the natural gas system can have outsized impacts
methane leakage and other greenhouse gas emissions
natural gas supply system includes production of raw gas,
processing of the raw gas to make it suitable for pipeline
literally thousands of places where leaks of methane
included more than half a million producing wells,several
than a million miles of local distribution mains, and
methane leak rates for the global warming impact of
Figure 7. The U.S. natural gas supply system.8
Each Stage in the Natural Gas Supply System is aVast Infrastructure

Figure 8. U.S. natural gas processing plants. 9
Figure 9.The U.S. natural gas transmission system (as of 2009). 10
There are Hundreds of Natural Gas Processing Plants in the Country
Hundreds ofThousands of Miles of GasTransmission Pipelines Cover the U.S.

Figure 10. U.S. natural gas storage facilities. 11
Natural Gas Storage Facilities Exist Across the Country

Box 1: Shale Gas
4
The
production of some water with shale gas from these formations, a complication not present for most wells in other
Figure 11. Shale gas formations in the lower-48 states. 12

Table 2. Mean estimate by the U.S. Geological Survey of undiscovered technically recoverable shale gas resources by basin. 13
Gulf Coast
Haynesville Sabine
Eagle Ford
Maverick Basin Pearsall
Mid-Bossier Sabine
Interior Marcelllus
Northwestern Ohio
Western Margin Marcellus
Devonian
Foldbelt Marcellus
Shublick
Brookian
Delaware-Pecos Basins Barnett
Delaware-Pecos Basins Woodford
Midland Basin Woodword-Barnett
Woodford
Fayetteville-High Gamma Ray Depocenter
Fayetteville Western Arkansas
Chattanooga
Caney
Greater Newark East Frac-Barrier
Extended Continuous Barnett
Woodford
Thirteen Finger Limestone-Atoka
Gothic, Chimney Rock, Hovenweep
Cane Creek
Total
Trillion cubic feet*
60.734
50.219
8.817
5.126
81.374
2.654
2.059
1.294
0.765
38.405
2.184
17.203
15.105
2.822
10.678
9.070
4.170
1.617
1.135
14.659
11.570
15.973
6.850
6.490
4.530
* One trillion cubic feet of gas contains about one quadrillion BTU (one quad).

but only with the development of hydraulic fracturing
production, the proppant stays behind and keeps the
“Fracking” was originally developed for use in vertically
drilled wells, but shale gas production only began in
earnest with the development of horizontal drilling,
which when combined with fracking, enables access
to much more of the volume of the thin, but laterally
horizontal holes are typically drilled from a single well
pad, reducing overall drilling costs and enabling access
to much more of a shale formation from a small area on
Figure 13. Hydraulic fracturing combined with horizontal drilling
allows accessing more of a thin shale formation.
Figure 12. Shale gas production in the U.S. has grown rapidly. 14
Seven States Accounted for 90 Percent of Shale Gas Production in 2011
*Not to scale

Box 2: The Global Warming Potential of Methane
2 4
2
2
2
, according to the
2
2
molecule in the atmosphere
4 2 4
depends on the time period over which the
but
2
suggest that varying time frames for

Table 3. The global warming potential for methane falls as the time horizon for its evaluation grows.15
A 20-year GWP of 72 for methane
means that 1 kilogram of methane gas in the atmosphere will cause the equivalent warming of 72 kilograms of CO2
over a 20 year period.
The GWP values here are consistent with those shown in Figure 14.
Figure 14. The global warming potential (GWP) of methane relative to CO2
for a pulse emission at time zero.This assumes a characteristic
lifetime in the atmosphere of 12 years for methane and a lifetime for CO2
as predicted by the Bern carbon cycle model.15
(See Alvarez et al.18
)
20-year GWP 100-year GWP 500-year GWP
GWP of CH4
(methane) 72 25 7.6

2. EPA Estimates of GHG
Emissions from the Natural
Gas Supply System
,
2
b
2
Methane leakage from the natural gas supply
system also contributesc
2
leaks in the natural gas supply system accounting for
2
d
2
corresponding to an estimated methane leakage rate
involved in estimating the national methane leakage
wide variety of data sources and by applying a multitude
Figure 15. U.S. greenhouse gas emissions as estimated by the Environmental Protection Agency.21
b
The EPA inventories use 100-year global warming potentials (GWPs) for non-CO2
gases taken from the Third Assessment Report (1996) of the
Intergovernmental Panel on Climate Change (IPCC), not from the most recent (2007) IPCC Assessment.The methane GWP value used by EPA in
this inventory is 21. See Box 2 for discussion of GWP.
c
Some naturally-occurring underground CO2
is also vented to the atmosphere in the course of producing, processing, and transporting natural gas.
EPA estimates these are much less one-tenth of one percent of the CO2
-equivalent emissions of methane.23
d
U.S. natural gas consumption in 2010 was 24.1 trillion standard cubic feet according to the U.S. Energy Information Administration. Assuming the
methane fraction in this gas was 93.4 percent,the value assumed by EPA in its emissions inventory,23
and taking into account the fact that one standard
cubic foot (scf) of methane contains 20.23 grams (or 20.23 metric tons per million scf), the total methane consumed (as natural gas) was 455 million
metric tons. Considering a GWP of 21 for methane (as the EPA does), this is 9,556 million metric tons of CO2
-equivalent.The ratio of 215 (Table
4) to 9,556 gives a leakage estimate of 2.25 percent of methane consumed.The leakage as a fraction of methane extracted from the ground is
L = 1 - where x is the leakage expressed as a fraction of methane consumption. For x = 0.0225, or L = 0.0220, or 2.2%.1
(1 + x)
Methane was an Estimated 10 Percent of U.S. Greenhouse Gas Emissions in 2010

Figure 16. U.S. methane emissions in 2010 (in million metric tons of CO2
equivalents) as estimated by the Environmental
Protection Agency.21
Table 4. EPA estimates of methane emissions in 2010 from the natural gas system in units of million metric tons of CO2
-equivalent (for a
methane GWP of 21). Figures are from the 201222
inventory and the 2013 inventory.20
Liquids unloading
Pneumatic device vents
Gas engines
Shallow water gas platforms
Completions and workovers with hydraulic fracturing
Other production sources
Reciprocating compressors
Centrifugal compressors (wet seals)
Gas engines
Other processing sources
Centrifugal compressors (wet seals) (transmission)
Reciprocating compressors (transmission)
Engines (transmission)
Reciprocating compressors (storage)
Other transmission and storage sources
Natural Gas Distribution
Meter/regulator (at city gates)
Leaks from main distribution pipelines
Leaks from service pipelines connected mains and users’ meters
Other distribution sources
2012 Inventory 2013 Inventory
85.7
12.8
5.6
5.6
3.8
12.5
8.3
4.9
3.5
0.3
15.7
12.8
4.7
3.7
1.9
5.0
12.5
9.3
4.3
2.4
million metric tons of CO2
-equivalent
5.4
16.7
Leaks in the Natural Gas System are Estimated to be OneThird of Methane Emissions

Box 3: EPA’s Methodologies for Estimating Methane Leakage
from the Natural Gas Supply System
emissions per day will vary from one compressor to another24
26
Figure 17.
Differences in data sources and methodologies account for the differences in estimated emissions. 27

2.1 Gas Production
“completions and workovers with hydraulic fracturing”
was the smallest contributor to production emissions
considerations in the latter include the number of
times each year that the average well is unloaded,
the average volume of gas that is entrained with the
e
A shale gas operation in Greene County, PA. (Nov 2010).
Credit: Mark Schmerling via FracTracker.org.
e
One pound of methane vented to the atmosphere has a GWP of 25, considering a 100-yr time horizon (see Box 2). If instead the 1 lb of methane
were burned, 2.75 lbs of CO2
would be produced.This amount of CO2
reduces the global warming impact of the emission by a factor of 9.

well to maintain its productivity at an acceptable
workovers during their producing life, with some wells
estimate of the number of wells that were hydraulically
fractured and a decrease in the assumed percentage
2.2 Gas Processing
f
to make it suitable for entry into the gas transmission
29
smallest contribution to methane emissions among
occur during gas processing are the result of leaks
based on the number of compressors and engines
earlier determined the emission factors and the
emission factors, and the number of compressors and
these by the ratio of gas produced in the inventory
2.3 Gas Transmission and Storage
The natural gas pipeline transmission system in the
f
Processing typically removes“condensates” (water and hydrocarbon liquids),“acid gases” (H2
S, CO2
, and others), and sometimes nitrogen. On average
the volume of gas after processing is 7 percent or 8 percent less than before processing.
Natural gas processing plant Natural gas transmisison lines

from the transmission and storage stage come from
compressors and engines,with only a small contribution
with the large seasonal movements of gas in and out
of storage reservoirs were not considered when
measurements were made, and this may introduce
2.4 Gas Distribution
distribution are for local pipeline distribution systems
the main transmission pipelines and through which the
most electric power plants and about half of large
industrial customers, which are connected directly to
a main transmission pipeline and account for perhaps
g
Figure 18. There are more than 1400 compressor stations in the U.S. natural gas transmission pipeline system. 30
g
In 2012, 36 percent of all gas used for energy was used in electric power generation and 33 percent was used in industry. Assuming all of the gas
used for electric power and half of the gas used by industry was delivered via transmission pipelines, then approximately half of all gas used in the U.S.
was delivered to users via transmission pipeline.
Natural gas meters in the distribution system.
Compression Stations ExistThroughout the Natural GasTransmission System

as it is transferred from a transmission line into a
connections in total, and it assumes no leakage occurs
leakage of methane is at the metering/regulating
factor for each type of station is multiplied by the
estimated number of that type of station in operation
account for most of the rest of the estimated methane
unprotected steel, protected steel, plastic, and copper
Table 5. Pipeline methane emission factors and pipeline
mileage in EPA’s 2013 inventory.20
h
Protected steel refers to carbon steel pipes equipped with a special material coating or with cathodic protection to limit corrosion that can
lead to leakage. (Cathodic protection involves the use of electrochemistry principles.)The use of cast iron and unprotected steel pipes, which are
susceptible to corrosion, is declining. Nevertheless, there are still an estimated 100,000 miles of distribution pipe made of cast iron or unprotected
steel and more than 4.2 million unprotected steel service lines still in use.23
steel pipes are assumed to have high leak rates, based
h
inventory also estimates the number of miles of each
type of pipe in the distribution system and the number
of each type of service connection to customers based
Distribution mains
Cast iron
Unprotected Steel
Plastic
Protected steel
Transmission pipelines
Annual Leak Rate Miles of Pipe
239,000
110,000
9,910
3,070
33,586
64,092
645,102
488,265

3. Other Estimates of GHG
Emissions from the Natural
Gas Supply System
included a provision to separately calculate emissions
develop greenhouse gas emission estimates for natural
have appeared, with emissions
diversity of natural gas basin geologies, the many
steps involved in the natural gas system, the variety of
technologies and industry practices used, and, perhaps
most importantly, the lack of measured emissions
data, a large number of assumptions must be made to
authors come to different conclusions about the
some conclude that upstream emissions per unit
energy for shale gas are higher than for conventional
gas Many of
the authors rely on the same two information sources
for many of their input assumptions,
few key assumptions mainly responsible for differences
Table 6. Estimates of upstream methane and CO2
emissions for conventional gas and shale gas, with comparison to EPA estimates for the
natural gas supply system as a whole.* (Emissions from gas distribution are not included here.)
2
Well pad construction
Well drilling
Hydraulic fracturing water
Chemicals for hydraulic fracturing
Well completion
Fugitive well emissions
Workovers
Liquids unloading
Transmission emissions
Total upstream methane emissions
2
Flaring
Lease/plant energy
Vented at processing plant
Transmission compressor fuel
2
emissions
2
UPSTREAM EMISSIONS
Conv
1.8
6.6
8.6
1.2
2.3
0.2
0.4
Shale
1.3
1.8
4.6
7.8
1.2
2.3
0.2
0.4
Conv
0.3
0.4
0.9
1.6
0.5
1.7
2.8
0.2
Shale
0.3
0.3
1.6
0.9
3.1
0.5
1.7
2.8
0.2
Conv
5.0
0.6
5.6
0.4
6.8
4.1
0.6
Shale
8.6
5.0
15.1
0.4
6.8
4.1
0.6
All
6.8
0.9
2.4
* Methane leakage has been converted to kgCO2
e using a GWP of 25. Numbers in all but the EPA column are taken fromTable SI-5 in the supplemental
information for the paper by Weber and Calvin.49
Numbers in the EPA column are my estimates based on the 2012 inventory (Table 4, but adjusted to
GWP of 25) and total 2010 U.S. natural gas end-use consumption for energy.54
CO2
emissions in the EPA column include estimates from the EPA 2012
inventory23
plus emissions from complete combustion of lease and plant fuel in 2010 that I have estimated based on EIA data.55
0.2
1.8
0.1
2.0
1.5
Conv
3.4
2.5
5.9
1.5
1.9
0.4
3.7
1.0
0.4
Shale
0.1
0.2
0.3
0.1
1.0
3.4
5.1
1.5
1.9
0.4
3.7
1.0
0.4
Shale
4.7
2.1
4.7
11.5
0.6
1.8
Conv
1.6
3.6
5.9
11.1
0.8
0.9
0.4
4.3
0.8
0.3
Shale
1.0
0.8
3.6
1.5
6.9
0.8
0.9
0.4
4.1
0.8
0.3
Jiang47
NETL33
Hultman46
Stephenson48
Burnham43
Howarth17
Best49
EPA
Conv
0.16
0.23
0.18
2.70
3.80
7.1
1.8
1.9
0.6
3.2
1.2
0.4
Shale
0.16
0.2
0.26
0.07
1.2
2.70
1.20
5.8
1.8
1.9
0.6
3.2
1.2
0.4

3.1 Leakage During Gas Production,
Processing, and Transmission
49
encapsulates
well the diversity of estimates of upstream emissions
that have been published relating to the gas production,
and took care to normalize estimates from each study
to eliminate differences arising from inconsistent
assumptions between studies, such as different values
Table 6 shows their normalized estimates in units of
2
i
assuming a methane
consider their best estimate based on their analysis,
including a Monte Carlo uncertainty analysis, of all of
numbers fromTable 6,and shows estimated uncertainty
2
e/MJ
being markedly
represent the highest and lowest estimates, with the
2
e/MJ
2
emissions are due to combustion of natural gas used
numbers in Table 6 suggest that the global warming
2
emissions accounts for about
2
plus methane,
Figure 19. A diversity of estimates exist in the literature for GHG emissions associated with natural gas production, processing, and delivery.
This graph, from Weber and Calvin49
(and consistent with numbers inTable 6, but using different sub-groupings) shows upstream emissions
in units of grams of CO2
e/MJLHV
of natural gas, excluding emissions associated with natural gas distribution. Ranges of uncertainty are also
indicated. “Best” refers to Weber and Calvin’s own estimates.
i
The energy content of a fuel can be expressed on the basis of its lower heating value (LHV) or its higher heating value (HHV).The difference between
the LHV and HHV of a fuel depends on the amount of hydrogen it contains.The heating value of a fuel is determined by burning it completely under
standardized conditions and measuring the amount of heat released. Complete combustion means that all carbon in the fuel is converted to CO2
and
all hydrogen is converted to water vapor (H2
O).The heat released as a result of these oxidation processes represents the LHV of the fuel. If the water
vapor in the combustion products is condensed, additional heat is released and the sum of this and the LHV represents the HHV of the fuel. For fuels
with low hydrogen content, like coal, relatively little water vapor forms during combustion, so the difference between LHV and HHV is not especially
large.The high hydrogen content of methane,CH4,
methane, has an HHV that is about 11 percent higher than its LHV.
j
Category groupings in Figure 19 are different from those inTable 6, but overall totals are the same.
Estimates of Upstream Emissions in the Natural Gas SystemVary Widely

considered, methane would have a higher impact, and
2
k
2
emissions for the
moment, it is possible to remove the complication
the methane emissions in physical terms as a percent
methane leakage during production, processing, and
transmission,as estimated in the various studies,ranges
lower and upper ends of the uncertainty ranges for the
nearly as high as the upper end of the uncertainty
ranges for any of the other shale gas results shown in
who estimate
m
They estimated well completion emissions by assuming
that the amount of gas brought to the surface with
gas handling is represented by an assumption that, on
2
acknowledge the uncertainties in this latter assumption,
proportion of gas produced during well completions is
et al.,
Table 7. Upstream methane leakage (excluding leakage in distribution systems) as a percentage of methane production for the studies
shown inTable 6 and Figure 19.*
k
For example, with GWP = 72 (20-year time frame), CO2
emissions would be less than 15 percent of total CO2
-equivalent emissions in most cases.
l
The paper by Howarth, et al.17
gives total estimated system leakage fractions (including leakage in distribution), of 3.6 percent to 7.9 percent. I have
estimated the range for distribution leakage, based on discussion in that paper, to be 0.35 percent to 0.9 percent and removed this from the original
Howarth et al.
m
O’Sullivan and Paltsev also made estimates for wells in the Fayetteville, Marcellus, and Woodford formations.
Production
Processing
Transmission
TOTAL
Conv
1.7
0.2
0.4
2.4
Shale
1.5
0.2
0.4
2.2
Conv
0.3
0.1
0.3
0.7
Shale
0.6
0.1
0.3
1.0
Conv
1.1
0.1
1.4
2.6
Shale
3.0
0.1
1.4
4.5
All
1.37
0.19
0.48
2.02
* Based onTable 6 and (for all but the EPA numbers) energy contents of produced gas per kg of contained methane reported byWeber
and Calvin:49
Jiang (50 MJLHV
/kgCH4
), NETL (48.8), Hultman (48.2), Stephenson (47.3), Burnham (48.6), Howarth (50.0), and Best
(48.8).The EPA estimate assumes a gas energy content of 51.5 MJLHV
/kgCH4
for consistency with EPA numbers inTable 6.
Conv
1.2
0.3
0.4
1.9
Shale
1.0
0.3
0.4
1.7
Shale
2.2
0.1
0.3
2.7
Conv
2.2
0.2
0.2
2.5
Shale
1.3
0.2
0.2
1.7
Jiang NETL Hultman Stephenson Burnham Howarth Best EPA
Conv
1.4
0.4
0.4
2.1
Shale
1.1
0.4
0.4
1.9

inventory, the emissions are more than triple this value
of the methane that might otherwise be vented or
general agreement that methane leakage in the gas
in the entire natural gas system,a conclusion supported
by some recent measurements of the concentrations
of methane in the air above gas wells, including
a reported leakage rate of 9 percent from oil and gas
down” measurements, involve large uncertainties,
but draw attention to the need for more and better
measurements that can help reduce the uncertainty of
variations in overall estimates from one study to
another: i)
well, ii) the well completion and workover emissions
factor, iii)
iv) the rate of fugitive emissions
at the wellhead, v) the fugitive emissions during gas
processing, vi)
Emissions that occur only once over the lifetime of a
into an estimate of emissions per unit of gas produced
by dividing the estimated emission by the total gas
the shale gas industry is still young, there is a limited
have noted that
there is “appreciable uncertainty regarding the level
accurately represent leakage per unit of gas production
is compounded by the large and inherent variability in
62
for wells in different
Table 8. Comparison of estimates for methane leakage during completion of shale gas wells in two different formations.
4
per well completion
35.1
151.3
4
per well completion
252
4638
Barnett formation
Haynesville formation

3.2 Leakage from Gas
Distribution Systems
concerned primarily with gas leakage in connection
commercial buildings and smaller industrial facilities
million service pipelines connecting the mains to users,
stations found at the interface of transmission and
number of leakage measurements that have been made
suggest that there could be large uncertainties in the
leaky steel or plastic in recent decades, but there are
conservatively estimated an average annual leak rate
n
being pursued to try to improve estimates of leakage
patterns and other variables to try to estimate what
concentrations above urban streets in Boston,64
66
The growing use of natural gas for power generation
in place of coal makes it particularly important to
understand methane leakage and its global warming
others with varying conclusions due in large
certainty about actual methane leakage rates, it is
especially informative to understand the prospective
global warming impact of different overall leakage rates
4. Natural Gas vs. Coal in
Electricity Generation
n
Comgas subsequently implemented an effort to place plastic inserts in their cast-iron distribution mains to reduce leakage.The extent to which such
sealing robot) that add sealant to jute-packed joints by self-navigating through distribution mains, thereby reducing the need for more costly excavation
to repair or replace pipes.65,63

o
the contribution from methane leakage corresponding
p
Green represents
2
gas used as fuel at gas processing plants and in the gas
2
that originated
underground and was removed from the natural gas
The left and right graphs include the same physical
leakage the choice of time horizon affects the global
2
impact of leakage approach the level of combustion
impact of methane leakage is triple the impact from
Going a step further, we can calculate emissions
earlier,natural gas contains much less carbon per unit of
67
Representative
o
Assuming complete combustion of natural gas containing 14 kg of carbon per GJHHV
.This corresponds to an assumed natural gas composition by
volume of 97.01percent methane, 1.76 percent ethane, 0.47 percent nitrogen, 0.38 percent CO2
, 0.26 percent propane, and 0.11 percent n-butane
and an elemental composition by weight of 74.0 percent C, 24.4 percent H, 0.8 percent N, and 0.7 percent O.The average molecular weight is 16.57
g/mol, and the LHV and HHV are 47.76 MJ/kg and 52.97 MJ/kg, respectively.
p
The methane leakage (in kgCO2
e/GJHHV
) as a function of the percentage of production leaked is calculated, using the natural gas characteristics in
footnote o, as follows: = GWP * * 14 * *
q
Upstream CO2
emissions include those reported by the EPA for the natural gas system23
plus emissions from combustion of “lease and plant fuel”
(which EPA excludes from its inventory for the natural gas system to avoid double counting). Lease and plant fuel emissions are estimated by assuming
complete combustion of lease and plant fuel energy used in 2010 as reported by the Energy Information Administration.54
Figure 20. Estimates of greenhouse gas emissions from natural gas production, processing, delivery, and end-use for different assumed rates
of upstream methane leakage.
kgCO2
e
GJHHV
% leaked
100
kgC
GJHHV
16gCH4
molC
1molC
12gC
Even Small Methane Leaks Can Have a Large Global Warming Impact in the ShortTerm

Figure 21. Estimates of greenhouse gas emissions from electricity production from natural gas for different assumed rates of upstream
r
r
Based on emissions shown in Figure 20 and power plant fuel consumption of 7172 GJHHV
/kWh a natural gas combined cycle (corresponding to
67
11736 GJHHV
67
and 10019 GJHHV
/kWh for a
68
Upstream CO2
emissions for the subcritical and supercritical coal plants are 8.34 kg/MWh and 7.48
kg/MWh, respectively, and upstream methane emissions are 3.20 kgCH4
/MWh and 2.76 kgCH4
/MWh, respectively.68,69
Bitumous coal
power plants
Natural gas combined cycle power plants with varying
upstream methane leakage (% of produced methane)
Bitumous coal
power plants
Natural gas combined cycle power plants with varying
upstream methane leakage (% of produced methane)
With Methane Leakage Natural Gas Power Generation Can Have a Similar or Higher
Global Warming Impact as Coal Power Generation

69
estimates of the “upstream” emissions associated with
coal electricity, including estimated methane emissions
emissions for a kwh of electricity from a natural gas
kwh still has a lower global warming impact than the
about 2 percent for the natural gas kwh to have half
is no better for the climate than the kwh from an
in comparing the global warming impact of electricity
et al. have proposed a
method for assessing the climate impact of a switch
dependent global warming potential of technology
2
,this method yields a ratio,for any
time horizon of interest, that represents the relative
global warming potential of switching from technology
Figure 22. Global warming impact of shifting electricity generation from a coal power plant to a natural gas power plant in year zero and
continuing that generation from gas each year thereafter, assuming different methane leakage rates in the natural gas system. Natural gas is
friendlier for the climate for values less than 1.0. s
s
Assumed heat rates for electricity generation are 7172 kJHHV
/kWh (6798 BTU/kWh) for NGCC and 10550 kJHHV
/kWh (10000 BTU/kWh) for existing
coal plants. Upstream emissions for coal are as described for subcritical coal in footnote r.
Upstream methane leaked
(% of production)
{

horizon due to the different atmospheric lifetimes of
2
et al.
with our leakage assumptions, Figure 22 shows the
global warming impact of replacing the electricity from
and then maintaining that natural gas generation for
for different assumed total methane leakage rates
NGCC electricity has half as much global warming
Many authors have suggested that switching from
coal to gas electricity halves the global warming impact
switch to gas would still be better for the climate than
coal over any time period considered, although barely
Figure 22 represents the impact of shifting one
power plant worth of electricity generation from coal
be the global warming impact of shifting over time the
to know that the average rate at which coal electricity
percentage rate of reduction has been rising in recent
been predominantly replaced by increased generation
from gas plus coal grew an average of less than half of
et al.to
electricity generated from coal and a corresponding
increase in electricity generated from gas,t
with total
electricity production from coal plus gas remaining the
u
prospective global warming impact of switching from
coal to natural gas electricity at different annual rates
enough time horizon, all of the cases will approach the
reached more slowly when coal replacement occurs
percent reduction in warming potential is achieved by
t
For a constant annual percentage conversion of coal electricity to gas electricity, the fraction of original coal electricity converted to gas each year is
[r * (1 - r)(t-1)
] where r is the annual percentage reduction in coal electricity and t is the number of years from the start of the conversion process.
(Conversion begins in year t = 1.)
u
et al.18
(Equation 2 in their paper, with L/Lref
= 1) is used here to calculate the reduction
in Global Warming Potential from substituting a unit amount of coal-generated electricity with gas-generated electricity in a given year and continuing to
produce that unit amount of electricity from gas in subsequent years.(Figure 22 shows the result of this calculation.)When the amount of electricity made
from natural gas is not constant every year but increases year to year (as coal electricity generation decrseases year to year) the climate impact of each
new annual increment of gas electricity is assessed using theTWP.Then, the climate impact of the electricity generated from coal and gas in total in any
year is the sum of climate impacts caused that year by each new increment of gas-generated electricity added from the start of the counting period up
to that year plus the impact of the reduced amount of coal-generated electricity being produced in that year. Mathematically, the climate impact in total
from the start of a shift from coal to gas over some number of years, N, is calculated as: [r (1 - r)(t-1)
*TWP(N + 1 - t)]dt + {1 - SN
t=1
[r (1 - r)(t-1)
]dt}
where r is the annual percentage reduction in coal electricity and TWP(N + 1 - t) is given by Equation 2 in Alvarez et al.
N
t=1
N
t=1
N

Figure 23.
annual rates. In all cases the assumed methane leakage is 2 percent of production.
gas replaces coal at
average % per year
>>>>
2% methane leakage rate
Table 9. U.S. coal and natural gas electricity generation 2002-2012 (left)6
and annual percentage reduction in coal electricity generation
when averaged over different time periods (right).
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
Coal
1,933,130
1,973,737
1,978,301
2,012,873
1,990,511
2,016,456
1,985,801
1,755,904
1,847,290
1,733,430
1,517,203
Natural Gas
691,006
649,908
710,100
760,960
816,441
896,590
882,981
920,979
987,697
1,013,689
1,230,708
Coal + Gas
2,624,136
2,623,645
2,688,401
2,773,833
2,806,952
2,913,046
2,868,782
2,676,883
2,834,987
2,747,119
2,747,911
2002 - 2012
2003 - 2012
2004 - 2012
2005 - 2012
2006 - 2012
2007 - 2012
2008 - 2012
2009 - 2012
2010 - 2012
2011 -2012
-
2.4 percent
2.9 percent
3.3 percent
4.0 percent
4.4 percent
5.5 percent
6.5 percent
4.8 percent
9.4 percent
12.5 percent
-
Reduction in
Coal Electricity

Figure 24.
Figure 25.
average % per year
>>>>
average % per year
>>>>

The same analysis can be carried out for a different
leakage, the impact of switching from coal to gas is not
6 percent, there would initially be negative impacts of
Figure 26. Additional gas required each year (compared to preceding year) under different scenarios. The solid lines represent the new gas
in 2012 was 1517TWh. Gas generation that replaces coal is assumed to require 7,172 kJ of gas per kWh generated, corresponding to a
heat rate of 6,798 BTU/kWh.)The black line is the new gas supply (for all gas uses) projected by the Energy Information Administration in its
2013 Annual Energy Outlook (Early Release) Reference Scenario.7
(There are approximately 1.1 EJ per trillion cubic feet (TCF) of gas.)
these graphs,then the amount of additional gas supplies

warming impact from substituting gas for coal out to
electricity consumption and/or increased electricity
supply from nuclear, wind, solar, or fossil fuel systems
2
capture and storage to provide some of the
This analysis considered no change in leakage rate or
increase if leakage were reduced and/or natural gas

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