April 2017
Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2015:
Update to Storage Segment Emissions
Incorporating an estimate for the Aliso Canyon Leak
In fall of 2015, a well in a California storage field began leaking methane at an initial average rate of
around 50 metric tons (mt) of methane (CH4) an hour, and continued leaking until it was permanently
sealed in February of 2016.1 After considering approaches and stakeholder feedback, the EPA included
this emission source in its 2017 GHGI (which includes estimates from 1990-2015).2
The EPA applied the California Air Resources Board (ARB) estimate of the methane release from the
leak.3
ARB calculated emissions for two phases of the leak. The first phase corresponds roughly to the first
month of the leak (late October-late November 2015), a time of heightened activity at the well. ARB's
estimate for emissions in Phase I is 48,450 {+/- 8,810) metric tons of methane. Phase II covers the
remainder of the leak period (around 2.5 months), when less site-level activity was occurring. ARB's
estimate for emissions in Phase II is 51,200 {+/- 2,970) metric tons of methane. The resulting total
estimate of emissions for the duration of the leak is 99,650 {+/- 9,300) metric tons of methane.
For the 2015 estimate in the 2017 GHGI, the EPA calculated emissions from Aliso Canyon as the sum of
Phase I emissions (48,444 metric tons) and the portion of Phase II emissions that occurred in 2015.
Table 1. Use of ARB leak calculations to develop 2015 estimate for U.S. GHGI4
Phase
Measurement
Leak Rate
Assumed number
Leaked methane
Flight Date
(mt CH4 per
of days at this leak
for this period
hour)
rate
(mt CH4)
Phase 1
Three flights over
11/7-11/28/2015
46.9-60.4
37.0 (10/23-11/27)
48,445
Phase II 2015
11/28/2015
54.7
3.0 (11/29-12/1)
3,938
12/4/2015
48.8
7.0 (12/2-12/8)
8,198
12/12/2015
38.0
9.5 (12/9-12/18)
8,664
12/23/2015
28.1
13.5 (12/18-12/31)
9,104
1 For more information, please see Ensuring Safe and Reliable Underground Natural Gas Storage, Final Report of
the Interagency Task Force on Natural Gas Storage Safety, available at
httpi//www.energv.gov/sites/prod/files/2016/10/f33/Ensuring%20Safe%20and%20Reliable%20Underground%20
Natural%20Gas%20Storage%20-%20Final%20Report.pdf.
2 In this memo, "2016 GHGI" refers to the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990- 2014,
published April 15, 2016, and "2017 GHGI" refers to the Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2015, to be published by April 15, 2017.
3 For more information on the leak measurements and calculations, please see California Air Resources Board.
Determination of Total Methane Emission from the Aliso Canyon Natural Gas Leak Incident. October 21, 2016.
https://www.arb.ca.gov/research/aliso canvon/aliso canyon methane emissions-arb final.pdf.
4 Adapted from California Air Resources Board. Determination of Total Methane Emission from the Aliso Canyon
Natural Gas Leak Incident. October 21, 2016. Table 4: Methane Emission Estimates of the Aliso Canyon Natural Gas
Leak.
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April 2017
Phase II 2015
29,905
total
emissions
Sum of Phase
78,350
1 and Phase II
2015
emissions
Phase II 2016
1/8/2016
22.7
10.0 (1/1 -1/10)
5,448
1/12/2016
20.7
6.5 (1/11-1/17)
3,229
1/21/2016
19.1
7.0 (1/17-1/24)
3,208
1/26/2016
19.0
7.0 (1/24-1/31)
3,192
2/4/2016
22.5
11.5 (1/31-2/11)
6,210
Phase II 2016
21,288
total
emissions
The EPA applied this estimate of 78,350 metric tons of methane to its estimate for 2015. This resulted in
a total 2015 storage well emission estimate of 92,590 metric tons of CH4 or 2.3 MMT C02e (using AR45
GWP of CH4, consistent with GHGI), and total transmission and storage segment emissions of 42 MMT
C02e, compared with the 2014 estimate of 40 MMT C02e.
Table 2. Update to storage well emissions estimate in the 2017 GHGI, mt CH4
1990
2000
2005
2010
2014
2015
Storage wells
13,565
15,433
14,910
14,772
15,714
92,590
Storage well emission estimates in the 2018 GHGI
The EPA plans to include in the 2018 GHGI both the estimate of the Aliso Canyon leak for 2015, and an
estimate for the portion of the leak that occurred in 2016. It is estimated that 21,288 (99,638 total
minus 78,350 in 2015) metric tons of methane were emitted from the Aliso Canyon leak in 2016. This
value would be applied to the 2016 estimate using the same approach selected for the 2015 estimate.
January 2017 request for stakeholder feedback
The EPA initially sought feedback on the questions below in the stakeholder version of this memo
released January 2017. The EPA continues to welcome stakeholder feedback on these questions for
potential updates to future GHG inventories.
1. The EPA seeks feedback on including this source under the storage well category.
5 AR4 refers to the Fourth Assessment Report by the Intergovernmental Panel on Climate Change. Climate Change
2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change [Core Writing Team, Pachauri, R.K. and Reisinger, A. (eds)]. IPCC,
Geneva, Switzerland, 2007.
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April 2017
2. The EPA seeks feedback on whether adjustments to the estimate are needed to take into
account other sources of methane potentially included in the measured data (calculated to be
<0.1% of total).
The measurement results from studies conducted to assess the well leak at Aliso Canyon also
included emissions from other sources at the storage site. In addition to other storage wells,
the Aliso Canyon site had various CH4-emitting equipment. The data reported to GHGRP from
the Aliso Canyon site included equipment counts of 100 pneumatic controllers (intermittent
bleed), 10 reciprocating and 3 centrifugal compressors, and meters, valves and connectors.6
Adding the estimate from the Aliso leak measurements to the existing storage well estimate
would result in some minor double counting, for the fraction of emissions that are likely
included in the ARB leak estimate.
The EPA has assessed 2015 GHGRP subpart W data reported by the facility to develop an
estimate of other emissions occurring at the time of the leak. Total GHGRP CH4 emissions
reported by the facility in 2015 were 1360 metric tons.7 For the duration of the leak, no
compressors were used as no natural gas was being injected into the storage field. Deducting
the emissions estimates for compressors (1065 metric tons of CH4) from the total reported to
GHGRP results in emissions of 295 metric tons of CH4, and includes emissions from meters,
valves and connectors. Apportioning these GHGRP-reported non-compressor CH4 emissions to
the 70 days of the leak in 2015 results in 57 metric tons (295 x 70 / 365).
The EPA could reduce the Aliso Canyon estimate by 57 tons, assuming that those tons are
accounted for in the other estimates in the GHGI, or could make no adjustment, due to the small
size of the emissions (calculated to be <0.1% of the total) and the uncertainty range of the Aliso
leak. The EPA seeks comment on these approaches.
6 The GHGRP subpart W data used in the analyses discussed in this memorandum are those reported to the EPA as
of August 13, 2016.
7 The facility's actual 2015 emissions were higher than those reported to the GHGRP. The GHGRP provides well-
vetted average emission factors to calculate emissions. The facility followed these GHGRP calculation methods for
2015 reporting. Actual emissions differ from those reported due to the Aliso Canyon leak event.
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