Document ID: EPA-HQ-OAR-2009-0923-0086
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2010-06-03T04:00Z

Date: May 27, 2010

Listed below is further information about methods used by the U.S.
Inventory of Greenhouse Gas Emissions and Sinks as well as Natural Gas
STAR presentations and documents that cite inventory information.

Question:  Can you elaborate on the reference for methane emissions pie
charts used in Natural Gas STAR Presentations?

Response:  Consult the Energy section (Section 3) and Sections 3.4 and
3.5 of the Annexes in the 2007 U.S. Inventory found at   HYPERLINK
"http://epa.gov/climatechange/emissions/usgginv_archive.html" 
epa.gov/climatechange/emissions/usgginv_archive.html .  The individual
source categories and their methane emissions begin on page A-143 for
natural gas and page A-151 for petroleum in Annex 3.  The Annex more
thoroughly covers the methodology used to arrive at the results in
Section 3.

Question:  Why are REC reductions greater than the entire inventory
number for well completions?

Response:  Presently, REC reductions reported in the Natural Gas STAR
body of work is larger than well completion venting in the inventory on
an annual basis.  Section 3 of the inventory discusses improvements and
future plans for each sector.  For the natural gas industry, the planned
improvements are discussed on pg. 3-46.  For the petroleum industry, the
planned improvements are discussed on pg. 3-51.

In a New York Times article published on October 15, 2009, it is
mentioned that the EPA is currently reviewing and revising methane
emissions from U.S. gas wells.  As a result, the EPA found that the
amount emitted by routine operations at gas wells is approximately 12
times larger than the existing estimate of 9 Bcf.  The NYT article can
be found here at   HYPERLINK
"http://www.nytimes.com/2009/10/15/business/energy-environment/15degrees
.html#" 
www.nytimes.com/2009/10/15/business/energy-environment/15degrees.html# .

EPA has included these revised emission numbers in the upcoming
Mandatory Reporting Rule (MRR).  Specifically, these new numbers are in
the Technical Support Document (TSD), explained in Appendix B (pgs.
79-85).  On page 82, the new 2007 U.S. emissions from well completion
and workover venting is approximately 120 Bcf.  The TSD can be found
here at   HYPERLINK
"http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-02
2310.pdf" 
www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-022310.pdf
.

Question: Why are Natural Gas STAR methane emissions reductions
accounted for entirely in the natural gas systems sector of the
inventory and not at all in the petroleum systems sector?

Response:  The methodology in Annex 3 of the inventory indicates how
Natural Gas STAR reductions are applied to the natural gas systems
sector.

Natural Gas STAR considers data at the level of the production sector,
rather than further dividing its reporting into natural gas and
petroleum.  At the same time, the inventory considers data at the level
of natural gas systems production & petroleum systems production.  Often
a Natural Gas STAR reduction activity straddles both natural gas systems
and petroleum systems and is therefore inherently an overall production
sector activity.  The Natural Gas STAR production sector reductions are
not further deconstructed and are thus applied to natural gas systems
production sector.  For example, vapor recovery units (VRUs) can be
installed on both condensate (gas) and storage (oil) tanks, but it is
not a Natural Gas STAR reporting requirement to specify which.

Question:  How can natural gas and petroleum systems methane emissions
be expressed as a percentage of total gas throughput entering and
exiting the national supply chain?

Response:  This percentage is not a published statistic but can be
roughly calculated from public sources such as the inventory and such as
EIA gas production data.  It is also important to understand numerical
treatment of gas imports and exports, NGL imports and exports, gas
reinjected, gas consumed on site, gas unaccounted for due to intrinsic
metering errors, and distinguishing between methane versus natural gas
data, which all have first order effects on the resulting percentage. 
Any result using this calculation method would not apply to any specific
site given the top-down nature of the calculation.

To estimate the percentage of gas lost from both the oil and gas
industry, two public sources are used: 1) information from the Energy
Information Administration (EIA) and 2) the Technical Support Document
(TSD) for the MRR.  The equation shown below illustrates how the
percentage can be calculated.

		

Where:

%gaslost	=	Estimated percentage lost from produced gas due to methane
emissions

ETotal,2006  =		2006 revised methane emissions for petroleum and gas,
all sectors (with reductions), from TSD which includes revised
estimation methods for well completion emissions

WGross,2006  =	EIA 2006 gross gas withdrawals	

EProduction,2006  =	2006 revised methane emissions for petroleum and gas
production (with reductions) from TSD which includes revised estimation
methods for well completion emissions

The term EProduction,2006 in the denominator of the above equation was
included because the gross withdrawals, WGross,2006 does not account for
the original amount of gas produced from a given well.  This is because
WGross,2006 is based on meter readings which typically occur as the gas
exits the production site, after it has encountered all production
equipment that contribute to method emissions.  Therefore, the emissions
from the production sector must be added back in to truly have the
entire amount of gas withdrawn from wells.

Both revised TSD terms in the equation include Natural Gas STAR
reductions and were calculated from consulting page 9 of the TSD and
subtracting out the 2006 reductions listed in Table A-123 on page A-151
of the 2008 U.S. Inventory Annex 3.  A summary table is shown below and
gives the value of each equation term along with the calculated
percentage of gas lost in the petroleum and gas industry due to methane
emissions.

Determination of Gas Lost Due to Methane Emissions in the Oil and Gas
Industries

Equation Term	Calculation of Term	MMtCO2e

ETotal,2006	257.2 MMtCO2e from gas1 + 26.3 MMtCO2e from oil1 – 45.8
MMtCO2e NGS reductions2	237.7

WGross,2006	23,535,018 MMcf3 converted to 9519.0 MMtCO2e	9519.0

EProduction,2006	0.66(257.2 MMtCO2e)4 + 0.97(26.3 MMtCO2e)4 – 30.8
MMtCO2e NGS production reductions5	164.5

 	 	 

	Percentage of gas lost	2.45%

1MRR. Technical Support Document. page 9.   HYPERLINK
"http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-02
2310.pdf" 
www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-022310.pdf

2EPA.  Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008. 
Annex 3: Section 3.4.  Table A-123: CH4 reductions derived from the
Natural Gas STAR Program   HYPERLINK
"http://www.epa.gov/climatechange/emissions/usinventoryreport.html" 
http://www.epa.gov/climatechange/emissions/usinventoryreport.html 

2,182 Gg x 109g/Gg x 21g CO2/g CH4 x Mg/106g x MMtCO2e/106Mg = 45.8
MMtCO2e

3EIA website:   HYPERLINK
"http://www.eia.doe.gov/dnav/ng/ng_prod_sum_dcu_NUS_a.htm" 
www.eia.doe.gov/dnav/ng/ng_prod_sum_dcu_NUS_a.htm 

23,535,018 MMcf x 106cf/MMcf x 19.26g/cf x 21g CO2/g CH4 x Mg/106g x
MMtCO2e/106Mg = 9519.0 MMtCO2e

4 MRR. Technical Support Document. page 9.   HYPERLINK
"http://www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-02
2310.pdf" 
www.epa.gov/climatechange/emissions/downloads10/Subpart-W_TSD-022310.pdf

-Gas production emissions account for 66% of total methane emissions
from gas industry = 169.8 MMtCO2e

-Oil production emissions account for 97% of total methane emissions
from oil industry = 25.5 MMtCO2e

5EPA.  Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008. 
Annex 3: Section 3.4.  Table A-123: CH4 reductions derived from the
Natural Gas STAR Program   HYPERLINK
"http://www.epa.gov/climatechange/emissions/usinventoryreport.html" 
http://www.epa.gov/climatechange/emissions/usinventoryreport.html 

1,469 Gg x 109g/Gg x 21g CO2/g CH4 x Mg/106g x MMtCO2e/106Mg = 30.8
MMtCO2e

Notes:

MMtCO2e = million metric tonnes carbon dioxide equivalent

MMcf = million cubic feet

Gg = gigagrams

Mg = megagrams

Question:  Are changes/updates to the national inventory tracked every
year?  Why would previous year results change?

Response:  Yes, for each annual inventory cycle, the entire time series
is considered and updated.  The entire time series of 1990 to the
present reporting year is updated annually because revised activity data
is applied as it is available.  It is not uncommon for EIA or other data
sources to issue revised estimates for years in the recent past, and
such revisions are tracked by the inventory.  Any changes in the data
sources used are be noted in the executive summary and introduction
sections of the inventory.

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