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factors are referred to as “potential emission factors” because they do not represent natural gas
that is directly released to the atmosphere, but they represent the volume of natural gas that can
be sent to flares and other environmental control equipment. NETL uses a potential emission
factor of 9,000 Mcf of natural gas per each episode of shale gas hydraulic fracturing, and a
potential emission factor of 3.6 Mcf of natural gas per each episode of liquids unloading (with 31
liquids unloading episodes per well-year). NETL’s model augments potential emission factors
with flaring, thereby reducing the amount of methane that is released to the atmosphere. These
emission factors are consistent with the findings of a survey jointly conducted by API and
America’s Natural Gas Alliance and released in September 2012.
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They also match the factors
used by EPA’s 2013 GHG inventory.
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NETL’s current model accounts for liquids unloading emissions from conventional wells,
but does not account for liquids unloading from unconventional wells. Applying liquids
unloading to the unconventional wells in this analysis increases the life cycle GHGs by 0.6% for
LNG export scenarios (using 100-year GWPs as stated in the IPCC Fifth Assessment Report).
This 0.6% was estimated by assigning the liquid unloading emissions from onshore conventional
natural gas to the upstream results for Marcellus Shale natural gas, followed by an expansion of
the boundaries to a life cycle context. Simply put, liquids unloading accounts for 11% of the
upstream GHG emissions from conventional onshore natural gas.
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When liquids unloading is
added to unconventional natural gas in the LCA model, it is scaled according to the unique
production rates and flaring practices of unconventional wells in addition to the subsequent flows
of natural gas processing, liquefaction, ocean transport, regasification, power plant operations,
260
Characterizing Pivotal Sources of Methane Emissions from Natural Gas Production: Summary and Analysis of
API and ANGA Survey Responses. Final Report (Sept. 21, 2012).
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U.S. Envtl. Prot. Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011 (Apr. 12, 2013).
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See NETL, Life Cycle Analysis of Natural Gas Extraction and Power Generation.
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and electricity transmission. Thus, while liquids unloading may account for a significant share
of upstream GHG emissions, none of the LCA GHG Report’s conclusions would change with
the addition of liquids unloading to unconventional natural gas extraction.
The potential emissions from unconventional well completions are modeled as 9,000 Mcf
of natural gas per episode. It is important to remember that this factor does not represent
methane emissions directly released to the atmosphere, but the flow of natural gas prior to
environmental controls. For unconventional natural gas, NETL’s model flares 15% of these
potential emissions (flaring converts methane to CO
2,
thus reducing the GWP of the gas) and
apportions all completion emissions to a unit of natural gas by dividing them by lifetime well
production (completion emissions occur as one-time episode that must be converted to a life
cycle basis by amortizing them over total lifetime production of a well). Further, the life cycle
GHG contributions from well completions are diluted when scaled to the subsequent flows of
natural gas processing, liquefaction, ocean transport, regasification, power plant operations, and
electricity transmission. However, in NETL’s model, life cycle completion emissions are
directly affected by the estimated ultimate recovery (EUR) of a well because the total amount of
natural gas produced by a well is used as a basis for apportioning completion and other one-time
emissions to a unit of natural gas produced. From an engineering perspective, wells with high
EURs are more likely to have a high initial reservoir pressure that increases the potential
completion emissions. A reasonable uncertainty range around the potential emissions from
unconventional completion emissions (9,000 Mcf/episode) is -30% to +50% (6,100 to 13,600
Mcf/episode). This uncertainty range matches the scale of uncertainty around the Marcellus
Shale EUR used in the LCA GHG Report (see Table 5-4 of the LCA GHG Report). This -30%
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to +50% uncertainty around potential emissions from unconventional completions causes a -2%
to 3% uncertainty around life cycle GHG emissions for the export scenarios of this analysis.
The New Source Performance Standards (NSPS) rules for the oil and natural gas sector,
which EPA amended in a final rule published on June 3, 2016,
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will achieve significant
methane emission reductions primarily by requiring all new or modified wells to capture and
control potential emissions of VOCs during natural gas well completion. In addition to well
completion emissions, the NSPS rules target other point sources of VOC emissions from new
and modified sources at natural gas extraction and processing sites, but they do not address
liquids unloading.
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The LCA GHG Report does not account for the potential effects of the
NSPS rules on natural gas emissions because the scope of the LCA accounts for GHG emissions
from natural gas being produced today. EPA’s Regulatory Impact Analysis estimated that the
final NSPS rule would reduce annual methane emissions in 2015 by 18 million metric tons,
meaning that this rule will have the effect of reducing life cycle emissions from natural gas
systems as new wells are developed and existing wells are modified. The likely effects of the
NSPS rule therefore suggest that the conclusions of the LCA GHG Report are conservative with
respect to the life cycle GHG emissions of natural gas produced in the United States.
Sierra Club contends that NETL’s documentation, including the 200-page supporting
LCA document, does not clearly cite EPA’s Subpart W document. NETL’s Report has three
references to Subpart W, cited as EPA 2011a, 2011b, and 2011c. These three references should
263
U.S. Envtl. Prot. Agency, Oil and Natural Gas Sector: Emission Standards for New, Reconstructed, and
Modified Sources; Final Rule (40 C.F.R. Part 60), 81 Fed. Reg. 35,824 (June 3, 2016); available at:
https://www.gpo.gov/fdsys/pkg/FR-2016-06-03/pdf/2016-11971.pdf.
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U.S. Envtl. Prot. Agency, Oil and Natural Gas Sector: New Source Performance Standards and National
Emission Standards for Hazardous Air Pollutants Reviews (40 C.F.R. Part 63) (Apr. 17, 2012); available at:
http://www.epa.gov/airquality/oilandgas/pdfs/20120417finalrule.pdf.
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