American Oil and Gas Reporter - January 2015 - 223

study, the UT researchers say they made
more extensive measurements the following year, and confirmed a majority of
emissions come from a small number of
wells that vent frequently.
Collectively, UT says, liquid unloadings
from those wells are estimated to emit
14.2 Bcf, or 14 percent of methane emissions from the natural gas supply chain.
According to the study, because older
wells typically produce less gas as they
near the end of their life cycles, liquid
unloadings happen more often in those
wells than in newer wells. Researchers
say a key identifier for high-emitting
wells is how many times the wells unload
in a given year.
Plunger lifts are one factor in emission
results, the study says. Emissions for unloading events for wells without plunger
lifts averaged 21-35 Mcf per event, while
wells with plunger lifts averaged 1-10
Mcf for each event. Although emissions
per event were higher for the first category,
the study points out those wells have, on
average, fewer events than wells with
plunger lifts. Wells without plunger lifts
averaged fewer than 10 unloading events
annually, while wells with the lifts averaged more than 200 events a year. Overall,
the study estimates wells with plunger
lifts accounted for 70 percent of emissions
from unloadings nationally.
Researchers say they also found that
the Rocky Mountain region, with its large
number of wells with high frequencies
of unloadings that vent to the atmosphere,
accounts for about half of overall emissions
from liquid unloadings.
For the well testing, researchers say
the unloading of 32 wells sampled without
plunger lifts had durations of less than
five hours, and vented methane volumes
that ranged between 550 and 135,000
standard cubic feet per event. For the 50
plunger lift wells, events lasted less than
four hours and vented methane volumes
that ranged from 200 to 49,000 standard
cubic feet per event.
Annual methane emissions from
plunger lift wells are estimated at 10 Bcf
a year, with 80 percent of those emissions
associated with wells that vent more than
100 times a year, the UT study says. For
wells without plunger lifts, the researchers
estimate that 26,438 wells vent a total of
177,000 times a year, with total emissions
of 4.4 Bcf a year.
Erica Bowman, vice president of research and policy analysis for America's
Natural Gas Alliance, says the two studies
confirm that greater use of natural gas
reduces greenhouse gas emissions.
"Natural gas producers have reduced
methane emissions by 25 percent since

1990, even as production has grown by
37 percent," she says. "The studies show
emissions to be lower than EPA estimates,
and that a small number of sources are

responsible for the majority of emissins.
Both reports are available through the
ACS
Publications
website
at
http://pubs.acs.org.
Ì

Methane Emissions Overstated
In Most Studies, OIPA Claims
OKLAHOMA CITY-A study by the
Oklahoma Independent Petroleum Association challenges many of the assumptions that link pneumatic controllers
used in oil and natural gas production
to high methane emission rates.
The study uses improved quantification methods to assess up-to-date controller sample data, the association says,
and examines controller emissions for
a range of facility characteristics, including age, production type, and applicability to state emission permits.
OIPA says its research finds an average pneumatic controller's emission
rate of 1.05 cubic feet an hour is significantly lower than previous studies,
which estimate emissions five to 27
times the levels found by the association.
"Previous studies greatly overestimated pneumatic controllers' emissions,"
says OIPA Regulatory Committee Chairman Kim Hatfield. "This study used
improved methods to collect and analyze
data, giving regulators a better perspective
on emissions from the oil and natural
gas industry."
Those earlier studies labeled pneumatic controllers as one of the top emission sources for the oil and natural gas
industry, but Hatfield says because of
incorrect previous estimates, the emission
numbers exist only on paper.
OIPA randomly selected 172 oil and
gas production sites from the Oklahoma
assets of eight association members. It
says the sites contain 205 producing
wells and 680 pneumatic controllers. It
points out sample sizes for previous
studies ranged from several dozen to
several hundred controllers. Data collected for each controller included:
* Controller make and model;
* Supply pressure;
* Actuator make and model;
* Actuator physical dimensions; and
* Actuation count over 15 minutes.
OIPA says the data collected were
augmented with manufacturer specifications such as continuous bleed rates
and gas volumes contained within each
actuator.

Assumptions And Calculations
Its study uses assumptions for missing
data and complex emission scenarios,
which OIPA says resulted in conservatively high emissions. The assumption
most influential for calculating emissions
is default actuation frequency, and OIPA
says its study teams assumed controllers
with zero observed actuations over the
15-miunute observation interval underwent actuation once every 15 minutes.
OIPA says its sample contains, on
average, 3.83 intermittent vent controllers
and 0.12 continuous bleed controllers
on each site. On average, it reports, intermittent vent controllers emit 0.40
cubic feet an hour and continuous bleed
controllers emit 21.54 cubic feet an
hour.
The results also follow a pattern seen
in other emission studies, the association
points out: A small number of sources
are responsible for the majority of emissions. OIPA says 24 observed controllers
emitted more than 6.0 cubic feet an
hour, which accounted for 73 percent
of all emissions.
In comparing its work with earlier
emissions studies, OIPA says previous
work used lower counts for intermittent
vent controllers and much higher emission factors than the randomly selected
examples in the OIPA study. For continuous bleed controllers, it says other
studies used higher controller counts
and an emissions factor that was too
high, but of the appropriate magnitude
to represent the sample.
"This study was the first known effort
to investigate pneumatic controller differences between new, old, oil, gas, permitted and permit-exempt sites," the report points out. "The most significant
difference was between controller counts
at old and new sites. New sites had 2.2
more controllers on average because
new sites had more process units. This
characteristic was further emphasized
in the difference of 4.2 controllers per
site between permitted and permit-exempt
sites. The group of permitted sites generally represented the most recently constructed and most complex sites."
Ì
JANUARY 2015 223

American Oil and Gas Reporter - January 2015

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