Methane Emissions from Process Equipment at Natural Gas Production Sites in the United States: Liquid Unloadings

Methane
emissions from liquid unloadings were measured at 107 wells
in natural gas production regions throughout the United States. Liquid
unloadings clear wells of accumulated liquids to increase production,
employing a variety of liquid lifting mechanisms. In this work, wells
with and without plunger lifts were sampled. Most wells without plunger
lifts unload less than 10 times per year with emissions averaging
21 000–35 000 scf methane (0.4–0.7 Mg)
per event (95% confidence limits of 10 000–50 000
scf/event). For wells with plunger lifts, emissions averaged 1000–10 000
scf methane (0.02–0.2 Mg) per event (95% confidence limits
of 500–12 000 scf/event). Some wells with plunger lifts
are automatically triggered and unload thousands of times per year
and these wells account for the majority of the emissions from all
wells with liquid unloadings. If the data collected in this work are
assumed to be representative of national populations, the data suggest
that the central estimate of emissions from unloadings (270 Gg/yr,
95% confidence range of 190–400 Gg) are within a few percent
of the emissions estimated in the EPA 2012 Greenhouse Gas National
Emission Inventory (released in 2014), with emissions dominated by
wells with high frequencies of unloadings.