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Thesis Defense

Department of Atmospheric Science

Fri., Oct. 28, 10:30 am, EN6085

Methane Emissions from Production Sites in Dry vs. Wet Natural Gas Fields

Anna Robertson

University or Wyoming


The most recent U.S. greenhouse gas inventory from the Environmental Protection Agency (EPA) ranked the natural gas and petroleum industry as the primary source (33%) of anthropogenic methane emissions. Natural gas, which produces roughly half as much carbon dioxide as coal when burned in a modern power plant, has been proposed as a transitional, less carbon intensive energy source. However, atmospheric methane emissions along the natural gas supply chain can erode the climatic benefits of burning natural gas over other fossil fuels. This is because methane, the primary component of natural gas, has a 20-year global warming potential approximately 86 times greater than carbon dioxide. Emissions from unconventionally drilled (horizontal drilling and hydraulic fracturing) well pads have received added attention because unconventional drilling is the primary driver of the dramatic increase in U.S. production of natural gas over the last decade.
Large variations in atmospheric methane emissions from oil and gas operations (relative to production) have been observed in various regions of the country, but the cause of these variations remains uncertain, making implementation of effective control strategies difficult. In this study, methane emissions from production well pads were quantified using the Environmental Protection Agency’s (EPA) Other Test Method (OTM) 33a. With this method, measured methane concentrations are combined with Gaussian plume dispersion parameters and concurrent meteorological conditions to obtain a flux estimate. Measurements in this study were performed at production sites (well pads) in four major U.S. oil and gas basins: 1.) Upper Green River (UGR) Basin in Wyoming, 2.) the Denver-Julesburg (DJ) Basin in Colorado, 3.) the Uintah Basin in Utah, and 4.) the central Fayetteville (FV) gas play in Arkansas' Arkoma Basin. These basins range from wet gas basins that produce significant amounts of oil or liquid condensate (DJ, Uintah, UGR) to dry basins that produce virtually none (Fayetteville). Mass averages (MA) of methane emissions as a percent of production are computed for each basin, which give a rough estimate of basin-wide emissions from well pads. The median MA emissions for each of the four basins ranges from 0.09% in FV to 3.09% in Uintah, revealing the large variability basin-to-basin.  The relationship of leakage rate to how wet or dry a basin is will be discussed along with other potential explanations for the variation in leakage rate.

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