The validation of emission rate estimation methods
Date
2015
Authors
Wells, Bradley, author
Collett, Jeffrey L., advisor
Pierce, Jeffrey, advisor
Ham, Jay, committee member
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Abstract
Oil and natural gas production throughout the United States has been dramatically increasing in recent years, due in large part to hydraulic fracturing processes and horizontal drilling techniques that allow for extraction from unconventional wells. The rise in well drilling and completion activities raises concern over potential air quality impacts on nearby communities. Methane, other volatile organic compounds (VOCs), and nitrogen oxides (NOₓ) may be emitted into the atmosphere during well development and production activities. Methane is a greenhouse gas, VOCs and NOₓ act as ozone precursors, and some VOCs are classified as air toxics. For these reasons, there is a need to accurately quantify the rate of emissions of these gases into the atmosphere from oil and gas development and production. One such emission rate estimation technique is the tracer ratio method (TRM). The TRM requires access to a well site and involves the release of a passive tracer gas as close to the source of emissions as possible. This known emission rate is multiplied by the ratio of the downwind concentrations of emission gas to the tracer gas (both in excess of background) to derive an estimate of the emission gas emission rate. Another technique, recently developed by the Environment Protection Agency, utilizes a simplified point source Gaussian plume (PSG) dispersion model. This approach requires only one mobile downwind measurement location for both concentration and meteorological measurements, without the need for site access; it does not require a tracer gas. In order to evaluate the effectiveness of these techniques, a series of experiments were conducted at Christman airfield in Fort Collins, Colorado. These experiments involved releasing both acetylene, as a tracer gas, and methane (to simulate an emission source) at controlled flow rates to compare the predicted emission rate of methane to its actual emission rate. A vehicle equipped with a PICARRO methane and acetylene analyzer traversed or remained stationary within the gas plume to provide real-time concentration measurements of both gases. A 3-D sonic anemometer was used to characterize local meteorological conditions. The TRM is evaluated using both a mobile transect and a stationary approach. There is an overall positive bias in both cases. Our best results are obtained when sources are co-located during a stationary analysis and changes in background methane concentrations are determined and corrected. In these cases the mean bias is +9% with σ=22% (standard deviation about the mean bias). The separation of tracer and emission gas sources in the mobile transect analysis is the largest cause for uncertainty. The mean bias when sources are separated is +83% (σ=99%), as opposed to transect analyses of co-located sources which have a mean bias of +33% (σ=31%). The PSG technique, which involves a 20 minute stationary analysis, contains more inconsistent results compared to the stationary approach performed by the TRM (mean bias of methane emission rate prediction +34%, σ=123%). Most interesting of note is that for nearly every sample the bias in the prediction of the emission rate of acetylene is more negative than the bias in methane emission rate predictions (mean bias of acetylene emission rate prediction -19%, σ=128%). This suggests possible biases in the acetylene release rate or concentration measurement; however, at this time the issue cannot be located. Regardless, a stationary TRM technique produces the best results, and its use is recommended when site access is available for tracer release.
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Subject
Gaussian plume
emission rates
tracer ratio method