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Browsing by Author "Zimmerle, Daniel J., advisor"

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    Using above-ground downwind methane and meteorological measurements to estimate the below-ground leak rate of a natural gas pipeline
    (Colorado State University. Libraries, 2023) Cheptonui, Fancy, author; Riddick, Stuart N., advisor; Zimmerle, Daniel J., advisor; Fischer, Emily, committee member
    Natural gas (NG) leaks from below-ground pipelines present a safety, economic, and environmental hazard, and triaging the severity of leaks remains a significant issue for pipeline operators. Typically, operators conduct walking surveys using hand-held methane (CH4) detectors which output CH4 concentrations to indicate the location of a leak, but quantification often requires excavation of the pipeline. Industry-standard CH4 detectors are lower-cost and have a higher detection threshold and lower precision than optical-cavity CH4 analyzers typically used to quantify emissions. It remains unclear whether coarser CH4 concentration measurements could be used to identify the large leaks that require immediate response. To explore the utility of industry-standard detectors, above-ground downwind CH4 concentration measurements made by the detectors as input to a novel modeling framework, the ESCAPE-1 model were used to estimate the leak rates from below-ground NG pipelines. Controlled below-ground emission experiments were conducted to test this approach over a range of environmental conditions. Using 10-minute averaged CH4 mixing/meteorological data and filtering out low wind/Pasquill Gifford Stability Class (PGSC) A events, the ESCAPE-1 model estimates small distribution leaks (0.2 kg CH4 h-1) to within -31 to +75% (95% CI), and medium distribution leaks (0.8 kg CH4 h-1) to within -73 to +92%(95% CI) of the actual leak rate. When averaged over a longer period (more than 3 hours of data), the average calculated leak rate was an overestimate of 55% for the small (0.2 kg CH4 h-1) leak and an underestimate of 6% for a medium distribution leak (0.8 kg CH4 h-1). Results suggest that as the wind speed increases, or the atmosphere becomes more stable both accuracy and precision of the leak rate calculated by the ESCAPE-1 model decreases. This is likely the result of a trade-off between the high enough wind to move the gas but not high enough that the plume becomes collimated and less homogenous. Optimizing this approach for oil and gas industry applications, this study suggests that CH4 mixing ratios measured by industry-standard CH4 detectors lasting at least 3 hours could be used as a guide to prioritize NG leak repair by estimating the below-ground leak rate from a pipeline within reasonable uncertainty bounds (±55%) in favorable atmospheric conditions.
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    Using controlled subsurface releases to investigate the effect of leak variation on above-ground natural gas detection
    (Colorado State University. Libraries, 2023) Mbua, Mercy W., author; Riddixk, Stuart N., advisor; Zimmerle, Daniel J., advisor; Fischer, Joseph von, committee member
    Leaks from underground natural gas (NG) pipelines pose safety and environmental concerns. Pipeline leak detection generally relies on measuring surface methane (CH4) enhancements during walking surveys and/or mobile surveys that attempt to identify CH4 plumes downwind of the pipeline. The likelihood of plume detection is dependent on the above-ground CH4 plume width. The size and shape of the plume is primarily dependent on environmental conditions but could also be complicated by leak characteristics. To investigate the effect of leak characteristics on CH4 plume width, this study uses controlled release experiments to observe above-ground plume width changes with changes in the gas composition, leak rate, and leak depth. Results show that plume width generally decreases with increased NG density, decreased leak rate and increases with depth between 0.6 and 0.9 m, but the above surface plume is undetectable above the background for leaks 1.8 m deep. The study established that the effect of adding heavy hydrocarbons to the NG mixture on plume width is equivalent to the effect of increased leak rate and depth on plume width multiplied by -0.04 and -0.89, respectively, with overall relative uncertainty of -42/ +14 %. This shows that reported leaks in areas with heavier hydrocarbons could currently be missed or underestimated. Further, this study shows that leaks from pipelines laid in covers meeting the Colorado Oil and Gas Conservation Commission minimum depth requirement of 0.9 m could be easier to detect compared to those buried at depths less than the minimum depth. Applying the findings to a real-world scenario, the study illustrates that a successful leak survey protocol tuned to NG leaks from Fayetteville shale (0.66 g/L NG density) may result in missed detections in the Permian, where NG is heavier (1.01 g/L) due to higher percentages of heavy hydrocarbons. Overall, this study illustrates that leak survey protocols for flowlines and gathering lines should be different from distribution pipelines and tailored to the compositions of the transported NG to report emissions accurately.