A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver-Julesburg Basin

摘要

Emissions of methane (CH_4) from oil and natural gas (OG) operations in the most densely drilled area of the Denver-Julesburg Basin in Weld County located in northeastern Colorado are estimated for 2 days in May 2012 using aircraft-based CH_4 observations and planetary boundary layer height and ground-based wind profilemeasurements. Total top-down CH_4 emission estimates are 25.8 ± 8.4 and 26.2 ± 10.7 t CH_4/h for the 29 and 31 May flights, respectively. Using inventory data, we estimate the total emissions of CH_4 from non-OG gas-related sources at 7.1 ± 1.7 and 6.3 ± 1.0 t CH_4/h for these 2 days. The difference in emissions is attributed to OG sources in the study region, and their total emission is on average 19.3 ± 6.9 t/h, close to 3 times higher than an hourly emission estimate based on Environmental Protection Agency's Greenhouse Gas Reporting Program data for 2012. We derive top-down emissions estimates for propane, n-butane, i-pentane, n-pentane, and benzene from our total top-down CH_4 emission estimate and the relative hydrocarbon abundances in aircraft-based discrete air samples. Emissions for these five nonmethane hydrocarbons alone total 25.4 ± 8.2 t/h. Assuming that these emissions are solely originating from OG-related activities in the study region, our results show that the state inventory for total volatile organic compounds emitted by OG activities is at least a factor of 2 too low for May 2012. Our top-down emission estimate of benzene emissions from OG operations is 173 ± 64 kg/h, or 7 times larger than in the state inventory.