North Front Range oil and gas air pollution emission and dispersion study
Collett, Jeffrey L.
Improved unconventional oil and natural gas extraction methods have facilitated the development of these resources in several areas, including the northern Front Range of Colorado. Increased activity has spurred questions concerning possible air pollutant emissions. Processes associated with oil and gas extraction have been identified as emitting a variety of air pollutants, but observations of the rates and types of compounds emitted are limited. This is especially true for emissions during completion (hydraulic fracturing and flowback) of new wells, activities which have not been closely examined for emission of atmospheric pollutants, but additional information is also needed for oil and gas production sites which have long operational lifetimes. This study was designed to characterize and quantify emission rates and dispersion of air toxics, ozone precursors, and greenhouse gases from oil and gas operations in the Denver-Julesburg Basin on the northern Front Range of Colorado. Based on a review of critical knowledge gaps and input from a study Technical Advisory Panel, particular focus was placed on quantifying emissions of individual volatile organic compounds (VOCs), methane, and ethane from oil and gas production sites and from hydraulic fracturing ("fracking") and flowback, important steps in the completion of new wells. Four oil and gas production companies were recruited to participate in the study and provided access to field operations for emission measurements. While some prior studies have measured VOC, ethane, or methane concentrations near oil and gas operations, ambient concentrations are strongly dependent not only on emission rates but also on sampling location and meteorological conditions, which greatly affect downwind dispersion and dilution. By characterizing emission rates directly, results from this study can be used to predict downwind concentration fields for any location of interest under a wide range of weather conditions. By using a similar measurement approach, this study was designed to complement a parallel effort examining methane, ethane, and VOC emission rates from drilling and completion of natural gas wells in the Piceance Basin in Garfield County, Colorado. Emission rates were determined using a tracer ratio method (TRM). In this method, the emission rate of a compound of interest (e.g., g s-1 of benzene) is determined as the product of a known tracer emission rate multiplied by the ratio of the background-corrected concentrations of the compound of interest and the tracer. Acetylene was selected as a tracer gas and its controlled release co-located with the main source of emissions on study sites. Real-time methane and acetylene concentrations and three-minute integrated whole air sample canisters for VOC and ethane analysis were collected downwind of the release location. Meteorological data were collected at two heights (3 m and 10 m) near the activity under study. Upwind acetylene, methane, ethane, and VOC concentrations were determined for background correction. The canisters were analyzed for ethane and a large suite of VOCs using gas chromatography with flame ionization detection. The study results provide novel information concerning emissions from oil and natural gas production and completion activities in the northern Front Range of Colorado. Overall, 18 emission experiments were conducted from 2014-2016. Several sets of canisters were collected at different times during each experiment, in addition to upwind background samples. Using the TRM, each canister in the plume provides an independent measure of ethane and VOC emission rates. Ethane and 47 VOCs are reported for each canister, along with real-time methane and acetylene data collected during each experiment. Using the TRM, the emission rates of methane, ethane, and individual VOCs are calculated and reported. Methane, ethane, and propane were the most abundant constituents in measured emissions. Generally, higher rates of VOC, ethane, and methane emissions were observed during flowback operations, although a wide range of emissions was observed for each type of activity studied. Methane emission rates were examined as a percentage of produced natural gas at the diverse array of production sites included in the experiment. These included large and small sites (between 1 and 18 horizontal and/or vertical wells) with a variety of different separation schemes. A positive relationship was observed with gas production rate; median and mean methane emissions measured across all production sites were 0.23% and 0.37%, respectively, with the 95th percentile of emissions at 1.03%. The emission rates and field observations were used to conduct air dispersion simulations (using EPA's AERMOD model) to: (1) evaluate AERMOD's accuracy in predicting observed, near-field dispersion of ethane and VOCs in the Colorado Front Range and (2) predict concentration fields, as a function of emission rate, for dispersion of benzene under a range of local meteorological conditions at a site with terrain similar to that observed in the Front Range of Colorado. While not perfectly designed for prediction of the short-term concentration fields measured in this study, AERMOD did a reasonable job predicting the observed extent of dispersion across several field experiments. Moreover, emission rate ranges determined by activity type in this study can be used in a wide range of future simulations with AERMOD or other models to simulate downwind concentration fields relevant to understanding potential local health and air quality impacts associated with oil and gas well completion and production activities on the northern Front Range.
PI: Professor Jeffrey L. Collett Jr.
Department of Atmospheric Science
Department of Atmospheric Science