Characterizing emissions from natural gas drilling and well completion operations in Garfield County, CO

This study was designed to characterize and quantify emission rates and dispersion of air toxics, ozone precursors, and greenhouse gases from unconventional natural gas well development activities in Garfield County, CO, located on top of a geological formation known as the Piceance Basin. Particular focus was placed on quantifying emissions of individual volatile organic compounds (VOCs) and methane during well drilling, hydraulic fracturing ("fracking"), and flowback. While some prior studies have measured VOC or methane concentrations near well development 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. Emission rates were determined using a tracer ratio method (TRM). In this method, the rate of emission 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 studied well pads. Real-time methane and acetylene concentrations and three minute integrated whole air sample canisters for VOC analysis were collected downwind of the release location. Meteorological data were collected at two heights (3 m and 10 m) near the well pad. Upwind acetylene, methane, and VOC concentrations were determined for background correction. The canisters were analyzed for a large suite of VOCs using gas chromatography with flame ionization detection. The study results provide novel information concerning emissions from natural gas drilling and completion activities in Garfield County, CO and are some of the first measurements of this type in any U.S. basin. Overall, 21 emission experiments were conducted from 2013-2015. Several sets of 2 to 5 canisters were collected at different times during each experiment, in addition to an upwind background sample per experiment. Using the TRM, each canister in the plume provides an independent measure of VOC emission rates. 28-48 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 and individual VOCs are calculated and reported.The emission rates and field observations were used to conduct air dispersion (using the EPA's AERMOD model) simulations to: (1) evaluate AERMOD's accuracy in predicting observed, near-field dispersion of VOCs in Garfield County, CO and (2) predict concentration fields, as a function of emission rate, for dispersion of a hypothetical compound under a range of local meteorological conditions at a site with terrain similar to that observed in Garfield County. While not perfectly designed for prediction of the short-term concentration fields measured in the 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 well development activities in Garfield County.

本研究旨在表征并量化美国科罗拉多州加菲尔德县（该区域位于被称为皮斯安斯盆地的地质构造之上）非常规天然气井开发活动所产生的空气有毒污染物、臭氧前体物以及温室气体的排放速率与扩散特征。研究重点聚焦于井钻井、水力压裂（又称"压裂"）以及返排阶段中，各类挥发性有机物（VOCs）与甲烷的排放速率量化工作。尽管此前已有部分研究对井开发作业附近的VOC或甲烷浓度进行了测量，但环境浓度不仅显著取决于排放速率，还与采样点位及气象条件密切相关——这些因素会极大影响污染物的下风侧扩散与稀释过程。通过直接表征排放速率，本研究结果可用于在广泛的气象条件下，预测任意目标区域的下风侧浓度场。本研究采用示踪剂比率法（TRM）确定排放速率：该方法通过已知示踪剂的排放速率，乘以目标化合物经本底校正后的浓度与示踪剂浓度的比值，即可得到目标化合物的排放速率（例如苯的排放速率，单位为克每秒）。研究选取乙炔作为示踪气体，并将其与研究井场的主要排放源同址进行受控释放。在示踪剂释放位置的下风侧，采集了实时甲烷与乙炔浓度数据，以及用于VOC分析的3分钟集成式全空气采样罐。在井场附近的两个高度（3米与10米）采集了气象数据；同时测定了上风向的乙炔、甲烷与VOC浓度，用于本底校正。采用带火焰离子化检测器的气相色谱法，对采样罐内的多种VOCs进行了分析。本研究结果提供了有关美国科罗拉多州加菲尔德县天然气钻井与完井活动排放特征的全新信息，亦是美国境内所有盆地中此类实测研究的首批成果之一。整体而言，研究团队于2013年至2015年间共开展了21次排放实验。每次实验期间，除采集一次上风向本底样品外，还在不同时段采集了多组（每组2至5个）采样罐。通过示踪剂比率法，污染物羽流中的每个采样罐均可独立计算VOC的排放速率。每个采样罐可检测到28至48种VOCs，同时附带每次实验期间采集的实时甲烷与乙炔数据。基于示踪剂比率法，可计算并报告甲烷与各类VOCs的排放速率。本研究将排放速率与现场观测数据相结合，采用美国环境保护署（EPA）的AERMOD模型开展大气扩散模拟，以实现两大目标：(1) 评估AERMOD模型在预测科罗拉多州加菲尔德县近场VOC扩散观测结果时的准确性；(2) 针对地形与加菲尔德县相似的区域，模拟在一系列局地气象条件下，以排放速率为变量的假设性化合物扩散浓度场。尽管AERMOD模型并非专为预测本研究中测得的短期浓度场而设计，但其在多次野外实验中对扩散范围的预测效果较为合理。此外，本研究基于活动类型确定的排放速率区间，可广泛应用于未来各类使用AERMOD或其他模型的模拟工作，以模拟与加菲尔德县井开发活动相关的潜在局地健康与空气质量影响相关的下风侧浓度场。