Using Ambient Concentration Measurements to Quantify Volatile Organic Compound Emissions from Unconventional Oil and Gas Operations.

Oil and gas (O&G) development in the U.S. has accelerated in the past two decades, aided by unconventional extraction techniques. Potential environmental and health impacts of volatile organic compounds (VOCs) originating from O&G activities have raised concerns, but emission estimates remain highly uncertain. This study offers new insights into operation-specific VOC emission rates during unconventional O&G development (UOGD). We utilize dispersion model simulations with a new emission inversion method to analyze four years (2019-2022) of weekly air canister samples, measuring 48 VOCs at 10 monitoring sites in Broomfield, Colorado, where several large multiwell pads were drilled, completed, and entered production during the study period. Emissions are characterized for well drilling, hydraulic fracturing, coiled tubing/millout, flowback, and production operations. Drilling using synthetic drilling muds and coiled tubing operations exhibit the highest NMVOC emission rates, with median values of 2.8 g/s and 1.1 g/s, respectively. NMVOC and benzene emission rates during flowback were 96% and 98% lower, respectively, than previously reported values, highlighting the effectiveness of improved management practices in reducing air pollutant emissions from what used to be often the most significant emission source during UOGD. Our findings provide the first report of VOC emissions from coiled tubing/millout operations and show that the EPA's nonpoint oil and gas emission estimation tool underestimates VOC emissions from drilling mud volatilization and flowback green completions.