Estimating Emission Rates from a Hydraulic Fracturing Well using Open-Path Dual-Comb Spectroscopy

Hydraulic fracturing has become a ubiquitous technique for oil and natural gas (ONG) extraction throughout the United States over the past two decades. The method has liberated previously inaccessible ONG reserves throughout the Denver-Julesburg basin, resulting in the Northern Front Range of Colorado becoming one of the largest natural gas producing regions in the US. A key concern arising from this development is the impact that fugitive emissions from ONG infrastructure may be having on regional air quality, ozone production, and atmospheric methane concentrations. Accurately constraining the magnitude of fugitive emissions is therefore critical for improving environmental, public health, and carbon flux models. Unambiguously determining the magnitude and source of (often transient) fugitive emissions in a chemically complex regional environment is a challenge that open-path dual-comb spectroscopy (DCS) is well suited to address. Open-path data provides integrated path concentrations of key natural gas constituents, including methane, ethane, propane, n-butane and iso-butane, with ~1 minute time resolution; these data can then be used in inversion models to identify the location and magnitude of fugitive emissions in the local environment over time. We report results from an open-path mid-infrared DCS case study of a hydraulic fracturing site located in Broomfield, Colorado. The site was observed throughout well development and production from September 2019 to April 2020. Data from the site is used to constrain a Gaussian plume model of site emissions. Transient emissions attributable to the well site reached an estimated maximum rate in excess of several hundred grams/second. Efforts underway include developing a Bayesian framework for constraining model uncertainties, as well as exploring more sophisticated dispersion models, such as AERMOD and STILT, to improve the accuracy of plume transport in the model.