Volatile Organic Compound Emissions from Natural Gas Facilities in the Denver-Julesburg Basin, the Uintah Basin and the Marcellus Shale

Because of the rapid advancement of horizontal drilling and hydraulic fracturing techniques, the production of crude oil and natural gas in US increased dramatically in recent years. Currently natural gas contributes to about 25% of total US energy consumption. Several recent ambient observation studies reported an increase in Volatile Organic Compound (VOC) concentrations in some gas and oil production regions. It has been suggested that natural gas facilities may be the source of these VOCs, which contribute to ozone formation, and affect regional air quality, public health and climate change. However, few studies have conducted on-site VOCs emission measurements or provided facility-level VOC emission rates from natural gas production.

In this study, we visited 46 natural gas facilities in the Denver-Julesburg (Weld County, CO), Uintah (Uintah and Duchesne Counties, UT) and the Marcellus  (Northeastern PA) basins  in March to May 2015 and May 2016. VOCs and methane concentrations were measured downwind of individual facilities with a mobile laboratory. In total 13 VOCs, including benzene and toluene, were measured with a SRI 8610C Gas Chromatograph. Preliminary results show that concentrations of VOCs species are generally less than 1 ppb. For many VOCs the concentrations measured downwind of facilities are not substantially higher than the upwind background concentrations. We identified two high VOCs emitters in the Denver-Julesburg basin, which emitted about 2-3 orders of magnitude higher than other sites.

In order to compare the VOCs emissions of different site types (gas well or compressor station) in different basins we calculated background-corrected VOC-to-methane ratios of well pads and compressor stations in each basin. We found that the VOC-to-methane ratios from individual shale gas facilities are highly variable, suggesting that a single VOC profile may not be suitable to characterize all natural gas production facilities. We also found that well pads in the Denver-Julesburg Basin had higher VOC-to-methane ratios than sites in Uintah basin. However, the total site count was small the result may not be representative of all sites in both basins.

In this work we also coupled VOC-to-methane ratios with methane emission rates to obtain VOC emission factors from natural gas production in each basin. These VOCemission factors will be scaled up using the natural gas production data to estimate the total VOCs emissions in each basin, and to estimate the spatial distribution of VOC emissions from natural gas production facilities. Lastly, we compare the total VOC emissions from natural gas production in each basin with other major regional VOC emission sources such as on-road vehicles.