Emissions of Methane, Volatile Organic Compounds, and Black Carbon from Natural Gas and Oil Development, and an Examination of the Fat-Tail Problem (Invited Presentation)

Horizontal drilling and hydraulic fracturing techniques have significantly increased the production of crude oil and natural gas in the US in recent years. Natural gas has been widely considered as a “bridge” fuel, as we transition from coal-based electricity generation to renewable or other non-fossil energy sources. In addition, the lower carbon density of natural gas (mostly methane) could provide climate benefits compared to coal or oil. However, methane emissions from the natural gas infrastructure can offset these benefits, or even worsen the climate impact of energy use. Oil and gas production facilities also emit Volatile Organic Compounds (VOCs), which can contribute to the formation of ozone and affect regional air quality and climate change. Flaring of associated gas from oil production can be a significant source of black carbon, which is also a climate warming agent.

Since 2013, we have characterized methane and other emissions from hundreds of facilities across the oil and natural gas supply chain. These include transmission compressor stations and gathering and processing facilities across the US; gas production well pads and processing facilities in the Marcellus, Uintah, and Denver-Julesburg basins; and associated gas flares in the Bakken. Methods have included direct onsite and tracer flux measurements for methane emissions; stationary downwind capture for VOCs; and airborne measurements for BC from gas flares.

In 2013, methane emissions from 45 transmission compressor stations (more than the three previous studies combined) were measured with a combination of direct onsite measurements and downwind dual tracer flux. Facility-level emission rates agreed within measurement uncertainties at most facilities, while the direct onsite measurements could not capture two super-emitters. The emissions distributions showed significant skewness with 50% of the methane attributed to 10% of the facilities, a phenomenon referred to as a “fat tail.”

In 2014 and 2015, tracer flux and other downwind techniques were used to sample methane emission rates from over 100 natural gas well pads, gathering compressor stations, and other natural gas facilities in the Marcellus shale, Denver-Julesburg, and Uintah basins. Thirteen VOCs, including benzene and toluene, were measured downwind of or near ~35 natural gas facilities in Colorado and Utah, with similar measurements to be conducted in the Marcellus Shale in late August 2015. Preliminary results show that VOC emission profiles from individual facilities vary significantly, which suggests that a single VOC profile may not characterize all natural gas production facilities. In the Marcellus, methane emissions as a function of average gas production ranged from 0.01% to 1% (median 0.09%) at unconventional production wells, and from 0.29% to over 70% (median 8.5%) at conventional wells.

In early 2014, black carbon from over 30 associated gas flares in the Bakken was measured using a single particle soot photometer (SP2) aboard the Purdue University Airborne Laboratory for Atmospheric Research (ALAR) platform. The average emission factor of 0.14 g-BC/kg-fuel is significantly lower than previous literature estimates (0.7-1.1 g-BC/kg-fuel), though several observations over 0.5 kg-BC/kg-fuel evidence a fat-tail distribution in these emissions as well.

In this talk, I shall present selected results from these different studies, and examine the distributions for different pollutants based on these measurements, which is informative for emissions inventories and a guide to emissions reduction policy.