21 October 2014
Oil and natural gas production fields can emit large amounts of air pollutants that affect climate and air quality – but tackling the issue has been difficult because little is known about what aspects of complex production operations leak what kinds of pollutants, and how much. Now a CIRES-led study in the journal Atmospheric Chemistry and Physics sheds light on just that, pinpointing sources of airborne pollutants.
The results have important implications for mitigation strategies in the nation's oil and natural gas production.
"Before you can stop a leak, you have to know where it is," said lead author Carsten Warneke, an atmospheric chemist CSD. "This study tells us where the largest emissions are coming from, and that, in turn, helps industry identify what they can do to reduce emissions as cheaply and effectively as possible."
Oil and gas production fields emit the greenhouse gas methane and also other air pollutants called volatile organic compounds (VOCs), which include the air toxics benzene, a carcinogen, and toluene. VOCs, present naturally in oil and natural gas, are chemical precursors for ozone pollution, which, at high levels, can harm people's lungs. The new study focuses on the emissions of VOCs in the oil and gas fields of the Uintah Basin in Utah, where the landscape is dotted with 8,000 gas wells and 2,000 oil wells in operation, and about 1,000 new wells are added each year.
The study shows that in the Uintah Basin, equipment located on well pads – such as condensate tanks, dehydrators, and pumps – are key sources of pollutants. It also found that well operations frequently emit high levels of benzene and toluene, and that emissions vary by production method.
The study is one of the first to use fast-response and highly sensitive instruments to measure VOCs from individual gas and oil well pads and other point sources. To collect the data, Warneke and his team, in February 2012, drove an instrumented van downwind of 38 gas wells, 12 oil wells, one newly producing well, one refractured well with a flowback pond and 17 other point sources such as evaporation ponds, storage tanks and compressor stations. The mobile laboratory approached closer than 300 feet of most sources, measuring the VOCs in the air.
The researchers detected high amounts of VOCs at almost all the locations, but large differences existed among sources. Specifically, they found:
The new findings are qualitatively similar to emissions "inventories," which are estimates of emissions based primarily on well counts and production data. Like the measurements, inventories identify well heads themselves, dehydrators and tanks as major VOC emission sources.
The researchers' measurements were part of a larger experiment to unravel the mystery of why the sparsely populated Uintah Basin experiences frequent wintertime exceedances of ozone air quality standards. The research shows that these exceedances trace back to oil and gas activities – and to the VOCs that Warneke and his colleagues have now detailed in the new study.
"To understand ozone pollution, we need to understand both the chemistry behind it and the major sources that start this chemistry, and we went right to the source of the emissions to study them," Warneke said.
Warneke, C., Geiger, F., Edwards, P. M., Dube, W., Pétron, G., Kofler, J., Zahn, A., Brown, S. S., Graus, M., Gilman, J. B., Lerner, B. M., Peischl, J., Ryerson, T. B., de Gouw, J. A., and Roberts, J. M., Volatile organic compound emissions from the oil and natural gas industry in the Uintah Basin, Utah: oil and gas well pad emissions compared to ambient air composition, Atmospheric Chemistry and Physics, doi:10.5194/acp-14-10977-2014, 2014.
Emissions of volatile organic compounds (VOCs) associated with oil and natural gas production in the Uintah Basin, Utah were measured at a ground site in Horse Pool and from a NOAA mobile laboratory with PTR-MS instruments. The VOC compositions in the vicinity of individual gas and oil wells and other point sources such as evaporation ponds, compressor stations and injection wells are compared to the measurements at Horse Pool. High mixing ratios of aromatics, alkanes, cycloalkanes and methanol were observed for extended periods of time and for short-term spikes caused by local point sources. The mixing ratios during the time the mobile laboratory spent on the well pads were averaged. High mixing ratios were found close to all point sources, but gas well pads with collection and dehydration on the well pad were clearly associated with higher mixing ratios than other wells. The comparison of the VOC composition of the emissions from the oil and natural gas well pads showed that gas well pads without dehydration on the well pad compared well with the majority of the data at Horse Pool, and that oil well pads compared well with the rest of the ground site data. Oil well pads on average emit heavier compounds than gas well pads. The mobile laboratory measurements confirm the results from an emissions inventory: the main VOC source categories from individual point sources are dehydrators, oil and condensate tank flashing and pneumatic devices and pumps. Raw natural gas is emitted from the pneumatic devices and pumps and heavier VOC mixes from the tank flashings.