Profiling Oil and Gas Well Emissions with Mobile PTR-MS

Natural gas production in North America has increased sharply in the last several years, enabled by horizontal drilling techniques and hydraulic fracturing. The environmental impacts of oil and gas well emissions remain a subject of considerable debate. 

Mobile PTR-MS

To study the impacts of oil and gas well emissions, researchers from Aerodyne Research recently deployed a Vocus 2R PTR-TOF to a U.S. production region.   The Vocus PTR-TOF was installed on the Aerodyne Mobile Laboratory (AML) and configured to sample alongside tunable infrared direct laser absorption spectrometers (TILDAS) measuring ethane and methane.  In addition to profiling the VOC emissions, spatially resolved PTR-MS measurements can help identify specific sources of methane, which can be difficult to trace directly because of its higher background  levels and large number of sources. Concentrations of the VOCs rapidly decrease as plumes travel from the wells, so accurate mapping of the VOCs requires a detector with ultra-low limits of detection, such as the Vocus PTR-TOF.

Tracer Release Method

In collaboration with the well operating company, the Aerodyne team set up dual tracer release points on the well pad.  Dual tracer flux ratio measurements allow quantification of chemicals leaking or released from a well by comparison to known amounts of chemicals released from a tracer release device.  [Allen et al., 2013; Lamb et al., 1995; Mitchell et al., 2015; Roscioli et al., 2017; Roscioli et al., 2015; Yacovitch et al., 2017].

The tracer release device comprises: a tank of a harmless, volatile gas to be released in a controlled and known quantity; a mass flow controller to precisely measure the quantity of released gas; and a GPS device that precisely recorded the coordinates of the release point. By driving the AML to follow the tracer release gas downwind from the well and continuously measuring how much dissipated and was diluted, the Aerodyne team could calculate the concentration of gases being released from different equipment on the well pad. For more on this approach, see previous Aerodyne publications on the subject here, here, and here.

Vocus PTR-TOF measurements of oil and gas well emissions were made from the AML, while driving at speeds less than 50 km/hour.  Data were saved at 1 spectrum/second, corresponding to spatial resolution of 14 meters or better. The Vocus PTR-TOF drew 5 standard liters per minute (SLPM) of air from an inlet situated on the front of the AML through a 2 m-long Teflon line. Data were processed with Tofware analysis software.  

At distances of 0.5 to 2 kilometers downwind of the well, the Vocus PTR-TOF could detect enhancements of benzene, xylene, toluene, and other VOCs in the 10-100’s of ppt.

Oil and Gas Well Emissions Results

The figure shows the result of a particular emission plume encounter at an example oil and gas production pad.  In this plume the molar emission ratios of benzene to methane and ethane are 0.8 and 3.8 ppt ppb-1, respectively.

Benzene plume detection at oil and gas production site. The upper left panel indicates the ground track of the Aerodyne Mobile Laboratory colored and sized by the 1-s ethane mixing ratio. The right panel depicts the time series for this plume encounter.

Benzene plume detection at oil and gas production site. The upper left panel indicates the ground track of the Aerodyne Mobile Laboratory colored and sized by the 1-s ethane mixing ratio. The right panel depicts the time series for this plume encounter.

References:  

US Energy Information Administration: Drilling Productivity Report, available at: http://www.eia.gov/petroleum/drilling/, last accessed: 1 December 2018

Allen, D. T., et al. (2013), Measurements of methane emissions at natural gas production sites in the United States, Proc. Natl. Acad. Sci. U. S. A., 110(44), 17768-17773, doi:10.1073/pnas.1304880110.

Lamb, B. K., et al. (1995), Development of Atmospheric Tracer Methods to Measure Methane Emissions from Natural-Gas Facilities and Urban Areas, Environ. Sci. Technol., 29(6), 1468-1479, doi:10.1021/es00006a007.

Mitchell, A. L., et al. (2015), Measurements of Methane Emissions from Natural Gas Gathering Facilities and Processing Plants: Measurement Results, Environ. Sci. Technol., 49(5), 3219-3227, doi:10.1021/es5052809.

Roscioli, J. R., S. C. Herndon, T. I. Yacovitch, W. B. Knighton, D. Zavala-Araiza, M. R. Johnson, and D. R. Tyner (2017), Characterization of Methane Emissions from Cold Heavy Oil Production with Sands (CHOPS) Facilities, J. Air Waste Manage. Assoc., submitted.

Roscioli, J. R., et al. (2015), Measurements of methane emissions from natural gas gathering facilities and processing plants: measurement methods, Atmos. Meas. Tech., 8(5), 2017-2035, doi:10.5194/amt-8-2017-2015.

Yacovitch, T. I., C. Daube, T. L. Vaughn, C. S. Bell, J. R. Roscioli, W. B. Knighton, D. D. Nelson, D. Zimmerle, G. Pétron, and S. C. Herndon (2017), Natural gas facility methane emissions: measurements by tracer flux ratio in two US natural gas producing basins, Elem. Sci. Anth., 5, 69, doi:10.1525/elementa.251.

 

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