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Open-Path Hydrocarbon Laser Sensor for Oil and Gas Facility Monitoring
Citation:
Farria, B., E. Thoma, P. Deshmukh, AND A. Yellen. Open-Path Hydrocarbon Laser Sensor for Oil and Gas Facility Monitoring. AWMA Air Quality Measurement Methods and Technology conference, Long Beach,CA, November 07 - 09, 2017.
Impact/Purpose:
This is a poster for the Air Quality Measurement Methods and Technology conference on some limited scope. prototype next generation emission measurement (NGEM) sensor development work performed with Colorado State University. This was a student lead-project.
Description:
This poster reports on an experimental prototype open-path laser absorption sensor for measurement of unspeciated hydrocarbons for oil and gas production facility fence-line monitoring. Such measurements may be useful to meet certain state regulations, and enable advanced leak detection and repair strategies for methane and volatile organic compounds (VOCs) that may have adverse health effects or act as precursors to ozone formation. Our initial design employs a single path measurement system though future implementations may use multiple paths for large scale facility monitoring. The sensor uses a compact mid-infrared laser source in the spectral region of ~3.3 μm to measure absorption of several hydrocarbon species over open-paths of ~1 km. Spectral simulations show that for typical conditions the hydrocarbons cause a transmission reduction of greater than 10% allowing a robust measurement. The initial prototype system uses a helium-neon (He:Ne) laser at 3.391 μm for which signal contributions from methane and non-methane hydrocarbons are comparable. Closed-cell tests were performed with diluted methane (~150-250 ppm) to validate the transmission signals and showed good agreement with expected (calculated) values to within ~10 %. The system employs a reference leg, with a 2nd detector (near the source) to normalize for laser power fluctuations. For improved signal-to-noise, particularly for small concentrations and transmission changes, we employ phase-sensitive detection with a mechanical chopper and software based lock-in amplifier. This detection scheme allows measurement of transmission signals with stability <0.5% (based on coefficient of variation over 60 s). The portable, field sensor system uses two refractive (Keplerian) telescopes (2” diameter optics), a transmitter and receiver co-located on a mobile optical breadboard, and a reflector dictating the path length. We have performed initial tests with path lengths up to ~25 m (one way) though the design should allow paths in excess of 100 m (one way). For initial field tests, methane was released at known flow rates near the center of the beam path. Transmission signals in rough agreement with expectation (given uncertainties in the wind and plume dispersion) were observed. The system should allow detection of leaks (emissions) for mass flows as low as ~1 g/s of methane (or equivalent optical signal from other species) for the case where the source is ~150 m from the beam path, for typical atmospheric conditions. The sensitivity to small leaks will be experimentally verified in upcoming field work.