IR Spectral Inversion of Methane Concentration and Emission Rate in Shale Gas Backflow
CHENG Xiao-xiao1, 2, LIU Jian-guo1, XU Liang1*, XU Han-yang1, JIN Ling1, XUE Ming3
1. Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics，Hefei Institutes of Physical Science, Chinese Academy of Sciences，Hefei 230031，China
2. University of Science and Technology of China, Heifei 230026, China
3. State Key Laboratory of Petroleum Pollution Control, CNPC Research Institute of Safety and Environmental Technology, Beijing 102206, China
Abstract：With the development of shale gas, the traditional handheld methane meter cannot cope with the complex shale gas production conditions due to the need for manual contact sampling. In view of the difficulty of real-time online monitoring of the emission concentration and rate of greenhouse gas methane in the process of shale gas development, we use the self-designed and built open Fourier transform infrared(FTIR) measurement system to measure the backflow liquid under various working conditions in the process of shale gas production. The FTIR resolution is 1 cm-1 and the optical path is 50 m. The light source passes directly above the backflow liquid and is received by the spectrometer. The measured infrared spectra are averaged several times, the hyperspectral mass is maintained, and the inverse calculation is carried out. The absorption cross-section calculated from the HITRAN database, considering the influence of environment and instruments, the measured temperature was modified, the appropriate methane absorption band was selected, and the absorption peak was superimposed with the absorption cross-section of water vapor to synthesize the standard spectrum. The methane concentration was calculated by fitting the measured spectrum with the standard spectrum using the least square method. The methane emission rate during shale gas exploitation was calculated according to the emission rate of the backflow fluid and combining the distance of the light path through the backflow pool and the infrared spectrum inversion concentration. The results show that spectral inversion concentration fluctuates obviously under different mining conditions. When the three separators were replaced, the methane concentration increased obviously. When the torch was lit, the methane concentration continued to be low, and the results of infrared spectrum inversion concentration were consistent with the methane emission in shale gas development and construction. The spectrum was averaged several times to improve the spectral quality, and methane in the backflow liquid was measured in a unit hour and continuous 80 hours. Within the unit hour, the methane concentration fluctuated within the range of 100～800 μmol·mol-1, and the variation trend was continuous and obvious. The emission rate of methane fluctuated between 50 and 300 m3·h-1. After 80 hour’ continuous measurement of the backflow fluid, the maximum methane concentration was 936.4 μmol·mol-1, and the maximum emission rate reached 535.1 m3·h-1. The lowest value was 36.82 μmol·mol-1, and its minimum emission rate was 18.63 m3·h-1. The result of inversion data shows that the backflow fluid is an irregular methane emission source in the development process of shale gas, and the change is very obvious in a short time. At the same time, the infrared spectrum inversion concentration was compared with the results of the traditional handheld methane measurement instrument, and the correlation coefficient was 0.743 6. Compared with the traditional handheld methane measurement instrument, the infrared spectral inversion method has the advantages of faster response, non-contact distance, real-time online measurement, etc.
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