Novel Coalbed Methane (CBM) Origin Analysis and Source Apportionment Method Based on Carbon Isotope Ratio Using Infrared Dual-Wavelength Laser Absorption Spectroscopy

Abstract

We report the application of an infrared dual-wavelength laser absorption spectroscopy system, which combines tunable diode laser absorption spectroscopy and wavelength modulation integrated cavity output spectroscopy technologies based on two spectroscopic sources operating at 6,029 and 2,310 cm−1, to simultaneously measure the gas concentrations (CH4 and CO2) and carbon isotopic signatures (13C and 12C) in coalbed methane. The Allan variance is used to analyze the optimum mean time (t) and the standard deviation (SD), with t = 450 s, SD = 0.41‰ for 13CH4 monitoring and t = 70 s, SD = 0.17‰ for 13CO2 monitoring. Furthermore, 47 groups of coalbed methane samples collected from the Huaibei-Qidong coal mine seam are analyzed based on the measured results for carbon isotopes and geophysical geochemistry. The δ13CH4 values of the Qidong coal mine fall between −44.57‰ and −61.57‰, and the δ13CO2 values fall between −21.65‰ and −17.06‰. The genetic types of methane may be divided into biogenic gas, thermogenic gas, and mixture gas. Carbon dioxide may be derived from coal pyrolysis, and it also has the characteristics of carbon-isotope changes of organic genetic gas. The new approach for analyzing rapid genetic types, methane migration pathways, gas disaster source warning, and coal chemistry studies is expected to be further studied in the future.