Isotope tracing the coalbed methane migration from the underground longwall coal face to the surface of mining area 

Abstract

Simultaneous methane emission studies in combination with the analysis of the isotopic composition can help to identify the genesis, transport process, and migration pathways of coalbed gases resulting from coal mining activities. This study was focused on the investigation of the influence of physical processes, site-specific conditions, and parameters of coal mine operations on variations in composition and distribution of coalbed CH4 during gas migration pathways. The flask air sampling was performed in the sequence of the return airways flowing along the longwall face of a coal seam at depths down to 700 m, across walkways of mine workings, and through mine ventilation shaft into the atmosphere. The analyses were conducted during undergoing mining operations in one of the hard coal mines in the Upper Silesian Coal Basin (USCB), Poland.The results of isotopic analysis of CH4 confirmed the relationship between gas migration from the coal seam into space of excavation and enrichment in 13C and 2H signatures. Free gas samples taken from the borehole in the coal seam, in the region of the longwall under analysis, consist mainly of thermogenic methane (86.8 – 92.1 %), with δ13C values between −50.9 and −50.7 ‰ and δ2H of −197.3 to −191.9 ‰. Samples collected along 145 m length of longwall coal face showed downward decreasing in CH4 concentration (range from 0.85 to 0.19 %) being diluted with the rise of ventilating air stream supplied to the longwall. The determined 13C- and 2H-enrichment in methane mixture flowing through the longwall from −48.9 to −46.6 ‰ for δ13C, and from −192.4 to −178.1 ‰ for δ2H, respectively, generally resulted from diffusion-controlled adsorption and desorption processes.In general, methane concentrations significantly decreased during upward migration across walkways of mine workings expressing a pronounced dilution effect with the increase in air velocity and distance from the exploitation longwall towards the ventilation shaft. The measured methane concentration inside of the exhaust ventilation shaft (surface channel) varied from 0.15 to 0.42 %, while mean isotopic signatures were estimated to be –48.8 ± 0.9 ‰ for δ13C and –188.2 ± 3.2 ‰ for δ2H. In addition, such isotopic mass balance approach can be used to determine the contribution of mine ventilation and other atmospheric methane sources contributions around coal mining areas. This work was funded by the Polish Ministry of Science and Higher Education under Grant No. 2022/44/C/ST10/00112. The isotopic analysis has been supported by the ATMO-ACCESS Project (grant agreement ID: ATMO-TNA-4--0000000041).