Research on Depressurization Gas Extraction Techniques and Identification of Gas Sources in Goaf Based on Carbon and Hydrogen Isotope Tracing

In the mining environment of coal seams, in order to accurately quantify the gas sources in the goaf of the protective layer working face and effectively implement gas extraction and control, this paper conducts experiments on gas source identification in the goaf in the Pingdingshan mining area. By collecting desorbed gas samples from the parent coal seam, a detailed analysis of the gas components (including methane, ethane, carbon dioxide) as well as the values and distribution characteristics of stable carbon and hydrogen isotopes was conducted. Based on these data, we established a calculation model for gas source identification in the goaf based on stable carbon and hydrogen isotopes and component averages, achieving a quantitative calculation of gas sources from various coal seams as the goaf advances, and implementing depressurization gas extraction techniques for coal seams at a greater distance upward. The results show significant differences in the stable carbon and hydrogen isotopes of desorbed gases from the three coal seams, although the overall trend is relatively consistent. As the burial depth of the coal seam increases, the carbon isotope values of methane, ethane, and carbon dioxide, as well as the hydrogen isotope values of methane, all show a trend of becoming heavier. In addition, the main source of gas in the goaf of the protective layer comes from the downward adjacent coal seam, accounting for 81% of the total source, while the average contribution from the same coal seam is 12%, and the contribution from distant upward coal seams is 7%. By implementing depressurization gas extraction techniques for distant upward coal seams, we also identified the optimal window period for depressurization extraction in the C1 coal seam to be between 85 and 100 days.