Carbon isotope evolution law and extraction effect evaluation of negative pressure extraction in complex low-permeability coal seams

To address the limited availability of methods for accurately evaluating coal mine gas extraction performance, a group of steeply inclined and closely spaced coal seams, including the M6-3, M7-2, M8, and M11 coal seams in Fengchun Coal Mine, was selected as the research object. Comparative experiments on combined gas extraction using cross-measure boreholes were conducted. Gas samples were systematically collected at different extraction stages to determine their gas compositions and carbon isotope characteristics. Based on the principle of mass conservation for chemical and isotopic compositions, a four-source gas mixing ratio calculation model was established. Furthermore, a carbon isotope evolution model was developed to clarify the underlying fractionation mechanism, and a quantitative relationship between carbon isotopes and the gas extraction rate was ultimately established. The results indicate that, with increasing extraction time, the gas extraction concentration gradually decreased, whereas the extracted gas volume ratio increased. Meanwhile, δ¹³C(CH₄) exhibited a linear enrichment trend, while δ¹³C(CO₂) showed nonlinear isotopic depletion. Differences in molecular bond polarity resulted in the preferential desorption of ¹²CH₄ and ¹³CO₂, which was identified as the primary mechanism governing carbon isotope fractionation during coal seam gas extraction. During the combined extraction of the coal seam group, the contribution of the M6-3 coal seam decreased substantially, whereas those of the M7-2 and M8 coal seams increased, and the contribution of the M11 coal seam remained relatively stable. The gas extraction rate of each coal seam increased with the progressive enrichment of the CH₄ carbon isotope, indicating that carbon isotopes can serve as a microscale indicator for characterizing coal seam gas extraction performance.