Carbon isotope evolution law and influencing mechanism of gas desorption-diffusion in coal seams with different water contents

Gas extraction is the primary technical measure for coal mine gas disaster prevention. Water content difference significantly affects gas desorption-diffusion behavior and carbon isotope fractionation process, while existing studies lack a systematic understanding of the carbon isotope evolution law and internal mechanism of gas under different water content conditions. Based on gas geochemistry and isotope fractionation theory, this study takes the C₇₊₈ coal seam of Pingqing Coal Mine in Yunnan as the research object, and systematically explores the evolution law and influencing mechanism of gas carbon isotope （δ¹³C） under normal pressure desorption and crushing release conditions. The results show that, during normal pressure desorption, the gas desorption volume ratio presents a two-stage feature of “rapid growth followed by stable growth”; the δ¹³C values of CH₄, CO₂, and C₂H₆ all become heavier in a negative exponential trend with desorption time and increase linearly with the desorption volume ratio, with CH₄ showing the most significant fractionation effect and C₂H₆ the weakest. Under crushing release conditions, the desorption volume ratio of coal samples with different water contents increases steadily; the δ¹³C value of CH₄ becomes heavier exponentially with desorption time （higher water content leads to heavier isotope value） and maintains a linear positive correlation with the desorption volume ratio. The core mechanism of carbon isotope fractionation is that the weaker polarity of ¹²C—H bonds than ¹³C—H bonds causes the preferential desorption of ¹²CH₄, while desorption dynamics, molecular forces, and methane water solubility jointly intensify this process. The research provides a theoretical basis and technical support for revealing the micro-mechanism of gas extraction controlled by water content and optimizing evaluation methods of gas extraction effect.