Evaluation of Gas-in-Place Content and Free Gas Ratio in Deep CBM of the Daning–Jixian Block: An Isotope Fractionation Method

This study focuses on deep coal samples from the Yichuan Well block in the Daning–Jixian area of the Ordos Basin. Samples were obtained through conventional coring and pressure-holding coring (PHC) techniques, and field degassing experiments were conducted to measure the carbon isotopic compositions of CH4 (δ13C1) and CO2 (δ13CCO2) in the degassing gas. The results indicate that during the deep CBM field degassing, carbon isotope fractionation for CH4 ranges from 3.3 to 24.9‰, exhibiting three distinct fractionation patterns: Type I (initially stable, then decreasing), Type II (continuously increasing), and Type III (initially decreasing, then increasing). Based on the four-stage general pattern of isotope fractionation observed in the complete shale gas degassing process (stable→decrease→increase→decrease again), Type I corresponds to the first and second stages (stable→decrease), attributed to pressure-driven seepage in the fracture-cleat-macropore system. Type II is the most common pattern observed in the field degassing experiments, corresponding to the third stage (increase) of the four-stage general pattern, reflecting the extensive desorption of adsorbed gas from the matrix pores. Type III appears exclusively in PHC samples, corresponding to the second and third stages (decrease→increase), indicating the simultaneous production of free and adsorbed gas. The carbon isotope fractionation pattern of CO2 differs significantly from that of CH4, possibly related to differences in diffusion coefficients and adsorption capacities between gas components, leading to desorption stage differentiation. Using the carbon isotope fractionation (CIF) model that considers the bidisperse pore structure and multiple gas transport mechanisms of coal developed by previous researchers, we established a quantitative evaluation method for gas-in-place (GIP) content and in situ free gas ratio in deep CBM, and validated the method’s accuracy through pressure-holding coring data. The evaluation results show that the GIP content of deep coal samples in the Daning–Jixian block ranges from 22.9 to 39.2 m3/t (with an average of 27.2 m3/t), and the in situ free gas ratio ranges from 16.4% to 29.0% (with an average of 23.4%). Comparative analysis with four global deep shale gas regions, one shallow shale gas region, three deep coalbed methane (CBM) regions, and one shallow CBM region confirms that the deep CBM in the Yichuan Well area of the Daning–Jixian block exhibits unique characteristics of “high GIP content and abundant free gas”. This study offers new perspectives and technical approaches for understanding the occurrence characteristics and flow production mechanisms of deep CBM.