The pore structure of deep coal reservoirs is the key to evaluating the potential of coalbed methane resources and optimizing the development strategy. In this study, the 8# coal sample of Benxi Formation in the DJ57 well of the Daning-Jixian block in the eastern margin of the Ordos Basin was taken as the research object. Based on the microscopic components and industrial analysis, the multi-scale quantitative characterization of nano-scale pores was conducted by using CO2 and N2 adsorption experiments. On this basis, the V-S (Volume-Specific Surface Area) and FHH (Frenkel Halsey Hill) fractal models were used to reveal the multi-scale fractal characteristics of pores. Combined with the CH4 isothermal adsorption experiment, the main controlling factors affecting the CH4 adsorption gas content were systematically discussed. The results show that: (1) The thermal evolution degree of deep coal in the study area is high, and the whole is in the stage of high rank coal (the average Ro, max is 2.44%). The material composition of coal rock is mainly vitrinite (average content is 81.0%), inertinite, and mineral group content is 12.5% and 6.5%, respectively. Industrial analysis shows that it has the characteristics of low moisture (the average Mad is 0.97%), low ash (the average Aad is 21.7%), low volatile (the average Vad is 7.36%), and high fixed carbon (the average FCad is 69.95%). (2) The micropores are highly developed, with an average CO₂ adsorption capacity of 17.52 cm³/g. The average total micropore volume and specific surface area are 0.05301 cm³/g and 180.57 m²/g, respectively. The pore morphology is primarily composed of cylindrical pores with openings at both ends and flat slit shaped pores with open cylindrical on all sides. (3) The coal samples exhibit strong CH₄ adsorption capacity, with an average Langmuir volume (VL) of 28.77 cm³/g. (4) The fractal characteristics of multi-scale pores are significant. The average pore fractal dimension (DC) based on CO2 adsorption is 2.4686, and the average pore surface fractal dimension (DN−1) and pore structure fractal dimension (DN−2) based on N2 adsorption are 2.5943 and 2.7202, respectively. (5) The degree of thermal evolution, material composition, pore structure, and heterogeneity of coal rock jointly control the CH4 adsorption capacity. With the increase of Ro, max, the content of vitrinite and the proportion of micropores increase, and the complexity and heterogeneity of pore structure increase, providing more CH4 adsorption sites, thus improving the adsorption capacity. There is a strong positive correlation between VL and DC, and DC can be used as a key parameter to evaluate the adsorbed gas content of deep coal in the study area. The research results have theoretical guiding significance for deep coalbed methane resource evaluation, “sweet spot” identification, and efficient development.
Chen et al. (Sun,) studied this question.