Pore Fractal Dimensions of Bituminous Coal Reservoirs in North China and Their Impact on Gas Adsorption Capacity

Coal pores not only serve as the storage space for coalbed methane but also provide channels for gas migration. The accurate characterization of coal pore structures is of significance to study the methane adsorption behaviors. In this work, the quantitative relationship between gas adsorption and pore characteristics was investigated in depth for bituminous coals. Results of scanning electron microscopy showed that the surface morphological characteristics of these samples differ greatly. Some typical pore types including cylindrical pores and conical pores were found in these samples. The remarkable hysteresis loop was observed, which is attributed to the bottle-shaped pores with poor connectivity. Fractal theory was introduced to quantitatively evaluate the surface roughness of coal. Pore fractal dimensions, D1 and D2, were calculated using low-pressure N2 gas adsorption data, and their values were in the range of 2.125–2.721 and 2.084–2.461, respectively. D1 was larger than the corresponding D2 for the same sample, suggesting that micropore structures in coal were more complex when compared with mesopores and transition pores. Both D1 and D2 were enhanced with increase in micropore specific surface area, but they were reduced with increase in mesopore specific surface area. Gas adsorption in coal was estimated from the perspective of fractal dimension. Judging from the fitting degree, the influence of D1 on adsorption capacity of coal was remarkably greater than that of D2. D1 is expected to be used as one of the major adsorption indicators in the future study.