Multiple Fractal Characterization of Microscopic Pores in No. 8 Coalbed of Benxi Formation in Central-Eastern Ordos Basin

The pore structure characteristics of coalbed methane reservoirs have influence on gas adsorption, desorption, diffusion and transportation processes. Pore heterogeneity characterization is instructive for coalbed methane exploration and evaluation. On the basis of the previous basic geological research and analytical tests, the multi-method characterization of the coal pore structure is carried out by combining CO2 adsorption, low-temperature N2 adsorption (LTNA), and mercury intrusion porosimetry (MIP). Multiple fractal theory is used to characterize the fractal characteristics, heterogeneity and connectivity of different pores, to investigate the influence of coal components, coalification, and pore volume on the differentiated evolution of pore structure. The research shows that the pore type of the No.8 coalbed in Benxi Formation is dominated by micropores, which have high α0 value (0.085) and H value (0.737). The pore structure is heterogeneous, but the micropores are well connected and have high transmission capacity between pores. The heterogeneity of macropores is weak, the pore distribution tends to be homogenized. However, the macropore connectivity is low, and the transmission capacity between pores is significantly reduced. As the pore volume increases, the quantity of micropores increases and the microporous connectivity becomes better. In contrast, macropores have increased heterogeneity, pores mostly show isolated distribution, and connectivity between macropores is low. With coalification, macropores and mesopores decrease, micropores develop, and the overall pore structure tends to be complicated. The heterogeneity of the micropores is significantly enhanced by strong coalization, but the pore connectivity is significantly improved. This research innovatively describes the different evolution characteristics of heterogeneity and connectivity in different pore structures of deep coalbed methane. To provide theoretical basis for the in-depth research on the seepage and transportation paths of coalbed methane, the characteristics of reservoir formation and enrichment, and the exploration of coalbed methane.