Pore Fractal Characteristics of Coal and Their Effect on Permeability: A Case Study of the Gujiao Block

The Shanxi and Taiyuan Formations in the Gujiao Block are rich in coalbed methane (CBM) and represent important targets for CBM development. To better understand the reservoir characteristics, this study investigates the pore–fracture system of medium- to high-rank coals using low-field nuclear magnetic resonance (NMR), field emission scanning electron microscopy (FESEM), and optical microscopy. A fractal model of the coal pore–fracture system was established to quantify reservoir heterogeneity and evaluate the CBM production potential. The results show that the pore structure of coal reservoirs can be classified into three types, among which Type III reservoirs are the most favorable for CBM exploration due to superior pore connectivity and permeability. The fractal dimension of adsorption pores ranges from 0.05 to 0.46 with strong fitting accuracy ( R 2 > 0.74), while seepage pores exhibit a broader range (2.55–3.00), reflecting the structural complexity of the pore system and microfracture networks. Both adsorption and seepage pore fractal dimensions display a significant negative correlation with permeability, likely controlled by the effective porosity and pore surface roughness. The reservoir quality index (RQI) ranges from 0.05 to 0.30 (average 0.16), effectively characterizing the CBM reservoir quality. Its negative correlation with adsorption pore complexity suggests that reservoirs with lower fractal dimensions possess better pore structures and a higher production potential. These findings provide a quantitative framework for evaluating reservoir quality and identifying favorable CBM targets in the Gujiao Block.