Multi-scale fractal characterizations of lignite, subbituminous and high-volatile bituminous coals pores by mercury intrusion porosimetry

Pore fractal models, including thermodynamics and classic geometry models, can help quantify pore structure. To evaluate the effect of the pore structure on coal permeability, we calculated the multi-scale fractal dimensions and discussed factors influencing pore fractals, including coal petrology and coal reservoir parameters. We investigated pore physical properties, including of the pore size/volume distribution, pore structure and pore heterogeneity for lignite, subbituminous and high-volatile bituminous coals (LSBC) in the Southern Junggar Basin, NW China. The multi-scale fractal dimensions of coal pores according to classic geometry models (Dc1, Dc2, Dc3 and Dc4) are in the range of 3.15-4.26, 2.42 to 3.92, 2.30 to 3.60 and 2.35 to 5.41, respectively. The abnormal fractal dimensions (D > 3) can be attributed to high pore heterogeneity, fractures that existed in coals and pore compressibility during a high-pressure mercury injection. The fractal dimension (Dc) and Ro, m generally presented an inverted 'U-shaped' tendency with maxima occurring at 0.6% Ro, m, which was mainly caused by variations in the macropore volume by the first coalification jump. It was found that the multi-scale fractal dimensions of classic geometry model is more appropriate for describing pore heterogeneity in LSBC. The micropore/macropore volumes and the fractal dimension (Dc3 and Dc4) were predominant indices of pores connectivity (IMS and EMW). A 'U-shaped' trend was obtained between the permeability contribution and macropore fractal dimension (2.6-2.7 for Dc4), similar to EMW/IMS vs. the macropore fractal dimension. These conclusions coincided with that of good pore connectivity representing high permeability and can have high feasible applications in CBM exploration and exploitation. High heterogeneity coals with developed macropores may contribute significantly to coal permeability in LSBC reservoirs.