Fractal characterization of seepage-pores of coals from China: An investigation on permeability of coals

To better understand the characteristics of seepage-pores (pore radius larger than 100 nanometers) and their influence on the permeability of coals, we have conducted fractal analyses for 34 fresh coal samples (mean maximum vitrinite reflectance Ro,max from 0.43% to 4.21%) from North, Northwest and Northeast China. Mercury porosimetry data indicate that the coals are fractal, with pore radius ranging from 0.1 to 50 μm. Calculated fractal dimensions of these coals range from 2.61 to 2.98, higher than those from other kinds of rocks such as sandstone, shale, and carbonate. The data suggest that the coals have more complicated and inhomogeneous pore structures than other rocks.The fractal dimension has a negative correlation with the petrologic permeability of coals, particularly for higher rank coals (with 1.47–4.21% Ro,max), from which a strong negative linear correlation (R2=0.85) between fractal dimension and permeability is observed. A ‘U-shaped’ trend between fractal dimensions and coal ranks is observed, with the minimum fractal dimensions occurring at 1.1–1.3% Ro,max. The sub-bituminous, high volatile bituminous, semi-anthracite, and anthracite have higher fractal dimensions. The effects of coal rank upon fractal dimensions are mainly due to the variety of micropore contents and aromaticity of coals with coalification.