Fractal characteristics of pore structure in high rank coals from Xinjing coal mine, Yangquan

Based on mercury intrusive data of high rank coal samples from Xinjing coal mine, fractal geometry model was built to describe the pore structure quantitatively. The results show that pores in coal samples mainly exist in nanoscale, the distribution of pore diameter, pore volume and specific surface area are also of nanoscale. The pore diameter over 65 nm are significantly fractal, the fractal dimension varies from 2.89 to 2.99, and the variation of volume increment are phase-based, indicating complicated pore structure. There is a linear relationship between specific surface area increment and pore diameter below 65 nm (which are hardly fractal) in double logarithm coordinates, indicating a rather simple pore structure. Pores are classified into two major types:diffusion pores(<65 nm) and seeping pore(>65 nm) and 6 secondary types based on the fractal characteristics and laws of gas motion. Fractal dimensions are negatively related to median radius and total volume of pores, while positively related to pore volume and pore specific surface area in pores over 65 nm. The fractal dimension can be utilized in CBM reservoir exploration as a comprehensive index for its rather complete characterization ability.