Characteristics of pore structure and fractal dimension of low-rank coal: A case study of Lower Jurassic Xishanyao coal in the southern Junggar Basin, NW China

- Characteristics of pore structure of low-rank were discussed.
- The poor development of mesopores in low-rank was observed for the first time.
- Characteristics of pore structure fractal dimensions were studied.
- The relationships between CBM accumulation and fractal dimensions were discussed.

In this study, a series of laboratory experiments were performed to firstly analyze the characteristics of the pore structure in low-rank coal (Ro < 0.70%), and then relationships between fractal dimensions and coal rank, coal composition and pore structure, and their impacts on methane adsorbability and seepage ability were studied too. The desorption isotherms of low-rank coal mainly belongs to Type B, which are primarily caused by ink-bottle-shaped and narrow-slit pores that are good for coalbed methane (CBM) enrichment, but bad for seepage. The seepage pores are dominated by primary plant macropores (>1000 nm), with a poor development of mesopores (100-1000 nm). The fractal features of adsorbed pores and seepage pores were defined and calculated using fractal dimensions D2 (2.563-2.926) and D4 (2.683-3.263), respectively. D2 has a positive correlation with moisture content, while D4 has a positive correlation with Ro and fixed carbon and a negative correlation with the volatile content. Both D2 and D4 have a weaker association with the ash yield and no apparent relationship with coal macerals, indicating that coal-forming materials and environments have little influence on the pore structure fractal characteristics. D2 has a strongly correlation with the BET surface area (R2 = 0.9026) and the average pore diameter (R2 = 0.9841), again proving that D2 can be used to characterize the pore structure fractal dimension of adsorbed pores. D2 shows a negative correlation with the Langmuir volume (VL), but no apparent relationship with the Langmuir pressure (PL), indicating that the pore structure of absorbed pores has little effect on methane desorption during the process of CBM exploitation. D4 has weak positive correlation with permeability. Overall, fractal analysis is beneficial for better understanding the pore structure, methane adsorbability and seepage ability of low-rank coal.