Full-scale pore characteristics in coal and their influence on the adsorption capacity of coalbed methane

Abstract

Coalbed methane (CBM) is primarily stored and transported through the pores in the coal matrix, making it essential to study the development of different scales of pores in coal to better understand the evaluation and exploration of CBM. In this study, four coal samples of varying ranks (R_o,max = 0.68%-2.86%) were selected, and different scale pores were obtained through low-pressure CO₂ and Ar adsorption (LP-CO₂/ArGA) and mercury intrusion porosimetry (MIP) experiments. A full-scale pore evaluation model was established, and the impact of pores on methane adsorption and restriction was analyzed and discussed through high-pressure adsorption experiments. Our results show that (1) at high pressures (> 100 MPa), the MIP technique caused pore compression and overestimated the pore size below 30 nm by up to 47.2%; (2) to obtain a comprehensive pore evaluation, we developed an accurate model that combines LP-CO₂/ArGA with NLDFT and BJH and NLDFT models to determine micropore (0.3-1.5 nm) and mesopore (1.5-30 nm) parameters. By combining this model with MIP test results, we can obtain a full-scale pore size in the range of 0.3 nm-200 μm; (3) coal rank affected the development of full-scale pore characteristics. As coal rank increased, the specific surface area (SSA) of micropores and adsorption capacity of methane first decreased, then increased. Micropores were found to be the most important storage space for CBM and control gas adsorption, with a microporous SSA and PV to total SSA and total PV ratio of 97.93% and 63.69%, respectively. (4) We also observed a significant linear relationship between the fractal dimension of micropores and the Langmuir volume (V_L) based on fractal theory. As the fractal dimension increased, V_L also increased (R² = 0.8581), indicating that V_L is controlled by the complexity of micropores, which is consistent with the comprehensive evaluation index (D_t) and V_L (R² = 0.8744). Based on our predicted model, V_L can be estimated using the SSA of micropores and D_t. Our findings shed light on the relationship between pore morphology and CBM occurrence and have practical implications for fields such as catalytic synthesis, E-CBM, and gas purification.

Keywords: Adsorption; Fractal; Full-scale pore; Methane; coal bed.