Pore structure of deep coal of different ranks and its effect on coalbed methane adsorption

To study the influence of coal metamorphism on the pore system development characteristics of deep coal reservoirs, coal samples of different ranks were collected as research objects. Based on maceral identification and industrial analysis, the pore structure of the coal samples of different ranks was characterized on multiple scales by combining low-pressure CO2 adsorption (L-PA), low-temperature N2 adsorption (L-TA) and high-pressure mercury intrusion porosimetry (MIP) experiments. In this paper, the distribution and variation in pores in coal samples of different ranks were investigated, the relationship between the pore structure characteristics and coal metamorphism was examined, and the influence of coal metamorphism on the pore structure was analysed. CH4 isothermal adsorption experiments of the coal samples of different ranks were conducted to reveal the relationship between the Langmuir parameters and Ro,max. According to the shape of the hysteresis loop of the nitrogen adsorption–desorption curves, the pores in the low-rank coal samples mainly included slit pores and cylindrical pores, while those in the medium- and high-rank coal samples mainly included slit pores, semiclosed wedge pores and ink-bottle pores. There were significant differences in the pore structures of the coal samples of different ranks. With increasing metamorphism, the pore volume (PV) and specific surface area (SSA) first decreased and then increased according to a U-shaped trend, reaching minimum values at a Ro,max of 1.7 %. According to the characterization results of the coal sample pore diameter, the pore size distribution (PSD) type was mainly the unimodal micropore-dominated type. Coal micropores with pore diameters smaller than 1.5 nm largely contributed to the SSA and PV. In addition, the coal rank linearly increased with increasing Langmuir volume (VL), while the Langmuir pressure (PL) first decreased and then increased, indicating that the CH4 adsorption capacity increased with increasing metamorphism.