Pore structure of low-permeability coal and its deformation characteristics during the adsorption–desorption of CH4/N2.

The pore structure of coal plays a key role in controlling the storage and migration of CH₄/N₂. The pore structure of coal is an important indicator to measure the gas extraction capability and the gas displacement effect of N₂ injection. The deformation characteristic of coal during adsorption–desorption of CH₄/N₂ is an important factor affecting CH₄ pumpability and N₂ injectability. The pore structure characteristics of low-permeability coal were obtained by fluid intrusion method and photoelectric radiation technology. The multistage and connectivity of coal pores were analyzed. Subsequently, a simultaneous test experiment of CH₄/N₂ adsorption–desorption and coal deformation was carried out. The deformation characteristics of coal were clarified and a coal strain model was constructed. Finally, the applicability of low-permeability coal to N₂ injection for CH₄ displacement technology was investigated. The results show that the micropores and transition pores of coal samples are relatively developed. The pore morphology of coal is dominated by semi-open pores. The pore structure of coal is highly complex and heterogeneous. Transition pores, mesopores and macropores of coal have good connectivity, while micropores have poor connectivity. Under constant triaxial stress, the adsorption capacity of the coal for CH₄ is greater than that for N₂, and the deformation capacity of the coal for CH₄ adsorption is greater than that for N₂ adsorption. The axial strain, circumferential strain, and volumetric strain during the entire process of CH₄ and N₂ adsorption/desorption in the coal can be divided into three stages. Coal adsorption–desorption deformation has the characteristics of anisotropy and gas-difference. A strain model for the adsorption–desorption of CH₄/N₂ from coal was established by considering the expansion stress of adsorbed gas on the coal matrix, the compression stress of free gas on the coal matrix, and the expansion stress of free gas on micropore fractures. N₂ has good injectability in low-permeability coal seams and has the dual functions of improving coal seam permeability and enhancing gas flow, which can significantly improve the effectiveness of low-permeability coal seam gas control and promote the efficient utilization of gas resources. Highlights: The pore structure characteristics of low-permeability coal were obtained by fluid intrusion method and photoelectric radiation technology. The multistage and connectivity of coal pores were analyzed. A simultaneous test experiment of CH₄/N₂ adsorption–desorption and coal deformation was carried out. The deformation characteristics of coal were clarified and a coal strain model was constructed. The applicability of low-permeability coal to N₂ injection for CH₄ displacement technology was investigated. [ABSTRACT FROM AUTHOR]