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The engineering activities of deep stratigraphic resources development and space utilization are often subjected to complex stress conditions. In terms of gas-bearing coal resource mining, the dynamic mechanical properties of coal at complex loaded stress and gas environment were researched in this paper. Through the split Hopkinson pressure bar (SHPB), the dynamics experiments were performed, in which the factors of axial static load, circumferential confining pressure, internal gas pressure, and outside impact load were considered. Based on the stress and strain results of gas-bearing coal, the dynamic compressive strength and peak failure strain characteristics with strain rates were obtained. Under different loaded conditions, both dynamic compressive strength and peak failure strain added linearly along with strain rates increasing. In fact, high strain rates were the results of comprehensive loading conditions, so the effects of them to coal mechanical properties were discussed. And the improved dynamic material model of gas-bearing coal at high strain rates was proposed, which considered pre-damage of static load, hardening effects of strain rate, and weakening role of gas. Through using this model in numerical simulation, the plastic deformation and stress distribution of coal specimens subjected to impact failure were researched, which verified the results of dynamics experiments. Dynamics achievements of gas-bearing coal at complex loaded environment will provide guides for disaster prevention and promote green mining. [ABSTRACT FROM AUTHOR]

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]