Molecular dynamics simulation of helium adsorption-diffusion behavior in nanopores of anthracite from Permian Shanxi Formation in Qinshui Basin, China

To understand the occurrence and migration mechanisms of helium in coal-measure gas, this study investigated anthracite from No. 3 coal seam of Lower Permian Shanxi Formation in the Jincheng mining area of the southern Qinshui Basin, China, through experiment analysis and molecular simulation. Firstly, low-temperature CO 2 and N 2 adsorption experiments were performed to characterize the multi-scale pore structure of anthracite, and accordingly, a 1–10 nm slit pore model of anthracite was constructed. Using grand canonical Monte Carlo, molecular dynamics and non-equilibrium molecular dynamics simulation methods, the effects of pore size, temperature, pressure, pore water, and associated gases (CO 2 , CH 4 , N 2 , H 2 ) on helium adsorption, diffusion, and pressure-driven flow were systematically examined. The results show that helium exhibits weak enrichment near wall in nano-scale slit pores and coexistence of free gas in the pore center. Its interaction with coal wall is significantly weaker than those of CO 2 , CH 4 and N 2 , and is relatively close to that of H 2 . Pore size, temperature and pressure all affect helium diffusion and flow behaviors. Smaller pores reduce helium diffusion coefficient and increase its viscosity, whereas larger pores facilitate helium migration. Pore water weakens continuous gas-phase pathways through film formation, permeation and interfacial friction, causing a sudden drop in helium diffusion coefficient at high water content stage. Strong adsorption of associated gases (CH 4 and CO 2 ) will further suppress helium diffusion, while H 2 as a lightweight carrier can enhance the diffusion ability of helium and H 2 itself to a certain extent. The research results provide theoretical references for understanding the microscopic migration and occurrence characteristics of helium in coal-measure gas.