Gas displacement characteristics during the water wetting process of gas-bearing coal and microscopic influence mechanism.

Gas and dust posed a threat to the safe working environment of miners. The key to the effectiveness of coal seam water injection in controlling gas and dust was the wetting of the coal body by water. The gas displacement characteristics were crucial for evaluating the wetting effect of coal. To investigate gas displacement characteristics during the water wetting process of gas-bearing coal, this study employed an experimental equipment of water wetting gas-bearing coal to test the gas displacement quantity under varying gas pressure conditions, and the microscopic influence mechanism was revealed. During the water wetting gas-bearing coal, there was an increase in the accumulated displacement gas quantity corresponding to rising adsorption equilibrium pressures, whereas the accumulated displacement gas rate decreased. Water wetting progressively reduced the system's Helmholtz free energy, reaching a minimum at equilibrium, indicative of a balanced wetting state. The displacement gas quantity curve initially rose linearly, reflecting rapid infiltration, then curved upwards more gradually as gravity and viscosity slowed the process, until reaching a relatively stable state. The curve mirrored the coal's saturation progression. The molecular formula of anthracite was CHNO. Elemental compositions of carbon, hydrogen, oxygen, and nitrogen matched elemental analysis results. The bridge carbon ratio of this molecular model was aligning with the bridge carbon ratio ascertained from the C NMR spectrum analysis. Above the coal-water interface, the relative concentration of water molecules increased with the increase of gas pressure. However, below the coal and water interface, the relative concentration of water molecules increased with the decrease of gas pressures. As the gas pressure increased, the adsorption degree of water molecules weakened, the degree of gas displacement in coal decreased, the diffusion coefficient of methane molecules decreased, the dispersion degree of water molecules increased, the aggregation degree weakened, and the diffusion coefficient of water molecules increased. The study results laid a theoretical foundation for revealing the mechanism of coal seam water injection wetting gas-bearing coal, achieving collaborative gas disaster prevention and pre-wetting dust reduction.