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 C123H58N2O5. 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 13C 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.

Anisotropic Damage Mechanism of Coal Seam Water Injection with Multiphase Coupling.

After coal seam water injection, coal mechanical properties will change with brittleness weakening and plasticity enhancement. Aiming at the problem of coal damage caused by the coal seam water injection process, based on nonlinear pore elasticity theory and continuum damage theory, a nonlinear pore elastic damage model considering anisotropic characteristics is proposed to calculate and analyze the gas-liquid-solid multiphase coupling effect with the fully coupled finite element method during the coal seam water injection process. The research results indicate that the wetting radius of calculated results by the model agrees well with the in situ test results, and the relative errors are less than 10%. Water saturation and induced damage of the coal body in the parallel bedding direction are greater than that in the vertical bedding direction during the coal seam water injection process, which exhibits significant anisotropic characteristics. With the increasing water injection time, the induced damage of the coal body also increases near the water injection hole. Considering the inherent permeability arising with damage, it has a significant impact on both water saturation and induced damage, which also indicates that there is a strong interaction between water saturation and induced damage. The theoretical model reveals the coal damage mechanism of gas-liquid-solid multiphase coupling after coal seam water injection and provides a theoretical prediction of coal containing water characteristics in engineering practice.

Desorption Characteristics of CH4-C2H6 Mixed Gas in Heavy Hydrocarbon-Rich Coal Seams.

The gas desorption characteristics of coal are closely related to the gas content of the coal seam. The gas in heavy hydrocarbon-rich coal seams contains CH4 and C2H6 heavy hydrocarbons. However, most current research on the gas desorption characteristics of coal seams focuses on CH4 analysis, ignoring the influence of the C2H6 heavy hydrocarbon gas. To accurately determine the gas content of a heavy hydrocarbon-rich coal seam, methods based on CH4 analysis are inadequate and the desorption characteristics of CH4-C2H6 mixed gas must be clarified. This work experimentally and theoretically studies the desorption characteristics of single-component gas and CH4-C2H6 mixed gas from coal samples. The results show that increasing the adsorption-equilibrium pressure was found to increase the desorption quantity and desorption speed of single-component gas and increase the desorption quantity, desorption ratio, and diffusion coefficient of mixed gas. Under the same adsorption-equilibrium pressure, the desorption quantity and rate of single-component CH4 gas exceeded those of C2H6. The quantity and speed of mixed gas desorption increased with rising CH4 concentration and decreased with rising C2H6 concentration. The change in the mixed gas concentration during desorption reflects the distribution characteristics of light hydrocarbon components on the outer surface and heavy hydrocarbon components on the inner surface of coal. From the desorption characteristics of mixed gas, desorption models of mixed gas were obtained at different concentrations, laying a theoretical foundation for accurate determinations of gas contents in heavy hydrocarbon-rich coal seams.

Experimental Study on Coal and Gas Outburst Risk under Different Water Content Rates in Strong Outburst Coal Seams.

To investigate the alleviation potency of coal seam water infusion on coal and gas outburst, this paper focuses on the Qidong coal mine outburst coal seam, where outburst accidents have occurred many times, and obtains the impact of water content on outburst prediction parameters by studying the features of outburst parameters and gas desorption law under different water content rates. How water content affects outburst was also researched through the use of a self-made outburst simulation test system, and the relationship between water content and outburst intensity and critical gas pressure was studied. It can be concluded that with the rise of water content, the initial velocity of gas diffusion, the gas desorption index of drilling cuttings, and the adsorption constant a decrease, but the firmness coefficient (f) increase, and these indicators are exponentially related to the water content. Meanwhile, as the water content raises, the outburst pressure threshold increases, the outburst intensity gradually decreases, and the less likely outburst occurs. Under 0.5 MPa pressure, as the water content arose from 2.02 to 5.14%, the outburst intensity was significantly weakened, while no outburst occurred as the water content reached to 10.25%. Fitting analysis of the influence curve of outburst parameters and comparing the vital values of outburst prediction indexes finally determined that the water content rate of 5.14% could be used as a key index for water injection measures for coal and gas outburst prevention coal seam in Qidong coal mine no. 9. This research offers a guiding significance for the outburst prevention measures of water infusion in outburst coal seams and gives a feasible scheme for the safe mining of outburst coal mines.