Molecular dynamics simulation of water absorption and mechanical weakening in coal rocks based on Monte Carlo methods

Abstract

Aiming at the problem of roadway destabilization in water encountered in the roadway of the back mining roadway of an extra-thick coal seam in Xinjiang Dabei Coal Mine, this paper used molecular dynamics simulation to study the interaction between coal and water and analyzed the adsorption characteristics of water molecules and the change of mechanical properties of coal. Firstly, the model of C₁₈₁H₁₃₈N₂O₂₄ bituminous coal was constructed by test, the coal-water adsorption was simulated based on the Monte Carlo method, and the coal-water adsorption configuration was analyzed. The results showed that: the saturation adsorption capacity was about 60 water molecules/cell, and the water molecule adsorption was concentrated in the vicinity of oxygen-containing groups and hydrogen atoms; the increase of water molecule content led to the decrease of heat of adsorption and diffusion capacity, and the heat of adsorption decreased by 9.1 %, and the diffusion coefficient of the early stage of adsorption was about three times of that of the final stage; the mechanical parameters of the coal body were significantly decreased, and the bulk modulus, Young's modulus, and shear modulus were respectively decreased by 21.90 %, 36.76 %, and 38.87 %, Poisson's ratio increased by 14.81 %, Poisson's ratio variability was low, volumetric modulus variability was medium, Young's modulus and shear modulus variability was high. The decrease in strength after coal water adsorption is due to the significant volume expansion of the coal body, the saturation expansion rate reaches 12.47 %, and at the same time, the total energy of the coal model decreases, where the weakening effect produced by the changes in the bonding and non-bonding energies results in the decrease in the mechanical strength of the coal molecules, and the weakening of the stability of the coal rock. The results of the study reveal the deformation and damage mechanism of the softening and deformation of the back-mining roadway in contact with water in Xinjiang Dabei Coal Mine, which provides a basis for the subsequent disaster prevention and control.