Molecular dynamics simulation of the impact of small molecules in coal on CH4 adsorption, diffusion, and micromechanical properties

The presence of small molecular compounds (SMCs) within the macromolecular structure of coal significantly influences CH4 adsorption, diffusion, and micromechanical properties. However, the quantitative effects of SMCs on isolated pores and micromechanical characteristics have not been thoroughly investigated. In this study, we developed a model of long-flame coal incorporating SMCs and employed molecular dynamics simulations to analyze the extraction process. This approach enabled us to quantitatively evaluate the impact of SMCs on the micropore structure and micromechanical properties of coal. The experimental results indicate that the dissolution of SMCs leads to the compression of smaller pores within the coal matrix, while simultaneously causing the expansion of larger pores and the formation of new pores. Collectively, these changes result in an 18.9% increase in micropore volume, which enhances CH4 adsorption energy by up to 6.67%. Furthermore, SMCs exhibit significant blocking and separating effects on the microporous structure. The dissolution of SMCs results in a 31.1% increase in connected pore volume, a 23.1% reduction in isolated pore volume, and an 11.9% improvement in micropore connectivity, leading to a 20.15% enhancement in CH4 diffusion capacity. The interaction of SMCs with the coal macromolecular structure is primarily mediated by van der Waals forces. Notably, the dissolution of SMCs significantly reduces the deformation resistance of the coal matrix, resulting in decreases of 21.2%, 11.8%, 5.9%, and 9.1% in bulk modulus, shear modulus, transverse wave velocity, and longitudinal wave velocity, respectively. Additionally, the dissolution process induces a rearrangement of the coal macromolecular skeleton, altering the shape of the macromolecular units and decreasing the degree of directional ordering by 10.1%. Our findings provide valuable insights into the effects of SMCs on the adsorption capacity and micromechanical properties of coal, highlighting their critical role in influencing gas storage and transport in coal seams.