Macromolecular structure model of anthracite in southern Qinshui Basin and its methane bearing mechanical properties

The coal structure affected by geological factors can affect the gas production capacity of CBM wells,and under the actual geological conditions,coal reservoirs often contain methane,so the mechanical properties of methane bearing coal bodies are closely related to the effect of reservoir fracturing.By means of industrial analysis,element analysis,13C NMR and XPS,the molecular structure characteristics of Sihe anthracite in Qinshui Basin were tested and analyzed,including element composition,atomic ratio,type and distribution of functional groups.The macromolecular structure model was established,and the carbon content and density of the model were in good agreement with the measured values.The adsorption capacity,adsorption site and adsorption heat of methane in anthracite were simulated by Monte Carlo method,and the adsorption configuration of methane in anthracite was obtained.The results show that the saturated adsorption capacity of methane in anthracite is 22.4/cell,and the Langmuir pressure is 1.12 MPa.Aromatic carbon,pyridine nitrogen,pyrrole nitrogen and carboxyl group in anthracite model are the main adsorption sites of methane molecules.The isothermal adsorption heat decreases logarithmically with the increase of adsorption pressure,which indicates that methane occupies the high energy adsorption sites on the surface of anthracite at low pressure.The adsorption heat of anthracite is between 22.65-25 kJ/mol,far less than 42 kJ/mol,which belongs to physical adsorption.Molecular dynamics method was used to simulate the mechanical properties of anthracite.The effect of gas content on bulk modulus,Young’s modulus,shear modulus and Poisson’s ratio of anthracite was quantitatively studied.The results show that the bulk modulus,Young’s modulus and shear modulus of anthracite decrease logarithmically with the increase of gas content,while Poisson’s ratio increases linearly with the increase of adsorption capacity.Compared with the anthracite without methane,the bulk modulus,Young’s modulus and shear modulus decrease by 38.5%,24.4% and 27.1% respectively,which indicates that the adsorption of methane can significantly reduce the mechanical strength of anthracite.The mechanism is that the volume and expansion rate of anthracite increase exponentially with the increase of adsorption capacity,which makes the interaction force between anthracite matrix decrease,and then the strength of anthracite decreases and the ability to resist deformation is weakened.The van der Waals energy of anthracite decreases most in the process of adsorption of methane,which indicates that van der Waals energy is the dominant factor to maintain the stability of coal structure and mechanical properties.