54 / 2023-05-15 21:55:13

Effect of methane adsorption on expanding the cell volume for coal-water molecules and tensile mechanical properties

methane adsorption swelling; tensile mechanical properties; coal-water molecular cell volume; diffusion coefficient; molecular simulation

Gas decompression for coal matrix is one of the most effective measures to prevent coal and gas outburst, interaction effects between methane and coal are the main drive force for swelling and deformation of coal matrix. Considering that there exists water in coal matrix itself, it has a certain impact on coal pressure relief. To discuss the effect of methane adsorption swelling and tensile mechanical properties, in this manuscript, according to the method of molecular simulation, two types of coal molecular models (CW-CM and RW-CM) with the same number and different shapes of water molecules are established to compare the change trends of methane adsorption capacity and space; The relationship between adsorption and swelling are clarified and influences of adsorption swelling on the volume expansion for coal-water molecular cell are investigated; Tensile mechanical properties of coal-water molecular cell are studied. The results demonstrated that as water molecular contents grow and for the same number of water molecules, the order of methane adsorption capacity and space is CW-CM＞RW-CM. Combining adsorption potential theory and for overall consideration of adsorption system, the relationship between adsorption space and potential under any water molecular contents and pressures can be expressed as piecewise linear and logistic regression of “four parameters” fitting, respectively. And adsorption potentials do not follow the change of water molecular contents. In CW-CM, no matter what water molecules exist or not, adsorption swellings increase continuously with the growth of pressure, which always present second order polynomial fitting function; In RW-CM, when water molecular contents are 0%-2%, after adsorption reaches saturation, their relationships are still second order polynomial fitting function; When water molecular contents are 3%-4%, after reaching saturation, adsorption swelling increase slowly, and their relationships are exponential fitting function; When water molecular content is 5%, only the contribution of adsorption to swelling is considered when the pressure is below 6 MPa, and its relationship tends to be linear fitting function; When water molecules are pre-existed into coal molecules, methane diffusion behavior is blocked, adsorption swelling and cell volume expansion weaken; And the higher the g(r) peak is, the stronger the repulsion to methane adsorption is and the more unfavorable adsorption swelling and volume expansion effect of coal-water molecular cell are. However, interaction total energy does not completely determine the volume expansion of coal-water molecular cell by adsorption swelling; Cell tensile strength is roughly divided into three stages, and the formation of a single atom (mainly sulfur) and some fragments. Whenever before and after methane adsorption and water molecular contents exist, the cell tensile strength in CW-CM is always totally greater than that in RW-CM. The decrease of strength is affected by the small pore volume and porosity in cell. And RW-CM provides a strong relaxation space for coal macromolecular cell structure. The research contributes to select the area with certain moisture contents and lower methane adsorption capacity, swelling and pressure, which can be conducive to the release of stress in coal matrix under macro conditions and offer enlightenment for methane replacement in some regions of working face for coal mine.