Study On The Characteristics And Influence Mechanism Of Coal Seam Gas Diffusion Under Temperature-stress Conditions

Coal mine methane is a clean energy source with methane as the main component,which is coexisting with coal in the coal seam,and the efficient utilization of methane can produce triple benefits of safety,energy,and environment.With the increase of the mining depth of coal resources,the significantly changing geological conditions such as stress,environment temperature and gas pressure in coal seams compared with shallow coal seams seriously hinder the safe production of coal mines and the efficient utilization of coal seam gas.Therefore,it is significant to master the gas diffusion characteristics under different temperature-stress conditions for the development of coalbed methane resources and disaster prevention and control.In this thesis,the dynamic characteristics and influence mechanism of gas diffusion under different temperature-stress conditions are analyzed by using experimental and numerical simulation methods with the coal of No.7 coal seam in Linhuan coal mine as the research object,and the main conclusions are as follows:(1)The pore and crack structure of the coal body was observed and characterized by a combination of optical observation method and fluid intrusion method.The gas adsorption capacity of low-pressure section under different temperature conditions was measured,and the ultimate gas desorption capacity under the corresponding conditions was calculated,while the exponential factor was introduced to modify the Langermuir equation,and the isothermal adsorption curves under different temperature and pressure conditions were derived by the modified equation to achieve a well-fitting result.(2)Based on the morphology of the experimental coal samples,spherical and flat matrix units were selected,and the diffusion models applicable to coal grains and cylindrical coal were established.Under the condition of non-stress constraint,the classical single-hole model cannot accurately describe the whole process of gas desorption,and the classical single-hole model is modified by introducing time-varying diffusion coefficients to achieve a well-fitting effect.For the gas diffusion process in the coal under stress constraint,the desorption process has a stable desorption rate,and the diffusion process can be well described by the plate-type diffusion model.Analyzing the applicability of the model with the simulation results,the result shows that the diffusion coefficient calculated by the measured data is the apparent diffusion coefficient considering the seepage process,and when the stress increases to medium-high stress,the seepage effect has very little effect on the apparent diffusion coefficient,which can be approximated as the apparent diffusion coefficient equals to the effective diffusion coefficient.(3)Using high-stress gas desorption device to carry out gas desorption experiments in coal under different temperature,stress and gas pressure conditions,the differences in gas diffusion dynamic characteristics under different stress conditions were compared by characteristic parameters such as desorption volume,desorption rate,and diffusion coefficient.The results show that the effective diffusion coefficient grows exponentially with increasing ambient temperature,and the change process can be described by the Arrhenius formula,increases linearly with the increase of gas pressure,and decreases exponentially with the increase of in-situ stress on the gas diffusion in the coal.The experiments of unstressed constrained gas desorption were carried out,and the experimental results showed that the complex pore and crack structure of the coal body had a great influence on the gas desorption process.Without stress constraint,the changes in temperature have a large effect on the structure of coal,and the effect of gas pressure change on the structure of coal is not obvious.(4)By analyzing the effects of different temperature,stress and gas pressure conditions on gas molecules and on the physical structure of the coal,we investigated the mechanism of the effects of different in-situ conditions on the dynamic characteristics of gas diffusion: the increase of gas molecules’ energy triggered by the temperature increase is the key factor of the temperature-promoted diffusion process.The increase of gas pressure affects the gas diffusion situation,improves the activation ability of gas molecules,reduces the effective stress on the coal body and leads to the expansion and deformation of matrix adsorption,and the above factors jointly affect the diffusion process.In-situ stress increases compressing the pore and fissure space in coal,thus affecting the diffusion process.(5)Through the coal deformation equation,gas flow equation,temperature-dependent gas diffusion coefficient equation,and in-situ coal permeability evolution law,the coupled model of gas transport is constructed to be applicable to different deep in-situ locations.The accuracy of the model was verified by comparing it with the experimental results,based on which we analyzed the in-situ stress as the main controlling factor of the diffusion process,and the sensitivity of the gas diffusion process to the temperature and gas pressure decreases with the increase of stress.For deep coal seams,depressurizing the coal seam and increasing the seam permeability is the best way to improve extraction efficiency.For the coal seam after decompression,heat injection can significantly promote the gas diffusion process to increase the gas recovery rate.This thesis has 53 figures,16 tables,and 100 references.