Mechanism Of Coal Fracturing And Permeability Enhancement Under Resonance Excitation

Artificial permeability enhancement technology for low-permeability coal seams is an important means to improve the recovery rate of coalbed methane and safe production in mines.With the exploration of more environmentally friendly and effective permeability enhancement methods,low-frequency vibration excitation reservoir resonance permeability enhancement technology has also received widespread attention.Low-frequency vibration is generated by a mechanical vibration source,which excites the coal to generate forced vibration and generate a large response near its natural frequency,promoting the permeability of the reservoir.The research on the natural frequency of coal and the structural changes of coal under resonance are currently fundamental research topics that need to be addressed urgently.The fracture structure of coal is the main seepage channel of reservoirs,and its structural change mode and degree have an important impact on the permeability enhancement effect.Therefore,this paper uses a combination of theoretical analysis,laboratory experiments,and numerical simulation to study the mechanism of resonant fracturing and permeability enhancement of coal excited by low-frequency vibration,in order to provide better theoretical guidance for the application of low-frequency vibration technology to increase permeability in coal seams.Based on the research of this thesis,the following results have been achieved:1)By analyzing the dynamic response of coal under forced vibration and mechanism of vibration induced fracturing,the dynamic essence and mechanism of resonance induced fracturing of coal are revealed.Firstly,the displacement and energy response laws of coal under low-frequency vibration are theoretically studied,and the effects of excitation parameters and inherent properties of coal on the forced vibration response characteristics of coal are analyzed.The phenomenon of amplitude amplification under resonance conditions is pointed out and the resonance frequency band range is defined.The results show that the displacement response and energy response of coal exhibit significant resonance amplification effects in the resonance frequency band.The smaller the damping ratio,the more obvious the resonance amplification effect.Based on the theory of fracture mechanics,the fracture failure criterion and damage mechanism of vibrating coal are analyzed.Based on the vibration fatigue theory,the initiation and propagation laws of coal fractures under resonance excitation are analyzed,and the physical mechanisms of resonance induced fracturing are summarized as fatigue failure,friction effect,and temperature rise effect.2)The experimental testing method based on the hammer method and the ANSYS harmonic response analysis method are used to carry out the relevant research on the natural frequency of coal.The modal characteristics of coal were systematically studied considering conditions such as coal size,moisture,and stress loading,the results show that within a sampling frequency of 500 Hz,coal generally have two order natural frequencies and their values are relatively low.The increase in water content decreases the natural frequencies of coal bodies by 15.6% to 34.3%.Combined with the axial pressure loading system,it was found that the main frequency of coal vibration gradually increases with the increase of axial pressure and the rate of change changes from fast to slow,but the natural frequency of the loading system has a significant impact on the test results.In addition,the magnitude of the vibration excitation amplitude does not affect the test results of the natural frequency.The mode shape is related to the geometric shape and boundary conditions of the coal,but has little correlation with the heterogeneity of the mechanical parameters.3)The experimental system of coal excited by low-frequency vibration was set up,and the experimental study of resonant fracturing coal was carried out.Based on the study of natural frequency of coal,the resonance fracturing experiment of coal was carried out,and the change characteristics of fracture structure of coal under resonance excitation were studied by CT scanning,and the quantitative analysis of fracture evolution was carried out by MATLAB.The results show that the resonance excitation promotes the extension of the fracture and gradually forms the cross network of "Y" type and "well" type.Under the resonance excitation,the coal damage accumulates gradually and the excitation damage in the resonance frequency band shows the divergent type.There are three distinct characteristics of fracture initiation in coal under resonance excitation,namely,slip induced fractures,phase interface induced cracks,and formation of voids.In addition,the evolution of fracture structures is accompanied by the evolution of natural frequencies.The damage of fractures gradually reduces the natural frequency of coal.4)The influence of excitation frequency and excitation time on coal seepage characteristics was studied through low-frequency vibration seepage experiments.A dual-porosity permeability model of coal under low-frequency vibration was theoretically established,and the deformation and seepage control equations of the vibrating coal were derived.The heterogeneity of the coal was considered and solved using COMSOL numerical simulation software.The experimental results show that the permeability increase of coal exhibits a change rule of first increasing and then decreasing with the increase of excitation frequency,and the increase is small under excitation far from the resonance frequency band.The permeability increase characteristics exhibit significant resonance correlation.The permeability of coal gradually increases with the increase of vibration time,indicating that the damage of coal under vibration excitation is a cumulative damage that evolves with time.The simulation results show that the fracture structure of coal has a significant impact on the stress distribution of coal under vibration,and the tip of the fracture produces a stress concentration effect.The magnitude of the vibration excitation amplitude affects the average stress on the coal,and the resonance effect promotes the flow of gas.The dissertation includes 104 figures,22 tables and 215 references.