Stress Response Mechanism Of Coal Seam Induced By CO₂ Phase Transition Fracturing

CO₂ phase change fracturing technology was invented in the United States in 1914.For over a hundred years,this technology has been widely used in coal mining and rock excavation in urban environmentally sensitive areas in over 100 countries around the world.Since 2012,it has been widely used for efficient coal gas control in China’s coal mines,and has achieved great success in the field of gas outburst prevention.It has made contributions to solving the technical problems of gas control under the conditions of"high ground stress,low permeability,difficult extraction,and serious outburst"in China’s coal mines,and has become an irreplaceable proprietary technology in the field of coal mine gas control technology.However,there is little research on the stress wave propagation characteristics and stress response characteristics of CO₂ phase change fracturing coal and rock,which leads to the fact that the theory of seam making,pressure relief and permeability increase of coal seams under the application condition of this technology can only refer to the conventional explosive blasting theory,and cannot profoundly reveal the basic theory of CO₂ induced dynamic damage to rock bottom,nor can it directly guide the further high-level innovative development and popularization of this technology.In view of this,it is urgent to establish a dynamic load rock breaking theoretical model that matches the foundation of CO₂ fracturing technology,revealing the dynamic response characteristics and crack development laws of coal and rock after the application of this technology,and providing theoretical support for innovative high-level CO₂ phase change fracturing technology and technological progress in coal mine gas control.This thesis comprehensively adopts laboratory experiments,theoretical analysis, numerical simulation,and on-site experiments in coal mines.Using a self-made large-scale CO₂phase change induced cracking stress wave testing platform,the impact tests of different strength columnar specimens such as coal and similar materials were systematically completed under 120MPa and 185MPa cracking pressure conditions.Tested and analyzed the strain time history curves of the impact specimen near the jet hole end（near end）,in the middle part（middle section）,and far away from the jet hole end（far end）,introducing a stress wave attenuation coefficientα,Stress wave energy intensity I and energy transfer coefficientηThe laws of stress wave propagation and energy attenuation at the near,middle and far ends are studied and analyzed.Based on the industrial CT and Field Emission Electron Microscopy（FESEM）test results of coal samples from Gaohe and Yangquan in Lu’an,the development patterns and failure mechanisms of multi-scale cracks in coal under stress wave control were revealed.Finally,the time evolution and spatial distribution of strain and stress in CO₂ phase change induced cracking coal seams in underground coal mines were monitored for a long time using quasi distributed optical fiber and actively coupled stress gauge testing technology.The main achievements are as follows:（1）During the process of CO2 phase transition fracturing coal rock,the arrival time of the first peak compressive strain response of coal and similar materials is 245-830μs and 250-1050μs.The attenuation coefficients of stress waves are 0.68～1.01 and 1.03～1.31,respectively.The peak strain arrival time of coal and rock under the action of explosive stress waves（40-50μs）Compared with the stress wave attenuation coefficient（2.58-2.81）,the time is extended by5-20 times and the attenuation coefficient is reduced by 1.9-2.5 times.（2）The stress wave energy intensity I at the near,middle,and far ends of the coal specimen rapidly decays with the increase of propagation distance,which is consistent with the stress wave attenuation coefficientαThe evolutionary laws are consistent.Based on the macroscopic failure characteristics of the specimen and the evolution laws of strain at the near,middle,and far ends,it can be concluded that the stress wave is mainly caused by the compression stress wave formed by high-pressure CO2 fluid jet impact at the near jet hole end,forming a fragmentation zone;The tensile and shear failure zone formed by alternating stress waves in the middle;The compressive and shear failure zone formed by compressive waves and interface reflection waves away from the end of the jet hole.（3）Axial stress wave energy transfer coefficientηIt can characterize the development degree of internal cracks in coal specimens after CO2 phase transformation induced cracking,η.The smaller the area,the greater the energy dissipation in that area,and the more developed the cracks.The energy transfer coefficients of the fracture zone,tension shear zone,and compression shear zone areη0<η1<η2.At the micro scale,three regions have formed new microstructures,and the fragmentation zone is characterized by"damage mark"and"three wing"fractures formed by supercritical and gas-phase mixed jets;The tension and shear zone is characterized by alternating stress waves forming a serrated shapeμm tensile fractures;The coal matrix in the compression and shear zone is crushed,the crack opening is smaller but more complex,providing a channel for gas migration and improving the permeability of coal.（4）By analyzing the time history curves of stress and strain in the middle of the coal specimen,it is found that the CO2 phase transition fracturing coal specimen belongs to the medium strain rate loading condition,the strain rate is 14.12～24.18s^-1.The dynamic constitutive equation of CO2 phase transition fracturing（10²MPa pressure and ms level acting time）coal is constructed.The simulation results indicate that under the action of long duration and multi pulse loads caused by CO2 phase transition fracturing,a fragmentation zone,a crack zone,a stress wave vibration crack development zone,and an undisturbed original zone are gradually formed centered around the borehole.The long-slit effect of CO2 fracturing technology is impossible to achieve by explosive blasting,and it is also a unique advantage of large-scale seam pressure relief and efficient outburst elimination in coal mine gas control.（5）Under the action of dynamic and quasi-static loads,the original cracks in the coal seam near the center of the borehole are reopened,forming radial multi-scale,tensile fractures centered around the borehole,which can fully homogenize the stress concentration area and cause irreversible damage to the coal seam,releasing the elastic potential energy of the reservoir,with a stress disturbance range of 15～18m.There is no secondary stress concentration area in the coal seam,which fundamentally solves the problems of low permeability,difficult pumping and high ground stress of the coal seam.（6）The on-site gas extraction effect of CO₂ phase change fracturing technology in coal mines shows that the effective extraction radius of 42 days of fracturing drilling can reach 6-7m,which is more than twice the extraction radius of 2-3 meters of conventional drilling.The average gas extraction volume of 100 meters of drilling is 257 m³/d,which is 91%higher than the pure gas extraction volume of hydraulic hole drilling（134 m³/d）.