Study On Dynamic Mechanical Response And Permeability Enhancement Mechanism Of Coal Under High-pressure Gas Impact

The low-permeability of coal seam is the main bottleneck restricting the high-efficiency extraction of coalbed methane.As a modern waterless physical expansion cracking measure with simple operation,safety and efficiency,economic and environmental protection,high-pressure gas impact cracking technology is widely used in the field of low permeability unconventional gas reservoir permeability enhancement.However,the theoretical research on cracking coal caused by high-pressure gas impact is still in the initial stage,and the relevant research is not perfect.Therefore,focusing on the dynamic response induced by high-pressure gas impact on coal and pore fracture evolution and antireflection mechanism of coal,this paper systematically studies the dynamic mechanical response and antireflection mechanism of coal under high-pressure gas impact by comprehensively using the methods of literature research,theoretical analysis,indoor test and numerical simulation.The main research contents are as follows:（1）Based on theories such as gas impact dynamics,elastic mechanics,and damage fracture mechanics,the flow characteristics of high-pressure gas impact in the crack hole were analyzed,and the calculation model of the blasthole wall pressure under the action of high-pressure gas was deduced.The crack criterion conditions of coal under single and multiple high-pressure gas impacts are determined.The stress field distribution and fracture damage mechanism of the specimen under high pressure gas single impact were expounded.The cumulative evolution process of damage and deterioration of coal under high pressure gas multiple impacts was described.（2）Indoor simulation test of high-pressure gas single impact on coal was carried out.The characteristics of borehole wall pressure under high pressure gas impact were obtained using a PVDF sensor.The strain,surface vibration velocity,and crack propagation velocity in different areas of coal under high pressure gas impact were measured using dynamic strain gauges,blasting vibration collectors,and detonation velocity meters.The propagation of stress wave,vibration response and crack growth in coal under high pressure gas impact were analyzed.The dynamic mechanical response and fracture failure characteristics of coal under the coupling action of confining pressure and high-pressure gas impact were investigated.The fracture mechanism of coal under high pressure gas impact was revealed.（3）Indoor simulation tests of high-pressure gas multiple impacts on coal were carried out.The strain and wave velocity of coal under high-pressure gas multiple impacts were tested using dynamic strain gauge and an ultrasonic testing acquisition instrument.The influence of the impact times on the strain propagation law,acoustic evolution characteristics and failure forms were analyzed.Scanning electron microscope（SEM）and mercury intrusion test（MIP）were used to analyze the change rule of impact times on pore-fracture structural parameters from the microscopic point of view.The dynamic evolution process damage and stress wave propagation of coal medium under high-pressure gas multiple impacts were studied.The fracture and cumulative damage mechanism of coal medium under high-pressure gas multiple impacts were revealed.（4）The evolution law of spatial geometric topology structure in different regions of coal under high pressure gas impact was studied.Based on the simulation test of the high-pressure gas single impact on coal.The 3D visualization models and ball-stick equivalent network models of the samples at different regions from the blasthole after high-pressure gas impact were constructed by using CT scanning and 3D reconstruction technology.The evolution characteristics and laws of pore fracture structure in different areas of coal media under high pressure gas impact were analyzed by using porous media theory and topology.Based on the change of porosity,the disturbance range under high pressure gas impact was preliminarily determined.（5）High pressure gas impact test of natural coal was carried out,the microscopic scale visualization models of natural coal before and after impact were established.The visualization CH₄ seepage simulation in the topology structure of connected pore and crack space at the microscopic scale of coal was realized.the formation mechanism and distribution characteristics of seepage channels in the representative units before and after high-pressure gas impact were analyzed.The migration and diffusion process and distribution law of gas in the microscopic porous medium structure were explored.The mechanism of gas seepage and migration was revealed from the microscopic perspective.There are 86 figures,29 tables,and 227 references in this dissertation.