Research On Mechanism And Application Of Liquid Carbon Dioxide Phase Change Jet Fracturing Coal Seam To Increase Gas Permeability

Coal mine gas is an associated resources produced during the coal-forming process and is a major factor affecting the safe production of coal mines.It is also a non-renewable resource with reserves and calorific value comparable to natural gas.Therefore,realizing large-scale coal seam gas drainage is not only the fundamental guarantee for preventing mine gas disaster,but also the premise of comprehensive utilization of non-renewable resource.In recent years,with the increase of the depth of coal mining in China,the risk of dynamic disasters in deep coal mines has increased.How to realize the efficient extraction of deep coal seam gas resources has become an important issue to ensure the safe production of coal mine enterprises in China,and the low gas permeability coal seam stimulation is the core technology and hot issue.This thesis focuses on the mechanism of liquid carbon dioxide phase change high pressure gas jet to crack coal to increase gas permeability.Based on the theoretical basis of rock mechanics,seepage mechanics,aerodynamics and fracture mechanics,based on theoretical research,experimental research,numerical simulation research,field research and other methods.The liquid CO₂ phase change high pressure gas jet coal mass cracking experimental device was independently researched and developed.The dynamic characteristics of liquid CO₂ phase change high pressure gas jet impact kinetics were studied.The mechanism of liquid CO₂ phase change high pressure gas jet impact coal rock mass cracking and crack propagation mechanism was studied.The experimental study on the cracking and fracture propagation law of coal-bearing rock with liquid CO₂ phase change high pressure gas jet is carried out,and the mechanism of liquid CO₂ phase change high pressure gas jet cracking,pressure relief and permeability enhancement in low permeability coal seam is studied.On the basis of the above theoretical research,the research and development of liquid CO₂ phase change high pressure gas jet coal rock fracturing technology equipment has been carried out,and the application and industrial application in the Baijiao Coal Mine and the Shamushu Coal Mine of Sichuan Coal Group have gained valuable application effects.The main research results of this thesis are as follows:?1?The parameters such as coal rock material composition,microstructure characteristics,gas adsorption characteristics and basic mechanical properties of coal samples at the experimental area were obtained.Based on the theory of elastic mechanics,a Kaiser effect method in-situ stress calculation method which avoids the error caused by sampling angle deviation is proposed.The field test of stress relieving method is used to verify the Kaiser effect test results,which indicates that the calculation method proposed in this thesis is reasonable and reliable.?2?Focusing on the“kinetic characteristics of liquid CO₂ phase change high pressure gas jet impact”.The theoretical model of the exit velocity and mass flow of the high pressure gas impinging jet of liquid CO₂ phase change is obtained.And a theoretical model of quantitative liquid CO₂ phase change high pressure gas impingement jet outlet pressure is established.The self-developed"liquid CO₂ phase change high pressure gas jet coal mass fracturing experimental device"was carried out,and the experimental study on the impact dynamics of liquid CO₂ phase change high pressure gas jet was carried out,and the relationship between jet velocity and initial pressure of the system,jet impact force and system initial pressure,target distance and target angle were revealed.?3?Focusing on the"liquid CO₂ phase change high pressure gas impact cracking crack propagation mechanism"research,the coal rock body liquid CO₂ phase change high pressure gas impact cracking pressure and cracking model research,obtain the grounding stress condition of the inclined drilling hole wall cracking pressure theoretical equations,the method for judging the advantage cracking direction of inclined borehole under ground stress conditions is proposed.The variation curves of the impact shear fracture zone and the tensile damage zone radius with jet time and jet cross section are calculated.Theoretical studies have obtained an extended theoretical model of open cracks in gas-bearing coals under ground stress under the action of liquid CO2 phase change high pressure gas jet.The judgment basis of crack compression fracture under the action of liquid CO₂ phase change high pressure gas jet of gas-bearing coal rock under ground stress is established.The mechanical mechanism of liquid CO₂ phase change high pressure gas jet impact rock breaking and crack propagation is revealed.?4?Based on the self-developed"liquid CO₂ phase change high pressure gas jet coal mass cracking experimental device"for the"liquid CO₂ phase change high pressure gas jet impact cracking crack expansion law",the system carried out the coal rock body liquid CO₂ phase change high pressure gas jet impact damage macroscopic micro characteristic experimental study,liquid CO₂ phase change high pressure gas jet cracking and crack propagation law under triaxial stress conditions.Experimental study on the damage of coal rock with liquid CO₂ phase change high pressure gas jet and its macroscopic and microscopic characteristics shows that the threshold pressure of experimental coal sample is 17 MPa,and the area of the fracture-damage area increases with the increase of jet pressure.The number and size of pores and fractures caused by the impact of liquid CO₂ phase change high pressure gas jet increase with the increase of jet pressure,which can increase the pore volume of coal sample by 188.51%and increase the porosity of coal sample by 163.01%.The study of liquid CO₂ phase change high pressure gas jet cracking and crack propagation under triaxial stress shows that the technology can be used for coal seam cracking,and the size of the cracking crack is exponentially related to the initial pressure of the jet.As the initial pressure of the jet increases,the torsional tendency of the main crack decreases,the degree of undulation of the main rupture surface of the test piece decreases,the surface scratches decrease,and the number of internal micro-cracks increases.Influenced by the three-dimensional in-situ stress distribution,liquid CO₂ phase change perforation cracks will expand toward the direction of larger principal stress.Influenced by the bedding structure,the fractures of the through-holes mainly extend along the soft structural plane of the bedding,while the fractures of the bedding bores undergo a large change in direction when they extend to the vicinity of the bedding.The fracture extension of fractured coal rock mass is affected by the spatial location of the borehole and fracture.When the fracture surface intersects with the fracture hole,the generated high-pressure CO₂ gas enters the fracture surface,so that the test piece eventually breaks along the fracture surface,forming a complex fracture network;When the fracture surface is far away from the fracture hole,the failure process of the test piece is not affected by the fracture.?5?Based on the analysis of the mechanism of liquid CO₂ phase change high pressure gas jet cracking pressure relief and permeability enhancement in low permeability coal seam.The"triaxial servo-controlled seepage equipment for thermo-fluid-solid coupling of coal containing gas"was used to carry out the experimental study on pressure relief and permeability enhancement of gas-bearing coal mass.The theoretical study has established a model of coal mine rock mass based on cubic structure.The verification shows that the model can effectively reflect the gas seepage law during the process of pressure relief damage of coal mass.The coal seam gas pressure distribution model of the through-hole drilling extraction process is established,which indicates that the coal seam gas permeability,comprehensive compression coefficient,gas drainage time and drainage flow are the main factors affecting the coal seam gas pressure drop velocity.?6?The related technical equipment was developed around the application research of liquid CO₂ phase change high pressure gas jet cracking and anti-reflection technology in low permeability coal rock mass.The field test of Baijiao Coal Mine shows that the technology can effectively increase the gas drainage concentration and flow rate by 9¹2 times,and reduce the attenuation coefficient of gas drainage flow in the test area by 92%.A grid-type gas drainage method for increasing the permeability of coal seam by liquid CO₂ phase change high pressure gas jet cracking is proposed,which can improve the tunneling speed of roadway by 4-5 times.The application of the S3012working face of the Shamushu coal mine shows that this technology can improve the gas drainage efficiency of the coal seam by 15.71%compared with the conventional dense drilling method,and realize the zero overrun during the recovery of the high-burst dangerous working face of the soft coal seam in the stress concentration zone of the inclined axis.