Study On The Induce-crack Mechanicals Of Coal Rock Mass By Tube-expansion Under Hydraulic Drive

Gas disasters seriously affect the safety of coal mine production,and gas extraction is the key to gas disaster management.Among them,the reform of coal seam structure is the main means to improve the gas drainage rate.Aiming at the reform of coal seam structure,this paper proposes a technology of coal seam tube expansion and crack penetration enhancement.That is,the hydraulic pressure causes the high-pressure expansion tube to physically expand,and the expansion force is concentrated on the coal layer of the borehole wall,causing it to rupture,thereby achieving the purpose of increasing permeability.In this paper,the new technology of"coal seam hydraulically driven pipe expansion and cracking"is studied from two aspects:theoretical analysis and experimental research.The main research process and results are as follows:?1?Based on the mechanism of coal rock pipe expanding and cracking,combined with the theory of elastic mechanics and fracture mechanics,on the basis of analyzing the stress distribution around the borehole wall before the coal rock pipe expanding and cracking,the fracture failure criterion of the maximum tensile stress theory is adopted to study the initiation pressure of coal rock pipe expanding and cracking under different strength and different stress combinations Under the influence of the main stress,dip angle,tensile strength of coal and rock mass,expansion pressure in the hole,radius of the hole and other factors,the fracture initiation pressure calculation model of coal and rock mass pipe expansion crack at the hole wall is as follows:P_W=? ?₁+?₃?+2??₁-?₃?cos2?+T?2?Using a self-designed tube expansion and cracking test system,using low-density foam concrete similar materials to simulate coal and rock masses,a similar simulation test of tube expansion and cracking was carried out,revealing the law of tube expansion and cracking propagation of coal and rock masses under different stresses:Under no confining pressure conditions??₁=0,?₃=0?,The flatness of the crack increases with the increase of the strength of the specimen,The main performance is as follows:for the specimens with low strength,the cracks are mostly rough curves.With the increase of specimen strength,the cracks become more and more flat to a certain extent,and the failure type tends to brittle fracture;high-strength specimens generally produce two symmetrical through cracks with the expansion tube as the center,and low-strength specimens generally produce three star-shaped cracks with the expansion tube as the center;under confining pressure,When ?₁ and ?₃ are basically the same,the cracks of the specimen are generally star-shaped toward the four corners of the specimen with the expansion as the center.As the difference between ?₁ and ?₃ gradually increases,the crack propagation direction of the main crack is mainly vertical In the direction of the minimum principal stress,a certain turning and bifurcation phenomenon will occur when it extends to the edge of the specimen,and it will be accompanied by the generation of secondary cracks.?3?Using Acoustic Emission?AE?to study the law of fracture and failure during coal fracture,And take the sound emission ring count as the damage variable,The damage curve of the coal rock mass during tube expansion and cracking was fitted,which revealed the damage degree of the coal rock mass in different periods.Under the condition of no confining pressure??₁=0,?₃=0?,The acoustic emission ringing of coal and rock masses is mainly concentrated in the initiation stage of tube expansion and cracking.At this stage,the damage degree of coal and rock mass is about 0.67⁰.71,which is the main stage of fracture and failure of coal and rock mass.Under confining pressure conditions,with the increase of the difference between ?₁ and ?₃,the damage degree of coal and rock mass in the initiation stage gradually increases,the damage degree in the cracking stage gradually decreases,and the damage degree in the extension stage is small fluctuation.In summary,the tube expansion and cracking technology can efficiently apply hydraulic energy to the surrounding rock of the borehole,with high cracking efficiency and good permeability enhancement effect.The research results of this paper will provide theoretical and experimental support for the application of this technology.