Tests And Numerical Simulation Study On Shale Crack Propagation Under Impact Load

In the process of shale gas deflagration fracturing,the distribution of main fractures,secondary fractures and micro fractures in shale fracture network affects the shale gas exploitation efficiency.Therefore,it is urgent to master and control the fracture propagation and the final formation of fracture network in shale during fracturing.The study on fracture propagation law of shale under high strain rate impact is an important research content to solve the formation and development of cracks in the process of cracking.The research on shale fracture propagation under impact is carried out from the aspects of fracture propagation mechanism,shale mechanical properties and initial fracture distribution,which is of great significance to obtain the law of shale fracture initiation and propagation under different load levels,so as to realize the efficient and reasonable exploitation of shale gas in practical engineering.Through the static splitting test and dynamic splitting test of specimens with different loading angles,the crack propagation law and mechanical properties of shale are analyzed.Combined with the test,the numerical simulation research on the influence of different factors on the crack propagation of shale under impact is carried out,and the following conclusions are obtained:(1)In the static experiment,specimens with different loading angles are split tensile failure.The specimens with loading angles of 0° and 45° are the main cracks formed by the initiation and propagation of the initial crack tip,and the specimens with loading angle of 90° are the main cracks formed by the initiation and propagation of the intersection of the initial crack and the diameter loading direction.The specimen with loading angle of 0° has secondary crack propagation due to the stress concentration at the loading end.The specimen with loading angles of 45° and 90° has secondary crack propagation due to the tensile stress at the edge of the specimen during the main crack propagation.The load-displacement curves of specimens with different loading angles are collected by static test.The peak load of specimens with 0° loading angle is the largest and the failure time is the latest.The peak load of specimens with 90° loading angle is the smallest and the failure time is the earliest.The splitting strength of the specimen is negatively correlated with the loading angle.(2)The crack propagation law of specimens under 0.1MPa loading pressure in dynamic splitting test is similar to that in static test.With the increase of air pressure,more secondary cracks initiate on both sides of the main crack of the specimen,resulting in more serious damage to the specimen.However,the propagation law and mechanism of the main crack and secondary crack of the same specimen are the same.The existence of initial cracks reduces the amplitude of transmitted wave.The stressstrain curves of specimens with different loading angles were obtained.The splitting strength and dynamic fracture toughness of shale increase with the increase of strain rate,showing strain rate correlation.(3)Through the numerical simulation of shale fracture propagation under impact,it is found that there are resident phenomena and velocity oscillation phenomena in the process of shale fracture propagation.The dynamic stress intensity factor decreases with the increase of initial crack length,initial crack number and initial crack angle.When the initial crack length is 23 mm,the initial crack number is 3 and the initial crack angle is 45°,the crack propagation of the specimen has less damage to the specimen,and can form more cracks,which is conducive to the formation of the crack network.