Research On Fracture Mechanism And Crack Characteristics Of Shale Under Liquid-Electric Effect

High voltage discharge in water generates shock waves that destroy structures and has been maturely applied in engineering practices,such as machining of machine parts,in vitro crushing,rock crushing,rock drilling,metal descaling,and reservoir fracturing of oil and gas wells.In view of the poor permeability of my country's unconventional natural gas(shale gas,coalbed methane)reservoirs and the low efficiency of traditional hydraulic fracturing,some scholars have applied underwater high-voltage electrical pulse technology to unconventional natural gas extraction.It is found that this technology has the technical advantages of voltage controllable,repeated discharge,sufficient cracking,and safety and reliability.However,for the shock wave formation process,the relationship between the mechanical energy carried by the shock wave and the peak pressure still needs more detailed research.In addition,in the study of rock failure,the research on the mechanism of crack evolution is not thorough enough,not only did not fully consider the impact of dynamic impact load,the mechanical properties of the rock and the complexity of the reservoir on the crack propagation,but also did not analyze the law of crack evolution from a quantitative perspective.In the paper,Firstly,the composite fracture criterion of shock wave failure rock is derived by analyzing the process of shock wave generation and the propagation characteristics of shock wave in different media(water,rock)under liquid-electric effect.Secondly,the shock wave generated by discharge in water is tested,which not only analyzes the shape of the shock wave,but also studies the effects of discharge voltage(6?9 kV),water pressure(0?3MPa)and measuring point distance(0.5,1 m)on the mechanical energy conversion efficiency,wavefront energy and pressure peak.Then,through the drill water pressure(3MPa)discharge fracturing shale test,the influence of discharge conditions(3kV,5kV)on the crack geometry was quantitatively analyzed.Finally,the evolution of a single crack is simulated by the extended finite element(XFEM)method in ABAQUS,which not only analyzes the type of crack evolution,but also studies the quantitative influence of single crack angle,ground stress change and drilling fluid viscosity on the results of crack evolution.The research results are as follows:(1)When the discharge voltage increases,the arc channel radius increases,the resistance decreases,the ratio of arc energy to total electric energy decreases,and the mechanical energy efficiency of arc energy conversion decreases,but the ion channel deposition energy increases,which promotes the increase of ionization degree,and increases the energy of the shock wave front Which increases the peak pressure of the shock wave.As the water pressure increases,the arc channel becomes thinner,the arc resistance increases,and the deposited energy increases,and the mechanical energy and efficiency of the converted shock wave increase.The distance measurement increases,the shock wave transmission distance increases,the mechanical energy consumption increases,and the shock wave pressure decreases,but its conversion efficiency does not change.(2)In the experiments of underwater discharges induced cracking shale at 3kV and5 kV,it not only found different extended morphology,location,and quantity of cracks,but also found that the longer any single crack length,the smaller its average width.In addition,with the higher discharge voltage,not only the number of fractures increases,but also the extension width,extension length,fracture area and fracture rate of the fractures are larger,thus making the rock fracturing effect better.(3)Through the theoretical calculation and numerical simulation calculation of the fracture intensity factor of the compound crack,it is found that when the preset crack angle is 0° or 90°,the crack is a type I crack,and the crack initiates and fails under tensile stress,and the dislocation displacement and opening displacement of the crack Small.When the predetermined crack angle is between 0° and 90°,the crack is a composite I-II type crack,and the crack occurs under tensile shear stress.As the crack angle increases,crack dislocation displacement and crack initiation and opening displacement increase.In addition,the crack evolution shape is sawtooth,which also indicates that the crack is a tensile-shear compound crack.(4)Rock under the same water pressure and discharge voltage,crack initiation pressure increases,length decreases,width increases,area increases and fracture rate increases with the increase of preset single crack angle.In addition,the crack eventually extends along the direction of maximum in-situ stress.At the same crack angle,after 3kV and 5 kV of high voltage pulse discharge in water,the crack extension direction and morphology are basically the same,but the crack extension length,crack width,crack area and crack rate increase with the increase of discharge voltage.(5)When the preset crack angle is 0°,as the in-situ stress difference(4?0MPa)decreases,the crack initiation pressure increases,the extension length decreases,the crack widens,and the crack area decreases.At the same time,it is found that the key parameters of the crack evolution are greatly affected by the minimum main stress perpendicular to the crack direction,and the maximum main stress parallel to the crack direction has a relatively small impact on the crack evolution.During the increase of liquid viscosity from 1 to 40 m Pa·s,the crack initiation pressure and crack growth area are less affected,only slightly increased,the crack length is shortened,and the crack width becomes larger.