Research On The Fracture Regularity Of Sandstone Excited By Electrohydraulic Shock Wave

The electrohydraulic shock wave(EHS)generated by the high voltage and high current pulse discharge in liquid has the characteristics of strong mechanical effect and repeatitive loading.When EHS repeatitive applied on the rock formation,it stimulates the formation and development of the rock crack network,expands the seepage and gas permeation channels,improves the rock formation porosity and permeability and has broad application prospects in the fields of oil and gas exploitation,rock cracking,etc.The law of crack propagation in rock formation stimulated by EHS mainly depends on the coupling mechanism between EHS load and rock physical properties(mechanics,acoustics and other characteristic parameters).At the same time,the effect of repeatitive EHS has certain cumulative expansion characteristic,and the process is relatively complicated.And it is difficult to determine the optimal discharge parameters and EHS loading method according to the physical properties of rock formation.Therefore,this thesis takes sandstones or sandstone-like samples as the research object to study the coupling mechanism of repetitive EHS and rock physical properties,and the excitation and propagation law of microcracks.It provides theoretical guidance for the application of EHS in rock fracturing and and fracture making.The main research contents and conclusions of this thesis are as follows:Firstly,considering the propagation and attenuation law of shock wave under the coupling action of EHS and rock physical properties,an EHS load model is established.An improved method for fitting the peak value of EHS is proposed,which can accurately characterize the peak value of EHS under different discharge energies and propagation distances.Based on the measured EHS waveform,a double exponential function fitting method considering the pre-peak time is proposed,which can accurately fit the EHS load waveform.Theoretical and shock wave test studies show that pre-peak time increases with the distance,and the greater the discharge energy,the smaller the growth rate of pre-peak time.On this basis,considering the shock wave transmission characteristics of the liquidsolid interface,an EHS loading model of liquid-solid composite medium is proposed.It can accurately calculate the peak value,pre-peak time,waveform and initial value of liquidsolid interface transmission wave for 10～120 mm near-field EHS.Secondly,the law of excitation and cumulative propagation of microcracks under the coupling action of EHS and rock physical properties is revealed.Using the PBM(Parallel Bond Model),the numerical simulation of the cross-scale cumulative failure process from microscopic microcracks to macroscopic large cracks in rock is realized.The experimentalnumerical simulation study of EHS on cement materials shows that the excitation and accumulation process of microcracks can be divided into three stages: microcrack excitation stage,type-I main crack propagation stage and microcrack deceleration accumulation stage.The research on the propagation of sandstone pre-cracks shows that as the pre-crack inclination angle decreases from 90° to 0°,under the same discharge energy,the accumulated number and total length of microcracks first decrease and then increase.And microcrack development pattern gradually changes from the microcrack network forming the liquid-solid interface to type-I crack forming the two ends of the pre-crack.Thirdly,an EHS cracking range model combining discharge parameters and stress intensity factor is proposed,and the influence mechanism of discharge energy on the distribution and propagation of crack network is revealed.The crack space network is affected by the discharge energy,and its expansion distribution is divided into dense area,crack area and non-rupture area from near to far.The EHS has a specific peak intensity threshold interval and a shock wave peak rising rate,which can excite the rock to produce only crack area.Increasing the duration and rising rate of the shock wave peak can effectively promote the expansion of the crack network.Experiments and three-dimensional numerical simulation studies have shown that after the dynamic initiation and expansion of microcracks in the rock are stimulated by the EHS,the formed type-I crack space network is radially dispersed to the cumulative fracture damage of the rock,which is beneficial to improve the overall damage of the rock mass.Finally,the law of crack propagation in rock excited by EHS under static pressure is revealed.Through the numerical simulation of sandstone,the inhibition law of static pressure on the expansion of sandstone pre-crack and internal crack space network is explained.Under the static pressure environment,the cumulative damage degree of rock mass shows the characteristics of "continuous deceleration growth",which is divided into "initial growth stage" and "deceleration flat stage".The cumulative damage model of rock is proposed,which can accurately characterize the relationship between damage and EHS loading times.The influence rule of the increase of static pressure on the damage degree is to lead to the slowdown of the cumulative damage in the "initial growth stage";the increase of the discharge energy can effectively promote the growth effect of the cumulative damage degree in the "deceleration flat stage".In this thesis,by studying the excitation and propagation law of sandstone or sandstone-like material microcracks,the distribution law of crack network and the influence law of static pressure environment on crack network propagation under EHS load,the theoretical support for the engineering application of EHS technology on rock fracturing is provided.