Research On The Characteristics Of Electrohydraulic Shockwaves And Rock Breaking Mechanism

At present,the exploration and development of oil and gas resources have gradually shifted from shallow formation to deep formation.The rock in the deep formation often has the characteristics of high hardness and high drillability,which leads to short bit life,long tripping time and low penetrate rate.In order to solve those problems,the technology of rock breaking by electrohydraulic shockwaves is proposed.Using this technology,a plasma channel is established in liquid medium and then powerful shockwaves are formed to break rock.In order to promote the development and field application of this technology,in this paper,we have studied electrohydraulic shockwaves and rock breaking mechanism by electrohydraulic shockwaves are studied,adopting theoretical analysis,laboratory experiment and numerical simulation.The main work is as follows:1.Combining the plasma channel impedance equation,the LCR equivalent circuit equation,the plasma channel dynamics equation,the shockwave pressure attenuation equation and the energy balance equation considering the ablation of the thin layer of liquid on the channel wall caused by heat conduction and radiation energy,the formation and propagation model of the electrohydraulic shockwave is established.The relationships of channel current,channel resistance,channel heating power,channel deposited energy,channel radius,channel pressure,and far-field pressure of the shockwave with time are obtained,respectively.Those calculated results are compared with experimental results to verify the accuracy of the model.2.Based on explicit dynamics software LS-DYNA,two existing methods to indirectly simulate electrohydraulic shockwaves are introduced: Underwater explosion equivalence(include explosion energy equivalence and shock wave energy equivalence)and ideal gas equivalence.Then,a new method called "shock wave pressure history matching method" is proposed.Based on the above methods,corresponding finite element models are built to simulate the far-field pressure,and the simulated results are compared with the experimentally measured pressure.The influence of deposited energy on above methods is analyzed to explore the applicability of above methods at different deposited energy.The results show that the waveform of the shockwave calculated by “shock pressure history matching method” is in good agreement with the experimental waveform.Moreover,the pressure of the shockwave simulated by this method can overcome the effects of different deposited energy.3.Based on the self-developed electrohydraulic plasma experimental setup,the experiment on concrete breakdown by electrohydraulic shockwaves are carried out.The results are as follows: 1)After several impact loads,the corner area of the cubic concrete upper surface is the first to be broken,which may be caused by stress concentration in the corner area;2)With the increase of impact frequency,the shape and number of surface cracks on concrete samples change in two stages.In the first stage,the number of microcracks increases rapidly,cracks connect with each other,and existing pores become larger gradually.In the second stage,the growth rate of the number of microcracks gradually decreases and the width of the formed cracks gradually increases.3)Based on the ultrasonic test,the internal damage of concrete sample can be described by damage degree.With the increase of impact frequency,the damage degree increases sharply at first and then slowly increases.4.The numerical simulation on concrete breakdown by electrohydraulic shockwaves are carried out.The results are as follows: 1)The concrete sample subjected to the electrohydraulic shockwaves is affected by four aspects: bubble;transmitted wave and reflected wave;diffraction wave;elastic precursor wave and plastic loading wave.2)The fractures on the rock are mainly caused by rarefaction waves.3)The corner areas of the upper surface of the cubic concrete are the first to be broken,which is mainly caused by the superposition of the reflected rarefaction waves in the corner area.This numerical result is consistent with the observed experimental result.5.The numerical study on the granite breaking by electrohydraulic shockwaves under different effect factors(discharge distance,charging energy,discharge times,confining pressure)is carried out.The sequence of the affection on crack formation by different effect factors is given by using the orthogonal analysis method.The results show that increasing the confining pressure,reducing the charging energy,increasing the discharge distance and reducing the number of discharges can inhibit the fracture formation of the rock.Under the condition that the charging energy is 2 k J,4 k J and 6k J respectively,the discharge distance is 3 cm,2 cm and 1 cm respectively,the discharge times are 3,6 and 9 respectively,and the confining pressure is 0 MPa,5 MPa and 10 MPa respectively,the sequence of the affection on crack formation by different effect factors is confining pressure > charging energy > discharge distance > discharge time.