The Foundational Research On The High Voltage Pulse Discharge Detonation Fracturing In Rock Mass Under Hydrostatic Pressure

The high voltage pulse discharge technology in water has many outstanding advantages such as good repeatability,energy controllability and safety,and it has been widely used in many fields.Recently,it has been also gradually applied in petroleum,coal bed gas and shale gas exploitation.However,the basic research of this technology is still in initial stage.The mechanism of high voltage pulse discharge in water was deeply discussed.The discharge breakdown characteristics,dynamic-load fracturing characteristics and influencing factors were analyzed.Some high voltage pulse discharge tests were performed under different charging voltage and hydraulic pressure conditions,and considerable achievements were obtained.The theoretical and fundamental experiments related to high voltage pulse discharge technology in water were preliminarily completed using coal bed gas exploitation as background,which provided reliable guidance and reference for the engineering application in future.The major research contents and results in this study included several aspects as below:(1)The gap gasification model of water in discharge process:The acceleration energy of particles is supplied by the strong electric field.The accelerated particles impact on the water molecules at head of fluid column,leading to gradually increasing gasified water molecules.When encountering the electron impact,these gasified water molecules can be ionized.The"gasification-ionization" cycle is developed to another electrode when the field strength is enough.The fluid column is connected with electrode gap,thus forming plasma channel and finally leading to breakdown.This kind of discharge breakdown with long time delay is called as thermodynamic breakdown.The whole thermodynamic breakdown process can be classified into four stages according to the functional features,i.e.,fluid column formation and connection stage,pre-breakdown stage,discharge breakdown stage and arc detonation stage.The mechanism analysis and numerical simulation of these four stages were carried out.(2)The high voltage pulse discharge characteristics in water were studied.The equivalent resistance and equivalent radius theories of initial channel were proposed and quantified to Zdx and RD/z_dx,which could be calculated by the detailed values.Wherein,Zdx was in order of 10?,while Rdlz_dx was in order of 0.1 ?m.When the hydrostatic pressure was fixed,the breakdown delay significantly decreased with increasing charging voltage(Um).It was found that the initial channel section was not greatly affected by charging voltage in range of 9-15 kV.The increase of charging voltage enhanced the ionization extent in initial channel.The velocity of "ionic current" was accelerated,resulting in the decrease of breakdown delay.When the charging voltage was fixed,the breakdown delay significantly increased with increasing hydrostatic pressure.The equivalent radius(Rdlz_dx)of initial channel was greatly affected by the hydrostatic pressure.The influence of increasing hydrostatic pressure on the initial channel section was the main factor causing increased breakdown delay when the charging voltage was fixed.(3)Strong shock waves can be generated by pulse discharge in water.In this research,the shock wave pressures at several sites were measured by experiment under different conditions of hydrostatic pressure,discharge energy,and propagation distance.The experimental results indicated that:An increase in hydrostatic pressure had an inhibiting effect on discharge breakdown;the shock-wave peak pressure increased with increasing discharge voltage at 0.5 m from the electrode.It increased rapidly at first and then decreased slowly with increasing hydrostatic pressure;and shock-wave attenuation slowed down with increasing breakdown energy and hydrostatic pressure during shock-wave transfer.The research investigates the effects of hydrostatic pressures and perturbation of wave source on normal reflection of one-dimensional unsteady flow shock waves.The results indicated that:Increased perturbation lead to increased peak pressure and velocity of the reflected shock wave,whereas increased hydrostatic pressure obviously inhibited superposition of the reflection waves close to the rigid wall.The perturbation of wave source influence on the reflected wave was much lower than that on the incident wave,while the hydrostatic pressure obviously affected both incident and reflection waves.The reflection wave from the rigid wall in water exhibited the characteristics of a weak shock wave,and with increased hydrostatic pressure,weak shock wave characteristics became more obvious.Bubble pulsation is directly reflected by the pulsation efficiency,which is mainly embodied in the transformation efficiency from the high voltage discharge energy to the bubble expansion energy,as well as the residual rate of bubble pulsation energy.The relationship between bubble pulsation efficiency with discharge voltage and hydrostatic pressure was analyzed by combining the experimental results of bubble pulsation expansion.(4)With theory of rock fracturing,similar experiments,the numerical simulation of coal-bed gas exploitation,analysis experimental specimen crack characteristics under different discharge voltages and confining pressures.Combined with the characteristics of the experimental actual CBM well numerical simulation calculation by ANSYS/LS-dyna,get discharge fracturing approximate rule.