| Rock breaking technology is the core of drilling engineering,and the efficiency of rock breaking determines the drilling speed and engineering cost.Although mechanical drilling has made significant progress in rock breaking technology,it can not avoid the problem of bit wear during drilling.When drilling hard formations,severe bit wear leads to frequent tripping,which is time-consuming,laborious,and significantly increases drilling costs.As the exploitation of oil,gas,and geothermal resources continues to advance to deep strata,the problem of hard rock fragmentation becomes more prominent,and it is increasingly important to find new and efficient drilling rock breaking methods.The rock breaking technology of pulsed discharge plasma drilling has many advantages,such as high rock breaking efficiency,no wear on the drill bit,strong controllability,safety and environmental protection.It has been highly expected since it was proposed.However,plasma drilling technology has always been unable to be commercialized.An important reason is that the mechanism of pulse discharge breakdown rock breaking has not been mastered,which can not provide strong theoretical support for the development of technology.In this paper,considering the whole physical process of electrical crushing,using multidisciplinary theory,using the combination of physical model experiment,simulation and theoretical modeling,the mechanism of rock breaking by pulse discharge breakdown is studied.The main work and achievements are as follows :1.A set of high-voltage pulse discharge rock breaking experimental device with pulse steepening function was developed independently,and a systematic electrical breakdown rock breaking experiment was carried out.The influence of high-voltage pulse discharge breakdown on rock breaking was comprehensively summarized.Studies have shown that discharge voltage,electrode spacing,and rock porosity are the main factors affecting electrical breakdown.Higher discharge voltage,faster pulse speed and higher rock porosity are conducive to the occurrence of electrical breakdown.There is an optimal value of electrode spacing to make the best electrical breakdown effect.When designing the electrode drill,it is necessary to optimize the electrode spacing to ensure the effectiveness of pulse discharge.2.The maximum of energy utilization rate measured by the experiment is 36.1 %,the minimum is 18.0 %,and the average is 26.6 %.The energy utilization rate increases first and then decreases with the increase of electrode spacing,and is optimal when electrode spacing is25 mm.The energy utilization rate increases with the increase of porosity,indicating that rock pore defects have an important influence on electrical breakdown rock breaking.The energy utilization rate increases first and then decreases with the increase of discharge voltage,and is optimal between 40 k V-60 k V.The selection of discharge voltage should take into account the success rate of electrical breakdown and energy utilization,as well as the insulation,life,safety and cost of drilling equipment.3.The experimental results show that the breakdown probability,energy utilization rate and elastic modulus are weakly correlated.The greater the elastic modulus,the more favorable the electrical breakdown and energy utilization efficiency of the rock.Rock strength has little effect on electrical breakdown and energy utilization efficiency.The mechanical test of the rock after discharge breakdown was carried out.The average compressive strength of the rock decreased by 39.68 %,and the average tensile strength decreased by 44.74 %.It shows that even if there is no obvious apparent damage trace,electrical breakdown will cause internal damage to the rock,resulting in a decrease in strength.Therefore,how to ensure the success rate of rock electrical breakdown in engineering applications is very important.4.A mathematical model and simulation method of electric field distribution of porous rock under strong electric field are established.The electric field distribution law of rock under high voltage electric field excitation and the distortion effect of pores on electric field are analyzed.It is found that discharge voltage,electrode spacing,rock thickness,lithology,insulating liquid medium and rock pore all affect the distribution of electric field.Rock pore is the main factor of electric field intensity.Rock pore defects will cause electric field distortion.The electric field intensity inside the pore is 1.4 times that of the rock matrix.Discharge breakdown should first occur in the rock pore.5.From the point of view that the rock pores are first broken down,it is considered that the rock electrical breakdown conforms to the theory of weak point breakdown.In order to explore the physical nature of rock electrical breakdown,based on the Thomson discharge theory and streamer theory,the critical electric field model of rock pore breakdown under electric field is established,and the discharge breakdown criterion of porous rock is established.Based on the theory of partial discharge,the thermodynamic model of rock pore discharge is established,and the mechanical behavior characteristics of pore discharge in elastic rock is analyzed.Combined with the characteristics of rock pore structure,a discharge breakdown mode of porous rock is proposed.6.The model of plasma rock breaking is established,and the power characteristics,wave dynamics characteristics and rock stress state of plasma shock wave are analyzed theoretically.Studies have shown that the larger the discharge energy and the faster the deposition rate,the greater the peak pressure of the shock wave.The discharge energy and discharge time can be controlled,and the discharge system with short oscillation period can improve the rock breaking efficiency.The plasma rock breaking is dominated by shock wave.After the shock wave acts on the edge of the channel,the energy decays rapidly and transforms into a mechanical stress wave.During the propagation process,the tangential stress of the stress wave will change between compressive stress and tensile stress,resulting in tensile fracture failure of the rock.Accordingly,the rock failure zone is divided into compression fracture zone,tensile fracture zone and compression deformation zone.7.The rock breaking experiment of plasma shock wave was carried out,and the failure process,fracture effect and failure law of rock were observed.Under the action of shock wave,the shock wave first causes the compression fracture zone at the edge of the near hole.After the shock wave is transformed into a stress wave and propagates outward,a corrugated plane seam is first generated at the stress concentration at the bottom of the hole,and then a radially distributed corrugated longitudinal seam is generated along the hole circumference.With the increase of shock wave energy,radial cracks increase and expand,and the tensile fracture zone expands,resulting in rock failure and disintegration.The wavy fracture surface observed in the experiment can be used as the direct evidence of the tensile failure of the wave stress.The rock failure state is basically consistent with the theoretical analysis conclusion,which confirms some conclusions of the theoretical model research. |
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