| The impact dynamic effect of the underwater pulsed discharge has been widely used in mechanical processing,pile foundation reaming,geological exploration,extracorporeal lithotripsy and other fields.In recent years,Some scholars have carried out some practice and exploration by using the underwater pulsed discharge in plug removal of oil wells and fracturing reservoirs of CBM wells.In view of the low permeability of domestic CBM wells’ reservoirs and the not obvious effect of the traditional fracturing technology,the impacts of the underwater pulsed discharge shock wave which are controllable and repeatable show great technical advantages.But the current studies of fracturing coal and rock mass by pulsed discharge were mostly in the environment of no triaxial geostress and no hydrostatic pressure in the borehole.The influence mechanism of hydrostatic pressure on the generation,transmission and fracturing of the discharge shock wave was not considered.The effect of different shock wave energy on the fracture effect was also ignored when studied the effect of shock wave waveform on the fracture effect of rock mass.By means of theoretical analysis,physical experiment and numerical analysis,this paper analyzed the generation,propagation mechanism and fracturing mechanism of pulsed discharge shock wave with hydrostatic pressure.Experimental studied on the characteristics of pulsed discharge shock waves and the regularity of fracturing coal and rock mass by the shock waves.And numerically analyzed the fracturing law of the homenergic shock waves with different waveform.This paper had laid a foundation for the application of high-voltage pulsed discharge fracturing technology in CBM exploitation.The main results of this paper are as follows:1.The process of fracturing coal and rock mass by underwater high-voltage pulsed discharge is classified into four stages: "Discharge breakdown" → "Channel expansion" → "Shock wave transmission" → "Impact fracture".Hydrostatic pressure has an inhibitory effect on the first two stages,and promote the latter two stages.By the analysis,the fracture of coal and rock mass by pulsed discharge was due to the coupling of three kinds of destruction: "Dynamic-Quasi static-Static".2.Developed a loadable hydrostatic pressure of high-voltage pulse discharge impact fracturing experiment system.This system consists of a shock wave characteristics experiment platform and a pulsed discharge fracturing test bench.The shock wave characteristics experiment platform can complete the test of shock waves characteristics which produced by pulsed discharge underwater with hydrostatic pressure.And the pulse discharge fracturing testbench can complete the rock fracturing experiment which coupled hydrostatic pressure and confining pressure.3.The characteristics of pulsed discharge shock waves in different hydrostatic pressure and discharge voltage were studied.The results show that:the increase of discharge voltage would always show the effect of intensifying the pulsed discharge process and strengthening the shock wave characteristics.The increase of hydrostatic pressure showed the dual function of impeding the pulsed discharge process and strengthening the shock wave transmission.4.Experimental studied on the effect of geostress,hydrostatic pressure and discharge parameters on the fracturing effect of concrete samples by pulsed discharge.The results show that: the existence of geostress would restrict the crack initiation and expansion.The level of the hydrostatic pressure would not affect the cracks’ propagation direction,but it could obviously accelerate the cracks’ propagation speed and increase the length of the cracks.Under the same discharge energy,the cracks’ propagation length and penetrating effect of high voltage(14kV)discharge were slightly better than that of low voltage(10kV)discharge.5.Using the numerical simulation software of PFC2 D,the effect of rock mass fracturing under homenergic shock waves with different waveforms was studied.under the action of the different wave energy shock wave effect of crack rock mass are studied.The results show that: the cracks’ propagation rules and vibration characteristics under single impact with different waveforms weresignificantly different.There had the optimal shock wave form under single impact in fracturing rock mass.The effects of multiple impacts can be very good to strengthen the fracturing effect of single impact by this shock wave,and the cracks’ propagation limit exists in all kinds of multiple impacts.The precrack would get more excellent propagation with the increase of hydrostatic pressure under low peak homenergic shock wave,but the hydrostatic pressure would almost has no help for cracks’ propagation under high peak homenergic shock wave. |
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