Research On Crack Propagation Law Of High Voltage Pulse Cracking Coal

At present,coal is still the main body in China's energy structure.The mining of coal is accompanied by the production of coalbed methane,which is both a harmful gas that causes the greenhouse effect and a clean fuel.In recent years,as a clean energy source,coalbed methane has been highly valued by various countries.China's coalbed methane resources are abundant,but because of the low permeability of most coal reservoirs,the extraction of coalbed methane is severely restricted.Therefore,it is imperative to propose a reasonable way to increase production of low permeability coal reservoir permeability.High-voltage pulsed discharge cracking and antireflection technology is a new technology emerging in recent years.It is a promising antireflection technology and has important practical value for improving the extraction rate of coalbed methane.This paper draws on the application of high-voltage pulse discharge technology in engineering fields such as petroleum mining and geological exploration,and uses a combination of theoretical analysis,similar simulation test and numerical analysis to analyze the high-voltage pulse discharge for different strength,joints and pore-wall prefabricated crack The cracking effect of similar coal samples is studied,and the crack propagation law and internal damage of the borehole during pulse discharge are studied;and use PFC2 D discrete element particle flow simulation software to apply different peak pressure shock waves to different strength coal bodies,and analyze the crack growth law and vibration speed attenuation law;The main research results are as follows:(1)The high-voltage pulse discharge test system is used to conduct the pulse discharge cracking test.The test shows that the crack propagation process under the action of high-voltage pulse discharge can be divided into three stages: preparation stage,development stage,and destruction stage;for samples of different strengths,with the increase of the number of discharges,the length of the crack propagation of the sample gradually increases until it completely penetrates;The width of the crack also gradually increases with the number of discharges;under the effect of ground stress,the crack always expands in a direction perpendicular to the minimum principal stress.(2)Compared with the complete sample,the number of pulse discharges required for through cracks in jointed samples is less.As the number of pulse discharges increases,the number and width of cracks on the surface of jointed samples increase significantly.The internal sample is more damaged than the complete sample,this indicates that when there are joint discontinuities around the borehole,the shock wave will generate reflection and superposition effect when it encounters it,and the peak pressure of the shock wave will increase,which will enhance the cracking effect on the specimen.(3)When the prefabricated cracks of the hole wall are 30 ° and 45 ° azimuth,it has a certain guiding effect on the cracking of the impact secondary cracks of the sample,under the effect of pulse discharge,the specimen cracked at the tip of the prefabricated crack and expanded along the direction perpendicular to the minimum principal stress.The 60 ° azimuth has no guiding effect on the initiation of the secondary cracks impacted by the specimen.The cracks start directly from the hole wall instead of the prefabricated crack tip.(4)Different coal bodies have different cracking effects under different peak pressures;under a single impact,as the peak pressure of the shock wave continues to increase,the number of cracks and rupture area show a trend of increasing first and then decreasing.The amplitude of vibration attenuation of fat coal and coking coal at a peak pressure of 15 MPa is relatively gentle and the duration is relatively long,which is more conducive to the desorption of coalbed methane;the attenuation of anthracite at 35 MPa is more gentle,which is more conducive to the desorption of coalbed methane.