| China has extremely rich reserves of coalbed methane(coal gas),but the permeability of coalbed methane is low,resulting in low mining efficiency and production.In order to improve the extraction efficiency of coalbed methane,a lot of artificial seam construction technologies have been developed,especially the high-pressure pulse hydraulic fracturing and anti-reflection technology,which has been recognized by the industry for its safety,efficiency and environmental protection.There are a lot of experts and scholars have been studied on this technology,and the crack distribution in coal and rock shape had been accurately described,but most of the method is only applicable to coal rock crack analysis of different strata,and confined to the 2d level,if want to further study on the 3d level and research on the spatial distribution of crack form,also need to use 3d reconstruction technology.Qualitative and quantitative evaluation of crack propagation network in threedimensional internal space of the structure is carried out and the law of crack propagation is studied.So in this paper,first of all,the high voltage discharge in water crack and the evolution of crack propagation theory of coal or rock is analyzed,combined with high voltage electrical pulse splitting test and 3d reconstruction technology of coal or rock analyses the same under the condition of hydrostatic pressure,discharge voltage increased,effect on the coal and rock formation of the three-dimensional crack extension,qualitative description of the spatial distribution pattern and characteristics of internal crack group,Volume,surface area,number of nodes,damage factor,crack rate and fractal dimension were used to quantitatively evaluate the crack propagation.Finally use the finite element software ABAQUS,the numerical simulation experiment content,validate the result of the test analysis,and further analysis of the different elastic modulus and poisson’s ratio on high voltage electrical hydraulic fracturing of coal rock mass,the effect of hydraulic fracturing for high voltage pulse application of this technology in the practical engineering of coal or rock provide certain theoretical basis,the main conclusions are as follows:(1)The difference between the fracturing pressure required by hydraulic crack propagation and the friction coefficient of crack surface and the ground stress is positively proportional to the fracturing degree of coal mass.In the fracturing process,the first crack is initiated vertically or parallel to the drilling direction.When the defect weak structural plane or original micro-crack exists in the coal mass,the crack propagation turns.Under this behavior,the weak structural plane or the original microcrack is connected again,and finally develops into a very complex crack network structure;(2)Through three-dimensional reconstruction,it is found that most of the cracks in coal and rock mass are sheet structures connected by numerous micro-cracks,and these sheet cracks cross and cross to form a complex spatial network of cracks.The coal core cracks are connected differently from top to bottom,and there are more cracks in the middle and upper part than in the lower part.The reason is that the shock wave generated by the high voltage electric pulse in water produces loss in the process of the coal core specimen propagating from top to bottom,and the energy decreases gradually,which delays the crack propagation;(3)The crack mode,distribution area,quantity statistics Angle,has close relationship with Angle,crack rate with the increment of discharge voltage.With the increase of discharge voltage,the statistical angles of crack distribution of each specimen are 18,11 and 20 respectively,and the maximum crack rates are 0.6474,0.518 and 0.579 respectively,which decrease first and then increase.The average crack rate changes in coal and rock crack first crack centrally by symmetrical two angles,gradually to the concentrated distribution in the direction of the development of small angle,Finally,it is distributed in a360 degree circular direction,forming a multi-angle crack network,which is beneficial to the permeability and circulation of coalbed methane;(4)The box dimension method is used to calculate the fractal dimension of each crack surface inside each specimen,and the values are stable between 1 and 3,indicating that they all have good fractal characteristics and crack development state is good,which can provide good dredging and migration conditions for coalbed methane.With the increasing voltage,the average fractal dimension values of each specimen are 1.979,1.993 and 2.012,respectively.Voltage affects the bending and tortuous degree of internal crack surface to a certain extent;(5)With the increase of discharge voltage,coal core specimen of internal crack surface area,volume,complexity,are in the trend of steady growth.The initiation time became earlier(0.021 s,0.008 s,0.007s),and the number of main crack groups increased(3,4,5).And the degree of crack extension and also increases gradually,the stress and the width of crack propagation along the borehole wall to the outer boundary gradually decreasing;(6)Analysis,the numerical simulation of the crack of coal rock mass,the change of elastic modulus,poisson’s ratio did not affect the cracking time,increases as the elastic modulus,poisson’s ratio decreases,material brittleness increases gradually,the shock wave and static water pressure to form a thick under the function of the main crack,and the width of crack propagation,penetration degree increases gradually,and then cross join to form a good network of crack. |
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