Study On Initiation Mechanism And Morphological Characteristics Of Pore Edge Crack In Coal Under Electrical Pulse

With the national strategy of green development put forward,energy saving and emission reduction,pollutant control,coping with climate change has become one of the national core development goals and strategies.China’s coal-bed methane（coal gas）reserves are rich,and currently China has huge investment in coal-bed methane exploitation technology.This measure not only solves the problems of low efficiency,low output and difficult commercial exploitation of deep coal-bed methane,but also alleviates the problems of natural gas shortage and mainly relying on large amounts of imports.And to prevent and reduce coal mine gas outburst problem and achieve the purpose of environmental protection.At present,in order to improve the mining efficiency of coalbed methane,different fracture inducing methods are usually used to create artificial fractures.Hydraulic fracturing is a conventional means to increase the production of oil,natural gas or geothermal reservoirs.High voltage electric pulse hydraulic fracturing is a new method based on hydraulic fracturing proposed in recent years.Its load is relatively special,which belongs to the impact rock breaking in water.In this paper,the crack propagation law and pore edge crack morphology law of coal rock mass under the action of hydraulic electric shock wave are studied by combining theoretical derivation,laboratory physical experiment and numerical simulation of discrete element particle flow simulation software PFC^2D.Firstly,based on the principle of hydraulic electric effect,energy conversion,spherical water shock wave propagation between water,coal and rock mass and interface,the cracking and spreading mechanism of high voltage electric pulse load on coal and rock mass is analyzed.Then the water shock wave waveform and peak pressure of high voltage electric pulse water shock wave under different hydrostatic pressure and different discharge voltage are obtained through the water shock wave loading test.Then,the relationship between crack initiation and ground stress,hydrostatic pressure and high-voltage electrical pulse load in coal rock mass is studied by setting up the true triaxial high-voltage electric pulse fracturing test platform.The crack initiation and propagation laws around the borehole are analyzed.Finally,the particle flow discrete element software PFC^2Dis used to carry out the numerical simulation calculation of drilling water pressure and electric pulse cracking of coal and rock mass.The stress wave data measured by the water shock monitoring system is imported into the intermediate water injection hole wall of coal and rock mass through the code in the form of waves to simulate the impact of water shock waves on the hole wall of coal and rock mass.The impact of water shock waves on the hole wall of coal and rock mass is studied under different ground stresses and loads.Crack initiation and pore side crack distribution of different coal and rock types（different elastic modulus）.Through the study of the above means,the distribution pattern and propagation law of cracks around boreholes induced by hydraulic electric pulse were studied and analyzed,and the following conclusions were drawn:（1）Failure zone,plastic zone and elastic zone are generated in coal rock mass under the action of high-voltage electric pulse in water.With the increase of voltage,the number of microcracks around borehole gradually increases,the fracture network becomes more complex,and the area of failure area around borehole becomes larger and larger.Moreover,cracks in coal rock mass are easy to appear in the form of symmetric cracks under the action of high-voltage electric pulse load.（2）The closer it is to the discharge electrode（the front section of the coal core sample）,the easier it is to generate multiple pairs of symmetric cracks,the more sufficient the crack propagation,the more intense the tortuous change,and the longer the extension length.With the distance from the discharge electrode（the rear section of the coal core sample）,the logarithm of symmetric cracks decreases,the width of crack growth Narrows gradually,the change of zigzagging degree slows down,and the extension length becomes shorter.（3）Under different in-situ stresses,the increase of the difference between vertical and horizontal in-situ stresses will lead to the enhancement of the deflection trend of cracks,and the deflection trend of cracks is mainly to the direction of large principal stresses.Under the same load,the stress state of coal rock itself has an important effect on the crack shape and direction.（4）Cracking tests on coal and rock masses with different elastic moduli show that the number of major cracks is closely related to the elastic moduli.In coal and rock masses with smaller elastic moduli,high-voltage electric pulse water pressure cracking load has stronger cracking capacity and can penetrate more areas along the radial direction,providing a good channel for coalbed gas migration.According to the above rules,some practical suggestions can be provided for the application of high-voltage electric pulse water shock pressure cracking technology in coal and rock mass in coalbed methane mining,such as higher voltage discharge cracking effect will be better;Because the crack propagation trend in the direction of high principal stress is much greater than that in the direction of low principal stress,the distribution in the direction of high principal stress should be more sparse than that in the direction of low principal stress.Different discharge parameters are used for coal and rock masses with different elastic modulus.Different discharge times should be used for coal and rock masses with different elastic modulus.