Research On The Influence Law Of The Existence Of Cracks On The Coal Rock Mass Caused By Water Shock Wave Under Three-dimensional Stress

Coal-measure energy provides a solid material foundation for the rapid development of my country’s economy,and coalbed methane,as a gas energy associated with or symbiotic with coal,is widely praised for its clean and efficient characteristics.However,most of the CBM reservoirs in our country belong to the medium and low permeability reservoir structure,and its characteristics of "low gas saturation,low permeability,and low pressure" seriously restrict the development and utilization of CBM.It is imperative to improve the permeability of gas-storage coal seams by using seam reconstruction technology.High-voltage electric pulse fracturing and permeation enhancement technology is a new technology in recent years.It has gradually entered people’s field of vision because of its convenient and efficient characteristics.However,its research content is not perfect because of its late development.This thesis further explores the technology of high-voltage electric pulse fracturing and permeation enhancement by combining theoretical analysis,numerical simulation and indoor similar simulation test.First,the mechanism of high-voltage electric pulse infiltration and cracking in water was analyzed from a theoretical point of view,and then the original stress state of the specimens with different crack occurrence states under three-dimensional stress was analyzed through numerical simulation experiments.The high-voltage pulse discharge test bench is used to conduct cracking tests on specimens with different fracture occurrence states.Finally,based on the results of a series of tests,the occurrence,propagation law and permeability enhancement effect of cracks after high-voltage electric pulses are analyzed in coal rock mass are discussed.The main conclusions are as follows:(1)Compared with a single coal-rock mass without any geological structure,cracks are more likely to occur in a reservoir with a fractured structure after being fractured by a high-voltage electrical pulse water shock,and the crack network of the final coal-mass-like sample covers a wider area.,the effect of cracking and increasing permeability is better.(2)When there are fissures,joints and faults in the coal body,the uniformity of the entire reservoir structure is poor.Therefore,after being fractured by high-voltage electric pulses,the cracks will preferentially occur near these geological structures,and the structural weak points are farther from the fractured wells.The higher the priority,the higher the priority;at the same time,the existing cracks will also preferentially expand and extend near these structural points,and the distribution of the crack network will show the characteristics of "generated from points and extended to points",and finally shows "topography with points" cracked network structure.(3)The fracturing results of similar coal specimens containing fractures with different dip angles show that when the minimum principal stress angle between the joints,faults and other geological structures and the reservoir structure is about 30 °,the reservoir structure is fractured by high-voltage electric pulses The effect of post-infiltration is the best.(4)The artificial prefabricated fissures in the series of tests are all symmetrical to the central boreholes of similar coal samples,and the structures of the samples passing through when the shock wave propagates in the south and north directions are also similar during fracturing.The final fracturing results show that: The distribution of crack network after fracturing is basically symmetrical to the borehole in the center of the specimen,that is,under similar reservoir structure conditions,the stimulation effect of high-voltage electric pulse fracturing technology on the reservoir structure is also similar.(5)When geological structures such as fissures,joints and faults in the coal body contain filler materials,the properties of the fillers will affect the effect of shock wave propagation and fracturing during electric pulse fracturing.Among the several types of interlayers created by different filling media in the series of experiments in this thesis,the order of shock wave propagation efficiency is hard interlayer > water interlayer > air interlayer,and the order of the three types of interlayers hindering shock waves is air interlayer > water interlayer > hard interlayer.