| China is rich in coalbed methane resources,ranking the third in the world.The prospect of exploration,development and utilization is very broad,and it has entered the reserve sequence and become a relatively realistic exploration field.However,95% of CBM reservoirs in China are deficient in low saturation,low permeability and low formation pressure gradient,leading to low extraction efficiency of CBM Wells in China.Therefore,in order to accelerate the exploration and development process of low permeability coalbed methane in China,and make it become the replacement field of conventional natural gas as soon as possible,it is necessary to further study the cracking and anti-reflection technology and fracturing mechanism of low permeability coal seam,and carry out quantitative analysis on the law of fracture expansion,distribution and damage evolution of coal-rock mass.In recent years,the application of new high pressure electric pulse hydraulic fracturing technology with dynamic and static loading combined in the field of coal and rock fracture and anti-reflection has gradually attracted the attention of engineering technicians and scholars.The crack anti-reflection technology of coal and rock mass basic principle is: the high voltage pulse discharge in water to produce liquid electric effect,resulting in shock,shock waves through the water medium role in coal and rock,transformation of coal and rock mass structure,make its internal cracks initiation and propagation and well versed in,eventually forming complex fracture network for coalbed methane migration channel.However,the evolution law of meso-damage of coal and rock mass in the process of fracturing has not been well explained,and the final fracture effect has not been quantitatively evaluated.Therefore,in view of the damage evolution law and cracking effect of coal and rock mass under high-pressure electric pulse hydraulic fracturing technology,this paper focuses on the research of the damage evolution law of coal and rock mass under high-pressure electric pulse hydraulic fracturing technology and the quantitative evaluation of the effect after cracking through theoretical analysis,physical similarity test and numerical simulation test.The main research results are as follows:(1)Based on the knowledge and research results of shock wave and dynamic damage mechanics,the mechanical properties of rock under dynamic load are theoretically analyzed.The tensile and shear failure criteria for determining the failure state of natural fractures and the propagation mechanism of fractures are introduced.(2)the control variable method is used for high voltage electrical pulses similar hydraulic fracturing of coal and rock physics experiment,the results show that under the condition of same water pressure,along with the higher discharge voltage,internal fracture geometry parameters of the coal samples are many times,the growth of the coal sample internal crack cracking and penetration degree are more fully;Under the same water pressure,with the increase of discharge voltage,the fractal dimension of internal cracks and the proportion of large cracks increase linearly,that is,the crack extension degree of internal cracks in coal samples becomes more complex.Under the same water pressure condition,with the increase of discharge voltage,the distribution range of crack width in the coal sample gradually increases,that is,the width of crack in the coal sample is wider and the distribution range is wider.It can be seen that high voltage electric pulse hydraulic fracturing technology has a good cracking effect on coal body,and the better the cracking effect is when the discharge voltage increases.(3)RFPA-Dynamic numerical analysis software was used to study the influence of high-pressure electric pulse hydraulic fracturing technology on the damage evolution law of coal and rock mass.The simulation results show that the shock wave with different peak intensity formed by different discharge voltages has a significant effect on the propagation of stress wave and acoustic emission phenomenon in coal and rock mass.At the same time,the degree of crack propagation is different under the peak intensity: the number of cracks produced by the lower peak intensity of the shock wave is relatively small and the length is shorter,while the higher peak intensity of the shock wave is more conducive to the initiation and length expansion of the crack. |
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