Study On The Evolution Law Of Turning Fracturing Fracture Process Zone Into Macroscopic Hydraulic Fracture

The existence of hard roof seriously affects the safe and efficient production of coal mine.Hydraulic fracturing technology can effectively crack hard roof and prevent the problems such as rock burst and large deformation of surrounding rock caused by hard roof.However,due to the influence of ground stress in hard roof,hydraulic fracture is easy to turn and expand to affect the effect of directional fracturing,and the influence of fracture process zone on the fracture of rock materials can not be ignored in limited scale.In this thesis,by carrying out three-point bending fracture test,exploring the theoretical framework of cohesive fracture model simulation combined with extended finite element method and numerical simulation of turning propagation of hydraulic fracture in hard roof,the evolution law of turning fracturing fracture process zone into macroscopic hydraulic fracture is studied.It is hoped to provide a theoretical basis for improving the design of hydraulic fracturing engineering.The semicircular three-point bending fracture test(SCB)is carried out,and the development characteristics of the fracture process zone of tension-shear mixed mode fracture of rock are analyzed based on the digital image correlation method(DIC).The results show that the specimen has to go through three stages from the beginning of stress to the occurrence of failure: the elastic deformation stage,the fracture process zone begins to develop to the complete formation stage,and the fracture instability extends to form the real fracture surface stage.Whether the specimen is subjected to pure I load or tension-shear mixed load,the fracture form of the specimen is tensile;there are two forms of shear effect: first,the opening displacement strength of tensile fracture does not change obviously with the increase of shear load strength,and the shear load will change the direction of the maximum tensile stress,affect the development direction of the fracture process zone,and then affect the initiation direction of the crack.Second,with the increase of the dip angle,the length of the fracture process zone increases,and there is a linear correlation between them,and the correlation coefficient is 0.977.It is proved that the theoretical framework and modeling process of hydraulic fracturing steering expansion are simulated by combining extended finite element method and cohesive fracture model method.The results show that the extended finite element method is an extension of the traditional finite element method based on the concept of unified partition,and the local enrichment function can be easily incorporated into the finite element approximation,which is suitable for simulating fracture propagation.the cohesive fracture model can be used to simulate the hydraulic fracturing of rock strata,which can be used to simulate the initiation and expansion of fractures along any path in rock materials,and the simulation method is effective.The cohesive crack model method combined with extended finite element method is used to realize the numerical simulation of turning propagation of hydraulic cracks in hard roof.The results show that the whole process of turning propagation of hydraulic fracture can be divided into five stages: pre-fracture preparation stage,holding pressure initiation stage,turning expansion and steering completion stage after hydraulic fracture initiation,horizontal maximum principal stress expansion stage and expansion end stage.With the increase of the dip angle of the preset fracture,the turning propagation behavior of the hydraulic fracture is restrained,and the turning propagation behavior of the hydraulic fracture tends to be expanded step by step,in which the first step steering behavior characterizes the essential effect of the preset fracture inclination angle on the steering of the hydraulic fracture.the high injection rate will restrain the turning propagation ability of hydraulic fractures,but the inhibitory effect is not prominent in the simulation experiment of turning propagation of hydraulic cracks in hard roof.With the increase of the horizontal principal stress difference,the turning behavior of the hydraulic crack is intensified,and the turning expansion of the hydraulic crack is particularly sensitive to the change of the horizontal principal stress difference.There are 59 pictures,13 tables and 109 references.