Study On The Mechanical Behavior During Unloading-Induced Slip And Flow Characteristics Of Shale Joints

The uniaxial mechanical properties of shale,the seepage characteristics of the jointed shales,the mechanical behavior of induced slip and the variation of permeability in the process of unloading slip were studied by ways of laboratory tests and numerical simulation.The effect of confining pressure,joint roughness coefficient and shear displacement on the critical hydraulic gradient were explored in the simulated shear seepage test of artificial dislocation joint surface.The effects of unloading rate and joint inclination on fault slip behavior and seepage characteristics in the process of slip were explored in the unloading induced slip test.The research results provide some references for the application of hydraulic fracturing technology in shale gas exploitation.The main contents and conclusions are as follows.The uniaxial compression tests of shale samples with bedding angle of 90°were carried out,and the surface crack and fragment quality of broken shale samples were counted,and the mass fractal dimension F_s and crack distribution fractal dimension F_bwere calculated.The results show that the value of F_s ranges from 1.3049 to 2.0605,and the larger F_s indicates the larger number of fragments.The F_b representing the complexity of the cracks varies from 1.2486 to 1.4303,and a larger F_b indicates that the shales are more seriously damaged.Both the peak strength and elastic modulus can be predicted using F_s and F_b,following linear and power-law relationships,respectively.The splitted shale samples were staggered to simulate the shear state,and the effects of confining pressureσ₃,joint roughness coefficient JRC and shear displacement u_s on seepage characteristics were discussed.The results show that there is a nonlinear relationship between volume flow rate and hydraulic gradient,and the Forchheimer equation can accurately describe the nonlinear relationship between volume flow rate and hydraulic gradient.The formulas of critical hydraulic gradient J_c onσ₃,JRC and u_sare fitted by the experimental data.According to the formula,the J_c is positively correlated withσ₃ and JRC,and inversely correlated with u_s.The J_c has a power function relationship with theσ₃ and JRC.The J_c has an exponential function relationship with us.To explore the effects of joint inclination and unloading rate on the slip behavior,stepwise unloading confining pressure with constant axial stress triggers the slip start-up of jointed shales by using MTS815 rock mechanics test system.The results show that the average height of asperity decreased and the roughness increased after the unloading-induced slip test.The slip velocity of the samples increases with the increase of unloading rate and increases with the decrease of joint inclination.There is a critical value of unloading rate between 0.1～1MPa/min.When the sample slips at the critical unloading rate,the unloading amount of confining pressure is the smallest,so that the strain energy accumulated on the joint plane is smaller.The energy mainly acts on the asperity to break through the interlock effect during the slip release,so the slip releases the least energy and produces the minimum shear displacement,which is of great significance to the safety of rock mass engineering.The friction coefficient decreased for the sample withθ=30°and increased for the sample withθ=40°and 50°.The dynamic slip state of samples withθ=40°and 50°in triaxial shear test cannot follow the rate-state friction law,yet the slip state of samples withθ=30°as well as the creep slip and the quasi-static slip of samples withθ=40°and 50°follows the rate-state friction law well.The aperture of the sample during the slip process was calculated by the shear displacement and normal displacement data of the unloading-induced slip test.The aperture data was imported into the Comsol software to evaluate the permeability of the sample slip by applying water pressure.The results show that the sample has larger aperture and permeability in the process of slip with the increase of joint inclination.The 50°sample has higher efficiency to enhance the permeability and the failure of cutting joint surface occurred in the process of slip,which can promote the increase of fracturing effect and permeability.Under the unloading rate of 0.01MPa/min,the permeability of the sample is higher than that of 0.1,1 and 10MPa/min,but its efficiency of increasing permeability decreases continuously in the slip process,which is not conducive to the efficient exploitation of shale gas.Under the unloading rate of10MPa/min,the permeability of rock mass increases rapidly and efficiently.