The Mechanism Of Pore Pressure Gradient Effect And The Fracturing Effect Induced By Disturbing Stress During Hydrofracturing

The application of hydraulic fracturing in the gassy coal seam and preliminary principle experiment show that the breakdown pressure of hydraulic fracturing may increases with the increase of initial pore pressure,which cannot be explained by the traditional hydrofracturing theory.In this study,the mechanism of the pore pressure gradient effect of hydrofracturing was taken as the entry point,the laboratory experiment,theoretical analysis,and numerical simulation methods were adopted to systematically study the effect of initial pore pressure and its gradient on hydraulic fracturing,the mesoscopic fracturing mechanism of rock hydrofracturing,the stress disturbance induced by hydrofracturing,and the fracturing effect induced by disturbing stress of hydrofracturing.The main innovative results are summarized as follows.(1)The rock hydraulic fracturing experiment under different initial uniformly distributed pore pressure was performed.The experimental results indicated that with the increase of initial pore pressure,the breakdown pressure of hydraulic fracturing increases linearly,and the acoustic emission energy increases correspondingly at the moment of fracture initiation.The propagation range and opening degree of hydraulic fractures are also larger in the same pumping time.(2)The rock hydraulic fracturing experiment under different initial pore pressure gradient was performed.The experimental results indicated that the larger the pore pressure gradient is,the larger the deformation of the internal pore structure is,and the greater the pressure drop after the fracture initiation.When fracturing by the same pumping rate for the same time,the larger the initial pore pressure gradient,the greater the opening degree of the hydraulic fracture.(3)A mesoscopic fracture criterion for rock hydrofracturing considering pore pressure and its gradient was established.The rock is composed of particles and pores on the mesoscopic scale.Based on the mesostructure of the rock,the meso-mechanical analysis of the rock hydrofracturing process was carried out,and the mesoscopic fracture criterion for rock hydrofracturing considering pore pressure and its gradient was established.According to the experimental results,the reasonability of the fracture criterion was verified.(4)Hydraulic fracturing causes a change in the stress of the surrounding rock,that is,the stress disturbance effect of hydraulic fracturing.The stress disturbance effect of hydraulic fracturing includes pore pressure disturbance and matrix stress disturbance.With the hydraulic fracture propagation,the pore pressure and matrix stress at the front and sides of the hydraulic fractures fluctuate synchronously with the fluctuation of the pumping pressure,and the closer to the hydraulic fractures,the more obvious the fluctuation.(5)The pore pressure and matrix stress distribution at the front of fracture tips and both sides of the hydraulic fracture follow the exponential decay law.As the pumping rate increases,the decay rate decreases.The pore pressure decay rate on both sides of the fracture during the hydraulic fracture propagation is smaller than the front of the fracture tip under the same pumping rate.Therefore,the penetration range on both sides of the hydraulic fracture is larger than the penetration range at the front of the fracture tip.(6)The matrix stress at the point where the pore pressure increases also increased correspondingly and both gradually decay exponentially along the direction away from the fracture.The matrix stress decay rate is less than the pore pressure,therefore the range of the matrix stress disturbance zone around the hydraulic fracture is larger than the pore pressure disturbance zone(permeate zone).The relationship between the pore pressure and the matrix stress on both sides of the hydraulic fracture follow the natural logarithmic decay.(7)The stress disturbance effect caused by hydrofracturing has both advantages and disadvantages.In the engineering construction,according to the specific engineering background,based on the evaluation of stress disturbance,take corresponding measures to achieve profit and avoid disadvantages.A stress disturbance evaluation method based on stress gradient and stress growth rate is proposed to evaluate the stress disturbance in time and space.(8)When there is a weak structure such as natural fractures on the front or both sides of the hydraulic fracture,the disturbing stress generated by the hydrofracturing will increase the shear stress of the natural fractures.When the shear stress reaches the shear strength of the natural fractures,the disturbing stress of hydrofracturing can induce shear failure of natural fractures,that is,the fracturing effect induced by disturbing stress during hydrofracturing.