Abstract: In the process of hydraulic fracturing, the high-pressure infiltration of fracturing fluid on the fracture surface will cause the change of pore pressure field on both sides of the fracture, resulting in the change of mechanical characteristics of rock, and consequently affecting the propagation of hydraulic fractures. According to the macroscopic damage theory based on micro-fracture, the linear slip crack model was applied to develop the stress intensity factor model at the tip of the micro-crack considering the fluid pressure in the pores. Then the stress-strain constitutive model of saturated rock was established, and compared with the laboratory test results of saturated rock. The influence of pore pressure on the mechanical properties and damage-induced permeability was evaluated based on this constitutive model. The results show that the proposed constitutive model can better characterize the influence of pore pressure changes on rock mechanical properties. The pore pressure changes in micro-cracks have a relatively small effect on the rock elastic modulus and Poisson’s ratio, while it can greatly reduce the yield stress limit of plastic damage, weaken the compressive strength of the rock, reduce the energy loss of the plastic deformation of the rock on both sides of the fracture surface, and improve the energy utilization rate of hydraulic fracturing. When the pore pressure around the fracture is higher than a certain threshold value during fracturing process, it will promote the unsteady and rapid propagation of meso-cracks, thereby promot more cracks to be connected in series, form a complex fracture network, and enhace the rock permeability surrounding hydraulic fractures. The proposed constitutive model of saturated rock can provide support for the mathematical simulation of rock matrix damage and field practice.

Key words: hydraulic fracturing, pore pressure, stress-strain constitutive model, micro-crack