Effect of the intermediate principal stress on hydraulic fracturing in granite: an experimental study

The fracturing behavior of hard rock under fluid pressure is affected by maximum, minimum, and intermediate principal stress. To investigate the effect of the intermediate principal stress on the hydraulic fracturing behavior, including the fracturing pressure, fracture pattern, fluid pressure evolution, etc., a series hydraulic fracturing experiments of granite under true triaxial compression conditions was conducted. Constant minor and major principal stresses (10 and 30 MPa, respectively) and variational intermediate principal stresses (10, 15, 20, and 25 MPa) were considered. Acoustic emission was used to monitor the hydraulic fracturing process. The effect of different intermediate principal stresses, i.e., various horizontal principal stresses, on the fluid fracturing pressure and failure pattern was studied. With higher intermediate principal stress, the hydraulic pressure fluctuates in the hydraulic fracturing process, whereas at lower intermediate principal stress, the pressure fluctuation phase becomes shorter. The higher intermediate principal stress makes the fracture plane simpler, straighter, and more planar. When the intermediate principal stress equals to the minimum principal stress, the fracture plane tends to be non-planar. In this case, the hydraulic fracture is mainly influenced by heterogeneity of the rocks. With the increase of intermediate principal stress, the formation breakdown pressure of granite decreases, which is different from the effect of minor principal stress. A modified criterion is proposed to reflect the influence of intermediate principal stress on the formation breakdown pressure.