Real-time seepage and instability of fractured granite subjected to hydro-shearing under different critical slip states

In this study, a high-confining pressure and real-time large-displacement shearing-flow setup was developed. The test setup can be used to analyze the injection pressure conditions that increase the hydro-shearing permeability and injection-induced seismicity during hot dry rock geothermal extraction. For optimizing injection strategies and improving engineering safety, real-time permeability, deformation, and energy release characteristics of fractured granite samples driven by injected water pressure under different critical sliding conditions were evaluated. The results indicated that: (1) A low injection water pressure induced intermittent small-deformation stick–slip behavior in fractures, and a high injection pressure primarily caused continuous high-speed large-deformation sliding in fractures. The optimal injection water pressure range was defined for enhancing hydraulic shear permeability and preventing large injection-induced earthquakes. (2) Under the same experimental conditions, fracture sliding was deemed as the major factor that enhanced the hydraulic shear–permeability enhancement and the maximum permeability increased by 36.54 and 41.59 times, respectively, in above two slip modes. (3) Based on the real-time transient evolution of water pressure during fracture sliding, the variation coefficients of slip rate, permeability, and water pressure were fitted, and the results were different from those measured under quasi-static conditions. (4) The maximum and minimum shear strength criteria for injection-induced fracture sliding were also determined ( μ = 0.6665 and μ = 0.1645, respectively, μ is friction coefficient). Using the 3D (three-dimensional) fracture surface scanning technology, the weakening effect of injection pressure on fracture surface damage characteristics was determined, which provided evidence for the geological markers of fault sliding mode and sliding nature transitions under the fluid influence.