Characteristics of large-displacement based dynamic shearing-flow through the fractures in the granitic hot dry rock mass

Hydro-shearing is an important technical method for deep energy extraction, such as hot dry rock geothermal energy and shale gas extraction. In addition to improving reservoir permeability, the hydro-shearing will also induce earthquakes. The essence of both hydro-shearing induced permeability enhancement and injection-induced seismicity are shearing-seepage through the geological joints or faults. In order to study the evolution of shearing-flow properties of joints or faults due to different derived conditions, we employed a high confining pressure and real-time large-displacement shearing-flow setup designed by ourselves to study the fluid flow characteristics of man-made fracture in the granitic hot dry rock mass during shearing-slip which were derived by shearing creep, increasing shearing stress, increasing water-injected pressure and increasing displacement rate, and further analyzed the friction characteristics of fracture or fault in the process of induced seismicity using the rate-state equation. The results indicate that:(1) Under the triaxially hydrostatic stresses, there existed linear relationship between water-injected pressure and injection rate in the granitic fracture, and the equivalent opening and permeability of the fracture showed negative exponential decay with the increment of normal stress.(2) The stick-slip of fracture or fault caused by shearing creep, water-injected pressure, shearing stress rate and shearing displacement rate could induce a significant increment in the permeability. The greater the slip deformation and slip rate, the greater the permeability enhancement. Among the four derivations, the permeability increment caused by shearing creep was the smallest as shearing stress rate induced enhancement was the largest.(3) During the shearing flow process, the rate-state friction law could still be used to analyze the stability of fracture or fault. With the increment of stick-slip times, the friction parameters such as shearing stress drop, b value(That is the drop of frictional coefficient with time and/or displacement) and slip rate increased in the form of logarithmic function in the regular stick-slip which represented the increment of instability of fault slip. In the chaotic stick-slip, the above parameters conformed to the Langmuir equation and tended to zero which indicated that the instability of fault slip decreased.