Mechanical and hydraulic coupling of injection-induced slip along pre-existing fractures

A series of injection-induced slip experiments using pre-fractured granitic rock specimens was conducted to investigate the dynamic hydraulic responses of existing fractures during hydraulic stimulation. The results show a characteristic stepwise slip and temporal increase in fluid flow in the fractures during slip. This increase is considered to result from significant shear dilation that is sufficient to change the flow properties of rough fractures with large asperities. Our laboratory data show that the stepwise slip is caused by temporal drops in pore fluid pressure in the fractures associated with shear dilation, which in turn indicates that the geometry of fracture surfaces plays a significant role in terms of both slip behavior and changes in fluid flow during induced slip. These results raise the possibility that stepwise slip and temporal permeability changes occur in actual rough fractures. This hydro-mechanical coupling behavior of such fractures during fluid injection suggests that dynamic (temporary) permeability changes may occur in geothermal reservoirs subjected to hydraulic stimulations.