Friction stability and permeability response of shale fractures during shear slip

During the shale gas production,hydraulic fracturing is a basic stimulation technology to enhance the reservoir permeability.However,after a large amount of high pressure fluid injection into the underground,the integration,stability,and permeability of the reservoir may also change.Moreover,the fracture permeability is critical in determining the reservoir permeability and production.Therefore,three kinds of shale samples,i.e.,the Longmaxi shale in Southern Lower Silurian Formation,the Green River shale,and the Marcellus shale,were selected for the fracture shear slip experiments under various slip rates.The fracture permeability was concurrently measured by steady state flow methods.The rate-state friction law was used to describe the stability and permeability evolution of shale fractures during the slip process.Based on the experimental observations,a microscopic physical model of shale fracture slip was established to show the effect of mineralogical composition on the shear strength,stability,and permeability feedback of shear fractures during slip.The research results show that the friction strength of the shale fracture surface increases with the increase of tectosilicate content,and decreases with the increase of phyllosilicate content;the stability parameter (a-b) increases with the increase of phyllosilicate content,and stable creep is favored.In the shale reservoir with high tectosilicate content,the sudden increase of slip rate may result in the permeability enhancement due to the shear dilation,which accompanies with unstable dynamic slip.While in the shale reservoir with high phyllosilicate content,the slip rate increase may induce further permeability decay,which accompanies with stable creep slip.The results provide a basic understanding of injectioninduced fracture slip in shale reservoirs.