Permeability of hydrated shale rocks under cyclic loading and unloading conditions

The hydration of reservoir shale is obvious during shale gas extraction，resulting in some vague permeability evolution and mechanical characteristics. Thus the understanding on the hydraulic characteristics of shale during the shale gas extraction process is very important. Taking the reservoir shales under different hydrated days as a case study，the mechanical properties and permeability evolution under cyclic loading and unloading conditions have been investigated. According to the experiments results of the hydrated shale under loading condition，the peak strength is lower and the corresponding permeability is higher for multi-day hydration. Then，the damage theory is applied to ana⁃ lyze the shale permeability evolution resulted in by rock damage. Also，from the permeability variation under initial loading and unloading conditions， the permeability while loading decreases with the increase of confining pressure， however， it increases while unloading with the decrease of confining pressure， and the former is greater than the latter under same confining pressure，indicating that the irreversible deformation causes marked differ⁃ ence of the permeability in the process. In addition， the fitting curves based on the experimental data firstly show the negative exponential relationship between the permeability and confining pressure under initial cyclic condi⁃ tions and the power function for more cyclic processes， changing the permeability evolution into different fitting model，however，the permeability variation while unloading is in accordance with a power function. Furthermore，the loss ratio and recovery ratio under five cyclic loading and unloading conditions have been analyzed showing a permea⁃ bility decreasing with more cyclic processes. Finally，the permeability-stress sensitive coefficient has been investiga⁃ ted demonstrating the coefficient is more sensitive to the initial confining pressure and decreases with more cyclic processes. Especially it is more sensitive to lower confining pressure while loading and maintains lower while unloading，calibrating the shale permeability loss and recovery，which reveals the permeability evolution of the hydrated shale under cyclic loading and unloading conditions.