Experimental Study on the Time-Dependent Gas Permeability of Fractures in Shales

The gas permeabilities of shale fractures provide a critical basis for deeply understanding the subsurface fluid flow processes in many underground engineering activities in shales. However, the time-dependent behavior of shale permeability under formation stress has been rarely reported. In this study, two artificially fractured shale cores were used to experimentally investigate the time dependence of the fracture gas permeability and underlying mechanisms. Daily measurements of permeabilities were conducted at various gas pressures under multilevel confining stresses, where the confining stress was incrementally changed from 10 to 25 MPa and then reverted to 10 MPa. Numerical calculations were performed to determine the fracture apertures. The experimental results reveal a notable decline in gas permeability and aperture over time under each confining stress during the loading phase that proceeds from a decelerating decline stage to a steady decline stage. The gas permeability can be overestimated by at least a factor of 2 due to fracture creep. The magnitude of time-dependent permeability reduction is related to the fracture geometry, contact area, spatial distribution of apertures, and fracture stiffness. The observed significant permeability loss and limited time-dependent permeability recovery during the unloading stage indicate an irrecoverable process of creep-induced permeability reduction. The gas pressure-dependent permeability suggests notable gas slippage in fractures, which is influenced by creep and exhibits a power function decay with time. Considering the coupling effect of stress creep and gas slippage, a time-dependent gas permeability model is developed and validated using experimental data. This model is helpful to effectively predict the variation trend of fracture permeability during the implementation of underground engineering.