Abstract: To understand the permeability of shale under osmotic pressure and stress coupling, a servo-controlled triaxial rock testing system was employed to determine the complete strain-stress curves and permeability of shale under different confining pressures and osmotic pressures. The relationships between the deformation stage and permeability of shale samples were analyzed. The relationship between the compression zone and the permeability in the process of shale destruction was discussed. The results show that under different confining pressures, the initial permeability decreases with the increase of confining pressure, and the peak intensity increases with the increase of confining pressure. Under the same confining pressure, the permeability of the samples increases with the increase of the osmotic pressure, but the peak strength decreases to a certain extent. It is found that there is a localised compression zone in the shale, and the compression zone appears to inhibit the increase of permeability. The appearance of the compression zone is not the characteristic of the brittle to the ductile transition critical point, but the characteristics of the initiation, propagation and softening of pores. The shale samples appear hardening under high confining pressure. Under the low confining pressure, the formation of the fracture network in the shale is earlier than the samples under high confining pressure, and the failure mode is mainly dominated by the high-angle shear failure. This study on the permeability of shale under different stress conditions is significant to reveal the percolation mechanism of the process of shale gas development.

Key words: rock mechanics, shale, osmotic pressure and stress coupling, permeability, compression zone