Abstract: The deep rock engineering is characterised by high ground stress and high head pressure. To study the permeability evolution of rock under high confining pressure and high pore pressure conditions, the seepage tests for the variable pore pressure were conducted on tight sandstone under different confining pressure conditions. The results show that within the studied confining pressure range (0～50 MPa), the permeability presents three different variation trends with the increase of pore pressure: rapid growth phase (confining pressure at 10～20 MPa), slow growth phase (confining pressure at 30～40 MPa) and constant phase (confining pressure at 50 MPa). During the unloading process of confining pressure, irreversible deformation occurs in the sample due to high confining pressure, which results in a significant irreversible permeability and hysteresis effect on the restitution of permeability. During the seepage test, volumetric strain and the permeability evolution have a good consistency. In the loading and unloading process of confining pressure, the sensitivity and recovery capabilities of permeability to stress under high pore water pressure are stronger than those under low pore water pressure. Polarising microscope images reveal the inherent mechanism of irreversible deformation during the confining pressure loading and unloading process: the mutual extrusion and movement of skeleton particles cause the compression of the original micro-fissures, the decrease or even collapse of the pores, resulting in the irreversible permeability. After the seepage test, the P-wave velocity increases, which indicates that the compactness of the rock is improved and has a good agreement with the change of the internal microstructure.

Key words: tight sandstone, high confining pressure, high pore pressure, permeability, microscopic mechanism