Evolution of the permeability and pore structure of transversely isotropic calcareous sediments subjected to triaxial pressure and high temperature

Abstract

The pore structure and the connectivity of the pore space within transversely isotropic calcareous sediments play important roles in the hydraulic properties of underground engineering. Therefore, it is essential to clarify the pore structure evolution of rocks subjected to triaxial pressure and high temperature. This study used self-designed triaxial permeability testing equipment and a micro-CT experimental system to study the evolutions of the permeability and pore structure. The results show that when the pore pressure is less than 2.5 MPa, the permeability decreases gradually with increasing pore pressure, and when the pore pressure is greater than 2.5 MPa, the permeability increases gradually with increasing pore pressure. The dramatic decrease is due to the Klinkenberg effect. The relationship between temperature and permeability is complicated. A decrease in the permeability was observed in two temperature stages. The decrease in the lower temperature range occurs because the expansion deformation is restricted by the high axial and confining pressures. The second decrease occurs because the rate of fracture closing is higher than the rate of crack opening. As the temperature increases, the size and quantity of fractures clearly change. The variations in the MCT (microcomputed tomography) images are consistent with the permeability evolution, indicating that the permeability is closely correlated with the crack distribution and pore structure.