Characteristics of unloading damage and permeability evolution of sandstone under hydro-mechanical coupling

Abstract

A triaxial unloading mechanical test is conducted on sandstone considering different initial confining pressures and unloading rates under the effect of hydraulic coupling. The energy evolution of sandstone unloading failure is studied, and the influences of the initial confining pressure and unloading rate on the deformation characteristics and permeability evolution of sandstone are discussed. It is found that the initial confining pressure and unloading rate have a significant impact on the axial and radial strains of the sandstone, and the radial deformation is more sensitive to the unloading process. The energy dissipation of the sandstone sample increases with the increase of the confining pressure and is nonlinearly correlated to the unloading time. The permeability during the unloading deformation of the sandstone decreases with the increase in the initial confining pressure, indicating that the initial confining pressure of unloading has an inhibitory effect on the permeability of the sandstone. The permeability decreases with the increase in unloading rate, indicating that the faster the increase in the unloading rate, the greater the dissipated instantaneous elastic energy, which is accumulated and released during failure. The higher the degree of fracture in the sample, the larger the peak value of permeability.