Effect of crack density on crack initiation and damage for high confining pressure using wave velocities

Abstract The study of progressive failure of reservoir rocks is very important for the stability analysis of ultra-deep boreholes. The progressive failure can be characterized by the crack initiation stress (CI), crack damage stress (CD) and peak stress (Peak), which is controlled by the evolution of cracks in rock and is significantly affected by confining pressure. To explore the effect of high confining pressure and initial crack on the progressive failure, the initial crack density of 121 sedimentary rock samples was characterized by acoustic waves, and the progressive failure under different confining pressures was analyzed. Experiments show that the ratios of CI to Peak (CI/Peak) and CD to Peak (CD/Peak) increase significantly with confining pressure, which is agreement with the existing conclusion from low confining pressure (0–52 MPa), but those decrease for high confining pressure (68–85 MPa) above critical point of sandstone. The CI/Peak and CD/Peak decrease with increase of initial crack density, which is sensitive to confining pressure but independent of rock type and grain size. The CD of sandstone is pertinent to the friction coefficient of cracks. These results laid a foundation for analyzing the evolution mechanism of wellbore damage and predicting the timing of wellbore instability in ultra-deep wells.