Influence of the microstructure on stress-dependent P-wave anisotropy in sandstone

To understand the factors that affect the stress-dependent P-wave velocity anisotropy, a method is proposed to simulate anisotropic microcracks and minerals based on the discrete element method (DEM). Laboratory triaxial tests and numerical simulations were performed on sandstone samples with bedding orientations parallel and perpendicular to the maximum principal stress. The ellipse fitting method was applied to analyse the variation in P-wave anisotropy. The micromechanism of stress-dependent P-wave anisotropy was revealed. The evolution of microcracks is the main reason for the change in P-wave anisotropy under compression. As the confining pressure increases, the magnitude of the P-wave anisotropy is reduced. The weakening of the P-wave anisotropy results from the decrease in the number of open microcracks. Under deviatoric stress loading, the P-wave anisotropy of the bedding-parallel sample in the axial direction is strengthened. Anisotropy reversal occurs in the bedding-normal sample. The microcrack behaviour depends on the direction of maximum principal stress. The variation in microcrack anisotropy induced by stress controls the evolution of P-wave velocity anisotropy. The stress at which anisotropic reversal occurs depends on the preferred orientation mineral. The DEM model offers the unique ability to directly examine the variation in microstructure anisotropy that causes the change in P-wave anisotropy.