Influence of the microstructure on stress-dependent P-wave anisotropy in sandstone
To understand the factors affecting 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 evolution of the P-wave velocity was back-calculated based on the numerical simulation under confining pressure and deviatoric stress. The ellipse fitting method was applied to analyze the variation in P-wave anisotropy. Based on the numerical model, 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 microcrack anisotropy is not changed. 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. Microcrack behavior 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. When the P-wave anisotropy comes mainly from mineral anisotropy, the deviatoric stress has difficulty reversing the direction of P-wave anisotropy. The DEM model offers the unique ability to directly examine the variation in microstructure anisotropy leading to the change in P-wave anisotropy.
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