Influence of bedding structure on stress-induced elastic wave anisotropy in tight sandstones

Abstract

To understand the evolution of stress-induced elastic wave anisotropy, three triaxial experiments were performed on sandstone specimens with bedding orientations parallel, perpendicular, and oblique to the maximum principal stress. P-wave velocities along 64 different directions on each specimen were monitored frequently to understand the anisotropy change at various stress levels by fitting Thomsen's anisotropy equation. The results show that the elastic wave anisotropy is very sensitive to mechanical loading. Under hydrostatic loading, the magnitude of anisotropy is reduced in all three specimens. However, under deviatoric stress loading, the evolution of anisotropic characteristics (magnitude and orientation of the symmetry axis) is bedding orientation dependent. Anisotropy reversal occurs in specimens with bedding normal/oblique to the maximum principal stress. P-wave anisotropy ε′ is linearly related to volumetric strain Sv and dilatancy, indicating that stress-induced redistribution of microcracks has a significant effect on P-wave velocity anisotropy. The closure of initial cracks and pores aligned in the bedding direction contributes to the decrease of the anisotropy. However, opening of new cracks, aligned in the maximum principal direction, accounts for the increase of the anisotropy. The experimental results provide some insights into the microstructural behavior under loading and provide an experimental basis for seismic data interpretation and parameter selection in engineering applications.

Keywords

• Elastic wave anisotropy
• Stress-induced anisotropy
• Anisotropy reversal
• Bedding orientation
• Microstructure
• Tight sandstone