Analysis of the Effect of Geomechanical Stress on the Acoustic Response of Anisotropic Rocks

This paper presents the analysis of borehole acoustic logging response in anisotropic rocks under geomechanical stress. Several effects, which are relevant to the interpretation of sonic logs, are observed and discussed. First, it is shown that the crossover of flexural modes dispersion curves in stress-induced anisotropic formation do not necessarily occur. Second, it is demonstrated that the effects of the difference in orientation of principal stress and anisotropy axes directions, stress magnitudes, and degree of anisotropy on the flexural dispersions can compensate each other at least partially. Finally, the examples of measurable stress effects on the dispersion curves in anisotropic formations are presented. The implication of these observations for the formation stress evaluation from sonic logs in the case of anisotropic formation is that the stress estimation from sonic logs should focus not only on stress magnitudes, but also take into account formation anisotropy and stress orientation with respect to the borehole and anisotropy axes.