Stress-Associated Intrinsic and Scattering Attenuation from Laboratory Ultrasonic Measurements on Shales

Abstract

Seismic attenuation is sensitive to stress-induced subtle changes in the physical state of rocks. In this study, the stress- and frequency-associated attenuation is quantified through ultrasonic measurements on three differently oriented cylindrical shale samples under various axial stresses. As an improvement to the single-scattering model, the elastic Monte Carlo method is employed to investigate multiple-scattering attenuations by incorporating the boundary reflections and wave conversions. Our results show that, as the axial stress increases, the intrinsic attenuation decreases in all directions, while the scattering attenuation decreases slightly in the direction perpendicular to the bedding but increases largely and nonlinearly in other directions. These discrepancies result from different attenuation mechanisms. Both the intrinsic and scattering attenuation are found to be largest in the direction 45° to the bedding, but least in the perpendicular direction. The S-wave attenuation is larger and more sensitive to stress changes than P-wave attenuation due to its shorter wavelength. As expected from sandstone examples, the scattering attenuation in shales is significantly larger and more sensitive to stress changes than the intrinsic attenuation. The frequency dependence of scattering attenuation suggests that the peak frequency with the maximum scattering attenuation is independent of axial stresses, but varies in different directions of an individual rock with different heterogeneity and anisotropy scales. The peak frequency of S-coda is smaller and its peak scattering attenuation is larger than P-coda. In conclusion, the stress and frequency dependence of ultrasonic attenuations in shales differ largely in various directions, indicating significant anisotropy and heterogeneity.