Effects of bedding structures on the propagation characteristics of acoustic waves in shales at operating frequencies of acoustic logging

Abstract

Acoustic characteristics measured by acoustic logging are usually used to estimate the structures and mechanical properties of rock materials. However, it is problematic to measure acoustic characteristics of shales because of their structural anisotropy caused by the bedding structures. This study focused on the effects of bedding structures on the acoustic characteristics of shales at operating frequencies of acoustic logging. A numerical model was first established to estimate the acoustic characteristics of shales based on the viscoelastic theory. Then, acoustic transmission experiments were conducted to examine the acoustic characteristics of shales with different bedding angles. The model calculations were compared with experimental results, showing good agreement. Based on the model, the effects of the angle, density, and thickness of bedding planes on the acoustic characteristics of shales at operating frequencies of acoustic logging were analyzed in detail. The results indicate that the wave velocity and main amplitude increased with increasing bedding angle whereas the attenuation coefficient decreased. The wave velocity monotonically decreased with an increase in the bedding density and thickness when the acoustic waves were transmitted through the shale model, but the attenuation of acoustic waves exhibited an opposite trend with the slope. In comparison, the dominant frequency was relatively stable with variations in the bedding structures. In general, the effect of bedding thickness on the acoustic characteristics of shales was smaller than those of the bedding angle and bedding density. The findings of this study can be beneficial for the accurate measurement of shale acoustic characteristics in acoustic logging.