Oriented cores of Barnett shale were studied in detail with the help of ultrasonic and X-ray imaging methods. The velocities of compressional wave (VP) and two shear waves (VSh and VSv) were measured on 56 samples. As a result, we found a relation between the X-ray images and specific features in behavior of elastic wave velocities. The average splitting of shear waves observed for the Barnett shale cores at room conditions is 30%.
Commonly, shales are mentioned as elastic anisotropic rocks (Jonston and Christinsen, 1994; Vernik and Liu, 1997; Wang, 2002). Barnett shale is characterized by the VTI type of anisotropy (Bayuk et al., 2007; Dyaur et al., 2007). The latest investigations of Barnett shale shows changes in azimuthal anisotropy magnitude, therefore the shale can be related to slightly orthorhombic type of symmetry instead of hexagonal. We measured elastic wave velocities in different direction in the bedding plane (i.e., in the azimuthal directions). These results are presented in this paper.
The Barnett shale cores from a well were oriented in space. The bedding plane was horizontal. Subvertical fractures in the shale were open or mostly healed with calcite. Our study of velocity of P-, Sh- and Sv-waves in lateral (bedding) directions is necessary to characterize changes of elastic properties of shale along the well, spatially evaluate anisotropy of velocity using the splitting of shear waves and find the direction of influence of tectonic stress on velocity anisotropy.
During the first examinations of the Barnett shale core by the X-ray method, we found out that this method is a good tool to distinguish in composition and porosity of thin layers (Dyaur et al., 2007). The mineralogical composition was analyzed by Y. Mohamed. Now we obtained results for 56 core samples. All samples are characterized by X-ray images and P-, Sh- and Sv-wave velocities measured in each 25.4 mm (1 inch).
Data obtained with both these methods depend on mineralogical composition, density, porosity, and structure of rock. We used the same direction of propagation of elastic waves and transmission of X-ray beam to compare the velocities and X-ray images.
We employed the pulse transmissiontechnique. The digital oscilloscope to record the waveforms had sampling rate of 50 MHz and allowed us to have the picking time of 20 ns. The central frequency of P-waves and S-waves piezoelectric transducers was 1 MHz. The velocities of polarized Sv waves were also measured with emitted frequency of 0.5 MHz. The accuracy of determination of VP was less than 0.4%, and of VS – less than 1% - 2%, depending on the polarization of shear waves.
Two special devices were developed for ultrasonic P-waves and S-waves velocity measurements on core to control the azimuth of wave propagation. The devices allowed us to change the azimuth of measured velocity with an accuracy of 1º. The polarization of shear wave transducers was also controlled by the device. A special orientation of shear wave transducers was used for determination of Sh- and Sv-wave velocity in radial direction of core.