Summary

Rock strength and elastic properties are primary rock parameters. Many rocks are inherently anisotropic. Knowledge of strength anisotropy and elastic anisotropy has great practical application in stimulation design. Twelve oriented pyrophyllite core plugs were tested to analyze the effects of anisotropy. The orientation angles between core axes and the anisotropy axis range from 0° to 90° with 15° increments. Ultrasonic pulse transmission technique is employed to measure P-wave velocity and S-wave velocities. Uniaxial compression tests give rock strength and static modulus from stress-strain data. Static moduli are compared with dynamic moduli calculated from wave velocities.

The results indicate that compressive strength, wave velocities and elastic moduli are affected significantly by anisotropy. Pyrophylllite has an asymmetrical U-shaped strength anisotropy behavior. The greatest strength is measured parallel to fabric, 108.7 MPa. The lowest strength is 88.3 MPa found for samples cored at 45° orientation to the fabric. The strength anisotropy behavior of pyrophyllite can be predicted by knowing the strength values at 0°, 90° and the orientation angle of the minimum strength (approximately 45° in this study). P-wave and S-wave velocities, dynamic and static Young’s moduli are strongly anisotropic. The variation of measured P-wave and S-wave phase velocities agree with the predicted model (Thomsen 1986). Dynamic Young’s modulus varies from 38.6 GPa to 53.6 GPa while the static modulus varies from 13.9 GPa to 32.5 GPa. Dynamic modulus is generally higher than the static for all the orientations. The ratio of dynamic to static moduli ranges from 1.65 to 2.83. A cross-plot of strength anisotropy ratio versus static modulus anisotropy ratio for pyrophyllite and other rock types reported in the literature is presented. No obvious correlation is found between these two parameters, which may be due to the limited number of published data..

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