Inherent transversely isotropic elastic parameters of over-consolidated shale measured by ultrasonic waves and their comparison with static and acoustic<i>in situ</i>log measurements

Abstract

In terms of elastic anisotropy, sedimentary shales may be considered to have transverse symmetry. Experimentally determining the five independent elastic constants required for this case remains challenging. This paper proposes the use of ultrasonic waves in determination of the five constants. Arrays of specially constructed transducers with different modes of vibration were mounted on samples trimmed from natural cores to measure ultrasonic P-waves and S-waves along the horizontal, vertical and 45°-inclination axes on the samples. The elastic constants calculated from these wave velocities were compared to those determined from static tests and acoustic in situ logs. The static tests included drained triaxial compression and confined torsion tests. The acoustic logs were conducted in an open hole using monopole compressional and dipole shear transmitters and receivers. It was found that the elastic constants determined by the static tests were much lower than those determined from the ultrasonic tests and acoustic logs. Discrepancies among the elastic constants determined from these three different methods are discussed and explained.