Experimental studies of shale cores of different orientations have been performed to compare the angular dependence of dynamic moduli acquired at ultrasonic frequencies with static moduli (Young’s moduli and Poisson ratios) obtained during initial loading and during subsequent unload – reload cycles. The data show that the dynamic moduli by far exceed their static counterparts. Dispersion explains part of this behavior: Dynamic moduli measured at seismic (10 Hz) frequency are significantly smaller than ultrasonic moduli, but reasonably similar to stiffnesses obtained from finite stress cycles. The difference between moduli obtained during initial loading and during stress cycles is largely caused by non-elastic behavior.
While anisotropy has become state-of-the-art in geophysics for petroleum exploration and monitoring [1, 2], it is often neglected in geomechanical applications to petroleum engineering. For example, shales are known to exhibit pronounced anisotropy, which has significant impact on e.g. hydraulic fracture growth and borehole instabilities caused by failure of rock around the hole.
The incorporation of anisotropy in geomechanical analysis is however limited by availability of data: Full anisotropic characterization requires either laboratory tests on several cores with different orientations, or procedures to convert anisotropic velocity data from seismic or sonic log data to static moduli. This is hampered by the fact that static and dynamic moduli may be quite different [3, 4], and that there is as yet no established theory that enables us to link the two sets of mechanical parameters.