Characterization of porous rocks embedded with aligned fractures from shear wave responses

Abstract

SUMMARY A single shear wave passing through an elastic anisotropic rock can split into two quasi-shear waves (noted by S1 and S2) with different polarization forms if the particle vibration direction of the wave source does not lie in the symmetry plane of the rock. This study focuses on the properties of shear waves penetrating a porous rock containing a set of aligned permeable fractures. The polarization characteristics of shear waves were selected to describe the dynamic properties of the rock as they are sensitive to the parameters of fractures and saturating fluids. From a physics viewpoint, in addition to the compressional wave, the shear wave (splitting) is governed by a wave-induced fluid flow (WIFF) process due to the specific shear stress decomposition happening on the fractures. The polarization formulas of S1 and S2 were derived based on the frequency-dependent Christoffel equation, which are related to the properties of fractures, fluids and wave frequency. The influence of the properties of fractures and fluids on the velocity and attenuation anisotropies of shear waves were analysed. The results showed that the particle oscillations of two shear waves are not completely mutually orthogonal, and are affected by the pressure equilibrium magnitude between the fractures and the corresponding interconnected pores. The S2 (slow) wave (i.e. particle polarized on the plane approximately perpendicular to the fractures) is more sensitive to the saturated fluids and the WIFF process than that of the S1 (fast) wave (i.e. wave polarized on the plane approximately parallel to the fractures). A frequency factor was proposed for quantifying the effects of WIFF on shear wave polarization and attenuation. Measurements on the unique polarization and the anisotropy of shear waves can provide a generalized indicator to predict the properties of fractures and the migration of infilling fluids in the rock fracture systems.