Use of S-wave anisotropy to quantify the onset of stress-induced microfracturing

Abstract

Microfracturing and induced elastic anisotropy impart changes on body wave velocities with implications to seismic and wellbore testing methods and interpretation. We have conducted simultaneous triaxial stress tests and ultrasonic wave propagation monitoring to quantify S-wave anisotropy and microfracture development in Berea Sandstone and Silurian Dolomite. The onset of stress-induced microfracturing is detected at the beginning of appreciable S-wave anisotropy called the “S-wave crossover” (SWX). The SWX and subsequent increases in S-wave anisotropy evidence microstructural damage development well before quasistatic indicators such as the volumetric strain point of positive dilatancy (PPD) and yield/failure in all samples. X-ray microtomography confirmed fracture development and allowed for geometric assessment of fracture orientation. Stresses at the SWX and PPD are compared with peak axial stress to understand linkages between damage and ultimate rock strength. In Berea Sandstone, the SWX occurs at 40%–60% of the peak axial stress, whereas in Silurian Dolomite, SWX occurs at approximately 60%–80% of the peak axial stress. Results indicate that rock samples undergo irreversible microstructural changes before dilatancy manifests, and earlier than previously thought. Analysis of tangent elastic coefficients indicates that the ratio between the dynamic and static Young’s moduli can change significantly prior to SWX due to elastic and inelastic processes induced by deviatoric loading and ranges from approximately 2:1 to 4:1 for Berea and 2:1 to 7:1 for Silurian. Understanding damage development and the relationship between the dynamic and static responses of rocks provides opportunities to upscale stress-strain behavior to the wellbore environment and for improved geomechanical interpretation from dipole sonic and time-lapse well-log analyses.