Fracture Patterns in Laminated Mancos Shale

• https://pubs.geoscienceworld.org/gsa/geophysics/article/82/6/MR201/519934
• https://doi.org/10.1190/geo2016-0579.1

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.

• https://www.onepetro.org/conference-paper/ARMA-2017-0182

Abstract — Layering-induced anisotropy of shale formations increases uncertainty in determining in-situ mechanical properties and stresses, thus increasing the risk associated with implementing advanced drilling and hydraulic stimulation in shales. We conduct simultaneous triaxial stress tests and ultrasonic wave propagation monitoring to quantify static and dynamic stiffness anisotropy in Mancos Shale. Two case studies evidence the impacts of (1) confining stress and (2) presence of pre-existing fractures, on dynamic-static transforms of Young’s moduli and Poisson’s ratios with increasing deviatoric stress. The first case shows that confining stress more heavily impacts dynamic mechanical properties than static. The effect is most prominent at high deviatoric stresses, where stress-induced damage increases the difference between dynamic and static Young’s moduli. The second case shows that samples with pre-existing fractures exhibit even higher differences between dynamic and static Young’s moduli than non-fractured (intact) and damaged rocks. Fractured samples exhibit ratios of Edyn/Est between 5:1 and 7:1, whereas intact samples generally remain near the 3:1 ratio. Pre- and post-test X-ray microtomography imaging confirm that bedding planes and pre-existing fractures act as planes of weakness, while sample layering causes mechanical stratigraphy, where changes in lithology may cause fractures to reorient. Results highlight the limitations of tangent linear elasticity moduli to explain complex deformational behavior in shales and the need for better models that address the strain-magnitude dependence of rock properties.