Quantification of static and dynamic mechanical anisotropy in fractured and layered rock systems : experimental measurement and numerical modeling

Shales often exhibit mechanical anisotropy, which directly impacts the accuracy of seismic imaging and the geomechanical response to drilling and completions. Anisotropy is often caused by mineralogical layering, fractures, and rock fabric. However, the relative impact of each of these features on static and dynamic measurements is not well understood. We utilize simultaneous triaxial stress-testing and ultrasonic monitoring, in addition to CT and SEM imaging to highlight the impacts of rock type (sandstone, dolomite, shale), confining stress, stress loading path (isotropic and deviatoric), lithological heterogeneity and layering, and fractures (pre-existing and stress-induced) on static and dynamic mechanical properties and rock failure. We show that: (1) changes in shear wave anisotropy during deviatoric loading are evidence of the onset of microfracturing in isotropic rocks. This new dynamic method is valuable because it detects stress-induced damage before the typical static method (dilatancy), allowing for rock failure property estimation while still preserving sample competence for future tests. (2) The deviatoric stress-dependence of anisotropic static and dynamic mechanical properties can be estimated by combining measurements from several cores as a function of the % of peak stress. Samples exhibit strength variability due to layering orientation and heterogeneity. Therefore, using % of peak stress rather than the stress magnitude ensures that measurements are combined from samples undergoing similar deformational processes (i.e., primarily elastic vs plastic strains). (3) Rock mechanical properties vary with applied stress and the presence of fractures. Thus, dynamic-static transforms must not only account for anisotropy, but also stress-induced changes in anisotropy and rock damage. (4) The relative impacts of layering and fractures on shale velocity anisotropy can be decoupled and modeled by combining CT and SEM imaging to develop mineralogically and structurally heterogeneous velocity models. Quantifying the contribution of these features to overall anisotropy provide an avenue for evaluating subsurface variations in rock heterogeneity (i.e., spatial changes in natural fracturing). Overall, this study evidences the complexity of rock mechanical behavior, which dictates robust mechanical characterization to accurately predict a rock’s response to human-induced stress changes during hydrocarbon exploitation.