Fracture behavior of transversely isotropic rocks with discrete weak interfaces

Anisotropic rocks have gained increasing attention due to the development of unconventional oil and gas reservoirs. This type of reservoir requires complex engineering procedures from drilling to completion, and the mechanical properties of the rocks play a significant role. One of the most important characteristics of unconventional formations is the presence of well-defined layers, which greatly modify both elastic and fracture properties of the rocks. Rock elastic response is direction dependent, commonly described as vertically transversely isotropic (VTI), meaning that there is an isotropic plane of symmetry with respect to the vertical direction. Between layers, calcitic veins or ash beds alter the rock's properties substantially. These interfaces, which can have low or no cohesion at all, act as preferential planes for fracture nucleation and propagation, affecting the rocks strength. In this work, the fracture behavior of VTI rocks with weak interfaces is studied using numerical simulations of brazilian tests. A hybrid discontinuous Galerkin-cohesive zone model was used to simulate fracture initiation and propagation on discs in the presence of weak planes. With this approach, we determined the effective brazilian test strength of the rocks under different conditions, namely, relative angles between layers and the loading direction, densities, and cohesion of weak interfaces. Shear reactivation of the interfaces is analyzed using the Mohr-Coulomb failure criterion and Mohr circles.