Summary 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.

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In the Oil&Gas industry, hydraulic fracturing is a technique that makes unconventional reservoirs economically viable. Despite the high impact of the stimulation processes on the productivity of the wells, hydraulic fractures are modeled assuming vertical and planar propagation in most cases. In Argentina, Vaca Muerta formation is the most important unconventional reservoir. It is predominately in a strike‐slip stress regime with areas in thrust regime, and has also strong elastic anisotropy with widely spread sub‐horizontal mechanical discontinuities such as calcitic veins or ash beds, that could act as preferential propagation paths. The effect of these heterogeneities on the fracture propagation, typically underestimated, is a key issue to model the hydraulic fracture geometry, to estimate the stimulated volume, and to optimize production under these complex mechanical conditions. In this work, we use a coupled fluid ‐ rock mechanics model that accounts for elastic anisotropy and weak discontinuities to study the fracture height growth. Fracture initiation and propagation are modeled using a Discontinuous Galerkin (DG) finite elements formulation with a softening Cohesive Zone Model (CZM). We analyze Vaca Muerta formation representative mechanical properties and provide some fundamentals on typical field problems like vertical containment induced by lamination and layering or horizontal fracture propagation with their consequent loss of productivity. We find, for typical Vaca Muerta formation conditions, that not only the weak bedding strength controls the height propagation of the hydraulic fracture but also the degree of anisotropy plays an important role under harsh stress regimes and high heterogeneity of the formation.

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