Impact of Toughness Anisotropy on Fracture Initiation Pressure and Morphology in Shale Rocks

Hydraulic Fracturing (HF) is the prime technology for enhanced production from shale plays. Due to their laminations at different scales, shales exhibit transverse isotropic (TI) properties. In most cases the lamination planes are nearly horizontal with the symmetry axis being vertical, therefore, shales are referred to as TI vertical (TIV) rocks. This significantly influences the initiation and propagation of the induced fractures during fracturing operation. However, many studies on HF modeling in shales assume isotropic medium to simplify the problem. The anisotropic toughness is a distinct feature of layered formations such as shales, and its modelling is important to estimate the hydraulic fracturing initiation pressure, as well as the fractures geometry. In this work, to address the effect of TIV properties of the rock, we used the anisotropic toughness to develop the analytical models for fracture initiation pressure (FIP) based on data from the Bakken Shale, as a case study. In particular, we used ResFrac for calculations, which is a fully 3D HF and reservoir numerical simulator. Different scenarios of radial fracture for cases of single and multiple fractures were simulated for isotropic and anisotropic toughness conditions. The results showed that the hydraulic fracture initiation and propagation are strongly affected by the toughness anisotropy. An increase in the magnitude of anisotropy in toughness leads to an increase in the FIP. We also observed that the FIP varies with direction and consequently the fracture becomes more elongated in the direction of the minimum toughness and contained in the direction of the maximum toughness. In the case of multiple HFs, the combined effect of the anisotropy and the stress shadow was observed. This effect was stronger in the anisotropic case compared to the isotropic case. Hence, the isotropic approximation for TIV shale rocks can lead to inaccurate prediction of FIP and incorrect fracture morphology that can possibly affect the HF design.