Prediction of Fracture Initiation Pressure and Fracture Geometries in Elastic Isotropic and Anisotropic Formations

Abstract

Laminated formation structures in shale formations may have elastic anisotropic properties, including Young’s modulus and Poisson’s ratio, that impact hydraulic fracturing treatment execution. Fracture initiation pressures and geometries are affected by these properties, especially in cased and in perforated horizontal wells. After initiation, stress concentration around the wellbore may cause the creation of longitudinal fractures (LFs) in the near-wellbore zone that reorient to transverse fractures (TFs) beyond this region. In this case, severe fracture kinking may occur that may hinder the transport of proppants and reduce well productivity. We developed an analytical model based on linear fracture mechanics theory to study the effect of perforation geometries on the initiation fracture pattern. Elastic anisotropy and well deviations were incorporated into simulations. Our simulation results show that when the perforation depth is in a specific range under normal fault regime, the initiation pressures for LFs can be smaller than the maximum horizontal stress $\sigma_{H}$. This behavior is significant for a smaller $\sigma_{H} /\sigma_{v}$ ratio, but it vanishes for a larger $\sigma_{H} /\sigma_{v}$ ratio. With increasing formation elastic anisotropy ($K_{aniso}$), the initiation pressures for both LFs and TFs increase, and the critical perforation depth is decreased. Considering the well deviation, the well azimuth and inclination angles affect initiation pressures for both longitudinal and transverse fractures. Results show how perforation depth, shale elastic anisotropy, and well orientation affect fracture initiation patterns. This paper provides a framework for well completion designs and well orientation designs to minimize the fracture kinking in the near-wellbore region in shale formations.