Shear mechanism of fracture initiation from a horizontal well in layered shale

Abstract

Prediction of fracture initiation pressure (FIP) is an essential issue for safety drilling, fracturing evaluation and efficient production in shale gas exploitation. To quickly and accurately evaluate the FIP of a horizontal borehole drilled in anisotropic rocks, an analytical solution for borehole stress was first deduced based on the complex potential method and superposition principle. In combination with anisotropic tensile and shear fracturing criteria for the bedding planes and shale matrix, the FIP for a particular case was then evaluated using the analytical solution, and the initiation mechanism was also revealed. Finally, parametric studies were conducted to explore the main factors influencing the initiation pressure and fracture mode. The results showed that the elastic anisotropy significantly alters the borehole stress and thus influences the fracture pressure and initiation orientation. The fluid pressure needed for fracture initiating in shear mode is much lower than that in tensile mode. This is mainly resulted from the weak shear strength of the bedding planes. Fracture will probably initiate from the bedding plane in shear mode under certain in situ stress conditions, regardless of the bedding inclination. A lower in situ stress difference appears to induce fracture initiating in shear mode; a larger one, however, is more likely to produce tensile failure. By incorporating the elastic anisotropy, fracture all initiates in shear mode with the increase in elastic anisotropy. However, when the elastic anisotropy degree is larger than 2, fracture will be produced from the rock matrix due to shear failure. The weaker the bedding's shear strength, the easier the fracture will initiate from the bedding planes due to shear failure. The analytical solution developed in this study is extremely useful and convenient to assess the influence of mechanical anisotropy on borehole stress, borehole collapse, wellbore stability and fracture generation of horizontal wells in anisotropic rocks.