On the initiation of hydraulic fractures in anisotropic rocks

Abstract

This paper addresses hydraulic fracture initiation from an initially notch-free wellbore situated in an anisotropic host rock, where the rock mass is subjected to an in-situ stress state that is also anisotropic in nature. In such conditions, hydraulic fracture initiation is fully characterized by three parameters, namely initiation pressure, initiation orientation, and initiation length. While a strength-based criterion captures only two of these parameters, break-down pressure and orientation, the evaluation of initiation length requires a mixed criterion approach in which both stress and energy conditions are met. We extend the existing mixed criterion formulation for isotropic rocks to also include anisotropic material behavior. We then investigate how material anisotropy interacts with stress anisotropy to determine the break-down pressure as well as the size and orientation of the starter crack. The results show that material anisotropy can overshadow the stress anisotropy, in the sense that it can drive the crack towards the weakest plane. It is demonstrated from a fracture energy perspective that the initiation length cannot extend as large as the fracture process zone length even when the wellbore size is unrealistically large. The difference between initiation pressures predicted by the mixed criterion and strength-based theory rises as the wellbore size decreases. Based on these observations, applying the mixed criterion is essential especially in relatively small wellbores.