This study aims at analyzing the fracture initiation pressure (FIP) of horizontal wellbore in anisotropic shale by considering elastic and strength anisotropy. The FIP is deduced as a function of the anisotropic elastic properties, in-situ stresses, anisotropic tensile strength as well as bedding dip. The results show that the increase of the anisotropy of Young's modulus, Poisson′s ratio and horizontal in-situ stress induces a reduction of the FIP. While an increase of tensile strength ratio Tv/Th and bedding dip will increase the FIP. It is concluded that the influence of Eh/Ev on FIP is the highest, followed by σH/σh, and then Tv/Th, while the influence of Poisson's ratio is the lowest. For the effect of azimuth of horizontal well, the FIP increases first and then decreases when the horizontal borehole azimuth is far away from the direction of maximum horizontal stress.
Borehole fracture initiation occurs when the stress at borehole wall exceeds the tensile strength during drilling or hydraulic fracturing (Guo et al. 1992 and Ong & Roegiers 1995). The tensile failure of formation would induce lost circulation during drilling, which is a most costly problem in increasing the non-productive time (Ma et al. 2017). The wellbore pressure which initiates new fractures is known as fracture initiation pressure (FIP). In theory, the FIP can be calculated as the stress distribution around the wellbore and the tensile strength criterion of the formation are known (Ma et al. 2019). In a homogenous isotropic formation, the Hubbert-Willis model, Matthews-Kelly model, Haimson-Fairhurst model are commonly used models in calculation of FIP (Zhang & Yin 2017 and Sampath et al. 2018). However, the above models could cause errors as used in prediction of FIP in sedimentary rocks, which show distinct anisotropy in mechanical behaviors (Ma et al. 2022). Shale, as a typical sedimentary rock, shows inherently anisotropic behaviour due to the bedding planes and laminations (Li & Jia 2018). The horizontal drilling and multistage hydraulic fracturing play an important role in commercial development of shale gas. Reducing lost circulation for safe drilling and improving hydraulic fracturing operation, both demand an accuracy calculation of FIP.
