Abstract: In order to clarify the effect of anisotropy of layered shale on the wellbore fracture initiation mechanical behavior, the uniaxial compression testing and Brazilian split testing experiments with different angle were conducted for LMX shale in the Lower Silurian Sichuan Basin. The anisotropic characteristics of elastic modulus and tensile strength were specified, and a wellbore fracture initiation mechanical model of shale gas horizontal wells was established considering the anisotropy of shale. The wellbore fracture initiation behaviors under different influencing factors are also analysed. The results show that:modulus anisotropy, in-situ stress difference, tensile strength anisotropy and pore pressure all have an obvious effect on fracture-initiation pressure but Poisson's ratio anisotropy has slightly effect on it. Modulus anisotropy and in-situ stress difference also have a significant influence on wellbore fracture initiation location and inclination angle, Poisson's ratio anisotropy, tensile strength anisotropy and pore pressure, however, have slight effects on that. Wellbore fracture initiation mechanical behaviour is closely related to wellbore azimuth; when the borehole azimuth is close to the direction of maximum or minimum horizontal principal stress, the difference of wellbore fracture initiation mechanical behavior varies slightly with borehole azimuth changing, while when the borehole azimuth is far away from the direction of maximum and minimum horizontal principal stress, the difference of wellbore fracture initiation mechanical behavior varies greatly with the changing of borehole azimuth. The research results can provide theoretical basis and reference for well leakage prevention during drilling and hydraulic fracturing design of shale gas horizontal wells.

Key words: shale, horizontal wells, wellbore fracture, anisotropy, fracture-initiation pressure, fracture-initiation trajectory