Lost circulation is a classical wellbore instability problem that is encountered during drilling. In order to minimize or avoid lost circulation, the best measure is to prevent the initiation of fracture. However, due to rocks are naturally anisotropic, the classical fracture pressure model is not applicable for anisotropic formations. Some researchers had therefore investigated the influence of transverse isotropic material parameters, but the influence of anisotropic tensile strength was ignored. The present work is to propose a novel fracture-initiation pressure model of horizontal well to investigate the influence of anisotropy on fracture-initiation pressure. The present method predicting the fracture-initiation pressure was verified by a laboratory hydraulic fracturing test. The results indicated that both anisotropic modulus and tensile strength have very distinct influence on the fracture-initiation pressure. Fracture-initiation pressure declines obviously under the influence of anisotropic modulus and anisotropic tensile strength, thus the anisotropy of modulus and tensile strength cannot be ignored. The present model can provide a more accurate method for the predicting of fracture-initiation pressure for horizontal well in anisotropic formations.

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