Abstract: Physical modelling of large-scale true triaxial test on hydraulic fracturing of shale horizontal well is performed to study in detail the law of the initiation and growth of hydraulic fissures in Longmaxi Formation shale, the fracturing mechanics and the interaction between hydraulic fracture and natural fracture. On the basis of fracture mechanics and fluid mechanics theories, the mechanical model of hydraulic fracture initiation and expansion was established, and the corresponding fracture mechanics criterion was derived. Results indicate that: (1) The crack initiation mode can be divided into three categories, namely, tensile cracks started with shale, shear fractures started with natural fracture (bedding plane), and tensile cracks started with natural fracture. (2) During the extension of hydraulic fracture, the pump pressure decreases with the elongation of crack length and finally tends to a stable value, which is slightly bigger than the minimum horizontal principal stress. (3) According to whether the natural fracture is open, the interaction between hydraulic fracture and natural fracture can be divided into the following five types: penetration through the sheared natural fracture, penetration through the natural fractures directly, penetration through one end of the natural fracture, penetration through the natural fracture from two ends of the natural fracture synchronously, and penetration through the natural fracture from a weak surface. The research results offer strong technical support for horizontal well fracturing of shale gas.

Key words: Longmaxi Formation shale, hydraulic fracture, crack initiation and propagation, physical modelling, pump pressure-time curve, interaction, natural fracture