关键词: 位移间断法, 天然裂缝, 张拉型破坏, 剪切型破坏, 最大主应力

Abstract: Activation of the far-field natural fracture by stress field induced by the hydrofracture can change the rock stress field, reduce the difficulty in formation of interconnected fracture network, such as main, branch and stress relaxation fractures, and improve shale gas reservoir stimulation efficiency, and therefore, has broad prospects of application to the shale gas development. The failure characteristics of natural fractures in the vicinity of hydrofracture are one of the core issues to the activation of far-field natural fracture by stress field induced by the hydrofracture. Displacement discontinuity method (DDM) is used to construct a mechanical model for activation of natural closed fractures in the vicinity of hydrofracture. According to Mohr-Coulomb failure criteria, the constraint conditions in different natural fracture failure states are established; the calculation methods of natural fracture characteristics are formed; and the correctness of these calculation methods are verified by comparison with the existing calculation results of semi-analytical solution. On this basis, the opening, slipping and closing behaviors of natural fractures in the vicinity of hydrofracture are simulated; the natural fracture displacement discontinuity and maximum principal stress distribution characteristics under the action of stress field induced by the hydrofracture are analyzed; and the factors influencing natural fracture failure characteristics are studied. The research results show that the maximum principal stress at the backside surface of natural fracture is in bimodal distribution. As the angle between the natural fracture and the maximum horizontal principal stress increases; the maximum principal tensile stress increases. In the vicinity of hydrofracture, the tension-type failure length of natural fracture is less than its shear-type failure length. As the angle between the natural fracture and the maximum horizontal principal stress increases; the tension-type failure length and shear-type failure length of natural fracture decrease. The natural fracture failure length increases with the increase of net pressure and increases with the decrease of the distance from hydrofracture tip to natural fracture center. The shear-type failure length decreases with the increase of friction coefficient and cohesive force. The research results can provide theoretical guidance for shale gas fracturing technology.

Key words: displacement discontinuity method, natural fracture, tension-type failure, shear-type failure, maximum principal stress