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In this work, we proposed a hydraulic fracturing model for layered rock based on peridynamics and studied the mechanism of hydraulic fracturing in a layered rock mass, i.e., complex propagation behaviors at the interface such as crossing, kinking, bifurcation, and arrest. Furthermore, we explored the effects of stress difference, elastic modulus ratio, and interface dip angle on the propagation behavior of hydraulic fractures. We found that the stress difference has an important influence on the propagation of hydraulic fractures in a layered rock mass. When the hydraulic fracture design direction is perpendicular to the direction of the maximum principal stress in a layered rock mass, the fracture network will be induced.

In this paper, the DDM numerical method is used to calculate the initiation pressure and initiation length of the circular wellbore, or the further elongated and non-elongated ‘dog-ear’ breakout wellbore under different sizes and different stress states. An analysis is made about the transfer of the fracture initiation point for the further elongated breakout wellbore and its possible impacts. The numerical results show that for small-size wellbore ((Rwσt2)/KIC2 6.4), the initiation fracture is dominated by tensile strength. There is only small difference between the initiation pressure of the ‘dog-ear’breakout wellbore and the circular wellbore. With further elongation of the ‘dog-ear’ wellbore or increase of the curvature radius around initiation point, the initiation pressure fracture along the direction of the maximum principal stress also increases in most cases. The further elongated ‘dog-ear’ wellbore causes the initiation point of the wellbore to deviate away from the direction of the maximum principal stress, and the existence of at least four initiation points tends to cause X-shaped fracture initiation around the wellbore, which may promote the further increase of the initiation pressure. The research in this paper provides a new possible version of multi-fracture initiation around wellbore and abnormally high initiation pressure during fracturing operation.

A coupled hydromechanical model based on peridynamics is proposed to simulate the hydraulic fracturing in a complex fracture network. The proposed method is validated by comparing the numerical results with the analytical and experimental solution. Then, the influence of the crustal stress distribution, the friction force acting on the natural fracture (NF) surface and the NF network on hydraulic fracture (HF) propagation are investigated. The simulation results reveal that the crustal stress ratio mainly affects the HF propagation direction. When HFs and NFs are connected, the induced stress caused by the opening of NFs deflects the propagation direction of new fractures. The friction force on NF surface is an important factor affecting the interaction between HFs and NFs.

A fully coupled enriched-FEM (XFEM) model is proposed to investigate the interaction between hydraulic fractures and natural fractures in finite 2D porous media. The weak forms of both equilibrium and continuity equations are discretized in an enriched form in the space and by the Newmark scheme in the time domain. The Newton-Raphson method is adopted to solve these discretized equations. To account for the fluid flow in the porous matrix, 2D fluid leakage from the fractures to the matrix is considered by imposing internal flux boundary conditions on the fracture surfaces. A posteriori analysis is carried out to determine the equivalent leak off coefficient of the 1D Carter’s leak off law. The frictional contact of the fracture surface is modeled by the penalty method in the model. The junction ridge function is adopted to describe the weak discontinuities of the pressure field in the elements containing fracture intersection points. A crack initiation criterion based on the average tensile stress along the natural fracture is used to determine whether a hydraulic fracture can cross a natural fracture. The presented model is first validated against the analytical solutions of the KGD model and then several numerical examples are simulated to investigate the effects of the natural fracture properties, treatment parameters, matrix permeability, anisotropy stress and approaching angle on the interaction between hydraulic fractures and natural fractures. The numerical results suggest the acceptable accuracy and robustness of the presented model on modeling fracture interaction behaviors in the presence of fluid leakage into matrix.