Abstract:

In view of the inadequacy of existing research on the bedding fracture propagation model of shale gas volume fracturing and its calculation method, the mechanical characteristic parameters of shale bedding were obtained by three-point bending (TPB) test combined with digital image method. A pseudo-three-dimensional (P3D) mathematical model of hydraulic fracture propagation in shale gas volume fracturing was established by the utilization of elastic mechanics and line spring model, which was verified by laboratory experiments. A calculation program for geometric parameters of shale bedding fractures was developed to calculated and analyzed the influence of bedding parameters and fracturing engineering parameters on the distribution of hydraulic fracture. The results show that the amount of shear slip reaches a maximum value and a minimum value when the bedding stiffness is less than  10 GPa/m and greater than 30 GPa/m, and keeps basically unchanged. And the bedding stiffness is linearly negatively correlated with the shear slip when the bedding stiffness is in the range of 10–30 GPa/m. The main fracture will communicate with more beddings when the bedding density is in the range of 5–7. The hydraulic fracture is easy to penetrate the bedding and can lead to the bedding shear slip when the bedding strength is in the range of 5–8 MPa, thereby generating a complex fracture network. Moreover, the hydraulic fracture is also inclined to pass through the bedding when the pumping rate and viscosity of fracturing fluid are in the range of 9–12 m³/min and 2.5–5 mPa·s, respectively, and finally a cross-shaped fracture is formed, which is conducive to the formation of complex fractures. This study has a certain guiding significance for understanding the mechanical properties of shale and its influence on the propagation regulation of hydraulic fracture.

Key words: shale bedding plane, volume fracturing, three-point bending (TPB), pseudo-three- dimensional (P3D) model, shear slip