Employing multi-stage fracturing technology in horizontally accessed wells is a well-known way to successfully develop shale reservoirs. The interaction between natural fractures and hydraulic fractures has a significant impact on the fracturing effect. In this study, a coupled model of rock deformation and fluid flow was established using the cohesive zone method to simulate the propagation of hydraulic fractures under the synergistic effect of natural fractures and wellbore interference. The influence of in situ stress, fracture spacing, the number of fracture clusters, and the fracturing methods on the formation of fracture networks was analyzed. Studies on the fracture morphology and connectivity of fracture networks show that when the in situ stress difference is small, multiple fractures can easily form, and when the in situ stress difference is large, they can easily gather into a single fracture. An excessive reduction in fracture spacing may impede the optimal propagation and interconnection of hydraulic fractures. The findings reveal that augmenting the fracture spacing from 5 m to 8 m results in a significant 15.59% increase in the overall extent of fracture propagation. Moreover, it also adds to the complexity of the fracture network. Increasing the number of hydraulic fracturing clusters can improve the fracture length and fracture propagation complexity. When the number of fracturing clusters increased from two clusters to five clusters, the maximum fracture propagation width increased by 25.23%. Comparing sequential fracturing and simultaneous fracturing, the results show that simultaneous fracturing can form a more complex fracture network with better connectivity, which is conducive to increasing oil and gas production. The obtained results can provide a reference for horizontal well fracturing designs of shale reservoirs.

There are many natural fractures in shale reservoirs, changes in hydraulic fracture extension patterns. In the paper, a multi-fracture extension finite element model for horizontal wells in shale reservoirs under the disturbance of natural fractures is established by combining the actual geological parameters and construction parameters of a horizontal well multi-fracturing operation in X oilfield to analyze the effects of the difference in geostress, elastic modulus, angle of natural fractures, and the number of natural fracture groups on the hydraulic fracture extension. The results show that with the increase in ground stress difference and natural fracture angle, hydraulic fractures are more likely to penetrate the natural fractures; with the increase in elastic modulus, the fracture stress and tip stress increase, the volume of rupture unit, the fracture extension width and the pore pressure concentration area decrease, and it is easy to form a long and narrow fracture; with the increase in the number of fracture groups, the connectivity of reservoir fractures increases, the extension of fractures is stronger, and it is easier to form a complex fracture network.

Conventional fracturing design generally only considers the propagation of a single fracture in homogeneous storage, but the heterogeneity characteristics of the reservoir and the development of complex natural fractures will affect the fracturing effect. In order to study the horizontal well fracture propagation law under the synergistic effect of complex natural fracture reservoir heterogeneity, the globally embedded cohesive element finite model is established, and the heterogeneous reservoirs are built by Python programming. The influencing factors of multi-fracture propagation with heterogeneous reservoirs are analyzed, including the in situ stress difference, the injection rate of fracturing fluid, the perforation spacing, and the perforating clusters number. The results show that the heterogeneity and the natural fracture of the reservoir will affect the in situ stress distribution and propagation direction. When the in situ stress difference is low, the reservoir heterogeneity and natural fractures have a great influence on the fracture morphology. With the increase in injection rate, the fracture length and fracture width increase. With the in situ stress difference increase, the fracture deflection angle decreases. With the increase in fracturing fluid injection rate, the fracture length increases. The number of hydraulic fractures communicating with natural fractures increases with the natural fracture number and the number of perforation clusters under the synergistic effect of complex natural fracture and reservoir heterogeneity.