Research on the propagation laws and influencing factors of hydraulic fractures in deep shale reservoirs based on the three-dimensional discrete lattice method

Unconventional shale gas reservoirs are an important part of oil and gas exploration and development. Generally, large-scale hydraulic fracturing is required to form a large number of complex fracture networks in the reservoir, so as to connect the reservoir volume and enhance reservoir permeability. However, during fracture initiation and propagation, unclear propagation patterns and controlling factors often cause issues such as perforation cluster failure to initiate or propagate effectively, hydraulic fracture merging, and unexpected propagation under natural fracture influence, making it difficult to form an effective complex network. To address these issues, we establish a three-dimensional fracture propagation model for deep shale gas horizontal wells using the discrete lattice method based on geological parameters from the deep shale gas block in southern Sichuan. Numerical simulations are conducted to investigate the effects of injection rate, cluster spacing, horizontal stress difference, and natural fracture approach angle on fracture propagation behavior. The results show that increasing the injection rate and cluster spacing can increase the number of initiated perforation clusters to a certain extent, thereby enhancing the number of hydraulic fractures and improving complexity. When the cluster spacing is 6 and 10 m, increasing the injection rate from 0.05 to 0.30 m3/s leads to an increase in over 40% of the number of effectively initiated clusters. When the injection rates are 0.15 and 0.20 m3/s, increasing the cluster spacing from 6 to 10 m results in an increase in over 20% in the number of effectively initiated clusters. However, when the cluster spacing is small, stress interference still prevents some central clusters from initiating and propagating properly even with higher injection rates, resulting in significantly smaller fracture volumes compared to others. Under low horizontal stress difference 8 MPa, fractures are prone to twist and form non-straight fractures and merge after initiation, affecting the fracture propagation distance. After increasing the horizontal stress difference to 14 MPa, fractures propagate to form more regular and straight hydraulic fractures, and the fractures can extend further. As the approach angle of the natural fractures increases, the propagation pressure of hydraulic fractures under the action of natural fractures gradually increases, and the hydraulic fractures transition from being captured by natural fractures to crossing them and eventually being arrested. Therefore, by increasing the injection rate and cluster spacing under higher horizontal stress differences, while promoting intersection with a small approach angle natural fracture, can enhance fracture network complexity and facilitate efficient hydraulic fracturing in deep shale gas reservoirs.