Simulation Investigation of Hydraulic Fracture Propagation Patterns at Lithological Interfaces Based on the Phase-Field Method

The propagation pattern of hydraulic fractures in the discontinuity during fracturing is complex and variable. To reveal the influence mechanism of lithological interfaces formed in reservoirs with different lithologies on the propagation of hydraulic fractures, this paper analyzed the propagation behaviors of hydraulic fractures in lithological interfaces with different lithological combinations based on the numerical model established by the phase field method. The results show that when the hydraulic fracture propagates to the lithological interface, arrested, diverted, crossing and first diverted and then crossed may occur, and the specific situation is affected by a combination of factors, such as the geostress, the mechanical properties of the rock layers on both sides of the lithological interface, the strength of the interface, and the angle between the lithological interface and the direction of propagation of the initial hydraulic fracture (the angle of intersection). Second, during the fracturing process, as the ratio of the elastic modulus of the lower reservoir to that of the upper reservoir and the interfacial elastic modulus increases, the hydraulic fracture gradually diverts from following the lithological interface to evolving directly crossing the lithological interface. The larger the intersection angle and the ratio of vertical stress to horizontal stress, the more the hydraulic fracture is inclined to cross the lithological interface directly, and the intersection angle of 90° is the optimal angle for the hydraulic fracture to cross the lithological interface. Finally, the propagation criterion of hydraulic fracture at the lithological interface under the effect of different combinations of influencing factors is established. The results of the research will guide the design and the optimization of fracturing in target reservoirs formed by different lithological combinations.