A mixed phase-field model for fracture propagation behavior in Gulong shale with complex wavy bedding

Shale reservoirs have abundant bedding structures, which deeply alter the mechanical properties of rocks, and thus affect the reservoir stimulation performance. Previous research mostly focuses on the effects of parallel bedding on fracture propagation, while the mechanical properties and mechanisms of fracture propagation remain unclear for rocks with complex wavy bedding (e.g. China’s continental-origin Gulong shale). Herein, a mixed phase-field fracture model of the wavy-bedding shale was applied, based on the local tension-compression decomposition phase field method (PFM) and geometric structure generation algorithm for the bedding with controllable morphological features. The parametric analysis of fracture propagation behaviors in the case of abundant complex bedding structures showed that with wavy bedding, the vertical fracture propagation rate is far higher than the horizontal propagation rate. Moreover, the development of branch fractures is suppressed during the fracturing process of the wavy-bedding sample, and the stimulated volume is limited, which is different from the characteristic of parallel bedding that promotes horizontal fracture initiation and propagation. The results showed that larger amplitudes, higher frequencies, higher inclination angles, and larger strengths of wavy bedding all promote the formation of vertical penetrating fractures and suppress the growth of branch fractures. Under such circumstances, it is hard to create a well-connected fracture network after fracturing. This research may provide a theoretical basis for understanding fracture behaviors in rocks with such complex wavy bedding.