Laboratory investigation across multi-scales on slip nucleation induced fracture bifurcation: Explaining swarm-like hydraulic fractures observed at hydraulic fracturing field test

Understanding the formation mechanism of swarm-like hydraulic fractures is crucial for efficient shale gas development, and such new phenomenon has recently been detected in field experiments. This study investigates the fracture bifurcation induced by the interaction between fractures and weak planes through multi-scale experiments. Experiments reveal that slip nucleation zone (SNZ) of the weak interface induces fracture bifurcation, a key mechanism of swarm-like hydraulic fractures. Specifically, the slipping of weak interface induces discrete fracture process zones where fracture bifurcates to form doublet fractures. This bifurcation occurs after fracture crosses the weak interface, consistent with the statistical data from the Hydraulic Fracturing Test Site (HFTS) project. At both the micro- and macro-scales, the larger SNZ leads to fracture bifurcation, forming doublet fractures, while the smaller SNZ only induces fracture kinks. Hydraulic fracture bifurcates with simultaneous spatial segmentation, forming doublet fractures marked by twist hackles and smooth plane, as observed in both physical experiments and core samples from HFTS. These research results enhance the understanding of the formation mechanism of swarm-like hydraulic fractures and provide valuable insights for shale gas development.