Staged propagation mechanisms and their implications for fracture interaction and fracture swarm formation

Hydraulic fracture swarms are widely observed in post-fracturing core samples, yet their formation mechanisms remain incompletely understood. In this study, we employ a visualized experimental system to directly investigate their evolution using polymethyl methacrylate (PMMA) blocks. High-speed imaging is used to facilitate real-time observation and quantitative characterization of hydraulic fracture propagation under perturbed conditions. The results show that fracture growth follows a repeated cyclic three-stage sequence: (ⅰ) local nucleation, (ⅱ) synchronously radial–circumferential propagation, and (ⅲ) circumferentially dominated propagation. Stress shadowing promotes multipoint nucleation, whereas fluid-energy competition governs fracture-path selection among interacting fractures. When fracture-front velocity exceeds fluid-supply velocity, fluid-lag zones form near the tip, causing temporary arrest and subsequent re-nucleation. In the final stage, stress redistribution induces mixed Modes Ⅰ–Ⅱ propagation, generating step-like and shell-like fracture morphologies. The coupling between stress interference and fluid competition drives repeated propagation cycles, producing high-density fracture swarms consistent with field-core observations. These findings provide a physics-based framework for understanding the formation and evolution of hydraulic fracture swarm.