Experimental investigation on the transport and plugging of lost circulation materials in large-scale vertical fracture coupled with wellbore system

Fracture plugging during lost circulation is a probabilistic process governed by the transport behavior of lost circulation materials (LCMs). Effective bridging depends on regulating LCM transport dynamics to improve plugging efficiency and mechanical strength. Plugging-slurry flow is a complex solid-liquid, two-phase process involving multiple coupled physical fields in the wellbore-fracture system. However, microdynamic plugging mechanisms in the coupled wellbore-fracture system remain poorly understood because high-fidelity experiments with scale-reproducible characterization are lacking. Using a self-developed apparatus to simulate squeeze injection in a coupled wellbore-fracture system, this paper characterized LCM transport and plugging behavior and elucidated the micromechanical mechanisms of fracture plugging. The results indicate that LCMs blockage at the narrow fracture entrances(“entrance blockage” phenomenon) is the primary cause of recurrent losses. Reducing the particle size of LCMs optimized based on the “D90 principle” by an additional 44–56% can ensure adequate entrance entry. The density differential between LCMs and drilling fluid determines the transition between “laydown type” sedimentary bridging and “anchoring type” interception bridging modes. Smaller density differences promote anchoring-dominated bridging. Irregular particle morphology enhances bridging stability by increasing interlocking effects. Approximately 10% LCMs loss in the wellbore disrupts optimal particle gradation and concentration, weakening fracture plugging efficacy. By integrating three dominant control factors: critical non-entrance-blockage, fracture geometry compatibility, and LCMs mass loss rate within wellbore, this paper developed a novel plugging slurry formulations design methodology that emphasizes matching geometric and physical properties of LCMs with fracture characteristics. This approach provides robust technical support for controlling drilling fluid losses in deep fractured formation.