Evolution mechanisms of plugging-zone structure and force-chain network under dynamic fracture deformation

Lost circulation is a critical drilling problem in deep fractured formations, where pressure fluctuations induce dynamic fracture deformation and destabilize plugging zones. In this study, a coupled CFD–DEM model incorporating a prescribed dynamic fracture-opening boundary was developed based on three-dimensionally scanned fracture geometries to simulate the formation, instability, and reconstruction of plugging zones in rough fractures during fracture opening. The evolution of the plugging zone structure and force chain network was systematically analyzed, and the effectiveness of a multi-stage injection method was evaluated. The results show that plugging-zone formation follows a distinct multi-stage process: coarse particles first form the bridging skeleton, while medium and fine particles fill interstitial voids and participate in load sharing, accompanied by the development of a dense interconnected force-chain network. Under the prescribed 10 ms fracture-opening condition, the plugging zone rapidly disintegrates as particle confinement weakens and the original force-chain network collapses, while secondary reconstruction after fracture deformation exhibits reduced structural stability due to the shift to less stable bridging modes and particle loss. Entrance pseudo-plugging is identified as a critical factor that creates a particle-entry deficit and severely hinders effective plugging-zone formation in the deeper fracture. Under the present numerical conditions, the multi-stage injection method resulted in approximately 46% and 97% higher retained-particle numbers in the initial and reconstructed plugging zones, respectively. Moreover, the peak outlet mass flow rate and cumulative fluid loss are reduced by approximately 17.9% and 57.9%, respectively, demonstrating improved hydraulic sealing performance. This study provides particle-scale insights and theoretical guidance for plugging design and lost circulation material selection in deep fractured formations subject to downhole pressure fluctuations.