A Coupled CFD-DEM Numerical Simulation of the Behavior of Particles Bridging and Destabilization in Natural Fractures

Lost circulation affects the safety and efficiency of drilling operations significantly, especially in naturally fractured formations. Adding granular lost circulation materials (LCMs) into the drilling fluid is the most commonly used to prevent/mitigate lost circulations. However, the bridging sealing mechanism of granular LCMs in natural fracture space is ambiguous at the meso-scale, which has limited its effectivity seriously. In order to investigate the bridging and destabilization behavior of rigid particles in natural fractures, we used a high-precision three-dimensional scanner to scan the natural fracture surface. The flow space of natural fracture was reconstructed with the combination of reverse engineering modelling technology and scan results. Based on the CFD-DEM method, the particle migration model of naturally fractured formation was established to investigate particles’ migration, bridging and destabilization behavior in the wedge fracture space. The results show that the particle bridging position and bridging form is closely related to the fracture aperture and the particle size. The typical particle bridging structures are single-grain and double-grain, and the fracture deformation could induce the particle bridging structures to vary from single-grain bridging to double-grain bridging. The seal process is a dynamic process of repeating bridging seal, bridging destabilization, re-migration, bridging and seal again. There are a large number of strong force chains at the bridging particles in the leading edge of the seal zone, which are the foundation for forming the seal zone. The force chain network gradually weakens from the front to the entrance direction. This research provides a better insight into the process of fracture sealing by granular LCMs, which indicate that irregularity of the natural fracture aperture has a crucial effect on the structure and position of particle bridging.