The drilling fluid loss or lost circulation via fractures is one of the critical engineering problems in the development of deep oil and gas resources. The conventional treatment is to introduce granular lost circulation material (LCM) into the drilling fluid system to plug fractures. In this work, a method incorporating the fracture surface scanning technique and coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) numerical simulation is proposed for the first time to investigate how the LCM particles plug rough fractures. The rough fracture model is built utilizing a high resolution and high precision measurement system. The LCM particle transport and plugging process in rough fractures are captured in the CFD-DEM numerical simulations. The results show that the local fracture aperture has a significant influence on LCM particle transport and the distribution of the plugging zone. The drilling fluid loss rate will decrease, and the drilling fluid pressure will redistribute during the accumulation of LCM particles in the fracture. The fracture plugging efficiency of nonspherical LCM is improved as a result of formation of multi-particle bridges. This study provides a novel approach and important theoretical guidance to the investigation of LCM particle transport in rough fractures.

The drilling fluid loss or lost circulation via near-wellbore fractures is one of the most critical problems in the drilling of deep oil and gas resources, which causes other problems such as difficulty in achieving wellbore pressure control and reservoir damage. The conventional treatment is to introduce granular lost circulation material (LCM) into the drilling fluid to plug the fractures. As the migration mechanism of the LCM in irregular fractures has not been completely figured out as of yet, the low success rate of fracture plugging and repeated drilling fluid loss still obstruct the exploitation of deep oil and gas resources. In this paper, the spatial data of actual rock fracture surfaces were obtained through structured light scanning, and an irregular surface identical to the rock was machined on a transparent polymethyl methacrylate plate. On this basis, a visualization experimental apparatus for fracture plugging was established, and the fracture flow space of this device was consistent with that of the actual rock fracture. Employing cylindrical nylon particles as LCM, a visualization experiment study was carried out to investigate the process of LCM bridging and fracture plugging and the influence of LCM injection methods. The experimental results show that the process of fracture plugging includes the sporadic bridging, plugging zone extension and merging, thickening of the plugging zone and complete plugging of the fracture. It was observed in the visualization experiment that a large number of small particles flow deep into the fracture in the traditional fracture plugging method, where all types and sizes of LCM are injected at one time. After changing the injection sequence, which injects the large particles first and the small particles subsequently, it is found that the large particles will form single-particle bridging at a specific depth of the fracture, intercepting subsequently injected particles and thickening the plugging zone, which finally increases the area of the plugging zone by 19%. The visualization experiment results demonstrate that modifying the LCM injection method significantly enhances both the LCM utilization rate and the fracture plugging effect, thereby reducing reservoir damage. This is conducive to reducing the drilling cost of fractured formation. Additionally, the visualized experimental approach introduced in this study can also benefit other research areas, including proppant placement and solute transport in rock fractures.