Plugging mode of flaky lost circulation materials within fractures and mechanism to enhance pressure-bearing capacity for the plugging zone

During drilling operations in deep fractured tight gas reservoirs, lost circulation of working fluid frequently occurs due to the formationʼs low pressure-bearing capacity. Adding lost circulation materials (LCMs) to drilling fluids is the most common method for controlling lost circulation. Among these, granular LCMs are widely used, but the application frequency of flaky LCMs has been increasing annually due to their unique morphology. However, the migration and plugging behavior of flaky LCMs within fractures, and the mechanisms enhancing the pressure-bearing capacity of the plugging zone are not well understood. Therefore, this study conducted visual plugging experiments and dynamic fracture plugging experiments to evaluate the plugging mode and pressure-bearing capacity of the plugging slurry with various particle sizes and concentrations of flaky LCMs. The experimental results demonstrate that the fracture plugging process can be divided into four stages: uniform flow stage of the plugging slurry, formation and development stage of the bridging area, formation and development stage of the plugging area, and pressure-bearing stage of the plugging zone. The inclusion of flaky LCMs notably reduces the duration of stages 1 and 2, while simultaneously increasing the proportion of the plugging zone and enhancing its surface porosity. Flaky LCMs reduce the effective fracture width through “interception” and “co-bridging” modes, thus improving plugging zone formation efficiency. Appropriate particle size and concentration of flaky LCMs increase the area and length of the plugging zone. This reduces the fracture width increment caused by injection pressure and enhances frictional force between the plugging zone and fracture surface, thereby improving the pressure-bearing capacity of the plugging zone. However, excessively high concentrations of flaky LCMs result in decreased structural stability of the plugging zone, and excessively large particle sizes increase the risk of plugging outside fracture inlet. The recommended concentration of flaky LCMs in the plugging slurry is 2%–3%, with a particle size 1.2 to 1.5 times that of the bridging granular LCMs and not exceeding twice the fracture width. This study provides a theoretical foundation for selecting LCMs and designing plugging formulations for field applications.