Research on the plugging mechanism in fractured formations and the drilling fluid system for while-drilling leak prevention

During drilling operations, when fracture-induced lost circulation occurs, the unclear sealing mechanisms often lead to reliance on empirical methods for plugging without targeted strategies. This lack of theoretical guidance results in poor sealing effectiveness and low pressure-bearing capacity. Therefore, the sealing process is first analyzed in this study to reveal the mechanisms of lost circulation control. It is clarified that the matching relationship between sealing particles and fracture dimensions, as well as the bonding forces between particles and fracture surfaces, are regarded as the primary factors influencing pressure-bearing capacity. Then, based on the perspectives of particle gradation optimization and bonding force enhancement, a drilling fluid system for while-drilling leak prevention is designed: Fresh water + 3% bentonite + 0.2% encapsulating agent + 0.8% fluid loss reducer + 2% anti-collapse agent + 0.8% anionic polyacrylamide + 0.7% wollastonite + 4% walnut shell powder + 2% sawdust. Compared to the target formation’s while-drilling system, fractures ranging from 200 to 600 μm can be effectively plugged by this optimized system. Furthermore, the pressure-bearing capacity is increased by 75%, and the leakage volume is reduced by over 60%. Clear theoretical guidance for plugging fractured formations is provided in this paper, which facilitates the selection of plugging materials and the optimization of particle size distribution during field operations.