Mechanism of Plugging Zone Formation in Deep POLIEs and Evaluation of Their Pressure-Bearing Stability

The efficient sealing of millimeter-width fractures is of critical significance for lost circulation prevention and reservoir protection during drilling and completion operations in deep fractured reservoirs. This study systematically investigates the synergistic plugging effects of spherical, flaky, and fibrous lost circulation materials (LCMs) through microscopic visualization experiments and laboratory-scale fracture sealing simulations. The formation process of fracture-plugging zones was elucidated, the formation mechanism of millimeter-width fracture sealing structures was revealed, key factors influencing structural stability were identified, and a pressure-bearing capacity evaluation method based on the Analytic Hierarchy Process (AHP) was proposed. Experimental results demonstrate that: ①The synergistic interaction of spherical, flaky, and fibrous LCMs enables the formation of fracture-plugging zones with high pressure-bearing capacity. Spherical materials distribute throughout the entire plugging structure with predominant accumulation in the front section, flaky materials predominantly occupy the front-middle section, while fibrous materials concentrate in the middle-rear section. ②Critical parameters including LCM blending ratios, particle size distribution, sphericity, and compressive strength collectively influence the structural configuration (LCM spatial distribution), mechanical strength (pressure-bearing capacity), and functional performance (fracture support efficacy) of the plugging zones. ③Three experimental parameters—pressure-bearing capacity, cumulative fluid loss, and pressure stabilization duration—were established as effective indicators for evaluating plugging stability. The AHP-based stability evaluation index enables comprehensive quantitative assessment, where higher scores correspond to enhanced pressure stability, with experimental validation confirming the methodology’s reliability. These findings provide a theoretical foundation for optimizing the pressure stability of fracture-plugging structures and improving fluid loss control efficiency in fractured reservoir applications.