High-Temperature-Resistant Composite Lost Circulation Materials for Oil-Based Drilling Fluids: Preparation, Performance, and Synergistic Mechanism

Lost circulation in oil-based drilling fluids (OBDFs) under high-temperature conditions remains a significant challenge in deep and ultra-deep drilling. In this study, a high-temperature-resistant composite lost circulation material (LCM) was developed based on a synergistic strategy combining rigid bridging–consolidation and flexible embedding–filling. Rigid self-consolidating particles were prepared by coating skeleton materials with modified thermosetting resin, while flexible oil-absorbing resin was synthesized via suspension polymerization. The materials exhibited excellent lipophilicity, thermal stability, and structural integrity at 150 °C, with oil absorption capacity up to 3.43 g/g. The optimized composite LCM showed superior plugging performance, achieving compressive strengths above 11 MPa in white oil and 5 MPa in base mud at 150 °C. Effective sealing of 1–3 mm pore structures was obtained with leakage volumes below 10 mL, and fractured formations could be successfully consolidated. Mechanistically, rigid particles provide structural bridging, flexible resin enables pore filling via swelling, and modified resin(thermosetting resin chemically modified to achieve self-consolidation) enhances consolidation and micro-pore sealing, resulting in a dense and high-strength plugging layer. This work provides a promising approach for designing high-performance LCMs for OBDFs in high-temperature drilling environments.