Synergistic Emulsifier System Based on Molecular Design for Ultra-Low Oil-to-Water Ratio Oil-Based Drilling Fluids

Formulating oil-based drilling fluids (OBDFs) with an ultra-low oil-to-water ratio (OWR ≤ 60:40) presents a formidable stability challenge due to the maximized interfacial area and intensified stress on the interfacial film under high-temperature, high-density conditions. To address this, we engineered a synergistic stabilization system through molecular and colloidal design. A novel hyperbranched polyamide emulsifier (epoxidized soybean oil polyamide) (ESOP), synthesized from epoxidized soybean oil, exhibits superior thermal stability and interfacial activity due to its hyperbranched architecture. Combined with calcium petroleum sulfonate (CPS) and hydrophobic nanosilica (HNs), it enables a high-performance OBDF with an ultra-low OWR of 60:40. The results show that the optimized formula achieves an excellent demulsification voltage of 1290 V, an ultra-low HTHP fluid loss of 1.5 mL, a yield point of 12.9 Pa, and a superior sag factor (SF) of 0.504, outperforming both base and commercial systems. Mechanistic studies reveal a multiscale stabilization strategy involving a dense composite interfacial film, Pickering stabilization, a 3D network, and a unique thermally triggered self-reinforcement effect. This work not only provides a cost-effective OBDF formulation but, more importantly, establishes a molecular topology engineering paradigm for stabilizing complex industrial fluids under extreme conditions.