Influence of Auxiliary Emulsifier Ionic Characteristics on Interfacial Film Stability and Performance of Water-in-Oil Emulsions for Oil-Based Drilling Fluids

Under high-temperature and high-salinity drilling conditions, maintaining the stability of water-in-oil emulsions is critical for oil-based drilling fluids, while the roles of auxiliary emulsifiers with different ionic characteristics remain unclear. In this study, Span 80 was used as the primary emulsifier, and nonionic OP-4, anionic SDBS, zwitterionic EAB40, and cationic CTAB were introduced as auxiliary emulsifiers to construct blended emulsifier systems. The HLB value was controlled at 5.2–5.4, and the total emulsifier concentration was fixed at 6.0 wt%. The effects of auxiliary emulsifier type on interfacial tension, rheological behavior, electrical stability, droplet morphology, and thermal stability were systematically investigated. The Span80/OP-4 system exhibited the lowest interfacial tension, smallest droplet size, and best overall emulsion stability. In contrast, the Span80/SDBS system showed poor electrical stability due to weakened effective interfacial adsorption in Ca2+ brine. After aging at 120 °C, EAB40 promoted interfacial rearrangement, whereas CTAB weakened interfacial order. Further verification in 1.50 g/cm3 weighted oil-based drilling fluids showed that the Span80/OP-4 system maintained high electrical stability, low HTHP filtrate volume, and good sedimentation stability after aging at 140 °C.