Thermoresponsive Amphiphilic Polymer Emulsifier for Low-Dosage, High-Stability Oil-Based Drilling Fluids

Reducing the oil-to-water ratio (OWR) in oil-based drilling fluids (OBDFs) offers significant economic and environmental benefits, yet severely challenges emulsion stability at high temperatures. To address this issue, we designed a thermoresponsive amphiphilic copolymer that dynamically reinforces the oil–water interface upon heating, thereby stabilizing low-OWR emulsions at elevated temperatures. Here, we report a thermoresponsive amphiphilic copolymer, poly(St-NIPAM-AMPS) (PSNA), synthesized via free-radical polymerization, as a primary emulsifier for low-OWR OBDFs. PSNA incorporates styrene (St) as a hydrophobic anchor, N-isopropylacrylamide (NIPAM) as a thermoresponsive unit, and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as an electrostatic stabilizer. Systematic optimization identified PSNA-2 (St:NIPAM:AMPS = 1:1:1) as the optimal composition. At an emulsifier loading of 6 wt % and an OWR of 70:30, the PSNA-2-based OBDF achieved an electrical stability of 879 V, a high-temperature, high-pressure filtration loss of 3.2 mL, and a sag factor of 0.513 after dynamic aging at 180 °C for 16 h, meeting all industry criteria. Mechanistic studies reveal a thermally triggered interfacial self-reinforcement mechanism: above its lower critical solution temperature (54 °C), the NIPAM segments collapse, densifying the interfacial film, while the AMPS groups provide sustained electrostatic repulsion. The PSNA-2 formulation also exhibits a nontoxic environmental profile (EC50 > 30,000 mg/L). This work offers a scalable, cost-effective emulsifier design for high-performance, low-OWR OBDFs.