Oil Displacement Performance and Synergistic Mechanisms of a Nonionic‐Gemini Sulfobetaine Composite System in Low‐Temperature Reservoirs

ABSTRACT To address the high viscosity and poor flowability of crude oil in low‐temperature reservoirs, we combined coconut oil fatty acid diethylamide (6501) with different betaine surfactants. The effects of surfactant type, blend ratio, concentration, temperature, and inorganic salts on crude oil–water interfacial tension were investigated to identify an effective formulation. To this end, we synthesized Gemini erucamide propyl hydroxysulfonyl betaine (GEHSB) and characterized its structure. Comparison of GEHSB with two conventional betaine derivatives (cocoyl amide propyl betaine and oleoyl amide propyl betaine) indicates that an increase in the length of the hydrophobic chains and the introduction of a twin structure considerably enhance the synergy with 6501. At a 6501:GEHSB mass ratio of 9:4 and a surfactant concentration of 0.25%, the interfacial tension remained on the order of 10 −4 mN m −1 (0.000873 ± 0.000029 mN m −1 ), even at 30°C. The introduction of salt ions (Na + , Ca 2+ , and Mg 2+ ) compressed the double layer and weakened hydrophilic group hydration, thereby reducing the interfacial tension of the system to 10 −5 mN m −1 , with Mg 2+ exhibiting the strongest regulatory effect. Emulsification tests showed that blending 6501 with GEHSB improved emulsifying ability and static emulsion stability. At 2500 ppm Mg 2+ , the emulsifying ability reached 95.57%. Moreover, optical microscopy showed more regular and uniformly dispersed droplets under the tested conditions. Core‐flooding tests showed that oil recovery increased from 46.00% ± 2.15% at the waterflooding endpoint to 63.53% ± 1.54% after surfactant injection and subsequent waterflooding, corresponding to an incremental recovery of 17.53% ± 0.89%. These results demonstrate the potential of the 6501/GEHSB formulation for enhanced low‐temperature oil recovery.