Self-assembly properties of ultra-long-chain gemini surfactants bearing multiple amide groups with high performance in fracturing fluid application

•Synthesis of gemini surfactants having ultra-long hydrophobic chains and cleavable groups.•Self-assembly properties are determined by surface tension method and cryo-TEM observation.•NH4Cl refines rheological behaviour of the gemini surfactant solutions.•C25-amide-C25 with multiple amides is a very promising candidate for fracturing in high temperature formations.

In this work, a series of cationic gemini surfactants (C25-alkyl-C25, C25-ester-C25 and C25-amide-C25), which possessed a special structure consisting of ultra-long hydrophobic chains and cleavable groups, were synthesized using a main feedstock source obtained from rapeseed and applied as viscoelastic surfactant (VES) fracturing fluids in high temperature formations. Surface activities, rheological behavior analysis, scanning electron microscope (SEM) and cryogenic transmission electron microscopy (cryo-TEM) observations, and fracturing fluid performance were performed to determine the solution properties of gemini surfactants in the presence of ammonium chloride (NH4Cl). The results showed that the order of the critical micelle concentration (CMC) value was C25-alkyl-C25 < C25-ester-C25 < C25-amide-C25. And their ΔGmic0 values were found to have negative values, indicating that their self-assembly processes were spontaneous. The three surfactant/NH4Cl fluids exhibit high viscoelasticity and three-dimensional (3D) network structure. The 20 mmol L−1 C25-ester-C25 fluid containing 0.19 mol L−1 NH4Cl meets the demands of a fracturing operation under 100 °C, so do C25-alkyl-C25/NH4Cl and C25-amide-C25/NH4Cl fluids. In particular, the highest applicable temperature of the C25-amide-C25/NH4Cl system can reach to 150 °C. Hence, C25-amide-C25 is a very promising candidate material for VES fracturing fluids.

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