Abstract:

In chemical flooding of high-temperature and high-salinity reservoirs, conventional surfactants suffer from interfacial activity attenuation and salt-induced precipitation, necessitating novel agents with combined temperature and salt tolerance. This work investigates four ethylhexyl extended sulfates （C₈P_xE_yS, x=10,15; y=5,10） in simulated formation water （TDS 29 145 mg/L）. Interfacial tensions （IFT） against n-Alkanes with varying carbon numbers were measured by spinning drop method during 30-86 ℃, and the alkane carbon number （ACN） scanning curve was employed to evaluate temperature effects on hydrophilic-lipophilic balance （HLB） and interfacial adsorption film structure. Results demonstrate that temperature does not exert a monotonic effect on IFT; instead, the response depends on PO/EO block lengths. With rising temperature, hydrogen bonds of EO segments break, reducing hydrophilicity and shifting HLB toward lipophilicity; while PO segments adopt helical conformations and EO segments transform from water-extending to interfacial spreading. These dual pathways collectively determine IFT variation. C₈P₁₅E₁₀S achieves an ultralow IFT of 10^-3 mN/m against n-Decane at 86 ℃, whereas C₈P₁₅E₅S exhibits excessive lipophilicity and fails to reach an ultralow IFT at high temperature. This study elucidates the microscopic mechanism by which temperature regulates IFT reduction via dehydration and conformational transition, providing theoretical guidance for designing ultralow IFT flooding systems applicable to deep high-temperature reservoirs.

Key words: extended surfactants, sulfate, interfacial tension, high temperature and high salinity reservoir, hydrophilic-lipophilic balance