Extended surfactants with the oxypropylene (PO) group and ethoxylated anionic surfactants with the oxyethylene (EO) group have a high salt tolerance capability. Most of the researches on extended surfactants and ethoxylated anionic surfactants focused on the microemulsion, solubilization, and interfacial tension (IFT) of concentrated surfactant solutions, whereas a few researches focused on the IFT of dilute surfactant solutions. Moreover, a previous work focused only on surfactants with PO numbers greater than 4 and copolymers of PO and EO. The effects of extended surfactants containing short PO chains and no EO groups have not been examined. We measured the IFT and optimal salinity between n-alkanes and dilute solutions of extended surfactants or ethoxylated sulfonates at 30 °C. The effects of the surfactant structure on the equilibrium interfacial tension (IFT_eq) and optimal salinity of the system were studied in detail. As for the effects on IFT, results indicate that the introduction of PO groups leads to their enhanced capability to reduce the IFT prior to cross-salinity and a reduction in the IFT between n-alkanes and surfactant solutions to ultralow values (smaller than 0.01 mN/m) near the optimal salinity. It was also found that extended surfactants with different alkyl chains also entail a cross-salinity; at values lower than the cross-salinity, the IFT reduction capacity of extended surfactants with a long alkyl chain (C16P3SO3) is better than that of extended surfactants with a short alkyl chain (C13P3SO3). As for the effect on the optimal salinity, it was found that the optimal salinity of extended surfactants is lower than that of ethoxylated sulfonates for the same oil phase. It was also found that the optimal salinity of extended surfactants first increased and later decreased with increasing PON. This finding is first proposed based on summarizing some researchers’ studies and our experiments.

Gel syneresis is a common problem in gel treatment for oil recovery applications. In this study, a stable gel was prepared in a soft brine by using a water-soluble phenolic resin as a crosslinker, nanoparticles as a stabilizer, and partially hydrolyzed polyacrylamide (HPAM) or copolymers with different contents of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) groups as polymers. The syneresis behavior of the gels formed in a soft brine was evaluated upon aging in hard brines. The results show that when the salinity of the hard brine is lower than 30,000 mg/L, the gel expands, and its strength decreases; when the salinity of the hard brine is higher than 50,000 mg/L, the gel exhibits syneresis, and its strength increases. The effects of various influencing factors on the gel syneresis behavior were also evaluated. It was found that optimizing the polymer structure and adding nanoparticles can effectively overcome gel syneresis and enhance gel stability. Based on the research described in this paper, some proposals for designing salt-resistant polymer gels are presented.

The properties of a reinforced gel with partially hydrolyzed polyacrylamide (HPAM) as the main agent, water-soluble phenolic resin (WSPR) as the crosslinker, and nano-SiO₂ as the stabilizer were evaluated in terms of gelation time, gel strength and thermal stability under the conditions of 110 °C and 12.124 g/L salinity in water. The results showed that the gelation time of the gel with high strength was adjustable from 3 to 23 h, remaining stable for more than 180 days under stratigraphic conditions, although with a certain degree of early dehydration in the gel. Cryo-scanning electron microscopy (cryo-SEM) and dynamic light scattering (DLS) analysis revealed that nano-SiO₂ improves the dispersion of the polymer in water, resulting in a more homogeneous structure of the formed gel and thus improving the strength of the gels. In addition, rheological tests and cryo-SEM showed that the interaction between nano-SiO₂ and the polymer could inhibit the degradation of polymer to a certain extent and improve the thermal stability of the gel. However, the oxidative degradation of the gel is still the main cause of early dehydration of water-soluble phenolic resin gel, and the addition of a small amount of hydroquinone to the gelants can significantly improve the antioxidative degradation properties of phenolic resin gel.