Hydrophobically Modified Polymer–Surfactant Self-Assembly System: A Study of a Self-Thickening Diverting Acid Solution

Studying an efficient acid additive is of great significance for acid diversion operations. This study synthesized a hydrophobically modified triblock copolymer ASN (acrylamide-styrene-<i>N</i>-vinylimidazole triblock copolymer, Mn = 0.49 × 10⁵). In the acidic solution, the surfactant octadecyltrimethylammonium chloride (STAC) was used to self-assemble with the polymer. By changing the self-assembly formula, the advantages of the self-assembled acid solution in diverting performance were intuitively observed, and the retarding performance of self-assembled acid solution was evaluated. A rheometer was used to test the temperature resistance, shear resistance, and viscoelasticity of self-assembled acids. The hydrodynamic radius and surface morphology of self-assembled acid were studied by using a dynamic light scattering laser particle size analyzer and environmental scanning electron microscope, and the mechanism of self-assembled acid thickening was explored. The results indicate that at ASN:STAC = 1.0:0.2 (molar ratio), the system achieved peak viscosity (250 ± 10 mPa·s at 100 s^{- 1}), 40% higher than other ratios, while maintaining 85% viscosity retention after rock reaction. Under high-temperature shear conditions at 160 °C and 170 s^-1 for 1 h, the self-assembled acidic solution maintained an apparent viscosity of 104 mPa·s. The self-assembled acid solution exhibits excellent temperature and shear resistance, achieving a retardation rate of up to 85% at 110 °C. Mechanistic investigations reveal that HCl consumption and Ca²⁺ precipitation during acid-rock reactions critically govern the acid-triggered self-thickening behavior of the self-assembled system, inducing viscosity fluctuations exceeding 80%.