Synthesis and salt thickening mechanism of salt-tolerant copolymers based on functional monomer synergy

To improve the utilization rate of thickening agents used for oil recovery in high-temperature and high-salinity reservoirs, a medium-molecular-weight water-soluble hydrophobic association copolymer (APDM) was prepared via the copolymerization of acrylamide (AM) with acrylic acid (AA), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), octadecyldimethylallyl ammonium chloride (DMAAC-18), and N-vinylpyrrolidone (NVP) using the free-radical polymerization method. The APDM copolymer was characterized and its salt resistance, viscoelastic properties, temperature resistance, and shear resistance were determined. Research has shown that the molecular weight of APDM was around 5 million. The apparent viscosity of a 0.5 wt% APDM solution was higher in 2 × 10⁴ mg/L NaCl solution (187 mPa·s) than in clean water but decreased to 87 mPa·s when the NaCl concentration increased to 20 × 10⁴ mg/L. In a high-salinity environment, the hydrophobic groups associated owing to the hydrophobic effect, causing the molecular chains to form a physical crosslinking network that increased the solution viscosity. Moreover, the steric hindrance stemming from the aggregation of hydrophobic groups prevented salt ions from approaching the hydrophobic groups, reducing the electrostatic interaction between salt ions and the hydrophobic long chains, endowing APDM with excellent salt thickening ability and salt resistance in NaCl solutions. At 140 °C and a shear rate of 170 s⁻¹, the apparent viscosity of a 0.5 wt% APDM solution with deionized water and 20,000 mg/L NaCl aqueous solution as a solvent was >50 mPa·s after shearing for 1 h. Owing to its good temperature resistance and shear resistance, the APDM copolymer can find application as a thickener in high-salinity reservoirs.