Preparation and Performance of a Multifunctional Acid Thickener for High-Temperature Carbonate Acidizing

To address the single functionality, insufficient temperature and acid resistance, and difficulty in balancing retardation and diversion of acidizing fluids for high-temperature carbonate reservoirs, a zwitterionic penta-copolymer hydrophobic associating thickener (MFAT) was designed and synthesized. Its backbone contains amide, carboxyl, and sulfonic groups, while its hydrophobic and cationic side chains contain numerous quaternary ammonium cations and di-cationic structures, enabling good molecular chain extension, hydrophobic association, and electrostatic synergy in strongly acidic and high-ionic-strength environments. MFAT has a viscosity-average molecular weight of 8.43×10⁶ g/mol and shows good thickening ability in 20% HCl as a gelled acid thickener. With viscoelastic surfactants, it forms a high-temperature-resistant gelled acid system with a retardation rate above 65% at 160 °C. MFAT and organic zirconium crosslinker construct a delayed crosslinked acid system with a crosslinking time of up to 157 s and a retardation rate above 75% at 160 °C. In 20% HCl, 0.2% MFAT exhibits the best drag reduction performance, and with surfactant assistance, the drag reduction rate exceeds 75%. Additionally, 0.1% MFAT significantly improves the temperature resistance of VES diverting acid; parallel core experiments show the maximum temporary plugging diversion pressure increases from 3.9 to 5.3 MPa, and the permeability improvement ratio of the low-permeability core increases from 8.53 to 12.63. These results indicate that MFAT constructs an acid- and temperature-resistant associating network through the synergy of "hydrophobic association–electrostatic interaction–hydrogen bonding," serving as a multifunctional acidizing thickener for gelled acid, crosslinked acid, drag-reducing acid, and diverting acid systems in high-temperature carbonate acidizing, with important engineering application value.