pH-Responsive Self-Assembly and Reversible Rheology of a Single-Component Amine Oxide Surfactant-Based Fracturing Fluid

To address the insufficient thermal stability, poor recyclability, and formulation complexity of conventional viscoelastic surfactant (VES) fracturing fluids under the high-temperature and strongly stress-sensitive conditions of deep coalbed methane (CBM) reservoirs, a single-component zwitterionic amine oxide surfactant, N,N-dimethyl-4-(tetradecyloxy)benzylamine oxide (P-ToDo), was developed as a pH-responsive VES fracturing fluid. The structure–performance relationship and pH-responsive thickening mechanism of P-ToDo were systematically evaluated through rheological measurements, pH-switching tests, coal core permeability experiments, and molecular dynamics (MD) simulations. The rigid benzene ring in P-ToDo contributed to enhanced thermal and shear resistance, while the amine oxide headgroup enabled protonation-regulated pH-responsive behavior and reversible viscosity control. Rheological results showed that the single-component 3 wt % P-ToDo system exhibited excellent thickening performance near neutral pH, maintaining an apparent viscosity of 34.54 mPa s after shearing at 120 °C and 170 s–1 for 1 h. Upon adjustment of the pH between 7.53 and 9.00, the system underwent reversible switching between a high-viscosity fracturing state and a low-viscosity gel-breaking state, indicating good recyclability. In addition, coal core permeability tests showed a low average damage rate of 10.85%. MD simulations further revealed that the protonation degree modulated intermolecular nonbonded interactions and aggregate compactness, thereby explaining the experimentally observed pH-responsive rheological behavior. These findings establish P-ToDo as a reusable, single-component VES platform that integrates pH-triggered rheological regulation, robust high-temperature performance, and simplified fluid formulation, offering a sustainable strategy for efficient fracturing stimulation in deep CBM reservoirs.