New Insights intothe Influence of Zwitterionic PolymerArchitecture on Shale Inhibiting Performance

Abstract During oil and gas drilling operations, water-based drilling fluids (WBDFs) can infiltrate formations under differential pressure, leading to the hydration and expansion of clay minerals and subsequent wellbore instability. The addition of inhibitors to WBDFs can mitigate clay hydration and expansion, thereby enhancing wellbore stability. Therefore, developing efficient inhibitors is crucial for addressing wellbore instability. In this study, four zwitterionic polymers with excellent solubility─namely P(ATAC-SSS), P(MATAC-SSS), P(ATAC-AMPS), and P(MATAC-AMPS)─were selected as the core research subjects, while P(ATAC-PEA) and P(MATAC-PEMA) served as control polymers. These polymers were synthesized via precise structural design and monomer optimization, and their structure–inhibition relationships were thoroughly investigated. The results indicate that the core polymers, which form homogeneous aqueous solutions, exhibit superior inhibition performance compared to the phase-separated control polymers. Specifically, P(ATAC-AMPS) demonstrated outstanding performance, achieving a rolling recovery rate of 77.85% at 1 wt % and reducing the linear swelling of montmorillonite (MMT) by 73.0% at 4 wt % relative to that in pure water. Mechanistic studies reveal that the absence of a methyl group on the vinyl moiety of P(ATAC-AMPS) promotes chain extension, increases accessible adsorption sites, and enhances interfacial adhesion through synergistic electrostatic and hydrogen-bonding interactions. Atomic force microscopy (AFM) quantified the adhesion forces, which follow the order P(ATAC-AMPS) > P(MATAC-AMPS) > P(MATAC-SSS) > aP(ATAC-SSS), in direct agreement with macroscopic inhibition performance. This work elucidates the critical role of molecular flexibility and functional-group accessibility in designing high-efficiency shale inhibitors, providing a structure-guided strategy for next-generation water-based drilling-fluid additives.