Dynamic Hydrophobic Crosslinking in Water-Responsive Polymer Emulsions: A Supramolecular Strategy for Controlling Severe Lost Circulation in Karst Formations

The development of effective lost circulation control materials for complex karst formations remains a critical challenge in deep and unconventional reservoir drilling. This study presents a water-responsive polymer synthesized through inverse emulsion copolymerization using hydrophilic monomers (acrylamide, acrylic acid, and 2-acrylamido-2-methylpropanesulfonic acid) and hydrophobic monomers (octadecyl acrylate). Advanced characterization techniques, including Fourier Transform Infrared Spectroscopy (FTIR), Gel Permeation Chromatography (GPC), Dynamic Light Scattering (DLS), Thermal Gravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM), revealed a water-triggered structural transformation from irregular morphologies to three-dimensional cocontinuous networks. This transformation is driven by hydrogen bonding and van der Waals interactions, which facilitate the formation of supramolecular architecture through intermolecular hydrophobic associations. The polymer emulsion demonstrates exceptional thickening performance, maintaining high apparent viscosity in both distilled water (2150 mPa·s at 2 wt %) and saline conditions (1680 mPa·s at 2 wt %). It also exhibits favorable shear-thinning and static-thickening behavior, with dynamic viscosity reducing from 5140 mPa·s to 280 mPa·s at 200 s–1 (at 5 wt %). Prolonged water exposure enhances the sealing efficacy, achieving 10% linear expansion after 48 h. Additionally, thermal stability testing at 140 °C for 16 h demonstrates that the sealing integrity is maintained in sand beds. Integrated with optimized conventional materials, the composite plugging system effectively seals 3 mm-wide fractures under 8 MPa. Molecular dynamics simulations confirm the adsorption mechanisms and surface retention behavior that govern the fracture sealing. This salt-tolerant, thermally stable polymer emulsion system overcomes conventional retention limitations in karst formations, enabling efficient one-time plugging with industrial scalability. This work establishes a new paradigm for the development of stimuli-responsive materials to control complex downhole fluid losses.