Ca2+-Triggered Reinforcement of Polyacrylamide/Sodium Alginate Composite Nanogel Spheres for Regulating Multiphase Transport in Low-Permeability Heterogeneous Porous Media

Low-permeability heterogeneous porous media commonly exhibit preferential flow through high-permeability channels and persistent residual-oil trapping in small pores and dead-end regions, limiting effective multiphase transport and residual-oil mobilization. Herein, Ca2+-responsive polyacrylamide/sodium alginate nanogel spheres (PAM/SA NSGs) were developed as an environment-responsive nanogel system for Ca2+-rich porous media. In simulated formation water, SA-Ca2+ coordination increased the apparent elastic modulus from 1.35 MPa for PAM NSGs to 3.45 MPa for PAM/SA NSGs and enabled the composite nanogel spheres to retain their morphology after hydrothermal aging at 60 °C for 14 days. The PAM/SA NSG emulsion dispersion reduced the crude-oil/water interfacial tension from approximately 32 to 12.6 mN·m-1 and produced smaller and more stable emulsified droplets than the PAM control. Microfluidic visualization showed that PAM/SA NSGs generated stronger pressure fluctuations, promoted flow redistribution toward less-swept pores, and increased displacement efficiency from 46.8% after water flooding to 65.3%. Post-displacement observations revealed stripe-like accumulation, surface deposition, and localized bridging without complete pore blockage, interfacial enrichment, and downstream migration of emulsified oil droplets, supporting the coupling of local flow regulation with in situ macroemulsion formation. Core flooding further showed that PAM/SA NSGs maintained stronger flow-resistance regulation after aging and achieved a final oil recovery of 48.7%, representing a 13.5-percentage-point increase over water flooding. These results establish an environment-responsive nanogel strategy in which Ca2+, conventionally regarded as detrimental to chemical flooding, is repurposed as an environmental trigger for nanogel reinforcement to couple mechanical stability, interfacial regulation, and multiphase transport in low-permeability heterogeneous porous media.