Selective Plugging and Flow Reallocation Characteristics of a Time-Controllable Silica-Reinforced Cationic Polymer Gel in High-Temperature, High-Salinity Heterogeneous Porous Media

Preferential flow through high-permeability pathways remains a central challenge for polymer-based conformance control in reservoirs where high temperature, high salinity, and strong heterogeneity compromise gel stability and selectivity. Here, a delayed-gelation organic–inorganic gel was formulated from a microbranched cationic copolymer, P(AM-co-DMDAAC), a urea–formaldehyde/phenol–formaldehyde prepolymer cross-linker, and silica nanoparticles. The precursor retained injectability and formed a grade H gel within 15 h at 90 °C in 240,000 ppm brine. Transmission electron microscopy and elemental mapping confirmed a continuous polymer network with uniformly distributed silica, indicating effective nanoscale reinforcement. Core flooding, computed tomography, scanning electron microscopy, and low-field nuclear magnetic resonance were used to correlate network formation with pore occupation and flow redistribution. After in situ gelation, steady-state water-flooding pressure drops increased from 0.052 to 0.28 MPa in a 100 mD core and from 0.015 to 2.25 MPa in a 1000 mD core, corresponding to plugging efficiencies of 81.4 and 99.3%, respectively. Imaging and relaxation responses revealed longitudinally extended gel zones, adherent layers on carbonate surfaces, and shorter pore-fluid relaxation times, demonstrating reduced mobility and enhanced confinement. In parallel cores with a permeability contrast of 10, the high-permeability/low-permeability flow ratio reversed from approximately 10:1 to 1:5. These results establish a structure–morphology–transport relationship for silica-reinforced cationic polymer gels and show how nanoscale network reinforcement enables selective plugging and controllable flow reallocation in heterogeneous porous media.