Constraining Hydrogel Migration via Surface Anchoring for Sustained Reservoir Plugging

Conventional polyacrylamide-based plugging hydrogels suffer from severe performance deterioration caused by dehydration-induced shrinkage and subsequent gel migration under high-temperature reservoir conditions. Most existing modification strategies mainly improve the bulk mechanical properties of hydrogels, while few attempts focus on immobilizing hydrogels from the rock–hydrogel interface. Herein, we report a surface-anchored polyacrylamide hydrogel constructed via covalent grafting of polyacrylamide chains onto sandstone rock surfaces, aiming to suppress gel migration and sustain long-term plugging performance in heterogeneous reservoirs. Dual-parallel core-flooding, visual-model observation, and rock–hydrogel adhesion measurements were jointly performed to evaluate plugging durability and enhanced-oil-recovery potential. Immediately after gel formation, free hydrogels and anchored hydrogels exhibited comparable initial absolute plugging efficiency. Nevertheless, obvious performance gaps emerged during high-temperature aging. After aging for 10, 20, 30, and 40 days, the free hydrogel achieved plugging efficiencies of 91.5%, 68.4%, 28.7%, and 0.9%, while the anchored hydrogel yielded plugging efficiencies of 96.2%, 86.4%, 70.9%, and 35.5%, corresponding to relative improvements of 4.7%, 18.0%, 42.2%, and 34.6%, respectively. Such improvement originates from the largely enhanced rock–hydrogel interfacial adhesion (from 284 N/m2 for free hydrogel to 827 N/m2 for anchored hydrogel), which restricts gel detachment and migration upon dehydration and shrinkage. Correspondingly, in heterogeneous core-flooding tests, after aging for 10, 20, 30, and 40 days, the systems with free hydrogel obtained absolute total oil recoveries of 15.66%, 11.35%, 9.81%, and 9.21%, while the anchored-hydrogel systems reached 16.63%, 13.22%, 11.44%, and 9.98%, giving incremental oil recoveries of 0.97%, 1.87%, 1.63%, and 0.77%, respectively. This surface-grafting anchoring strategy shows promise for future field evaluation as a conformance-control agent for high-temperature heterogeneous reservoirs.