Microscopic Depolymerization Mechanism Based on Free-Radical Degradation and Its Application in Gel Plug Removal After Polymer Flooding

Gel plugging in wellbores and near-wellbore regions caused by polymer adsorption and retention severely hinders fluid injection and production efficiency in post-polymer flooding reservoirs. To address this issue, a highly efficient chemical plug removal agent was formulated and selected. Under reservoir conditions of 65 °C and a salinity of 6864 mg/L, the system exhibited excellent dispersion stability, with a Zeta potential of −36.7 mV. It achieved a viscosity-reduction rate exceeding 95% for polymers across various molecular weights. The microscopic degradation mechanism was elucidated based on the published literature and experimental results. Fe2+ activation triggered the generation of highly reactive SO4•− and HO• radicals. Under acidic conditions (pH ≤ 6), the reaction was dominated by controlled SO4•− radicals, breaking down dense three-dimensional gel networks, which dramatically reduced the aggregate size from 500~2000 nm to 10~30 nm. Dynamic coreflooding tests demonstrated that injecting the system lowered the residual resistance factor from 6.5 to 1.0~1.5, achieving a permeability recovery rate exceeding 80%. Field trial results further verified that application of the composite plug removal agent and subsequent acid stimulation decreased injection pressure from 12.5 MPa to 5.5 MPa, increasing the apparent water-injectivity index to 2.4 times its initial value. This study provides crucial theoretical insights and technical support for efficient near-wellbore unplugging and residual oil mobilization in post-polymer flooding reservoirs.