Study on the Morphological Distribution and Formation Mechanisms of Residual Polymer in Polymer-Containing Wastewater

Polymer flooding is a crucial enhanced oil recovery (EOR) technique; however, postpolymer flooding, reinjected polymer-laden wastewater tends to trigger filter clogging and near-wellbore region permeability damage due to colloidal aggregation. This study investigates the microscopic morphological characteristics, aggregation mechanisms, and dominant factors of residual polymers through multiscale experiments. The basic properties, dispersion morphology, and stability of residual polymers in produced water were characterized by using iodine-starch colorimetry, scanning electron microscopy (SEM), zeta potential analysis, and dynamic light scattering (DLS). The effects of Na⁺, Ca²⁺, Fe³⁺, and clay on polymer morphology, size, and stability were systematically explored. Results indicate that the molecular weight of residual polymers in produced water decreases by ∼90% compared to preinjection polymers, accompanied by a significant viscosity reduction. Microstructurally, the polymer transitions from linear long-chain configurations to colloidal clusters exhibiting "deep-ravine" or flocculent structures. The zeta potential (0-10 mV) confirms the system's high instability, rendering it prone to rapid aggregation and blockage. Na⁺ induces molecular chain coiling through charge screening effects, while Ca²⁺ intensifies compression and dehydration processes, resulting in polymer molecular aggregation. Fe³⁺ triggers cross-linked flocculation via charge neutralization and bridging mechanisms. Concurrently, clay minerals adsorb polymer fragments to form encapsulating structures, expanding particle diameters by 3.8-6.8 times and significantly compromising system stability. Formation shear further fractures polymer chains, while synergistic interactions between clay and Fe³⁺ (amplifying particle sizes by 6.8 and 2.5×) dominate flocculation, the paramount cause of plugging. This study provides theoretical guidance for optimizing polymer-containing wastewater treatment processes and developing anticlogging technologies.