Development and Mechanism of a Bio-Based Water-Blocking Prevention Workover Fluid for Offshore Oilfields

In response to the severe water-blocking damage easily induced by invasive foreign fluids during workover operations in offshore low-to-medium permeability oilfields, this study developed a bio-based workover fluid system for water-blocking prevention based on modified rhamnolipids. The system involves the cationic modification of rhamnolipids with glycidyltrimethylammonium chloride (GTMAC) to synthesize a water-blocking prevention agent with low interfacial tension and wettability control capabilities. This agent addresses the issue of precipitation caused by traditional rhamnolipids in the presence of divalent cations such as calcium and magnesium. By combining it with anti-swelling agents and chelating agents, a complete workover fluid system was formulated. Through experiments including aqueous phase contact angle measurement, interfacial tension analysis, core flooding, low-field nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), the performance of the system was systematically evaluated, and its mechanism of action was elucidated. The results indicate that this workover fluid can increase the aqueous phase contact angle on core surfaces to 104.7°, achieving a reversal of wettability to hydrophobic/neutral. The oil-water interfacial tension can be reduced to the order of 10⁻² mN/m. Core flooding experiments demonstrated a permeability recovery rate of 109.79%, significantly higher than that of the blank sample (56.74%). Mechanistic analysis reveals that the cationically modified rhamnolipids strongly electrostatically adsorb onto rock surfaces via quaternary ammonium groups, with their hydrophobic tails self-assembling to form a low-surface-energy molecular film. Synergized with ultra-low interfacial tension, this effectively reduces capillary resistance, promotes the discharge of trapped aqueous phases, thereby preventing and mitigating water-blocking damage, and demonstrates promising application potential.