Mechanism and Performance Evaluation of a Solid-Free One-Step Flushing Fluid for Removing Drilling Fluid Filter Cakes

With the tightening of environmental regulations, eco-friendly water-based drilling fluids have been increasingly adopted in offshore drilling operations. However, the dense filter cakes formed by such fluids can severely impair the bonding quality at the cementing interfacial zone. In this study, the filter cake generated by BIODRILL water-based drilling fluid was investigated. Its composition and structural characteristics were examined using XRD, FT-IR, SEM, and TG analyses, which revealed that the cake is mainly composed of polymeric additives, weighting materials, and clay minerals. The resulting structure exhibits low porosity and high strength, making it resistant to removal by conventional methods. Based on the formation mechanism of the cake, a solid-free one-step flushing fluid was developed, formulated with oxalic acid, NaF, EDTA-Na, rhamnolipids, and TWEEN80. The system operates through a synergistic mechanism of acid dissolution, chelation, interfacial peeling, and physical scouring, enabling efficient disintegration and removal of the filter cake. Experimental results demonstrated that at an optimal pH of 5.5, the flushing fluid achieved over 80% cleaning efficiency within 15 minutes under both ambient and 85℃ conditions, with maximum removal rates exceeding 95%. Furthermore, the bonding strength of flushed sandstone cores recovered to more than 70% of that of clean cores, meeting field performance requirements. These findings not only elucidate the challenges and mechanisms of filter cake removal but also provide new strategies and theoretical support for efficient cleaning of cementing interfacial filter cakes and enhancement of bonding quality in complex drilling environments.