Microfluidic investigation of synergistic mechanisms of microsphere-microbial compound system for enhanced oil...

Field tests have shown that microsphere-microbial compound flooding improves the oil recovery factor. The compound system consists of surfactant-modified polyacrylamide nanospheres (initial diameter is 150 nm) and an enriched indigenous microbial consortium. However, the detailed mechanisms remain unclear. To address this gap, this study employs microscopic visualization and particle image velocimetry (PIV) to compare the compound system with single-component microsphere and microbial flooding. Results indicate that the compound system achieves recovery factors 7.26% and 4.54% higher than microbial and microsphere flooding, respectively, confirming a synergistic effect. Its displacement efficiency and sweep effectiveness are also superior. Mechanistic analysis reveals a stage-dependent advantage. During injection, surfactants released from microspheres alter wettability. In the shut-in stage, microbial metabolites further enhance displacement, producing dual benefits. Under dynamic conditions, blocking performance was quantified. PIV results demonstrate that the compound system significantly reduces velocity heterogeneity of the flow field. Single‑channel velocimetry reveals smaller velocity fluctuations (< 15%), indicating stronger shear stability and improved plugging performance relative to either agent alone. Microscopic observations further reveal aggregate structures in which the microsphere skeleton supports microbial adhesion. This enables stable spatial distribution under high shear and effective blockage of dominant channels. By integrating quantitative evidence from visualization and velocimetry, this study provides a useful approach for evaluating compound flooding performance aimed at improving oil recovery for mid-low permeability reservoirs.