Quantitative pore-scale evidence for viscosity-controlled tertiary flooding: Breakup cascade, size-conditioned mobilization, and a practical viscosity window

Residual oil after waterflooding is stored as disconnected ganglia, blobs, and droplets, and its remobilization under viscosity control governs the incremental benefit of tertiary flooding. In this study, we conduct five tertiary floods in a water-wet, rock-like glass micromodel using glycerol–water injectants at a fixed flow rate, with viscosity ratios ranging from 1 to 80. Time-resolved microscopy combined with common region of interest (ROI) image registration, automated segmentation, and cluster tracking yields type-resolved statistics and a dimensionless cluster-size measure. Replicated baseline tests establish a reproducible post-waterflood reference state. Increasing viscosity systematically truncates the large-size tails of ganglia and blobs and progressively confines the remaining oil to geometry-sheltered pores, while droplet sizes remain narrowly controlled by pore-scale geometry. State-resolved tracking reveals a viscosity-dependent pathway in which extended ganglia fragment into intermediate blobs and then into droplets prior to removal. We further propose a logistic size-conditioned mobilization law that links mobilization probability to cluster size and viscosity ratio, providing a compact description of size selectivity and its shift toward smaller structures as viscosity increases. The results provide a pore-scale, data-driven basis for identifying a practical viscosity window under the present controlled conditions, with dominant mobilization gains at low-to-moderate viscosity ratios (approximately 1–8), additional but reduced gains up to around 20, and progressively diminishing returns beyond this range, while the pressure-drop penalty continues to increase.