Pore-scale insights into mobilizing residual oil at the high-water-cut stage

Waterflooding is a widely used secondary recovery method that can substantially increase oil recovery. However, after decades of water injection, many reservoirs enter a high-water-cut stage in which large volumes of oil remain trapped, the oil fraction in the produced fluids becomes small, and overall waterflood performance deteriorates. Mobilizing this trapped oil requires a clear understanding of the types of residual oil, their spatial distribution, and the associated pore-scale formation mechanisms. Motivated by this requirement, in this work, we perform pore-scale simulations of drainage in a digital rock using the Navier-Stokes equations coupled with a volume-of-fluid method. The model and numerical implementation are validated by comparing simulated results with microchip experiments for an imbibition process. Based on the relative permeabilities obtained from the pore-scale simulations, we further derive an idealized Darcy-scale waterflood performance curve. To characterize residual oil morphology, we introduce two complementary metrics, the shape factor and Euler characteristic, and use them to classify residual oil into four categories: multipore ganglia, localized ganglia, singlets, and wetting films, and explain the corresponding formation mechanisms. Tracking the evolution of the volume fraction of each category shows that localized ganglia and singlets are the dominant trapped forms in the high-water-cut stage, together accounting for more than 80% of the residual oil. Finally, we investigate how surfactant and higher-viscosity fluid flooding mobilize residual oil in the post-waterflood regime. The additional recovery from surfactant and higher-viscosity fluid flooding is primarily associated with snap-off of multipore ganglia, driven either by reduced capillary resistance or increased viscous forcing. The volume fraction of the singlet increases markedly for both surfactant and higher-viscosity fluid flooding, and even becomes the dominant form of residual oil after higher-viscosity fluid flooding.