Pore-scale imbibition comparisons between capillary and gravity forces reveal distinct drainage mechanisms and residual oil distributions

Capillary and gravity forces are considered two important drivers for enhanced oil recovery (EOR) in low-permeability reservoirs. To better understand the residual oil distribution and oil displacement mechanisms of these two driven systems, core- and pore-scale imbibition experiments were conducted in this work. The core-scale results indicated that the imbibition of surfactin (capillary force-driven) and an anionic gemini surfactant (AG, gravity-driven) solutions were comparable (only 0.6% original oil-in-place (OOIP) difference). And pore-scale results showed that surfactin and AG systems exhibited different dominant forces, cocurrent imbibition and countercurrent imbibition. Regarding the residual oil during imbibition, the surfactin solution creates an obvious oil-water-solid interface in the microtube and pushes the residual oil to coalesce into a "cluster" near the outlet. In contrast, the AG solution leaves residual oil films on the microtube wall during imbibition. It was concluded that the interface mechanisms of these differences are revealed by the investigations of contact angles and solid-liquid interfacial forces, which indicated that the original hydrophilic solid wall is exposed by surfactin, leading to capillary force-dominated imbibition, while the AG molecules only absorbed on the oil film that attach to the pore wall, instead of peeling it, could not establish significant capillary forces and must depend on gravity for severe imbibition. These findings explain the micro mechanisms and residual oil distribution features of different imbibition systems, which is helpful to optimize oil producing processes at a given reservoir during given development stages.