Unraveling the Coupling Mechanism of Viscous and Capillary Forces in Tight Sandstone: An Integrated Study Using Online NMR and Phase-Field Simulation

For tight sandstone reservoirs, dynamic imbibition is critical for enhancing oil recovery but involves complex multiscale capillary-viscous interactions. This study integrates real-time online nuclear magnetic resonance (NMR) experiments with pore-scale phase field numerical simulations to elucidate these mechanisms in tight sandstone. The results indicate that critical nonmonotonic dependence of imbibition efficiency on displacement velocity, identifying an optimal regime showing positive correlation with permeability. It maximizes the synergy between capillary suction in microthroats and viscous displacement in macro-channels. Higher reservoir quality indices correspond to improved pore-throat connectivity, accelerated imbibition recovery equilibrium, and enhanced imbibition displacement efficiency. The progressive attenuation of matrix capillary forces combined with reservoir heterogeneity results in limited recovery enhancement during late-stage dynamic imbibition, whereas effective oil mobilization in poorly developed (micro and small) pores requires extended soaking time. Pore-scale analysis reveals distinct saturation anomalies driven by structural heterogeneity. In homogeneous matrices, a transient "reverse oil migration" into micropores occurs during early-stage displacement, which occurs when early-stage, high-velocity viscous forces create local hydraulic gradients in macro-pores that temporarily overpower the capillary entry pressure of adjacent micropores, forcing oil backward. Conversely, in fractured systems, while fractures enhance the fluid-matrix contact area and accelerate early recovery by 46.74%, they trigger a late-stage "water-locking" phenomenon. Microscopically, the high conductivity contrast promotes capillary snap-off in narrow throats, where deformed oil ganglia face severe resistance (the Jamin effect), leading to localized oil entrapment. The numerical models visually capture these counter-current flow topologies and interface evolutions, confirming the experimental saturation profiles. These findings provide a mechanistic understanding of dynamic imbibition, offering theoretical guidance for optimizing injection strategies in unconventional reservoirs.