Coupled effects of pore structure, wettability, interfacial tension, and viscosity ratio on cocurrent spontaneous imbibition in digital rocks

Spontaneous imbibition is known to control early fluid redistribution in heterogeneous porous media, whereas the relationship between rapid capillary percolation and effective pore-volume sweep remains to be fully elucidated. This study uses a multi-relaxation-time lattice Boltzmann method coupled with a color-gradient multiphase model to simulate cocurrent spontaneous imbibition in Ketton carbonate and sandstone digital rocks. The main contribution of this work is to distinguish breakthrough-controlled sweep from late-stage stable saturation and to identify the conditions under which pore structure enables capillary driving to activate additional connected pathways rather than merely accelerating outlet-spanning flow. The effects of wettability and interfacial tension were evaluated at breakthrough, whereas the influence of viscosity ratio was further examined until the wetting-phase distribution approached a stable state. The results show that Ketton carbonate tends to develop preferential invasion channels and pronounced bypassing because of its stronger pore-structure heterogeneity, while sandstone supports broader and more uniform front propagation. Increasing the interfacial tension promotes more cooperative pore filling in Ketton carbonate, leading to a transition from channelized percolation to multi-path invasion. This threshold-like response is less evident in sandstone, because the more uniform pore-throat network already favors distributed imbibition. Viscosity-ratio simulations indicate that lower absolute viscosity accelerates early imbibition, whereas higher absolute viscosity delays interface advancement but may enhance late-stage pore filling in heterogeneous media. The findings of this study provide a pore-scale framework for assessing early-time imbibition efficiency in heterogeneous rocks.