Pore-scale investigation of gas displacement by water invasion in low-permeability rocks using micro-computed tomography and lattice Boltzmann simulation

Gas displacement by water invasion in low-permeability gas-bearing rocks is commonly evaluated using gas saturation; saturation alone cannot identify whether the remaining gas is connected or isolated. The pore-scale controls on displacement efficiency remain unclear in three-dimensional pore structures, especially under initial-water conditions in low-permeability reservoirs and subsurface natural gas and hydrogen storage. Here, micro-computed tomography (micro-CT) experiments on tight cores are combined with three-dimensional lattice Boltzmann simulations on reconstructed digital rocks to link saturation decline with pore-size occupancy and residual-gas topology. Micro-CT images show that water invasion progressively converts a coarse-pore gas backbone into fragmented residual gas through limited early mobilization, selective removal from small-to-medium pore-throat regions, and late-stage fragmentation after water percolation. Simulations reveal that preexisting water is the leading control on displacement efficiency: increasing initial water saturation from 0 to 0.7 reduces efficiency by approximately 25%–56% across the digital rocks by weakening gas connectivity and promoting preferential flow along water-connected pathways. Capillary number governs capillary-threshold crossing, whereas Ohnesorge number mainly modulates local interface depinning and relaxation. Wettability and pore structure determine whether water invasion produces effective sweep, snapoff, or bypassing. Pore-size occupancy shows that small-pore gas responds early, while gas retained in medium-to-large pores controls sustained removal and late trapping. Topology analysis distinguishes droplet, columnar, multi-pore, and clustered gas, identifying whether remaining gas is isolated, partially connected, or preserved as bypassed bodies. This work provides a pore-scale topological basis for assessing gas mobility loss and trapping behavior in water-invaded reservoirs and related subsurface energy systems.