Spatial-temporal evolution and damage mechanism of pore-throat networks in unconsolidated sandstones during seawater reinjection based on in-situ CT

Seawater is widely used as an injection fluid in offshore subsurface systems because of its stable availability and low operational cost. However, direct pore-scale evidence of structural evolution and seepage degradation in unconsolidated sandstones during seawater reinjection remains limited under conditions representative of offshore operations. In this study, pore-scale structural evolution and seepage response of unconsolidated sandstone cores from the Bohai Sea were investigated using in-situ X-ray computed tomography, combined with three-dimensional pore network reconstruction and pore-scale flow simulation. The results show that seawater reinjection induces a staged and non-monotonic evolution of pore structure and permeability. During the early injection stage, clay swelling and fine-particle migration dominate pore–throat blockage, leading to pronounced loss of connectivity and rapid permeability decline. With continued injection, partial pore reopening occurs under hydrodynamic erosion, and porosity shows limited recovery. Nevertheless, permeability remains strongly constrained due to irreversible alteration of pore topology. Marked spatial heterogeneity is observed along the flow direction, with particle accumulation concentrated near the injection end and clay-related pore collapse more evident in downstream regions. Grey relational analysis indicates that pore topology exerts a stronger control on permeability evolution than clay content alone, highlighting the key role of connectivity degradation in sustained seepage impairment. These findings provide pore-scale insight into injectivity deterioration during offshore seawater injection and offer a microstructural basis for interpreting injection performance in offshore subsurface engineering.