Micro-CT based observations of pore structure in tight sandstone and residual oil under CO₂ and water flooding

Oil recovery efficiency in tight sandstone reservoirs is closely linked to fluid flow behavior, which is influenced by the intricate pore-throat structures and varying displacement methods employed. The objective of this study is to investigate the pore-scale distribution and recovery behavior of residual oil in tight sandstone during brine and CO₂ flooding. Two tight sandstone samples (in the Longdong area of the Ordos Basin) were analyzed using high-resolution in-situ micro-computed tomography (micro-CT) with a voxel size of 3 µm. Three-dimensional reconstructions were employed to quantify the morphology of residual oil, pore-throat structure, and recovery efficiency at various stages of flooding. The oil primarily appears in cluster and porous forms during brine displacement, breaking into smaller droplets and film structures as brine saturation progresses. In contrast, CO₂ displacement more effectively reduced cluster oil, but led to a higher proportion of challenging film residual oil (19%). Quantitative analysis revealed that CO₂ displacement achieved a higher final recovery rate of 52.35%, compared to 48.29% for brine. This study provides the first direct, quantitative comparison of recovery efficiency between CO₂ and brine flooding in tight sandstone using real-time micro-CT imaging. The findings clarify the morphological “cost” of CO₂ displacement and offer new experimental insights for validating pore-scale numerical models and optimizing enhanced oil recovery in low-permeability reservoirs.