Low recovery in mature oil fields, mostly in a high-water-cut stage, requires advanced methods to enhance oil recovery. However, incomplete understanding of the mechanisms of low oil recovery in pore-scale limits the application of proposed methods in industry. Digital core technology and high-resolution micro computed tomography (mu CT) scanning were deployed to investigate effects of pore structure and permeability scales on oil displacement efficiency and remaining oil-cluster morphology visually and quantitatively. In the study, brine displacing oil experiments were conducted on four naturally water-wet sandstone samples with different permeability scales. A mu CT equipment with a resolution of 3.78 mu m was used to image the samples at different water flooding stages by injecting brine water with different PV (pore volume): 0 (irreducible water saturation), 1, 5, 15, and 50PV. The complexity of the pore structures of four samples were quantified by fractal dimension analysis from mu CT images. The order of pore structure from simple to complex is samples 1, 2, 3, and 4. Then the distributions of cluster pattern of reminding oil were analyzed. Based on the cluster pattern distributions, microscopic remaining oil was classified into five categories by using shape factor and Euler number. Results show that large connected oil clusters can be more easily broken into smaller segments in high permeability samples. Sample 1 has the largest pore connectivity and shows the biggest change of distribution pattern. The clusters are concentrated on one to two distribution patterns. If the permeability gets lower and pore structure becomes complex, all distribution patterns coexist. Also, the lower permeability samples show a smaller displacement efficiency at all displacing stages. A large coordination number improves the displacement efficiency, but the large tortuosity and pore-throat ratio reduce the displacement efficiency. Results of this study can enrich the understandings of the low oil recovery and can inspire innovative methods to further tap those remaining oil.