Oil Displacement Characteristics and Pore-Scale Response Mechanisms of Water-Sensitive Low-Permeability Sandstone under Multiple Flooding Processes.

Low-permeability water-sensitive sandstone reservoirs face challenges of low recovery efficiency and injection difficulties with conventional water flooding, necessitating the exploration of more effective displacement technologies. A systematic comparative analysis was conducted on water flooding, gas flooding, water-alternating-gas flooding, surfactant flooding, and CO2-surfactant hybrid flooding, focusing on oil recovery efficiency, displacement pressure differential, pore-scale crude oil mobilization characteristics, and reservoir permeability damage through core flooding experiments. The results indicate that water flooding mainly mobilized crude oil in medium-to-large pores and was difficult to sweep small pores. Furthermore, injecting low-salinity formation water intensified damage and possible pore-throat blockage, which reduced recovery efficiency from 32.6 to 25.9% and increased permeability damage to 40.6%. CO2 flooding mobilized crude oil in small pores, achieving a recovery efficiency of 52.7%, but was prone to early breakthrough and gas channeling issues. Water-alternating-gas flooding delayed early gas breakthrough and improved sweep efficiency in small pores. At a gas/water slug ratio of 1:1, recovery efficiency increased to 66.5%, and permeability damage decreased to 12.8%. The addition of surfactant significantly reduced oil-water interfacial tension and altered rock wettability, improving the range of pore sizes mobilized during displacement, recovery efficiency was 29.9% higher than that of water flooding. However, increasing concentration raised permeability damage to 19.6%. The favorable performance of CO2-surfactant hybrid flooding may be related to CO2-induced oil mobility improvement and surfactant-induced capillary resistance reduction, leading to broader oil mobilization from micropores to large pores. It achieved the highest recovery efficiency of 74.6% among the evaluated schemes, with breakthrough at 0.39 PV and a relatively low displacement pressure differential of 0.083 MPa. These results indicate that maintaining sufficient injected-water salinity with KCl addition is important for controlling water-sensitive damage, while WAG flooding with a gas/water slug ratio of 1:1 and CO2-surfactant hybrid flooding showed more favorable relationships among recovery enhancement, injectivity improvement, and permeability-damage control under the tested conditions.