The study of microscopic remaining oil is of great significance for the effective development of reservoirs after water flooding. An observation and quantitative characterization method of occurrence states of remaining oil on pore scale is proposed in this manuscript. Core columns are frozen immediately after the displacement experiment with liquid nitrogen freezing technology, and the samples are ground to a thickness of 0.05 mm under frozen conditions. Distributions of oil and water in pores

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... are observed with the technology of ultraviolet fluorescence microphotography. The remaining oil content of different types is quantitatively calculated by analysis of characteristic parameters of the core image. Quantitative analysis of the laboratory displacement experimental results indicated that the average oil recovery reaches over 48% after water flooding. The main types of the remaining are throat state, cant state, thin film on pore surface, cluster state, interparticle adherence state and particle adherence state. Their relative contents account for 1.84%, 3.07%, 37.42%, 5.83%, 27.91% and 24.23% of total remaining oil reserves, respectively. Among them, the remaining oil in a thin film on pore surface, remaining oil of interparticle adherence, and remaining oil of particle adherence with high content are the development targets after water flooding. Based on determining the type and distribution characteristics of the microscopic remaining oil, the mechanism and influential factors of different types of microscopic remaining oil are analyzed, and the exploitation method for different types of remaining oil is proposed. This study is of great significance for guiding the development of remaining oil after water flooding and improving enhanced oil recovery.