Abstract: The seventh member of the Yanchang Formation (Chang 7 member) in the Ordos Basin is a typical interlayered shale oil reservoir in China. A national shale oil development demonstration base has been established in the Longdong area. Spontaneous imbibition and oil displacement are observed throughout the entire process from hydraulic fracturing to crude oil production, exerting a significant impact on shale oil output. Therefore, clarifying the characteristics of spontaneous imbibition and oil displacement of fracturing fluid in pores of different sizes and their controlling factors is crucial for enhancing shale oil recovery in Chang 7. This study took the interlayered shale oil reservoirs of Chang 7 in the Ordos Basin as a case study. A series of experiments was conducted, including porosity-permeability measurements, X-ray diffraction (XRD) analysis, contact angle determination, and nuclear magnetic resonance (NMR)-based spontaneous imbibition and oil displacement experiments with fracturing fluid. These analyses characterized the spontaneous imbibition and oil displacement behavior of fracturing fluid in pores of different sizes across different reservoir types and revealed their key controlling factors from the perspectives of reservoir physical properties, mineral composition, and wettability. The results showed that: (1) based on pore-type proportions obtained from NMR fractal analysis, the samples were classified into TypeⅠand TypeⅡreservoirs. In TypeⅠreservoirs, macropores made up an average of 85.1%, and in TypeⅡreservoirs, mesopores made up an average of 79.0%. (2) TypeⅠreservoirs exhibited higher reservoir quality factors and quartz content than TypeⅡreservoirs, while containing less clay minerals. Their contact angles ranged from 77.3° to 103.7°, indicating the development of both hydrophilic and lipophilic reservoirs. In contrast, TypeⅡreservoir samples had contact angles between 53.2° and 63.1°, showing strong hydrophilicity. (3) The average spontaneous imbibition and oil displacement ratio in TypeⅠreservoirs was 17.27%, primarily contributed by macropores (74.1% on average), with mesopores accounting for 25.5%. In TypeⅡ reservoirs, the average ratio was 40.74%, primarily attributed to mesopores (85.2% on average). A comprehensive analysis of all influencing factors indicated that mineral composition was the fundamental factor influencing the ratio of spontaneous imbibition and oil displacement, followed by pore type, wettability, and petrophysical properties.

Key words: Ordos Basin, interlayered shale oil, nuclear magnetic resonance, fracturing fluid spontaneous imbibition and oil displacement, key controlling factors