Abstract: Chang 7 shale oil in Ordos Basin is characterized by high initial output,rapid decline and low depletion development recovery efficiency,thus requiring urgent and effective enhanced oil recovery(EOR) methods.Based on core analysis results from high-pressure mercury intrusion(HPMI),scanning electron microscopy(SEM),casting thin sections(CTS),and low-field nuclear magnetic resonance(NMR),the pore structure of the target reservoir was classified and evaluated.Experiments were then carried out on oil displacement by fracturing fluid imbibition,fracturing fluid+surfactant imbibition displacement,and CO₂ huff-and-puff displacement,to investigate the crude oil production characteristics in different pore size ranges and the EOR effectiveness of the three patterns.An EOR method integrating fracturing and displacement was proposed to improve shale oil recovery.The experimental results show that the pore structure of the target shale reservoir can be classified into types I-Ⅲ,with reservoir physical properties deteriorating sequentially from I to Ⅲ.Among the three oil displacement patterns,CO₂ huff-and-puff achieves the highest recovery efficiency,followed by fracturing fluid+surfactant imbibition,and fracturing fluid imbibition the lowest.For a given pattern,type I shale yields the highest recovery,with types Ⅱ and Ⅲ decreasing in turn.For type I shale,ordinary slickwater can be used as the fracturing fluid;for type Ⅱ,adding a surfactant to the fracturing fluid is recommended;for type Ⅲ,CO₂ can be used as a pre-slug in the fracturing fluid.Compared with ordinary slickwater,adding an anionic surfactant(LAS) to the fracturing fluid increases the average daily oil output per well by 2.4 times.The research results can provide reference for improving shale reservoir development efficiency.

Key words: shale oil, fracturing fluid, surfactant, CO₂ huff-and-puff, pore structure, NMR