Microscopic Pore Structure Characteristics and Oil Transport in Nanoscale Shale

Shale reservoirs are characterized by mixed wettability and the widespread development of nano- to microscale pores. Investigating shale oil flow within these confined spaces requires a comprehensive approach that integrates real pore structure characteristics and multiple microscale effects. In this study, the large-field scanning electron microscopy (SEM-Maps) experiment was conducted to identify and classify microscopic pores, followed by statistical analysis of pore size distribution, eccentricity, elongation, shape factor, and other parameters to quantitatively characterize pore size and morphology. Based on the experimental results of pore structure, a zonal model of microscopic crude oil migration was developed, which incorporates both pore characteristics and microscale flow mechanisms to analyze the shale oil flow behavior at the microscale. The quantitative characterization of pore structures revealed that the mean pore eccentricity ranges from 0.6 to 0.9, while the shape factor falls between 1.1 and 1.9, indicating a general deviation from circularity. Additionally, the mean pore elongation ratio ranges from 1.7 to 2.8, suggesting a predominantly elliptical morphology. The investigation of microscopic shale oil migration demonstrated that pore wall wettability directly influences slip length, thereby significantly affecting fluid properties. Moreover, pore shape plays a crucial role in determining the proportion of near-wall fluid, while the flow enhancement factor is closely related to pore wall wettability, pore size, and pore morphology. This study quantitatively characterizes shale reservoir pore structures and develops a microscopic crude oil flow model that accounts for pore morphology. The findings provide valuable insights for the refined characterization of shale pore structures and the study of microscopic flow behaviors within nano- to microscale spaces.