Research on the micro-pore structure and multiscale fractal characteristics of shale under supercritical CO2 action: A case study of the Chang 73 submember in the Ordos Basin, China

Abstract

To elucidate the mechanism of supercritical CO₂ (ScCO₂) on the microporous structure of shale, this study focuses on the Chang 7₃ submember of the Yanchang Formation in the Ordos Basin. Utilizing a combination of organic geochemical and mineral composition analyses, low-temperature gas (CO₂ and N₂) adsorption experiments and nuclear magnetic resonance (NMR) scanning methods are employed—combined with multiscale fractal theory—the research comprehensively analyze the changes in shale microporous structure and its fractal characteristics under ScCO₂ treatment. The results show that after ScCO₂ treatment, the total organic carbon (TOC) content of the shale samples decreases, the quartz content increases, while the contents of clay minerals and feldspar decrease. Notably, TOC and mineral components are more sensitive to pressure changes compared to temperature variations. Additionally, shale pores are mainly distributed in the micropore (0–2 nm) and mesopore (2–50 nm) ranges, contributing significantly to the specific surface area, while macropores (>50 nm), though fewer, considerably contribute to the total pore volume. Following ScCO₂ treatment, the total specific surface area of shale samples decreases, whereas total pore volume, average pore diameter, and effective porosity increase. Specifically, total specific surface area and average pore diameter are more sensitive to temperature, while total pore volume and effective porosity are more influenced by pressure. The shale pores exhibit multi-scale fractal characteristics, with micropores displaying higher fractal dimensions than meso- and macropores. After ScCO₂ treatment, fractal dimensions at all scales decline, indicating an improvement in the complexity of the shale pore structure. A significant positive correlation exists between the fractal dimension of micropores and TOC content, whereas meso- and macropore fractal dimensions have a stronger correlation with quartz and clay mineral content. These findings indicate that changes in shale mineral characteristics are intrinsic factors affecting microporous structure, while ScCO₂ treatment conditions are important external factors. The interaction of both determines the evolution of shale pore structures, providing a valuable scientific basis and practical guidance for the optimal selection of carbon capture, utilization, and storage (CCUS) target layers.