Comprehensive Characterization of Shale Components and Pore Structures: An Analysis from Micro- to Nanoscale Perspectives

Abstract

The components and pore structures of shale are complex due to the heterogeneous distribution of organic and inorganic minerals. Understanding the pore structures properties of shale is essential for elucidating the fluid storage and migration mechanisms, which ultimately enhance recovery. This study employs X-ray diffraction to determine the mineral composition of shale. Focused ion beam scanning electron microscope (FIB-SEM) experiments are carried out to analyze the shale components and pore structures. In addition, fractures are described based on micro-computed tomography (CT) scanning techniques. Combining low-temperature nitrogen adsorption and high-pressure mercury injection experiments provides a quantitative description of the micro- and nanoscale pore structures of shale. The results show that clay, quartz, and carbonate content is high in shale samples. Coated, convoluted, and spongy pores exist in organic matter, with fractal dimensions of 1.12, 1.21, and 1.45, respectively. Fractal dimension increases with the maturity of organic matter. Due to complex shale components, the inorganic pores are highly heterogeneous, and the fractal dimensions are distributed in the range of 1.02–1.22. In organic pores, the pore volume in the diameter range of 800–1,200 nm accounts for a large proportion. For clay minerals, pores with diameters less than 100 nm constitute a substantial portion, and the pore volume exhibits a pronounced bimodal distribution. Fracture development is closely related to mineral composition, and the clay minerals restrict the fracture development. In addition, the multiscale pore structures of shale are further analyzed. The pore size of shale exhibits a bimodal distribution. Pores with a diameter of 20–50 nm and fractures with a diameter of about 50 μm occupy a large pore volume. The fractal dimensions of mesopores are distributed in the range of 2.16–2.59, and the fractal dimensions of macropores are distributed in the range of 2.908–2.976.