Multi-Angle Investigation of the Fractal Characteristics of Nanoscale Pores in the Lower Cambrian Niutitang Shale and Their Implications for CH4 Adsorption

Shale gas has received widespread interest due to its successful commercial development in China. Pore structures in shale can directly control its gas storage and migration properties. In this study, field emission scanning electron microscopy (FE-SEM), low-pressure N 2 /CO 2 adsorption and highpressure methane adsorption were used to investigate the nanoscale pore structures of the Lower Cambrian Niutitang Formation in the southeastern Upper Yangtze platform. The fractal parameters of the pore structures were also calculated using the Frenkel–Halsey–Hill (FHH) model. The relationships between the fractal dimensions and TOC content, mineral composition and pore structure parameters were also discussed. The results show that organic matter and clay minerals are primary factors affecting the nanoscale pore development. Slit-shaped pores and ink-bottle-shaped pores are the predominant pore types in the Niutitang shale. The Brunauer-Emmett-Teller (BET) surface areas vary from 4.91 m 2 /g to 34.33 m 2 /g, and the pore volumes range from 0.689 m 3 /100 g to 2.964 m 3 /100 g. Two fractal dimensions ( D 1 and D 2 ) of the Niutitang shale were obtained using the FHH model, with D 1 ranging from 2.605 to 2.684, and D 2 ranging from 2.681 to 2.865. D 1 adequately characterizes the surface roughness of the pore structures, while D 2 represents the complexity of the pore types. Inter-particle (InterP) pores commonly have greater shape complexities than OM pores and intra-particle (IntraP) pores, based on analyses using Image-Pro Plus software. In addition, the TOC content and clay minerals have great effects on the fractal dimension D 1 . Meanwhile, the fractal dimension D 1 increases with increasing BET surface area, but there is no definite relationship between the fractal dimensions and pore volumes. Both the fractal dimensions D 1 and D 2 are negatively correlated with pore sizes. Further investigation indicates that the fractal dimension D 1 exhibits a strong positive relationship with the methane adsorption capacity indicating that Niutitang shales with greater values of the fractal dimension D 1 have higher methane adsorption capacities.