Fractal characteristics of nanopore in shales

Abundant nanopores are present in shales, and the nanopore structure plays an important role on gas adsorption and migration. Taking nitrogen molecular as probe medium, we use the nitrogen adsorption fractal analysis method to measure the pore structures and roughness in shales. The fractal dimension was obtained from the nitrogen adsorption isotherm using the Frenkel-Halsey-Hill equation. The relationship between shale composition, organic matter content, pore structure parameters and fractal dimension were plotted. The impact of fractal dimension on gas adsorption and transportation was also discussed. Results show that the pore structure in shales is fractal, and the fractal dimension is related to organic matter and micropores. The more enriched organic matter and developed micropore are associated with the much bigger fractal dimension. The degree of fractal dimension indicates the micropore development. The smaller the average pore diameter, the larger the specific surface area, and the higher the fractal dimension. Fractal dimension increases with the increasing micropores. The higher the fractal dimension, the more complicated of pore structure, which is favorable for gas adsorption rather than gas transportation.