Application of Low Pressure N 2 Adsorption on Shale Nanoscale Pore Structure: Examples from the Permian Shanxi Formation of Transitional Facies Shale

Shale gas resources in transitional facies are important unconventional energy sources. In order to understand the seepage characteristics of shale reservoir, low pressure nitrogen adsorption experiment were applied to investigate the nanoscale pore structure and adsorption fractal characteristics of shale in Shanxi Formation. FHH fractal model was adopted to calculate the absorption pore fractal dimension values D and quantitatively confirm the relationship between fractal dimension values and pore parameters. Results show that distribution curve obtained by the desorption branch present pore size distribution ranges from 1.14 nm to 173.39 nm, micropores and macropores developed and concentrated, no obvious dominant peak in the mesopores (2-25 nm) with relatively uniform distribution. Fractal dimension values range from 2.42 to 2.81, indicating shale pore structure is complex and heterogeneous. Fractal dimension is positively correlated with specific surface area and total pore volume, and highly negatively correlated with the average pore diameter. Fractal dimension is also a characterization of buried depth, and there is a weak positive correlation between them. With the increase of buried depth, fractal dimension has an increasing trend. The study provides a new method to further understand the shale nanometer pore structural heterogeneity.