Nano-Scale Pore Structure and Its Multi-Fractal Characteristics of Tight Sandstone by N2 Adsorption/Desorption Analyses: A Case Study of Shihezi Formation from the Sulige Gas Filed, Ordos Basin, China

Fractal dimension is a critical parameter to evaluate the heterogeneity of complex pore structure in tight sandstone gas and other low permeability reservoirs. To quantify the fractal dimension of tight sandstone at various pore size classes and evaluate their implications on mineral composition and nano pore structure parameters, we conducted an integrated approach of N-2 adsorption/desorption experiment (N-2-GA), X-ray diffraction (X-RD), and field emission scanning electron microscopy (FE-SEM) on Sulige tight sandstone reservoirs. By comparing the nine types of fractal dimensions calculated from N-2 adsorption data, we put forward the concept of concentrated fractal dimensions and scattered fractal dimensions (D-N2, D-N3, D-N5, D-N7 and D-N8) for the first time according to its concentration extent of distribute in different samples. Result shows that mineral composition has a significant influence of a different level on specific surface area (SSA), pore volume (PV), and fractal dimensions (D-N), respectively, where the scattered fractal dimension is more sensitive to certain specific property of the reservoir, including mineral content and the specific surface area contribution rate (S-r) of type II mesopores (Mesopore-II: 10 similar to 50nm). In addition, three type of hysteresis loops were distinguished corresponding to different pore shape combination of N-2-GA isotherm curve, which reveals that pore structure heterogeneity is mainly controlled by inkbottle-shaped pores and the volume contribution rate (V-r) of mesopores in this study area. These findings could contribute to a better understanding of the controlling effect of pore heterogeneity on natural gas storage and adsorption.