Fractal characteristics of pore-throat structure and permeability estimation of tight sandstone reservoirs: A case study of Chang 7 of the Upper Triassic Yanchang Formation in Longdong area, Ordos Basin, China

Permeability estimation and quantitative evaluation of pore microstructure have always been hotspots and difficulties in tight sandstone reservoirs research. Taking the fractal dimension as the breakthrough, 10 core samples of Chang 7 section in the Upper Triassic Yanchang Formation were collected from different wells in Longdong area of Ordos Basin. The fractal dimension was calculated by mercury intrusion capillary pressure curve which can be obtained from high pressure mercury injection experiment, and samples were divided into four groups as group I, II, III and IV according to the value of fractal dimension estimated. The observation of casting thin sections and field emission scanning electron microscope show that group I mainly has dissolved-intergranular pores and original intergranular pores, group II is mainly dissolved-intergranular pores, group III mainly has dissolved-intragranular pores, and group IV is mainly micropores in clay matrix. Meanwhile, there is a highly correlation between the fractal dimension and the reservoir physical properties. With the fractal dimension of tight sandstone samples approaching 3, the compaction and cementation show a gradual increase, while the pore sorting, physical parameters and percolation capacity display a gradual decrease. Through multiple regression analysis, a set of empirical equations based on the fractal dimension and different pore throat radius were constructed, in which the equation based on maximum permeability contribution radius has the highest correlation. It shows that micropore (0.1–0.2 μm) contributes most to permeability in tight sandstone reservoir. 10 blind samples were included in regression analysis to improve the prediction accuracy of the new model. A total of 28 samples, including 8 referenced data, were used to verify the predictive accuracy of the new model. The new empirical equation can predict the permeability of tight sandstone relatively accurate. Thus, the fractal dimension can effectively predict the permeability and evaluate the quality of tight sandstone reservoirs.