Quantitative characterization of permeability heterogeneity of tight-sand reservoirs using nano-CT technology: A case study of the Yanchang Formation, Ordos Basin

The physical properties of hydrocarbon reservoirs are important factors affecting the percolation ability of the reservoirs. Tight-sand reservoirs exhibit complex pore throat connectivity due to the extensive development of micro- and nano-scale pore and throat systems. Characterizing the microscopic properties of these reservoirs using nondestructive, quantitative methods serves as an important means to determine the characteristics of microscopic pores and throats in tight-sand reservoirs and the mechanism behind the influence of these characteristics on reservoir porosity and permeability. In this study, a low-permeability sandstone sample and two tight sandstone samples collected from the Ordos Basin were nondestructively tested using high-resolution nano-CT technology to quantitively characterize their microscopic pore throat structures and model them three-dimensionally (in 3D) based on CT threshold differences and gray models. A thorough analysis and comparison reveal that the three samples exhibit a certain positive correlation between their porosity and permeability but the most important factor affecting both porosity and permeability is the microscopic pore throat structure. Although the number of pores in tight sandstones shows a minor impact on their porosity, large pores (more than 20 μm) contribute predominantly to porosity, suggesting that the permeability of tight sandstones is controlled primarily by large pore throats. For these samples, higher permeability corresponds to larger average throat sizes. Therefore, throats with average radii greater than 2 μm can significantly improve the permeability of tight sandstones.