An investigation into pore structure fractal characteristics in tight oil reservoirs: a case study of the Triassic tight sandstone with ultra-low permeability in the Ordos Basin, China

Pore structure, which not only determines the microscopic characteristics of reservoirs but also controls the macroscopic physical properties of reservoirs, has been difficult to study in reservoir research. Fractal theory is an effective method for quantitative analysis of the irregular and complex pore structures of rocks. On the basis of high-pressure mercury intrusion experiments, the fractal dimensions of tight sandstones in the Yanchang Formation of the Triassic in the Ordos Basin are calculated by two methods (the water saturation method and mercury saturation method), and the fractal characteristics of the pore structures in this tight sandstone reservoir are also analyzed. The results show that the pore structures of tight sandstone reservoirs are heterogeneous and can be divided into four types. The fractal dimensions calculated by the water saturation method are poorly correlated with reservoir quality, while the fractal dimensions calculated by the mercury saturation method have good correlation with reservoir quality, that is, reservoirs with low fractal dimensions have high reservoir permeability and large pore-throat radii, while reservoirs with high fractal dimensions have low reservoir permeability and small pore-throat radii. Therefore, the fractal dimensions calculated by the mercury saturation method are presented to characterize tight sandstone reservoirs. The pore structures with large pore-throat sizes (radius> r tr ) and small pore-throat sizes (radius< r tr ) are relatively independent, and this results in clear turning points on the fractal curves of most samples. More importantly, samples with different fractal characteristics can represent reservoirs with different physical properties and pore structures. In conclusion, fractal dimensions can be used as an effective parameter for the quantitative characterization of tight sandstone reservoir with ultra-low permeability.