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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>rtr) and small pore-throat sizes (radiustr) 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. [ABSTRACT FROM AUTHOR]

Tight oil is a significant component of unconventional oil and gas resources. The pore structures of tight oil reservoirs determine the seepage law, which is a significant factor in evaluating reservoir quality. However, because of the strong heterogeneity of the pore structure in tight oil reservoirs, it is difficult to characterize the pore structure by conventional methods. Therefore, on the basis of conventional reservoirs research methods, combined with high-pressure mercury intrusion (HPMI) and constant-pressure mercury intrusion (CPMI), the pore-throat size distribution of tight sandstone reservoirs and its control on reservoir physical properties were studied. The main pore types in the study area were residual intergranular pores, secondary intragranular pores, intercrystalline micropores and microfractures, and throat types were sheet-like and tube throats. The pore-throat size distribution characterized by HPMI ranges from 0.0025 to 2.79 μm, and pore-throats larger than 10 μm occurred less frequently. The pore size distribution characterized by CPMI was between 90 and 350 μm, and the peak value distribution of the pore radius of the different samples was relatively concentrated, while the throat size distribution was between 0.12 and 6 μm, indicating that the throat radius of the different samples showed a strong heterogeneity, which also led to a strong heterogeneity of pore-throat radius ratio. Although HPMI and CPMI had their own limitations in characterizing the pore-throat size distribution, their combination could effectively characterize the whole pore-throat size distribution of tight reservoirs, which ranged from 0.0025 to 350 μm. The permeability of the tight sandstone reservoirs is primarily determined by the relatively wider pore-throats with small proportions, and the permeability decreased as the proportion of narrower pore-throats increased. The nanoscale pore throats (radius < 0.1 μm) were widely developed in the tight reservoirs, which had little influence on permeability, but these pore throats could be used as oil and gas storage spaces and played an important role in improving reservoir porosity. • The pore-throat size scales spanned from nanometer to several hundred microns. • The combination of HPMI and CPMI could effectively characterize the whole-distribution characteristics of the pore-throat size. • The porosity was not only controlled by the wider pore throats, but the nanoscale pore throats (radius < 0.1 μm) also contributed, while the permeability was controlled by relatively wider pore throats with a small proportion. [ABSTRACT FROM AUTHOR]