Investigation of the Microscopic Pore-Throat Structure Difference of Tight Sandstone/Glutenite Based on a Multifractal Method

Themicroscopic pore-throat structure of a tight reservoiris directly related to its seepage capacity. Clarifying the differencesin the pore-throat structure of various tight rocks is conduciveto reservoir evaluation and flow interpretation. In this study, themicroscopic pore-throat structures of tight sandstone (YanchangFormation, Ordos Basin) and tight glutenite (Upper Wuerhe Formation,Junggar Basin) were characterized by a series of tests, includinghigh-pressure mercury injection, constant-rate mercury injection,and nano-CT. A quantitative analysis was carried out to describe heterogeneityand evaluate permeability based on the multidimensional fractal theory.The correlation coefficient of porosity and permeability of tightsandstone is 0.65, while that of glutenite is only 0.16. The pore-throatsize distribution of sandstone is relatively concentrated and well-sorted.The glutenite presents an obvious multipeak pattern, and the proportionof the throat with different sizes is approximate. Multidimensionalfractal fitting of sandstone can be divided into two linear segments,whereas glutenite can be divided into four segments. The key factorcontributing to this difference is the throat, which controls theflow in the rock. The throat of glutenite exhibits multiscale distributionfeatures, diverse connection modes, and significantly stronger heterogeneitythan sandstone, resulting in a poor correlation between porosity andpermeability and a complex seepage process. It is essential to considerthe influence of microstructure differences, especially heterogeneityand tortuosity when analyzing the flow capacity and clarifying theseepage mechanism of the two types of rock samples.

Automated summary

The microscopic pore-throat structures of tight sandstone and tight glutenite were characterized using high-pressure mercury injection, constant-rate mercury injection, and nano-CT. The fractal theory was applied to quantitatively analyze the heterogeneity and permeability of the rocks. The results showed that the pore-throat size distribution of sandstone was relatively concentrated and well-sorted, while glutenite exhibited a multipeak pattern with similar proportions of throat sizes. The key difference between the two rocks was the throat structure, which controlled the flow and resulted in poor correlation between porosity and permeability in glutenite.