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

Abstract

The microscopic pore–throat structure of a tight reservoir is directly related to its seepage capacity. Clarifying the differences in the pore–throat structure of various tight rocks is conducive to reservoir evaluation and flow interpretation. In this study, the microscopic pore–throat structures of tight sandstone (Yanchang Formation, Ordos Basin) and tight glutenite (Upper Wuerhe Formation, Junggar Basin) were characterized by a series of tests, including high-pressure mercury injection, constant-rate mercury injection, and nano-CT. A quantitative analysis was carried out to describe heterogeneity and evaluate permeability based on the multidimensional fractal theory. The correlation coefficient of porosity and permeability of tight sandstone is 0.65, while that of glutenite is only 0.16. The pore–throat size distribution of sandstone is relatively concentrated and well-sorted. The glutenite presents an obvious multipeak pattern, and the proportion of the throat with different sizes is approximate. Multidimensional fractal fitting of sandstone can be divided into two linear segments, whereas glutenite can be divided into four segments. The key factor contributing to this difference is the throat, which controls the flow in the rock. The throat of glutenite exhibits multiscale distribution features, diverse connection modes, and significantly stronger heterogeneity than sandstone, resulting in a poor correlation between porosity and permeability and a complex seepage process. It is essential to consider the influence of microstructure differences, especially heterogeneity and tortuosity when analyzing the flow capacity and clarifying the seepage mechanism of the two types of rock samples.