Heterogeneity of Pore and Fracture Structure in Tight Sandstone Using Different Fractal Models and Its Influence on Porosity–Permeability Variation

The study of pore structure in low-permeability sandstone uranium deposits has become a key factor in the profitability of uranium mining. In this paper, pore and fracture distribution in the target sandstone were determined by using mercury injection parameters. Single and multi-fractal models are used to calculate the heterogeneity of pore and fracture volume distribution. Moreover, the correlation between compressibility and the heterogeneity of pore distribution has been studied. The results are as follows. (1) All the samples can be divided into three types by using maximum mercury injection volume and mercury withdrawal efficiency. Type A is represented by a lower maximum mercury injection volume (less than 0.5 cm3·g−1) and a higher mercury withdrawal efficiency (larger than 25%). The volume percentage of pores whose diameter is less than 100 nm and 100~1000 nm in type A samples is larger than that of type B and C samples since in this type of sample, micropores are developed. (2) The fractal dimension value assessed using the Menger model has a good linear relationship with the thermodynamic model, which indicates that the abovementioned models have good consistency in characterizing the pore distribution of tight sandstone. Multi-fractal results show that the lower pore volume in the selected samples controls the heterogeneity of pore distribution in the overall sample. (3) As the effective stress increases, the permeability damage rate gradually increases in a power exponential equation. The correlation between porosity and compressibility is weaker, indicating that only a portion of the pore volume in the sample provides compression space. As the pore volume of 100~1000 nm increases, the compressibility decreases linearly, indicating that pore volumes larger than 1000 nm provide compression space for all the selected samples.