Methodological Study on the Full-Range Pore Structure and Fractal Characteristics of the Tight Reservoirs

Abstract

Scholars have used the method of splicing various experimental data to evaluate the full-range pore structure of tight reservoirs, but its applicability has not been discussed. When the fractal theory is used to characterize the heterogeneity of tight reservoirs, there is a lack of research on the controlling factors of segmented fractal pore size (the inflection points in the fractal curve) and the relationship between the fractal dimensions of different dimensions. In this paper, mercury intrusion porosimetry (MIP), N2 adsorption (N2-GA), and large-field splicing scanning electron microscopy (MAPs) were conducted to study the pore structure and full-range pore size distribution (PSD) of tight sandstone, and fractal theory was used to evaluate the heterogeneity of the reservoir. Combining the PSD of MIP and N2-GA, two parameters “cosine similarity” and “data similarity” were introduced to characterize the overlapping pore size range of the two experimental methods; then, the PSD of MAPs was used to verify the rationality of the data splicing. The results show that the characterization of the full-range pore structure should not only be based on simple data splicing but should focus on the data similarity of the overlapping pore size range. The study on the segmented fractal pore size of MIP shows that the segmented fractal pore size increases gradually with an increase in the main skeleton mineral content, the decrease in the clay mineral content, and the increase in the pore radius and porosity. For the same sample, the segmented fractal pore size is fixed and does not change with the calculation model of fractal dimension. Comparing the two-dimensional (2D) fractal dimension with the three-dimensional (3D) fractal dimension, calculating the 3D fractal dimension by +1 directly with the 2D fractal dimension is more applicable for the large pores, but it is not applicable for the small pores due to the influence of the extensive development of linear pores.