Investigation of pore-throat structure and fractal characteristics of tight sandstones using HPMI, CRMI, and NMR methods: A case study of the lower Shihezi Formation in the Sulige area, Ordos Basin

Abstract

The evaluation of pore-throat structure is essential for the exploration and exploitation of tight oil and gas reservoirs. In this study, various experiments such as casting thin section (CTS), scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR) are used to investigate the pore-throat structure and fractal characteristics of the tight sandstone from the Permian Shihezi Formation in the Sulige area, Ordos Basin. The type and size of tight sandstone pores and throats are qualitatively analyzed by using CTS and SEM. However, the structural parameters such as the size and distribution of tight sandstone pores and throats are quantitatively calculated by HPMI, CRMI, and NMR. According to the advantages and disadvantages of each method, the HPMI and NMR are combined to characterize the full-size pore-throat distribution (PSD) of tight sandstone. Based on the fractal theory, the fractal dimensions (D) of pore-throat of tight sandstone are evaluated by HPMI, CRMI, and NMR. The result shows the pores in the tight sandstone are mainly residual intergranular pores, dissolution pores, and inter-crystalline pores. There are few micro-cracks developed, and the throats are mainly tubular and curved sheets. The full-size PSD curve of tight sandstone presents the characteristics of the bimodal and unimodal distribution. It has a good agreement with petrophysical properties and movable fluid saturation. Different experimental methods can get different D. HPMI and NMR have different detection ranges and diverse principles for pore-throat evaluation, making sandstone's diverse fractal characteristics. The CRMI is more representative for studying the fractal characteristics of the throat. Moreover, there are two different types of throats: large throat and small throat. These throats have a double fractal feature. The relationship between D and the physical properties analyzes the parameters of the pore-throat structure, suggesting larger the D, the worse the physical properties of the reservoir. The development of the throat (especially the larger throat) controls the storage and fluid flow-ability of tight sandstone reservoirs. Mercury saturation, movable fluid saturation, and D are negatively correlated, indicating that the complicated pore-throat structure will reduce permeability and destroy the free fluid storage space.