Pore-throat structure and fractal characteristics of tight sandstone: A case study in Yanchang Formation, Southeast Ordos Basin

Abstract

The complex pore throat structure of tight sandstone reservoirs increases the difficulty of reservoir exploration and development, and becomes a key factor affecting oil and gas charging and reservoir oil content control. Quantitative evaluation and characterisation of the pore-throat structure is also important for maintaining and improving oil recovery. In order to determine the microstructure of pore throats, a series of experiments such as casting thin section, scanning electron microscopy, X-ray-rock diffraction experiment and high pressure mercury injection (HPMI) were carried out on the tight oil sandstone samples of the Chang 6 member in the south-eastern Ordos Basin, China. The relationship between each factor and pore throat complexity was investigated using factors such as reservoir physical properties, pore throat characteristic parameters and mineral composition. The results indicate that the main pore types of the sample are intergranular pores, feldspar dissolved pores, intergranular dissolved pores and micro-cracks. Meanwhile, the pore throat structure of each sample was identified as large pores and small pores by combining the result of HPMI with fractal theory. The small pores of Chang 6 tight reservoirs are the dominant factor affecting the pore throat structure. The development of small pores positively influences the maximum pore radius, median radius, sorting coefficient, maximum mercury saturation and mercury recovery efficiency, indicating that with the development of small pores, the properties of percolation, storage, pore throat connectivity and oil recovery would become better. Fractal theory was used to quantitatively characterise the complex and irregular pore throat structure of the reservoir. The total fractal dimension D ranged from 2.3601~2.7403 with an average of 2.5722. The average fractal dimension D1 was 2.8094 and the average fractal dimension D2 was lower than D1 with an average of 2.5325, indicating the complex structure and strong heterogeneity of the former. D2 has better correlations with the pore throat structure parameters than D1. The heterogeneity and surface roughness of small pores mainly influence the properties of the pore throat structure, especially the percolation and storage space of the reservoir. The development of small pores is a major factor influencing the pore throat structure of the Chang 6 tight oil sandstone. The fractal characteristic of small pores reflects how well the pore throat structure is suited for exploration and development of Chang 6 tight oil.