Pore throat structure and fractal characteristics of tight oil sandstone: A case study in the Ordos Basin, China

Abstract

Pore throat structure is a significant factor controlling the properties of reservoir. Evaluation of the pore throat structure is also important for the exploration and development of tight oil. In order to determine the micro-structure of pore throat, the high pressure mercury intrusion experiments (HPMI) were performed on the tight oil sandstone samples of Chang 7 member in Xin’anbian Block, Ordos Basin, China. HPMI were conducted to obtain the microscopic parameters of the pore throat structure, such as pore-size distribution, pore connectivity, and pore space topology.The pore throat network of the tight oil sandstone mainly consists of micropores, transitional pores, and mesopores. The mesopores of the Chang 7 tight reservoirs is the dominant factor affecting the pore throat structure. Mesopores development positively influences the maximum pore radius, median radius, sorting coefficient, skewness, maximum mercury saturation, and efficiency of mercury withdrawal, indicating that with the development of mesopores, the properties of percolation, storage, pore throat connectivity, and oil recovery would become better. Fractal theory was used to quantitatively characterize the complex and irregular pore throat structure of the reservoir. The fractal dimensions of mesopores, transitional pores and micropores (D1, D2, and D3) were calculated through the slopes of the trendlines of each segment obtained from the curve of log (1-SH) versus log (Pc). The total fractal dimension (D) was obtained, based on the weighted average of porosity of mesopores, transitional pores, and micropores. D ranging of 2.2520–2.7875 indicates that the pore throat structure of Chang 7 tight reservoir is complex and has a strong heterogeneity. D1, D2, and D3 have obvious negative correlations with porosity of the corresponding pores and only D1 has an obviously negative correlation with permeability. D has a downward trend, with the increasing porosity, but no correlation with permeability. D1 has better correlations with the parameters of pore throat structure than D2 and D3. The heterogeneity and surface roughness of mesopores mainly influence the property of pore throat structure, especially percolation and storage space of the reservoir. The development of mesopores is a main factor affecting the pore throat structure of the Chang 7 tight oil sandstone. The fractal characteristic of mesopores reflects how well the pore throat structure for the exploration and development of Chang 7 tight oil.