Identification and parameter characterization of pores and fractures in shales based on multi-scale digital core data

The characterization of carbonate microstructure is of great significance for the evaluation of carbonate oil and gas resources. However, due to the complexity and heterogeneity of the pore structure of tight carbonate rocks, high-pressure mercury intrusion, nuclear magnetic resonance (NMR) and other methods have different limitations in the characterization. This study takes tight carbonate core samples in the fourth member of the Ordovician Majiagou Formation in the Ordos Basin as the research object, and the rock physics experiments, computed tomography (CT), high resolution large-scale backscatter scanning electron microscopy (MAPS), quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and focused ion beam-scanning electron microscopy (FIB-SEM) was utilized to characterize the pore structure from micrometer to nanometer, revealing the main mineral composition, and systematically analyzing the relationship between different mineral and pore structures. The results show that the microscopic reservoir space in the study area is mainly composed of inter-crystalline pores, intra-crystalline pores and microfractures; there are obvious differences in the pore structure of different lithologies. The samples with more dolomite have the largest number of pores and throats, the largest coordination number, and the best connectivity; the samples with more calcite have the smallest pore radius. The presence of quartz is conducive to the preservation of pores. This multi-scale characterization method using digital core technology provides us with comprehensive pore characteristic, provides important clues for further understanding the pore structure of tight carbonate reservoirs.

Fractures play a crucial role as fluid conduits and reservoir spaces in reservoirs. The distribution and characteristics of fractures determine the presence of high-quality reservoirs. To accurately analyze and observe fracture parameters, three-dimensional (3-D) digital cores generated from computed tomography (CT) are utilized. However, the current process of extracting fracture properties from these digital cores is time-consuming and labor-intensive. This paper introduces a new, fast, and automatic workflow for extracting the apparent dip angle and direction of fractures from 3-D digital core images. The proposed workflow involves several steps. Firstly, two perpendicular cross-sections are obtained from the digital core and converted into binary images. Next, the coordinates of four fracture feature points within the core image are automatically extracted. The fracture plane is then fitted using the least squares method based on the extracted coordinates. Finally, the apparent dip angle and direction of the fracture are calculated using the plane’s normal vector. By comparing and analyzing the proposed workflow with the original method, it becomes evident that the method proposed in this paper allows for quick, automated, and accurate extraction of the apparent dip angle and direction of fractures. The application of this workflow to extract fracture attitudes in 3-D micro-CT and full-hole digital core images significantly enhances efficiency.