CT scanning and reconstruction of coal samples: principle, method and application

The internal microstructure of coal directly affects the mechanical properties and seepage properties of coal. It is the internal factor that induces coal and gas outburst, water damage, spontaneous combustion and other disasters. It is very important to analyze the microstructure of coal and its interaction with the permeability property. Among many microscopic analysis methods, CT scanning is widely used in the analysis and study of the internal microstructure of coal because of its unique advantages such as nondestructive and visualization. Based on this, this paper analyzes the research principle, method and reconstruction process of CT. The results of CT scanning on the internal structure of coal rock mass and its mechanical and permeability properties are discussed. On this basis, the main application and development direction of CT scanning in coal body property analysis are further discussed. CT scan imaging is related to the density of the material, and the denser the material is, the stronger the X-ray absorption ability, the brighter the image. CT scanning can be divided into high resolution (dm), ultra-high resolution (cm), micro tomography (mm) and nano tomography (nm) according to spatial resolution. However, the resolution of the scanning image is inversely proportional to the sample size. Before CT scan reconstruction, it is necessary to carry out noise reduction, artifact removal and segmentation to reduce the error of reconstruction results. Common processing methods include median filtering, mean filtering, intercepting analysis units, threshold segmentation, etc. Threshold segmentation is the most commonly used method in the process of numerical reconstruction. The threshold determination by means of NMR, mercury injection and XRD is helpful to improve the reconstruction accuracy. The application of CT scanning and the research and analysis of coal mechanics and seepage are mainly divided into two categories: one is the laboratory test analysis of mechanical seepage characteristics of coal, including conventional mechanical experiment, dynamic load experiment, fracturing experiment, seepage experiment, water-rock interaction experiment, liquid nitrogen freeze-thaw, microwave cracking, etc., which mainly analyzes the pore evolution during the experiment; the other is to import the model of coal body reconstruction by CT scan into the numerical analysis software. Then the corresponding simulation of loading, cracking and seepage was carried out. Finally, this paper discusses and looks forward to the problems that still need attention in the characterization of coal pores and minerals by CT scanning. It includes the accurate determination of pore and mineral composition threshold of coal body CT scanning, the determination of the reconstruction characterization unit of coal body CT scanning, and the coordination and determination of sample size and scanning resolution.