Establishment Of Coal Micro-pore Structure And Simulation On The Seepage Rules Of Low-pressure Water Based On Micro CT Technology

Coal seam is a porous medium containing a large amount of pores which form various structures through different sizes, shapes and arrangement patterns. The micro-pore structure influences many physical properties of coal, including its porosity, permeability, density,water absorption, softening and weakening performance of water injection. Therefore, the structure determines, to a great extent, the occurrence and flow characteristics of gas and liquid in coal seam. The development of high resolution micro-CT imaging and visual software technology provide the possibility for us to reveal the invisible physical and mechanical processes or mechanisms of coal micro-pore structure and seepage characteristics. Along with the deepening of research,revealing the pore characteristics of coal rock and internal fluid flow characteristics from the microscopic pore structure level has become a research hotspot in the field of rock mechanics and fluid dynamics, etc.In view of this, this paper, combines the methods of theoretical analysis,experimental test and numerical simulation, makes a deep research on the realization process of coal core, the extraction of coal micro-pore model based on filtering and gray level segmentation, and seepage characteristics of coal from micro level, thus gradually launched the research of "establishment of coal micro-pore structure and simulation on the seepage rules of low-pressure water based on micro CT technology". The extension of this direction provides a new train of thought and ways for the seepage law of coal micro-pore structure from micro-level.Firstly, the basic concepts of micro-CT technology and the experimental equipment of nano Voxel-2000 series X-ray 3D microscope used in this study are introduced, meanwhile, the imaging principle,reconstruction principle and components of imaging system of X-ray3D microscope are described in detail, which lays the theoretic foundation for the following micro-CT scanning experiment. Then four kinds of coal samples were scanned by nanoVoxel-2000 series X-ray 3D microscope, and a series of 2D slice and 3D gray distribution maps were obtained for each coal sample, the distribution of each component of coal can be clearly observed from the scanning image. The scanned data were saved as RAW format, which laid the foundation for the establishment of micro-pore model. Then, the median filter was finallychosen to denoise the original image by comparing three different filtering algorithms, the transition between the pore and rock matrix becomes natural and the boundary becomes smooth after filtering, at the same time, it also preserves the important feature information of the image as possible, which lays the foundation for the following image segmentation of pore and matrix. Based on the method of porosity image segmentation, the best segmentation threshold is calculated combined with the actual porosity, the images after filtering processing are then carried on binarization processing. The cubic element with a side length of 120 was selected to establish the micro-pore model and conduct numerical simulation on pore flow characteristics by the analysis of representative elementary volume (REV) based on porosity. Avizo software is used to realize the separation of pore and framework of the coal micro structure model, and is saved as a generic CAD format,which provides a pore structure geometric model based on real size and can represent the physical properties of coal samples for the construction of finite element model of coal micro-pore structure. Finally, based on the ICEM CFD software, the unstructured mesh are constructed and boundary condition are set up for the micro-pore model of CAD format,in which k-epsilon standard turbulence model was adopted and the influence of gravity was considered, then the numerical simulation on low-pressure water seepage of coal micro-pore structure were performed in unidirectional and multidirectional. The variation law of pressure,velocity, mass flow rate and other critical parameters of seepage field during the coal pore structure at the view of microscale is obtained,which makes a certain contribution to the perfection of related theory of coal microcosmic pore seepage characteristics.