To overcome the limitation of resolution and dimension of a single imaging method, a combination of X-ray computed micro-tomography (X-ray μ-CT) and focused-ion beam-scanning electron microscopy (FIB-SEM) tomography was employed to reconstruct the multi-scale 3D digital cores. First, macro-pore images were collected by X-ray μ-CT, and meso-pore images and micro-pore images were obtained by means of FIB-SEM at two resolutions, respectively. Second, the grayscale images of CT and SEM were converted into binary images, after image pretreatment mainly including contrast enhancement, sharpening and filtering, etc. Because it tends to develop similar pore structures in the same rock, we picked out the representative pore structures to make templates from meso-pore and micro-pore binary images. Next, each pixel on the macro-pore binary images were divided into i×i pixels according to the resolution ratio i of macro-pore images and meso-pore images, and then all the templates selected from meso-pore binary images were rotated to match the macro-pore binary images. Once the similarity of the search area and the rotated templates reach the preset threshold, the macro-pore images were modified by superposition method. In the same way, every pixel on the modified CT images were refined into j×j pixels again based on the resolution ratio j of meso-pore images and micro-pore images, and all the micro-pore structure templates were applied to update the modified CT images. Finally, the updated CT images were implemented to construct multi-scale 3D digital cores by the superposition method. Furthermore, the maximum ball algorithm was used to extract the pore-network models of digital cores, and the numerical simulations were completed with the Lattice-Boltzmann method (LBM). The results indicate that the constructed digital cores have anisotropy and good connectivity similar to the real rocks, and the porosity, pore-throat parameters and apparent permeability from simulations agree well with the values acquired from experiments. In addition, the proposed approach improves the accuracy and scale of digital core modeling by dealing with 2D images, which greatly reduces the amount of computation and saves time.
Keywords: X-ray μ-CT; FIB-SEM; Macro-pore Images; Meso-pore Images; Micro-pore Images; 3D Digital Cores
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