Characterization Of Multiscale Pores And Fissures In Coal Based On CT Scan

The development scale,morphology,structure and pore size distribution of thepores and fissures in the coal determine the gas adsorption capacity and permeability of coal reservoirs,and their size is also one of the most critical reservoir parameters to determine the development effect of coalbed methane(CBM).Therefore,the development characteristics of pores and fissures in coal are particularly concerned during the CBM development.The No.3 coal seam of Wuyang coal mine of Lu'an mining area and Chengzhuang coal mine of Jincheng mining area in the south of Qinshui Basin is taken as the research object.The CT scanning experiments are carried out on different coal cores with a diameter of 2 mm,2.5 cm and 10 cm respectively,and the three-dimensional model of each sample is established based on the FDK theory and the back projection reconstruction method.The pores and fissures in coal cores with different pore sizes larger than 1.24 ?m,16.13 ?m,and 74.44 ?m are identified effectively by using the digital terrain model(DTM)of gray threshold segmentation method.Then,using Avizo image analysis software,SPASS and Origin data analysis software,the distribution characteristics of the pores and fissures in the three-dimensional space are reconstructed and quantitatively analyzed.To verify the accuracy of CT identification of pores and fissures,the results of the scanning electron microscope(SEM)after argon ion polishing and the pore volume distribution measured by MIP are compared with the quantitative results of the CT scanning.The main conclusions are as follows:(1)Comparing and analyzing the maneuverability and accuracy of dividing the pores and fissures in the coal sample CT slice by the method of watershed,OTSU and DTM gray-level threshold segmentation,it is found that the DTM gray-level threshold segmentation method is more suitable.Comparing the distribution pattern and opening degree of fissurees identified by DTM method and SEM,it is found that their results are basically consistent,which proved that the DTM method can accurately identify the pores and fissures in coal samples.(2)Using Avizo software,a visual spatial distribution model of pores and fissures at different scales is established.Comparing the spatial distribution of different samples,it is found that the heterogeneity of the pores and fissures in coal is more obvious,and most of the pores are in a closed state and dispersed in the coal matrix The fissure connectivity is good,which basically intersects with the vertical or large angle of the scanning plane.(3)By comparing the CT scanning results of multi-scale coal cores with diameters of 2 mm,2.5 cm,and 10 cm,it is found that they can identify pores and fissures with pore sizes greater than 1.24 ?m,16.13 ?m,and 74.44 ?m,respectively.The volume of pores and fissures is quantitatively characterized based on the visual digital model.Compared with MIP results,it is found that the porosity of CT varies greatly among different samples.For samples with diameters of 2 mm and 25 mm,CT scanning can accurately and quantitatively characterize fissures with diameters greater than 15 ?m and 40 ?m respectively.(4)The development difference of pores and fissures in different samples is compared and analyzed,and the experimental results of permeability test prove that the development scale of the connectivity fissures is the key factor to determine the permeability of coal sample.