Three-dimensional modeling and analysis of macro-pore structure of coal using combined X-ray CT imaging and fractal theory

The porous structure of coal directly determines its gas transport property. The fluid flow behavior of coal is one of the key science questions that will influence the coal energy industry. In this study, the influence of real coal macropore structure on the fluid flow through coal was studied through 3-D coal structure reconstruction by the CT images. Based on the reconstructed coal structure, the micron-scale structure parameters were quantitatively analyzed. A newly programmed Matlab code was established to find the volume fractal dimension, obtain the relationship between porosity/permeability of coal and volume fractal dimension, and estimate the tortuosity fractal dimension by using the 3-D box dimension algorithm. The results show that the volume fractal dimensions of 6 coal samples range from 2.25 to 2.79 and the tortuosity fractal dimensions of capillaries range from 2.15 to 2.73. The 3-D coal structure cannot only quantitatively estimate the real porosity of the coal, but it can be used to characterize the complexity of coal's porous structure through mean deviation of surface porosity. It can be clearly seen from the reconstructed coal that coal specimen-C3 is highly heterogeneous because it has complex pore structure as well as wider pore size distribution and the highest mean deviation of the surface porosity. The volume fractal dimension can be used to quantitatively define the complexity of pores. The larger the porosity of coal, the greater the fractal dimension. The permeability and porosity of coal are negatively correlated with the volume fractal dimension. The tortuosity fractal dimension can effectively characterize coal permeability, but it weakly correlates with coal porosity. The outcome of this study helps to understand the structure-based flow characterization and gas transport behavior in heterogenous coal which will have the implication of the gas extraction from coalbed methane reservoirs and coal mine gas drainage.