Fractal Dimension Analysis of Pores in Coal Reservoir and Their Impact on Petrophysical Properties: A Case Study in the Province of Guizhou, SW China

Coal is a complex, porous medium with pore structures of various sizes. Therefore, it is difficult to accurately describe the characteristics of pore structure by using the traditional geometry method. The results from the present investigation suggest that the porous media system of the coal reservoir has obvious fractal characteristics at different scales. To study the complexity of the pores in the coal reservoir, 27 coal samples from Guizhou, SW China were studied. The fractal dimensions of coal pores were calculated, and the fractal dimension of a pore in a coal reservoir can be classified into two types: percolation and diffusion. The comprehensive fractal dimension can be obtained using the weighted summation method and the pore volume fraction of different fractal segments as the weight. The percolation fractal dimensions (D-p) of coal samples are between 2.88 and 3.12, the diffusion fractal dimensions (D-d) are between 3.57 and 3.84, and the comprehensive fractal dimensions (D-t) are between 3.05 and 3.63. The D-d values of all coal samples are all larger than the D-p values, which indicates that the random distribution and complexity of diffusion pores in coal are stronger than those of the percolation pores. The percolation fractal dimension decreases as the maturity degree increases, whereas the diffusion and comprehensive fractal dimensions increase. The diffusion pore volume fraction and total pore volume are all highly correlated with the comprehensive and diffusion fractal dimensions, respectively. The correlation between the comprehensive fractal dimension, diffusion pore volume fraction, and coal reservoir porosity is negative exponential, whereas the correlation between the total pore volume and coal reservoir porosity is positive linear. In comparison with the percolation and diffusion fractal dimensions, the comprehensive fractal dimension is better suited for characterizing the permeability of coal reservoirs. The fractal analysis of this paper is beneficial for understanding the relationship between the fractal characteristics of coal pores and properties.