Micropore Structure and Fractal Characteristics of Low-Permeability Coal Seams

Abstract

With the raw coal from a typical low-permeability coal seam in the coalfield of South Junger Basin in Xinjiang as the research object, this paper examined six kinds of coal samples with different permeabilities using a scanning electron microscope and a low-temperature nitrogen adsorption test that employed a JSM-6460LV high-resolution scanning electron microscope and an ASAP2020 automatic specific surface area micropore analyzer to measure all characteristic micropore structural parameters. According to fractal geometry theory, four fractal dimension calculation models of coal and rock were established, after which the pore structure characteristic parameters were used to calculate the fractal dimensions of the different coal seams. The results show that (1) the low-permeability coal seam in the coalfield of South Junger Basin in Xinjiang belongs to mesoporous medium, with a certain number of large pores and no micropores. The varying adsorption capacities of the different coal seams were positively correlated with pore volume, surface area, and the mesoporous surface area proportions, from which it was concluded that mesopores were the main contributors to pore adsorption in low-permeability coal seams. (2) The raw coal pore fractal dimension had a negative linear relationship to average pore size, a positive linear relationship with total pore volume, total surface area, and adsorption capacity, and a positive correlation with the mesoporous surface area proportion; that is, the higher the fractal dimension, the larger the pore volume and surface area of the raw coal. (3) The permeability of the low-permeability coal seam had a phase correlation with the micropore development degree; that is, the permeability had a phase negative correlation with the pore distribution fractal dimension, and there was a positive correlation between permeability and porosity. These results are of theoretical significance for the clean exploitation of low-permeability coal seam resources.