Microstructure Characteristics of Tectonic Coal and Primary Coal: A Case Study of Guizhou, China

The pore structure of coal has a great influence on the exploitation of coalbed methane. The primary coal and tectonic coal of the Faer Coal Mine and Wenjiaba Coal Mine in Guizhou, China, were selected for low-temperature liquid nitrogen and high-pressure mercury injection experiments. Based on the Frenkel-Halsey-Hill fractal model and multifractal model, combined with nano-CT system, the difference between primary structural coal and tectonic coal and their relationships with fractal dimension were analyzed. According to the results, coal can increase its pore volume and surface area by tectonic action. From the perspective of pore volume and surface area, micropores occupy the main part. According to the pore volume and pore surface area of micropores, that of primary structural coal was lower than that of tectonic coal. For the studied coal samples, their pores have fractal characteristics and complex structure. It can be seen from the fractional dimension that that of tectonic coal was higher than that of primary structure coal, indicating that the microscopic pore structure of coal tends to be complex after tectonic action. According to the results of multifractal and nano-CT analysis, the Hausdorff dimension of tectonic coal was slightly smaller than that of primary structure coal, but the multifractal parameter value was higher than that of primary structure coal. After tectonic action, the connected pores of coal increased by 142.86% compared with the connected pores of primary coal, and the adsorption capacity was further improved. It is further proved that the structure can affect the adsorption and desorption capacity by changing the distribution of pore structure.