The influence of coal pore structure on gas desorption-diffusion-seepage process

Fully understanding the mechanism of coal seam gas migration is the fundamental prerequisite for improving extraction efficiency. At present, research on the micro migration features of coal gas mostly focuses on the micro pore gas migration features of coal, ignoring the gas desorption-diffusion process. Taking coking coal as an example, the pore space structure of coal is accurately reconstructed and quantitatively characterized using mercury intrusion testing, nanoscale industrial CT scanning, and numerical simulation. The evolution process of gas desorption-diffusion-seepage is analyzed from a microscopic perspective, and the influence of coal pore space structure on gas migration is preliminarily explored. The results show the following points. ① The gas pressure is relatively high at the center of the pore, and desorption-diffusion proceeds from the center of the pore to the edge. The distribution of gas pressure varies significantly at different times and positions. The reason for the difference in gas pressure distribution is that the radius, length, shape, and connectivity of pores and throats in each representative elementary volume (REV) unit are different. ② The pore structure and topological advantages expand the range of gas desorption-diffusion-seepage. The large-sized pore structure can provide diversified movement space for gas molecules, weaken the influence of size effect on diffusion breadth, and promote the rate of gas desorption-diffusion. ③ In the strongly heterogeneous connected pore structure, gas seepage is dispersed and efficient, and the transformation of gas from diffusion to seepage can be achieved through extensive communication with the coal matrix, improving the efficiency of gas mass transfer. In weakly heterogeneous connected pore structures, the gas seepage path is single, the seepage lines are concentrated, the mass transfer resistance of the seepage is high, and the transformation efficiency of gas molecules from diffusion to seepage is low. It is not conducive to efficient gas migration. The research results enrich the theory of coal gas migration from a microscopic perspective and provide a theoretical basis for gas extraction engineering practice.