MOE Engineering Center of Mine Disaster Prevention and Rescue, Henan Polytechnic University
Collaborative InnovationCenter of Coal Work Safety and Clean High Efficiency Utilization, Henan Polytechnic University
Collaborative Innovation Centerof Coalbed Methane and Shale Gas for Central Plains Economic Region (Henan Province), Henan Polytechnic University
Henan Provincial Key Lab of Gas Geology and Control-Cultivation Base of Provincial and Ministry Joint State Key, Henan Polytechnic University

As one of the hot issues at the frontiers of science in the world, the multi-scale scientific question has occurredin the fields of natural science and engineering. The seepage in coal-rock, a branch of the multi-scale science, shows itsmulti-scale scientific question. Coal is a porous medium that contains multi-scale pores with the aperture from millimeterto nanometer. The pore size differential can reach one million orders of magnitude, which causes the multi-scale characteristicsin space and time for coal permeability. Therefore, the research on the multi-scale permeability of coal is a criticalscientific issue of the coal gas flow as well as an engineering extension of methane drainage. The unsteady diffusion-seepageexperiment is conducted for CH4/He with and without stress using a cylindrical coal sample, accompanied by steadystate seepage experiment. The experimental results show that the apparent diffusion coefficient of a cylindrical coal sampleattenuates with time. This apparent diffusion coefficient shows two different multi-scale characteristics in time, the smoothand dynamic attenuation and the dynamic attenuation in a two-stage step. A dynamic model for the apparent diffusioncoefficient is proposed, and it can accurately describe the complete unsteady flow process of gas in a cylindrical coalsample. The physical and mathematical models of the multi-scale pores in series are put forward. Then, the multi-scalestructure of pore in series is validated by the mercury injection experiment. After that, the multi-scale permeability modelis mathematically proved. Based on the Knudsen number (Kn), the continuous flow, slip flow, transition flow and free molecularflow are identified and introduced with the multi-scale pore size to build a multi-scale permeability model that reflectsthe effect of the effective stress and gas flow regime. The mechanism of the multi-scale seepage is revealed in thisstudy. The size and the number of pores in series connection are the critical factors to influence the multi-scale permeability.The multi-scale effect can reach tens of thousands orders of magnitude within measurable range. The gas outflowfirstly starts from the outside fractures, and then the inside small pores and finally the nano pores. With time goes on, thegradual increase in the number of pores in series connection leads to the gradual decrease in the equivalent pore size,which causes the equivalent pore aperture to get close to the minimum pore aperture. Therefore, the equivalent permeabilityquickly decreases with time, which is a reflection of the multi-scale space in coal. During the later stage of gas flow, theeffect of slip and transition flow regime is larger than that of effective stress with Kn increasing and dominates the permeability.The new experimental observation and modelling of the multi-scale permeability provides an experimental solutionfor the research of the multi-scale seepage and overcomes the shortcoming of single tube theory. The diffusion andseepage are apparently unified, and the micro-level distinguishment and macro-level union of the multi-scale permeabilityare realized.

国家自然科学基金资助项目(52174173); 河南理工大学博士基金资助项目(B2018b258, B2021b27)

李志强，陈金生，李林，等. 煤层瓦斯微纳米串联多尺度动态扩散渗透率实验−模型−机理及意义[J]. 煤炭学报，2023，48(4)：1551−1566

LI Zhiqiang，CHEN Jinsheng，LI Lin，et al. Experiment, modelling, mechanism and significance of multiscale and dynamic diffusion-permeability of gas through micro-nano series pores in coal[J]. Journal of China Coal Society，2023，48(4)：1551−1566