Experimental study on gas diffusion dynamics in fractured coal: A better understanding of gas migration in in-situ coal seam

In-situ coal seam is generally under the stress constraint condition. However, the powder coal is often adopted to study gas diffusion dynamics in the laboratory, in this case, the stress cannot be imposed on the coal sample. So the question is, can the laboratory test results with the powder coal reflect the gas diffusion behaviors in the in-situ coal seam? Does the confining stress affect the gas diffusion behaviors in fractured coal? To address these questions, in this work, we first investigated the effect of coal size on gas diffusion dynamics with powder coal and lump coal under unconstrained conditions. The results show that there exists an obvious scale effect for gas diffusion in coal, and a critical value of coal size has been found for the scale effect. When the coal particle size is smaller than the critical value, the effective diffusivity decreases with an increase of the particle size; and when the particle size is larger than the critical value, no obvious change can be found in the effective diffusivity. The critical value for gas diffusion corresponds to the size of the coal matrix. The essential reason for the existence of the scale effect is the differences among the pore structures of coals with various sizes. Based on the research results under the unconstrained conditions, a coal core was selected to study the effect of confining stress and pore pressure on gas diffusion under constraint condition. The results indicated that the confining stress and pore pressure have significant impact on gas diffusion in fractured coal. With an increase of the confining stress and a decrease of the pore pressure, the effective diffusivity reduces gradually. Therefore, to get an accurate understanding of the gas diffusion behavior in in-situ coal seam, during the test in the lab, both the scale effect and confining stress should be considered. The research results obtained in this work have important guiding significance to reveal gas migration in in-situ coal seams during CBM depletion, CO2-ECBM and geological sequestration of CO2.