Direct Measurement of the Effective Diffusion Coefficient of Coal

The effective diffusion coefficient of coal is critical in coal mining because it determines the magnitude and likelihood of gas-related dynamic failures such as outburst and, in some cases, coal burst. To assess the risk of failures using previously developed models, an accurate estimation of the diffusion coefficient is required. The significance of the diffusion coefficient was also highlighted in project C26066 where a mathematical model was developed for estimating the ejection velocities in the event of coal burst. It was shown that a significant source of uncertainty lies in the value assigned to the diffusion coefficient of coal. Project C26066 recommended that further research be conducted to better comprehend the apparent extreme variability of this coefficient in coal.

This report presents a methodology to calculate the bulk and local diffusion coefficients of coal. First, the counter diffusion method using helium and methane is used to determine the bulk diffusion for 20 coal samples at different time steps up to approximately 300 hours and under different stress conditions. Second, microcomputed tomography (micro-CT) X-ray imaging is used to determine the variations in the local diffusion coefficient for selected coal samples at different time steps. The obtained three-dimensional representation of the coal sample at each diffusion step allows the visualisation of the diffusion process as the gas moves from the coal matrix to coal fractures. The local diffusion is calculated for regions of interest within the sample, and a range of diffusion coefficients is calculated for each sample, indicating the heterogeneity of coal properties. The measured diffusion coefficients from Australian coal mines and the obtained values can be used for future modelling to estimate the risk of outburst and ultimately make mining operations safer.