Methane diffusion characteristics of Illinois coals

Movement of gas in a coalbed methane (CBM) reservoir starts in the coal matrix with diffusion towards the naturally occurring cleat network surrounding the matrix blocks. This study was aimed at predicting the gas deliverability of CBM reservoirs by evaluating the diffusion behavior of Illinois coals.; Diffusion experiments were conducted using coals from four Illinois seams, Herrin, Seelyville, Springfield, and Danville. The diffusion coefficient, D, was determined, followed by establishing the variation trend in its value with continued production. The results indicated that there was a negative correlation between D and pressure.; A simulation was performed to predict the long-term deliverability of CBM, and its dependence on the value of D. The results showed that, for low permeability wells, diffusion is not critical, but for high permeability coals, gas production during early stages is significantly higher. Since Illinois coals have medium to high permeability, these results favor CBM production.