Gas diffusion behavior of coal and its impact on production from coalbed methane reservoirs

This paper discusses the results of an experimental study carried out to study and evaluate the sorption and diffusion properties of methane in Illinois basin coals. As a first step, sorption results were modeled using the Langmuir isotherm model and the Langmuir Constants were estimated. Next, the diffusion coefficient, D, was estimated by modeling experimental data using the unipore diffusion theory and Fick's law of diffusion. The results clearly indicated a negative correlation between D and pressure for pressures below 3.5 MPa. The overall trend of the variation was found to be bi-modal, its value remaining constant at high pressures, followed by a sharp increase below this critical pressure. Finally, a comparison of the sorption and diffusion results revealed that D depended on the surface coverage, which exhibits a positive relationship with pressure. The trend of diffusion variation with pressure appeared to be in good agreement with the results reported in the past studies using the bi-disperse diffusion model. The practical implication of the results is that the movement of methane is eased significantly at low pressures. Given the low in situ pressure typically encountered in the Illinois basin, this is a positive finding. Finally, this behavior may be responsible, at least in part, for the increased gas production rates encountered in the San Juan basin after several years of production.

Research highlights
► Diffusion of methane in coal is eased significantly at low pressures.
► Diffusion is bi-modal, constant at high pressures, increasing at low pressures.
► Diffusion coefficient depends on surface coverage.
► Unipore model and bi-disperse models give very similar results.