| The transport of coalbed methane (CBM) through coal is through the cleat network, surrounding matrix blocks. The cleat aperture determines the ease of flow of gas and, hence, the permeability of coal, considered to be one of the most critical factors impacting the overall CBM production. This study was primarily aimed at establishing the permeability trend for Illinois coals.;Methane permeability experiments were conducted using coals from two major seams in the Illinois Basin, Herrin and Seelyville. One permeability experiment with coal from the well established CBM producing San Juan Basin was also carried out because history matching of laboratory trend with field trend is possible in this case. Finally, CO2 permeability for coal from Illinois Basin was conducted to gather valuable scientific data and conduct preliminary prediction of permeability trend for deep Illinois coal.;The permeability trend for the coals tested indicated an increase in permeability below a certain gas pressure for both coals. For coal from the Illinois Basin, a consistent increase in permeability was observed below 450 psi. For coal from the San Juan Basin, an initial decrease in permeability, followed by a subsequent increase below 370 psi, was observed. CO2 permeability for Illinois coal followed a trend similar to methane permeability for San Juan, with an initial decrease, followed by an increase in permeability. The findings of this study, therefore, predict a definite increase in permeability with reservoir depletion for the Illinois Basin.;The measured results were analyzed statistically to verify the difference in methane and CO2 permeability of coal at varying gas pressures. Empirical models were developed to describe methane and CO2 permeability as a function of mean gas pressure. As a final step, numerical simulation using the COMET3 simulator was carried out to study the impact of variation in methane permeability on long-term CBM production. The impact of the increase in permeability on production rate, cumulative production and overall recovery were quantified. The results showed a significant increase in the production rate and cumulative production due to increased permeability. |
