Evaluation of the non-Darcy effect in coalbed methane production

The non-Darcy factor, an indicator for the non-Darcy effect, is dependent on the properties of porous media and pore fluid including permeability, viscosity, density, flow velocity and a coefficient named as beta factor. Experimental results show that the beta factor can be expressed as a power law of permeability. For conventional gas reservoirs, this beta factor can be assumed as a constant as the permeability change is negligible. However, the constant beta factor may not be suitable for coal seams with remarkable permeability change and a variable beta factor as a function of coal permeability should be an alternative. Moreover, the coal permeability change is complex due to the competing effects of coal cleat compression and sorption induced coal shrinkage/swelling. Few studies have been done previously to incorporate the variable b factor as a function of coal permeability in reservoir simulations. In the present work, both the coal permeability change and the variable beta factor are coupled in a dual porosity model to study the non-Darcy flow behavior in coal seams. The simulation results illustrate that the evolution of non-Darcy factor becomes tortuous by using a variable beta factor, which differs from the monotonic behavior when constant b factors are applied. Furthermore, increasing the coal cleat compressibility and matrix shrinkage strain tends to intensify the tortuous behavior. The simulation results also indicate that using typical constant b factors, instead of the variable one, may significantly underestimate or overestimate the gas production rate for coalbed methane wells. (C) 2013 Elsevier Ltd. All rights reserved.