Permeability dynamic variation under the action of stress in the medium and high rank coal reservoir

The gas permeability of coals is altered as a result of the effective stress, matrix shrinkage and gas slippage which occur during the coalbed methane (CBM) production. In this work, the helium permeability change was primarily investigated on 15 middle and high rank coal cores under the varying effective stresses from 3.5 to 9.0 MPa in laboratory to reveal the control mechanism of effective stress and gas slippage. The results indicate that the coal permeability declines exponentially with the rise of effective stress. The higher rank coal is tighter and has the lower permeability simulated under the formation conditions. In the process of effective stress increases, the permeability loss rate ranges from 64.3% to 94.8% with an average of 79.7% while as the pore and fracture structures of the coals have been destroyed irreversibly, the permeability is difficult to fully recover even though the effective stress rebounds to the initial value. The permeability damage rate is 38.6%-52.5% (averaging 48.5%). The permeability damage/loss anisotropy demonstrates that the face cleat has the highest stress sensitivity with the largest damage rate and the permeability damage rate perpendicular to the bedding plane is minimal in high-rank coal while the middle-rank coals are in contrast. The cleat compressibility is quite similar for a same source coal regarding of the permeability anisotropy and decreases with the increase of effective stress for all the three direction coal cores. Gas slippage effect plays a positive role in improving permeability for all the cores, of which the slippage factor shows an exponentially decreased with the increase of the total permeability, and becomes much more influential for low initial permeability cores. The gas slippage contribution rate is observed remarkably in the direction of perpendicular to the bedding plane (>45%) while that of parallel to the face cleat is minimal. (C) 2015 Elsevier B.V. All rights reserved.