Permeability evolution in fractured coal - Combining triaxial confinement with X-ray computed tomography, acoustic emission and ultrasonic techniques

Cyclic loading of coals impacts permeability due to reversible deformation and irreversible damage and extension to pre-existing fracture networks. These changes in permeability influence the effectiveness of degassing of coal prior to mining, the recovery of coalbed methane by both conventional and enhanced methods and potential for sequestration of CO2. We explore these interactions of stress and damage that contribute to changes in permeability through imaging with X-ray computed tomography (X-ray CT), acoustic emission (AE) profiling together with the concurrent measurement of P-wave velocities. We use these techniques to examine the evolution of the 3D fracture network during stressing through failure. A total of five semi-anthracite/anthracite coal cores (~ 40 mm in diameter and 80 mm in length) are sequentially loaded to failure (~ 37.53 MPa) with concurrent measurements of permeability. Intermittent scanning by X-ray CT, AE profiling and measurement of the evolving P-wave velocity effectively determine changes in the 3D fracture network with applied stress. These results are correlated with the “V-shaped” variation of permeability with increasing axial stress under the imposed triaxial stress conditions. This is consistent with observations on hard rocks where increasing stresses initially close fractures before fractures ultimately dilate, propagate and coalesce as the peak strength is reached. The increase in fracture volume is non uniform within the sample and is largest at the end platens. The permeability evolution was similarly dynamic with coal permeability reduced by one to two orders of magnitude in some cores (0.18-0.004 mD) until increasing dramatically as failure is approached (14.07-37.53 MPa).