Effects of confining pressure on the permeability of three rock types under compression

Triaxial tests were conducted on three different rocks, under confining pressures of 1-15 MPa at 295 K using an ultra-compact triaxial cell. The samples were held for 24 h under the target consolidation pressure, and then constant strain-rate compression was applied measuring permeability. The structural changes were measured by thin section image analysis and micro-focus X-ray computed tomography. For the Shikotsu welded tuff, the permeability decreased monotonously with axial compression, mainly due to pore collapse. For the Kimachi sandstone, the permeability first decreased with increasing axial stress, then began to increase when the total lateral strain recovered its value before the application of confining pressure, and then maintained an almost constant value in the post-peak region. The minimum and final permeability decreased with increasing confining pressure mainly due to compaction and large plastic deformation of clay cementing materials. For the lnada granite, the tendency of permeability change during axial compression was almost the same as for the Kimachi sandstone. The final permeability was larger than that before compression, and the permeability decreased with smaller confining pressure mainly due to a decrease in the number and width of rupture planes and the absence of axial cracks from biotite. The permeability, however, increased under larger confining pressures mainly due to the formation of subrupture planes caused by the high stress concentration at the rough and stiff rupture plane under large confining pressure. (C) 2013 Elsevier Ltd. All rights reserved.