Development of the permeability anisotropy of submarine sedimentary rocks under true triaxial stresses

The true triaxial test, which can simulate in-situ stress fields of nature which have different scalar quantity along three principal axes of stress, is the most effective experimental method for obtaining the mechanical and permeability property of rocks under overburden and tectonic stresses. The intermediate principal stress (σ2) is a control on the fracture strength, mechanical behavior and crack distribution of the deformed rocks. Thus, understanding the effect of the stress on the development of permeability anisotropy is crucial. To evaluate the influence of the σ2 on the permeability anisotropy, the permeability measurement lines in the σ1 and σ2 directions were added into an upgraded Mogi-type true triaxial test apparatus. We measured the permeability of Berea sandstone and Amatsu mudstone in the σ1 and σ2 direction under the conditions of true triaxial compression (σ3 < σ2 < σ1) with a constant σ3. In the Berea sandstone, we found that the permeability reduced with increasing mean stress and the magnitude of permeability anisotropy correlated with the intermediate principal stress. Photomicrographs and X-ray computed tomography observations for post-failure Berea sandstones revealed a phenomenon related to permeability: under high σ2 conditions, the fault gouge generated along final fractures causes permeability reduction of σ1 direction. For the Amatsu mudstone is too soft to be subject to brittle deformation. Thus, the development of the permeability anisotropy with increasing mean stresses was more pronounced in the Berea sandstone than the Amatsu mudstone.