Permeability evolution during localized deformation in Bentheim sandstone

Strain localization in porous sandstones may significantly impact the regional fluid flow. Previous laboratory studies that investigated permeability evolution with deformation concentrated primarily on shear localization and distributed cataclastic flow. Here we focus on compaction bands, a localized mode of deformation characterized by compaction and negligible shear. In this study we used Bentheim sandstone with porosity 23%, where discrete compaction bands have been observed to develop subperpendicular to the maximum principal stress. To investigate coupling between strain localization and permeability, we conducted permeability measurements during triaxial loading at confining pressures ranging from 10 to 350 MPa and microstructural observation on failed samples. Two types of failure were identified: shear localization and compaction localization at low and high effective pressures, respectively. For both failure modes the bulk permeability decreased with deformation. A dramatic decrease of more than one order of magnitude was associated with compaction localization, where permeability reduction occurred over a relatively narrow range of axial strain with the onset of shear-enhanced compaction. Motivated by our microstructural observations, we modeled the permeability reduction of the failed sample as that of a layered medium with significant permeability contrast between the discrete bands and matrix. The model reproduced the experimental observations of permeability evolution during development of discrete compaction bands, with implications for the amount of localized strain and the permeability contrast. Permeability evolution during development of discrete and diffuse compaction bands suggests two different trends with strain, providing guidance for extrapolation of laboratory measurements to field settings.