Onset Of Shear-enhanced Compaction Research Articles

Compaction bands and induced permeability reduction in Tuffeau de Maastricht calcarenite

The mechanical behavior and permeability of the Tuffeau de Maastricht calcarenite were studied. Compactions bands were found to form in the “transitional” regime between brittle faulting and cataclastic flow. In order to predict the formation of compaction bands, bifurcation analysis was applied on a model developed by Lade and Kim. The numerical results proved to be in good agreement with the experimental ones where the localization point was identified to be the onset of shear-enhanced compaction (a threshold in differential stress after which significant reduction of porosity is induced). Before the onset of shear-enhanced compaction, permeability was primarily controlled by the effective mean stress, independent of the deviatoric stresses. With the onset of shear-enhanced compaction, however, coupling of the deviatoric and hydrostatic stresses induced considerable permeability and porosity reduction.

Effects of stress on the anisotropic development of permeability during mechanical compaction of porous sandstones

AbstractTo investigate the influence of stress on permeability anisotropy during mechanical compaction, a series of triaxial compression experiments with a new loading configuration called hybrid compression were conducted on three porous sandstones. The effective mean and differential stresses in hybrid compression tests were identical to those in conventional triaxial extension tests. Permeability was measured along the axial direction in both hybrid compression and conventional extension tests, which corresponds to flow along the maximum principal stress direction in the former case and the minimum principal stress direction in the latter case. Since their loading paths coincide, the comparison of permeability values from the two types of tests provides quantitative estimates of the development of permeability anisotropy as a function of effective mean and differential stresses. Our data show that the permeability evolution is primarily controlled by stress. Before the onset of shear-enhanced compaction C*, permeability and porosity reduction are solely controlled by the effective mean stress, with negligible stress-induced anisotropy. With the onset of shear-enhanced compaction and initiation of cataclastic flow, the deviatoric stress induces enhanced permeability and porosity reduction. The permeability tensor may show significant anisotropy. Our data indicate that the maximum principal component of permeability tensor k1 is parallel to the maximum principal stress σ1, and the minimum principal component k3 is parallel to the minimum principal stress σ3. During the initiation and development of shear-enhanced compaction, k1 can exceed k3 by as much as two orders of magnitude. With the progressive development of cataclastic flow, changes of permeability and porosity become gradual again, and the stress-induced permeability anisotropy diminishes as k1 and k3 gradually converge. Our data imply that permeability can be highly anisotropic in tectonic settings undergoing cataclastic flow, inducing the fluid to flow preferentially along conduits subparallel to the maximum compression direction. However, this development of permeability anisotropy is transient in nature, becoming negligible with an accumulation of strain of about 10%. The anisotropic development of permeability in a lithified rock is dominantly controlled by microcracking and pore collapse. This is fundamentally different from the mechanisms active in unconsolidated materials such as sediments and fault gouges, in which the permeability evolution is primarily controlled by the development of fabric and shear localization via the accumulation of shear strain.

Shear-enhanced compaction and permeability reduction: Triaxial extension tests on porous sandstone

Triaxial extension experiments were conducted to investigate the influence of radial stress on porosity and permeability (for hydraulic flow along the axial direction) in three porous sandstones. The effective mean stresses were sufficiently high that the samples failed by cataclastic flow, with development of strain hardening and shear-enhanced compaction. Comparison of the new data with triaxial compression data from a previous study shows that the critical stress states for the onset of shear-enhanced compaction are comparable for the two different loading paths. The initial yield stress data for each sandstone map out an approximately elliptic envelope in the stress space. Stress-induced permeability anisotropy was inferred from synthesis of the triaxial compression and extension data. Before the onset of shear-enhanced compaction, permeability and porosity reduction are primarily controlled by the effective mean stress and stress-induced anisotropy is negligible. With the onset of shear-enhanced compaction and development of cataclastic flow, coupling of the deviatoric and hydrostatic stresses induces considerable permeability and porosity reduction. The permeability for flow along the direction of the maximum (compressive) principal stress is greater than that along the minimum principal stress. Microstructural observations on the shear-compacted samples show appreciable increase of grain crushing and pore collapse, which explain the overall decrease in permeability. The damage from grain crushing is highly anisotropic, with the stress-induced microcracks preferentially aligned with the maximum principal stress direction. Because more microcrack conduits are available to focus the flow in this direction, the permeability is relatively enhanced.