Failure mode, strain localization and permeability evolution in porous sedimentary rocks

Laboratory study was undertaken to investigate failure mode, strain localization and permeability evolution in the brittle-ductile transition of porous sandstone and limestone. Conventional triaxial compression experiments and permeability measurements were conducted at high pressure and room temperature, and microstructural observations were performed on failed samples using optical microscopy.; Strain localization was investigated on Bentheim sandstone where besides shear localization, a new failure mode represented by discrete compaction bands was recently reported. To study the effect of stress heterogeneity on initiation and propagation of the compaction bands, triaxial experiments were conducted on dry cylindrical samples with a circumferential notch as a stress concentrator. Acoustic emission data were recorded to underscore the temporal aspect. Mechanical and microstructural data revealed sensitivity of initiation of compaction bands to stress concentration at the notch. The band propagated by sequential increments as “anti-crack” in direction perpendicular to the maximum principal stress. With increasing axial strain an array formed of parallel compaction bands. The effect of strain localization on hydraulic permeability was studied on cylindrical samples in triaxial compression for fluid flow parallel to maximum principal stress. The permeability decreased with deformation for both failure modes, shear and compaction bands. A dramatic decrease of more than one order of magnitude occurred over a relatively narrow range of axial strain when the first few compaction bands developed. Motivated by microstructural observations, the failed sample was modeled as a layered medium with permeability contrast between the compaction bands and the relatively undeformed matrix.; Unlike in sandstone, the failure in the brittle-ductile transition in limestone is affected by crystal plasticity of calcite. The interplay of crystal plastic and cataclastic mechanisms was investigated on dry samples of Indiana and Tavel limestones. Despite the difference in deformation mechanisms, failure modes in the limestones were similar to that of porous sandstones reported in literature: dilatant brittle faulting and compactioe ductile flow at low and high confining pressures, respectively. The observations were interpreted by two micromechanical models. Our data combined with published data characterize pore collapse in carbonate rocks with porosities between 3% and 45%.