Compaction localization in porous sandstones: spatial evolution of damage and acoustic emission activity

The coupled development of compaction and strain localization may significantly impact the stress field, strain partitioning and fluid transport. To investigate the phenomenon of compaction localization, mechanical deformation and microstructural observations were conducted on the Darley Dale, Rothbach, Berea, Bentheim and Diemelstadt sandstones with porosities ranging from 13 to 24%. The constitutive behavior, failure mode, acoustic emission (AE) activity and spatial distribution of damage were investigated. While our observations reveal a broad spectrum of geometric complexity associated with failure modes, two end-members can be distinguished: shear bands at relatively high angles and arrays of discrete compaction bands subperpendicular to the maximum compression direction, respectively. A hybrid localization mode involving high-angle shear bands and diffuse compaction bands was observed in sandstones with intermediate porosities. The discrete compaction bands in our laboratory samples are qualitatively similar to localization structures observed in the field. Whereas the development of high-angle shear bands is characterized by the continuous accumulation of AE, discrete bands are associated with episodic surges in AE that are characterized by an overall strain hardening trend punctuated by episodic stress drops.