Theory of Compaction Bands in Porous Rock

Compaction bands are narrow planar zones of localized purely compressive (without shear) deformation that form perpendicular to the most compressive principal stress. Such bands have been observed in high porosity rocks in the laboratory and in the field. Because compaction presumably decreases permeability, these bands can act as barriers to flow within reservoirs. Reexamination of the results of Rudnicki and Rice (J. Mech. Phys. Solids, 1975) for shear localization, with corrections by Perrin and Leblond (J. Appl. Mech., 1993), reveals that they admit solutions for compaction bands in a range of parameters that is representative of porous rock. Solutions for compaction bands are possible when the inelastic volume deformation is compactive and is associated with a “cap” on the yield surface. The expression for the critical hardening modulus (related to the slope of the shear stress vs. shear strain curve at constant mean stress) at which compaction bands are predicted to form differs from that for shear localization. For parameters representative of porous rock, axisymmetric compression is the most favorable deviatoric stress state for formation of compaction bands. Comparison of conditions for shear localization and compaction band formation suggests that either may occur depending on the stress path and magnitude of the confining stress.