Structural softening in poro-elasto-plastic media

Structural softening is a well-known phenomenon in single-phase materials. If we consider a single-phase material, assume an elastoplastic rheology, and perform a strain-driven loading under pure shear boundary conditions, the evolution of the integrated stress will exhibit a softening beyond its peak. This is true for non-associated flow rules considering an ideal plasticity model with, for example, Mohr-Coulomb or Drucker-Prager yield criteria. The post-peak softening is usually associated with the development of the localized shear zones. However, this phenomenon hasn’t been properly analyzed for the case of porous rocks modeled with the quasi-static Biot’s poroelastic equations.

In this contribution, we present numerical results considering the poro-elasto-plastic rheology with a focus on structural softening. We show that the post-peak structural softening might be significant and exhibit large stress drops. The most important outcome is that even a little fluid overpressure leads to much larger stress drops compared to scenarios without any kind of fluid overpressure. This result may shed some light on the phenomenon of large earthquakes associated with small injection rates in geothermal or CO2 sequestration fields.

 

References:

Vermeer, P. A. (1990). The orientation of shear bands in biaxial tests. Géotechnique, 40(2), 223-236.