Modeling the onset of bifurcation in porous sedimentary rocks

The mechanical response of porous sedimentary rocks is the outcome of a complex interaction between their granular skeleton and the cement matrix originated by lithification. Crustal movements and human activities alter this original microstructure, modify the hydro-mechanical properties and promote the development of localized deformation mechanisms. This paper focuses on the study of mechanisms of localized compaction. For this purpose, an elastoplastic constitutive model for porous sedimentary rocks is combined with a mathematical theory for predicting the initiation of bifurcation processes in elastoplastic continua. The attention is focused on the role played by the mechanical properties of the rock on failure and bifurcation. After discussing the mechanical response observed in typical sedimentary rocks, the paper illustrates the interplay between the material properties, the static-kinematic conditions associated with failure mechanisms and the bifurcation of the predicted response. It is shown that the potential for compactive bifurcation at high confinement evolves during loading and requires prior definition of the parameters related with the potential for pore collapse. The paper provides a modeling strategy to address technical problems involving porous rocks and identifies a set of material properties that can have a major effect on the geomechanical characterization of porous sedimentary formations.