The brittle-ductile transition in porous rock: A review

Abstract

Many of the earliest laboratory studies of the brittle-ductile transition were on porous rocks, with a focus on the evolution of failure mode from brittle faulting to cataclastic flow with increasing pressure. Recent advances in this area are reviewed. Porosity has been demonstrated to exert critical control on the brittle-ductile transition, and its phenomenology has two common attributes. Under low confinement, brittle faulting develops as a dilatant failure mode. Under high confinement, delocalized cataclasis is accompanied by shear-enhanced compaction and strain hardening. Plasticity models such as the cap and critical state models have been developed to describe such constitutive behaviors, and many aspects of the laboratory data on porous rock have been shown to be in basic agreement. Bifurcation analysis can be used in conjunction with a constitutive model to predict the onset of strain localization, which is in qualitative agreement with the laboratory data. However, recent studies have also underscored certain complexities in the inelastic behavior and failure mode. In some porous sandstones, compaction bands would develop as a localized failure mode intermediate between the end members of brittle faulting and cataclastic flow. In limestones (and selected sandstones) under relatively high confinement, cataclastic flow is accompanied first by shear-enhanced compaction which then evolves to dilatancy. Various techniques have been employed to characterize the microstructure and damage, which have elucidated the deformation mechanisms associated with the brittle-ductile transition. These observations have revealed a diversity of micromechanical processes, and fundamental differences were observed especially between sandstone and limestone with regard to inelastic compaction. Micromechanical models that have been formulated to describe these processes include the pore-emanated and sliding wing crack models in the brittle faulting regime, and the Hertzian fracture and cataclastic pore collapse models in the cataclastic flow regime. Numerical techniques based on the discrete element method have also been employed to simulate these processes. Comparison of the model predictions with laboratory and microstructural observations has provided useful insights into the mechanics of brittle-ductile transition in porous rock.