Résumé
We propose a model of the permeability behavior of clayrocks based on a review of plastic deformation observed in the field and permeability measurements in reactivation tests of fractures and faults. Firstly, we classify structures observed in the field according to their deformation mode and propose a model of clayrock deformation relative to usual mechanical concepts described in the literature. Secondly, we evidence controls on clayrock fracture/fault permeability by the stress conditions of the reactivated structures, the amount of shear displacement along them, and the mechanical properties of the clayrock. Finally, we formulate our model based on similarities in the concepts of clayrock deformation and the experimental evolution of clayrock fracture/fault permeability. This model links quantitative data on permeability evolution to deformation mechanisms, defining the permeability behavior of clayrocks, which was lacking in the literature until now. We highlight the dependence of this behavior on the stress and shear conditions, and demonstrate that the ratio of effective normal stress (σn′) to the Unconfined Compressive Strength (UCS) of the clayrock can be used to describe this behavior. This model can be used in the assessment of clayrock formation sealing capacity in the context of geological storage. Particularly, the use of the UCS has the advantage of integrating important geological parameters in this assessment, such as the clay content or consolidation history. According to this model, we recommend to preserve a σn′/UCS level larger than 0.5 in order to avoid a significant permeability increasing behavior associated with reactivation.
•Models of rock mechanics well describe clayrock deformations as observed in the field•Permeability increases the most in response to deformation under the lowest stress•Brittle, dilatant behavior is expected to cause permeability increase•σn′/UCS may be used as an index of the deformation-permeability change coupling•We recommend avoiding σn′/UCS < 0.5, allowing permeability increase of several orders