We present a simple geometric model for the evolution of fluid permeability during the sequential growth of deformation bands in an ideal elastic-brittle porous granular medium. The model is based on recent mechanical and microstuctural results from laboratory experiments on large (10 cm diameter) sandstone samples that reproduce field observation. The model assumes poroelastic compaction of the rock matrix in the prefailure stage, followed either by bulk shear-enhanced dilatancy or compaction in the postfailure stage, depending on confining pressure, and a constant-porosity shear zone that accumulates slip by sequentially increasing the number of discrete bands linearly with the inelastic strain. For large permeability contrasts between matrix and the deformation band, the model quantitatively explains the entire permeability cycle observed in bulk samples, including the apparent paradox of a negative correlation of bulk permeability with porosity during dilatant slip in the postfailure stage, as observed in recent laboratory tests.

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