Fault sealing during deformation-band growth in porous sandstone

We present a simple geometric model for the evolution of fluid permeability during the sequential growth of deformation bands in an ideal elastic-brit tle porous granular medium. The model is based on recent mechanical and mic rostuctural results from laboratory experiments on large (10 cm diameter) s andstone samples that reproduce held observation. The model assumes poroela stic compaction of the rock matrix in the prefailure stage, followed either by bulk shear-enhanced dilatancy or compaction in the postfailure stage, d epending on confining pressure, and a constant-porosity shear zone that acc umulates slip by sequentially increasing the number of discrete bands linea rly with the inelastic strain. For large permeability contrasts between mat rix 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 dilata nt slip in the postfailure stage, as observed in recent laboratory tests.