Permeability Evolution During Shear Zone Initiation in Low-Porosity Rocks

Christian Kluge (1) , Guido Blöcher (1) , Auke Barnhoorn (2) , Jean Schmittbuhl (3) , David Bruhn (1)

1 GFZ - German Research Centre for Geosciences - Helmholtz-Centre Potsdam
2 Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, P.O. Box 5048, 2600 GA Delft, the Netherlands
3 ITES - Institut Terre Environnement Strasbourg

Abstract Using an innovative experimental set-up (Punch-Through Shear test), we initiated a shear zone (microfault) in Flechtingen sandstone and Odenwald granite under in situ reservoir conditions while monitoring permeability and fracture dilation evolution. The shear zone, which has a cylindrical geometry, is produced by a self-designed piston assembly that punches down the inner part of the sample. Permeability and fracture dilation were measured for the entire duration of the experiment. After the shear zone generation, the imposed shear displacement was increased to 1.2 mm and pore pressure changes of $$\pm 5$$ ± 5 or $$\pm 10$$ ± 10 MPa were applied cyclically to simulate injection and production scenarios. Thin sections and image analysis tools were used to identify microstructural features of the shear zone. The geometry of the shear zone is shown to follow a self-affine scaling invariance, similar to the fracture surface roughness. The permeability evolution related to the onset of the fracture zone is different for both rocks: almost no enhancement for the Flechtingen sandstone and an increase of more than 2 orders of magnitude for the Odenwald granite. Further shear displacement resulted in a slight increase in permeability. A fault compaction is observed after shear relaxation which is associated to a permeability decrease by a factor more than 3. Permeability changes during pressure cycling are reversible when varying the effective pressure. The difference in permeability enhancement between the sandstone and the granite is related to the larger width of the shear zones.

Domaines: Géophysique [physics.geo-ph]; Géophysique [physics.geo-ph]

Christian Kluge, Guido Blöcher, Auke Barnhoorn, Jean Schmittbuhl, David Bruhn. Permeability Evolution During Shear Zone Initiation in Low-Porosity Rocks. Rock Mechanics and Rock Engineering, 2021, 54 (10), pp.5221-5244. ⟨10.1007/s00603-020-02356-0⟩. ⟨hal-03436279⟩