The Stress‐Memory Effect of Fracture Stiffness During Cyclic Loading in Low‐Permeability Sandstone

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

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

The hydraulic performance and mechanical stability of open fractures are crucial for several subsurface applications including fractured geothermal reservoirs or nuclear waste repositories. Their hydraulic and mechanical properties (fluid flow and fracture stiffness) are both strongly dependent on the fracture geometry. Any change in effective stress impacts aperture and thus the ability of fractures to promote flow. Here, we carried out flow experiments with shear displaced tensile fractures in pre-loaded, low-permeability sandstones with two different cyclic loading scenarios with up to 60MPa hydrostatic confining pressure. During “constant cyclic loading” (CCL) experiments, the fracture was repeatedly loaded to the same peak stress (up to 60MPa). During “progressive cyclic loading” (PCL) experiments, the confining pressure was progressively increased in each cycle (up to 15, 30, 45, and 60MPa). The matrix and fracture deformation was monitored using axial and circumferential LVDT extensometers to obtain the fracture stiffness. The fracture geometry before and after the experiment was compared by calculating the aperture distribution from 3D surface scans. Initial loading with confining pressure of the fracture leads to a linear fracture specific stiffness evolution. For any subsequent stress cycles fracture stiffness shifts to a nonlinear behavior. The transition is shown to be related to a stress memory effect, similar to the “Kaiser Effect” for acoustic emissions. Progressive loading of fractures possibly leads to less permeability reduction compared to continuous cyclic loading

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

Christian Kluge, Guido Blöcher, Hannes Hofmann, Auke Barnhoorn, Jean Schmittbuhl, et al.. The Stress‐Memory Effect of Fracture Stiffness During Cyclic Loading in Low‐Permeability Sandstone. Journal of Geophysical Research : Solid Earth, 2021, 126 (10), ⟨10.1029/2020JB021469⟩. ⟨hal-03436321⟩