The Stress‐Memory Effect of Fracture Stiffness During Cyclic Loading in Low‐Permeability Sandstone
Résumé
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 60MPa hydrostatic
confining pressure. During “constant cyclic loading” (CCL) experiments, the fracture was repeatedly
loaded to the same peak stress (up to 60MPa). During “progressive cyclic loading” (PCL) experiments, the
confining pressure was progressively increased in each cycle (up to 15, 30, 45, and 60MPa). 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
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