How to discover ancient stress-levels preserved within fractures using the stress-memory effect of specific stiffness

Stress changes have a large impact on the hydraulic and mechanical properties of fractures and can be caused by varying the fluid pressure in a subsurface reservoir or by tectonic movements. Innovative tools to assess the stress conditions are still of major importance for most subsurface applications. We derived an experimental procedure to reveal stress signals preserved in fractures in the laboratory.

In a set of complex experiments various fractured low-permeability rocks, two sandstones and two crystalline rocks, were cyclically loaded in a MTS tri-axial compression cell. The preconditioned cylindrical samples were split into two halves to generate an artificial tensile fracture and a rigid shear displacement was applied before installing the sample into the apparatus. Two different loading scenarios were applied: “continuous cyclic loading” (CCL) and “progressive cyclic loading” (PCL). During continuous cyclic loading samples were loaded from 2 to 60 MPa in several repeated cycles. In the progressive cyclic loading experiments the hydrostatic confining pressure was increased using a step-wise function (15, 30, 45 and 60 MPa) and was unloaded after every sub-cycle, while the pore pressure was kept constant at a low level. The mechanical fracture closure was monitored continuously during the experiments using axial and circumferential extensometers and the specific fracture stiffness could be calculated at a very high resolution. The fracture permeability was measured continuously using four high-pressure fluid pumps. A 3D surface scanner was used to analyze the fracture surface geometry before and after the experiments to reveal possible changes to the surface topography as well as to quantify changes in aperture and contact-area ratio.

The specific fracture stiffness was shown to be irreversible when a fracture was hydrostatically loaded once. Further, a “stress-memory” effect of fracture stiffness could be shown during progressive loading. It is characterized by a change from non-linear to linear stiffness evolution when a previous stress-level is exceeded. This phenomena can be used to identify previous stress states preserved within fractures. Additionally, this data is important for elasto-plastic contact theories of rough fractures. The impact of progressive loading on fracture permeability evolution showed varying results based on the heterogeneity and mineral composition of each rock type and the resulting fracture geometry.