An experimentally deformed sample of Opalinus Clay (OPA), which is being considered as host rock for nuclear waste in Switzerland, was studied by electron microscopy to image deformation microstructures. Deformation localised by forming micrometre-thick fractures. Deformation zones show dilatant micro-cracking, granular flow and bending grains, and pore collapse. Our model, with three different stages of damage accumulation, illustrates microstructural deformation in a compressed OPA sample.

Two time-lapse borehole ground penetrating radar (GPR) surveys were conducted during saline tracer experiments in weakly fractured crystalline rock with sub-millimeter fractures apertures, targeting electrical conductivity changes. The combination of time-lapse reflection and transmission GPR surveys from different boreholes allowed monitoring the tracer flow and reconstructing the flow path and its temporal evolution in 3D and provided a realistic visualization of the hydrological processes.

Hydraulic stimulation summarizes fracture initiation and reactivation due to high-pressure fluid injection. Several borehole intervals covering intact rock and pre-existing fractures were targets for high-pressure fluid injections within a decameter-scale, crystalline rock volume. The observed induced seismicity strongly depends on the target geology. In addition, the severity of the induced seismicity per experiment counter correlates with the observed transmissivity enhancement.

In this study, we present seismo-hydromechanical results from six hydraulic fracturing experiments executed in the framework of the In-situ Stimulation and Circulation (ISC) project. The well-characterized and extensively monitored target rock allows for the study of (1) the response of the rock mass, (2) the injection and pore pressure response, and (3) the geometry of newly created fractures and their interaction with the natural fracture network.