Development of an experimental approach to explore in situ fracturehydromechanics with ground surface tiltmeters and periodic fluidpressure perturbations

International audienceFractured bedrock reservoirs are of socio-economical importance, as they may be used for storage or retrieval offluids and energy. The characterization of such systems remains challenging, especially at scales where transportprocesses are critically controlled by the structure and properties of fracture networks. In particular, the hydromechanicalbehavior of fractures needs to be understood as it has implications on flow and fluid storage. It alsogoverns stability issues, that is to say medium failure and associated seismicity. Laboratory, numerical or field experimentshave brought considerable insights to this topic. Nevertheless, in situ hydromechanical experiments areless commonplace, mainly because of technical, instrumental or cost limitations. Here, we present the early stagedevelopment and validation of a novel approach aiming at capturing the integrated hydromechanical behavior ofnatural fractures. It consists in combining the use of surface tiltmeters to monitor the deformation associated withthe periodic pressurization of fractures at depth in crystalline rocks. Periodic injection and withdrawal advantageouslyavoids mobilizing or extracting significant amounts of fluid and may be applied to natural media wherehigh fluid pressures cannot be tolerated or achieved. In addition, the perturbation’s oscillatory nature is intendedto: 1) facilitate the recognition of its signature in tilt measurements; 2) explore various volumes of rock around theinlet and thereby assess scale effects typical of fractured systems. By stacking tilt signals, which is a broadly useddata processing method in seismology, we managed to detect small tilt amplitudes associated to pressure-derivedfracture deformation. Therewith, we distinguish differences in mechanical properties between the 3 tested fracturesbut we show that tilt amplitudes are weakly dependent on pressure penetration depth. Using an elastic model, weobtain fracture stiffness estimates that are consistent with published laboratory and field data. Our results shouldencourage further improvement of the method