An experimental demonstration of fault preconditioning for reduced seismic hazard

The mechanical stimulation of a fault or fracture in the case of an Enhanced Geothermal Reservoir (EGS) is generally reliant on inducing shear dilation of a targeted discontinuity. However, this same process can lead to the nucleation of a potentially-damaging seismic event. Here, a reservoir stimulation technique known as preconditioning is demonstrated experimentally for the first time. This technique consists of initially increasing the effective normal stress along the fault, in practice corresponding to a period of fluid production. Following this the fault is locally unloaded, corresponding to fluid injection. As the unloading continues, a slipping patch may form, eventually leading to dynamic rupture. However, the previously-induced high effective normal stress further along the fault acts as a fracture energy and reduced-stress-drop barrier, potentially resulting in rupture arrest. Here, a highly-instrumented (strain gauges, accelerometers, acoustic sensors, displacement sensors, load cells) biaxial apparatus is used to demonstrate this procedure, making use of the translucence of polymethylmethacrylate (PMMA) and a high-speed camera to image the development of propagating ruptures. It is demonstrated, as previously predicted from theory, that preconditioning has the ability to halt dynamic ruptures and may therefore be a viable stimulation technique resulting in reduced hazard in EGS stimulation. Specifically, experiments are performed at nominal normal stresses of 60, 90, and 120 bar, with preconditioning (or normal stress increase) corresponding to approximately 8, 16, and 24%; in addition to control cases with no preconditioning. Generally, preconditioning slows rupture propagation at 8% normal stress increase and completely halts it for larger values of preconditioning. It further results in a reduced shear stress drop, increased fracture energy, and reduced slip velocity. These results may one day have further implications for the potential of controlled stress release along natural faults.