Constraining the impact of cyclic hydraulic stimulation on granites

Subsurface engineering, such as geothermal energy extraction, requires knowledge of the rupture of geomaterials. Of particular importance is the time- and rate-dependence of material strength, which impacts fracture architecture and thus hydraulic conductivity and system permeability. Cyclic soft stimulation (CSS) techniques have been developed to maximise the efficiency of resource extraction whilst minimising large amplitude, fluid-injection induced seismicity. Here, we explore the benefits of cyclic stimulation experimentally, utilising novel “pulsed pumping” hydraulic fracture tests in which fluid pressure is cycled within the central borehole of a suite of 20x20cm cylinders of dense granite. The response is monitored at high-resolution by fibre-optic circumferential strain measurements, fluid pressure data and acoustic emission recording. Using cyclic high-low pressure square waves, we found that breakdown pressure was reduced by up to 15% compared to the monotonic case in which pressure was increased by applying a constant flow rate. Whilst peak pressure had the primary control on the number of cycles to failure, increasing the minimum pressure in the borehole (thus increasing mean pressure) further reduced breakdown pressure, suggesting that even small pressure fluctuations during hydraulic stimulation may reduce the largest stress drops, and hence the magnitude of induced seismic events. Strain measurements detected accelerating precursory deformation a few cycles prior to failure, hinting at the opportunity for responsive stimulation practices where activity can be monitored in real-time. These novel large-scale, high-resolution experiments were complemented by indirect tensile measurements at a range of strain rates, and by cyclic fatigue Brazilian disc testing at a range of peak loads and cycle amplitudes. These results further highlight the increasing contribution of time-dependent deformation during slower and cyclic loading, resulting in lower peak loads and reducing large magnitude fracturing events. The generated S-N curves demonstrate that weakening by cyclic hydraulic pressurisation mimics relationships defined by conventional fatigue testing of geomaterials. Such experimental constraints will be of great benefit to the development of cyclic stimulation practices for subsurface engineering.