A range of different geoenergy technologies require the injection or extraction of fluids into or out of the near subsurface (0 – 5000m). In some of these contexts, such as deep geothermal, the propagation of fractures is intentional to enhance permeability and well productivity, but the process must be controlled. In other contexts such as CO2 sequestration, the propagation of fractures in the caprock is a risk that must be mitigated against. Cyclical pulsing in the reservoir formation may lead to fatigue in the lower permeability cap rock.
Cyclic soft-stimulation (CSS) techniques aim to minimise large amplitude seismicity induced by fluid injection. Pulsed-pumping experiments involve simulating these techniques in the laboratory by repeatedly pressurising a borehole within a rock sample with values of fluid pressure which would be found in the field. Previous laboratory experiments at ambient conditions have demonstrated the ability to reduce breakdown pressure in Granites by up to 15% compared to monotonic fluid injection by controlling the maximum and minimum pressures of the pressure pulses. Here, we expand on those experiments by conducting the same tests under subsurface stress conditions in 20cm diameter Granite samples using the GREAT cell at the University of Edinburgh. This unique flexible-medium true-triaxial//polyaxial deformation rig allows for large samples and synchronous rotation of the stress-field during fluid transmission experiments. The circumferential and axial strains are measured around the sample throughout the experiments using high-resolution (spatial and temporal) fibre-optic strain gauges, allowing observation of the far-field stresses around the boreholes. These strains monitored around the sample edges demonstrate a measurable precursory response during the pulses prior to the eventual sample breakdown. These strains occur over a significantly longer period than is observed during monotonic fluid injection. This suggests the potential to control the breakdown further by limiting the pulse pressures once measurable strains start to develop.
