Subcritical crack growth can contribute to stimulation of permeability in geothermal reservoirs as it can lead to lower formation breakdown pressure and enlargement of fracture disturbed zone. This study examines tensile and shear failure due to fluid injection and axial compression in tight sandstones. The overall aim is to identify effects of subcritical crack growth on permeability enhancement. Subcritical crack growth theory and some key experimental data for sandstones were reviewed, and axial compression and fluid injection experiments are performed on a very tight sandstone in a triaxial deformation apparatus. Failure stresses for conventional (short term) experiments with monotonic increase in axial stress or pore pressure and long term experiments with more complex pressure history are compared. The comparison shows some variation in failure strengths that may point to an effect of subcritical crack growth, but can also be due to sample variability. If the effect of subcritical crack growth is confirmed in experiments of permeability stimulation by fluid injection, results may be used to optimize reservoir stimulation for tight sandstone hydrocarbon or geothermal reservoirs with lower injection rates and volumes. It thereby contributes to more efficient reservoir stimulation with lower risks of inducing felt seismicity.
The concept of "soft stimulation" has been advanced for stimulation of geothermal reservoirs, which aims to stimulate the permeability of tight reservoirs, while preventing large seismic events (Zang et al., 2013; Hofmann et al., 2018). One of the soft stimulation concepts is cyclic pumping ("fatigue hydraulic fracturing"), which involves cyclic increase and decrease of fluid injection rates (Kiel, 1977; Zang et al., 2013). In "conventional hydraulic fracturing" relatively short-term monotonic fluid injection at relatively high injection rates is usually performed, resulting in relatively fast pressure build-up to levels above the fracturing pressure. Fatigue hydraulic fracturing mainly differs from conventional hydraulic fracturing in that fluid is injected in cycles with alternating higher and lower injection rates at different cycle times, resulting in a cyclic pressure response that gradually reaches the fracturing pressure over successive injection cycles (Hofmann et al., 2018). Fatigue hydraulic fracturing has been applied in laboratory experiments (Stephansson et al., 2019) as well as in a few field cases (Zang et al., 2019; Huenges et al., 2020). In some studies a decrease in formation breakdown pressure and increase of fracture disturbed zone has been found (Kiel, 1977; Zang et al., 2019). These aspects are beneficial to reservoir stimulation as the likelihood of inducing seismicity may decrease due to lower injection pressures and volumes, while reservoir permeability and associated reservoir-to-well connectivity may increase due to the larger stimulated reservoir volume. An interesting aspect, compared with conventional hydraulic fracturing, is that the characteristics of injection cycles, such as duration, injection rate, frequency and number of stages, may be varied, yielding more operational parameters that can be optimized or tuned to different types of reservoirs. The effects on breakdown pressure and fracture network complexity may be attributed to progressive and enhanced microfracture development during stimulation and subcritical crack growth (Hofmann et al., 2018).