Numerical study of near-borehole coupled thermo-hydro-mechanical processes during stimulation of a synthetic geothermal reservoir

Soft stimulation technologies have been proposed as a means to reduce the breakdown pressure and mitigate the risk of induced seismicity during geothermal reservoir stimulation. Yet, the underlying mechanisms remain poorly understood due to the complexity of the coupled thermo-hydro-mechanical (THM) processes. In this work, a fully coupled THM model is developed to evaluate and compare the performance of different stimulation scenarios (monotonic, stepwise injection rate, cyclic injection rate or temperature, and stepwise combined with cyclic injection rate stimulation) on a synthetic, highly permeable reservoir with near-borehole clogging. Simulation results show that stepwise injection rate stimulation yields the most favourable outcomes, followed by the stepwise injection rate combined with cyclic injection rate stimulation. On the other hand, fatigue effects are seen to play a negligible role in the improved performance since the tensile stress at the fracture tip is relaxed with the continuous fracture growth. In addition, cyclic injection temperature stimulation is generally neither better nor worse than monotonic stimulation, but has slightly different characteristics, creating more local damage controlled by the period of the injection cycle. Cyclic injection rate stimulation can slightly reduce the peak pressure, compared with monotonic stimulation, but only when the injection rate is low. The reduction in peak pressure occurs due to the combination of thermally-induced stresses associated with cooling and incremental damage rather than any influence of fatigue. Stepwise or low-frequency cyclic injection rate stimulation are suggested rather than a high-frequency cyclic injection rate stimulation, while injection with cyclic temperatures is suggested when more local damage is wanted.