Cyclic fatigue effect on mechanical property change of hot dry rock in wellbores of enhanced geothermal systems

The fatigue effects induced by cyclic hydraulic fracturing and cyclic injection production in deep geothermal reservoirs are crucial for wellbore stability assessment of enhanced geothermal systems (EGS). Real-time high-temperature true triaxial fatigue compression tests were conducted on granite samples under thermal-mechanical coupled conditions with five representative temperatures (20 °C, 60 °C, 90 °C, 130 °C, and 180 °C) and in-situ stress conditions in the Gonghe geothermal field. The effect of temperature on fatigue strength, deformation, and fatigue life was investigated for the hot dry rock under high true triaxial stresses. The results indicate that high temperatures decrease the peak stress, fatigue strength, fatigue life, and fatigue deformation of the hot dry rock even under high stress. The hot dry rock remains show brittle failure under the tested high-temperature and high-stress true triaxial compression conditions. The ratcheting effect under cyclic loads is more significant in the σ3 direction than in the σ1 and σ2 directions. The phase transition from expansion to compression in the intermediate principal stress direction precedes fatigue macro failure. Microscopic analysis shows that high temperatures accelerate the nucleation of fatigue cracks in hot dry rocks. The results of this study contribute to the safe and efficient development of EGS projects, especially the wellbore instability under cyclic operations.