Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system(EGS).This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin,Qinghai,China.The grani...Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system(EGS).This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin,Qinghai,China.The granite samples were heated to 400℃ and then cooled in air,water,or liquid nitrogen.The physical and mechanical properties of the thermally treated granite were evaluated,andmicrostructural changes were analyzed using a scanning electron microscope(SEM)and computed tomography(CT).The results indicate that cooling with water and liquid nitrogen significantly enhancespermeability and brittleness while reducing P-wave velocity,strength,and Young's modulus.Specifically,liquid nitrogen cooling increased granite permeability by a factor of 5.24 compared to the untreatedsamples,while reducing compressive strength by 13.6%.After thermal treatment,the failure mode of thegranite shifted from axial splitting to a combination of shear and tension.Microstructural analysisrevealed that liquid nitrogen-cooled samples exhibited greater fracture complexity than those cooledwith water or air.Additionally,acoustic emission(AE)monitoring during damage evolution showed thatliquid nitrogen cooling led to higher cumulative AE energy and a lower maximum AE energy rate,withnumerous AE signals detected during both stable and unstable crack growth.The results suggest thatliquid nitrogen induces a stronger thermal shock,leading to more significant thermal damage andpromoting the development of a complex fracture network during EGS reservoir stimulation.This enhancesboth the heat exchange area and the permeability of the deep hot dry rock(HDR)in EGS reservoirs.The insights from this study contribute to a deeper understanding of thermal damagecharacteristics induced by different cooling media and provide valuable guidance for optimizing deepgeothermal energy extraction.展开更多
基金support from the National Natural Science Foundation of China(Grant Nos.52192622 and 52304003)the National Key Research and Development Program of China(Grant No.2023YFF0614102).
文摘Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system(EGS).This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin,Qinghai,China.The granite samples were heated to 400℃ and then cooled in air,water,or liquid nitrogen.The physical and mechanical properties of the thermally treated granite were evaluated,andmicrostructural changes were analyzed using a scanning electron microscope(SEM)and computed tomography(CT).The results indicate that cooling with water and liquid nitrogen significantly enhancespermeability and brittleness while reducing P-wave velocity,strength,and Young's modulus.Specifically,liquid nitrogen cooling increased granite permeability by a factor of 5.24 compared to the untreatedsamples,while reducing compressive strength by 13.6%.After thermal treatment,the failure mode of thegranite shifted from axial splitting to a combination of shear and tension.Microstructural analysisrevealed that liquid nitrogen-cooled samples exhibited greater fracture complexity than those cooledwith water or air.Additionally,acoustic emission(AE)monitoring during damage evolution showed thatliquid nitrogen cooling led to higher cumulative AE energy and a lower maximum AE energy rate,withnumerous AE signals detected during both stable and unstable crack growth.The results suggest thatliquid nitrogen induces a stronger thermal shock,leading to more significant thermal damage andpromoting the development of a complex fracture network during EGS reservoir stimulation.This enhancesboth the heat exchange area and the permeability of the deep hot dry rock(HDR)in EGS reservoirs.The insights from this study contribute to a deeper understanding of thermal damagecharacteristics induced by different cooling media and provide valuable guidance for optimizing deepgeothermal energy extraction.
