High-temperature rocks in the geothermal reservoir are susceptible to damage under periodic extraction and reinjection conditions, and variation of the pore structure threatens the efficient production of geothermal energy. We mainly conducted research on the influence of cyclic heating and cooling on the pore structure of granite by nuclear magnetic resonance technology and magnetic resonance imaging (MRI). The results show the significant increase of porosity and the sharp drop of ultrasonic velocity after cyclic heating and cooling. The total porosity and movable porosity of the sample increase successively with the increasing heating temperature, cooling rate and thermal cycles, while the fractal dimension of the pore structure shows the opposite tendency, this can be attributed to the fact that the dominant permeable pores weaken the pore complexity. The pore structure of granite with lower heating temperature or finer particles can maintain stability for more thermal cycles. MRI observations verified the extended and deepened micro-cracks in granite during thermal cycles, which can be attributed to the comprehensive effects of uneven expansion and shrinkage of the particles, the transformation of the mineral crystal structure, fatigue damage and the cooling-induced thermal shock.

地热储层高温岩体在周期性抽采回注条件下易受破坏，孔隙结构的变化威胁地热能的高效生产。我们主要通过核磁共振技术和磁共振成像（MRI）研究循环加热和冷却对花岗岩孔隙结构的影响。结果表明，循环加热和冷却后孔隙率显着增加，超声速度急剧下降。随着加热温度、冷却速度和热循环次数的增加，样品的总孔隙率和可动孔隙率依次增加，而孔隙结构的分形维数则呈现相反的趋势，这可以归因于占主导地位的渗透性孔隙削弱了孔隙复杂性。加热温度较低或颗粒较细的花岗岩的孔隙结构可以在更多的热循环中保持稳定性。MRI观察证实了花岗岩在热循环过程中微裂纹扩展和加深，这可以归因于颗粒膨胀收缩不均匀、矿物晶体结构转变、疲劳损伤和冷却热冲击的综合影响。 .