Direct Evidence of the Effect of Thermal Shocks on Permeability Evolution of Nanan Granite After Water Cooling Under Loading and Unloading Conditions

Enhanced geothermal systems (EGSs) are proposed to overcome the defects of low permeability and large buried depth to utilize the deep geothermal resources. Water is more successfully applied as the working fluid in the heat extraction from existing EGS projects due to its low cost and heat capacity so far. Permeability of reservoir rocks is critical for cyclic heat extraction from deep geothermal resources. In this research, the permeability tests were performed on Nanan granite subjected to cyclic water cooling under loading and unloading conditions. The two- and three-dimensional (3D) microcrack networks of granite after cyclic water cooling were presented to provide direct evidence for revealing the mechanisms of permeability evolution using optical microscopy and X-ray computed microtomography (CT). The testing results indicate that the permeability and gas volume flow rate of granite subjected to cyclic water cooling increase with the thermal cycle and decrease with confining stress regardless of the loading path. The permeability of Nanan granite subjected to cyclic water-cooling treatments under loading conditions is always larger than those under unloading conditions. The distribution density of 3D microcracks and the pore-throat networks developed in Nanan granite are gradually enhanced with thermal cycles based on AVIZO software, which is in accordance with the permeability evolution. The identified 3D microcracks are considered as the seepage channels and microscale direct evidence of the permeability evolution of granite subjected to cyclic water cooling based on microstructural and mesostructural analysis. The experimental results will make contributions to provide direct evidence for an in-depth understanding of mesostructural changes and subsequent heat and mass transfer behaviors by cyclic water through EGSs.