Permeability decides the mass transfer behavior and plays a key role in the heat extraction of enhanced geothermal systems (EGSs), and the aim of the injection of cooling water is to enhance permeability within the geothermal reservoirs. Direct evidence of how cool water affecting permeability evolution is still limited. Therefore, we explored the permeability evolution of Nanan granite after air and water cooling under different confining pressures combined with optical microscopy and X-ray CT observation. We found that there are lots of microdefects observed in Nanan granite after two cooling paths, which dominantly drives the evolution of permeability from a microscopic scale. The permeabilities of granite under water-cooling condition are always larger than those under air-cooling condition, and water-cooling treatment further enhances the permeability of specimens. The permeabilities of granite specimens after two cooling paths decrease with confining stress. More microcracks and better connectivity among microcracks produce a larger permeability within the specimen after water cooling. The observed microcracks are regarded as the seepage channels and direct microscale evidence of permeability evolution of granite after two cooling paths. Our results may suggest that water injection is an efficient way for permeability enhancement due to thermal microcracks propagation in thermal simulation.

A micro-CT instrument with an X-ray energy of 150 kV/60 mA (nano voxel-3000) was used to scan the small cylindrical specimens (F25×50 mm) after air and water cooling to increase the observed resolution.