Experimental investigation on the physical-thermal properties of Nanan granite after air and water cooling for deep geothermal heat extraction

Deep geothermal resources have been widely acknowledged as an alternative energy source. Investigation on changes in thermal characteristics of geothermal reservoir rocks after water-cooling treatment is of great significance of deep geothermal heat extraction through enhanced geothermal systems (EGSs). Therefore, experimental tests were carried out to study the physical-thermal properties of Nanan granite subjected to air and water cooling and the heated temperature ranged from room temperature to 600 °C. Meanwhile, the change mechanisms of physical-thermal properties of granite were revealed by microstructural observations. The relationships between physical-thermal characteristics of Nanan granite after exposure to two cooling paths were also discussed through multiple regression analysis. It was found that the physical and thermal parameters of granite after two cooling paths gradually degrade with temperature, and water cooling further deteriorates physical-thermal properties of granite. X-ray computed tomography (CT) and optical microscopy observation revealed that the degradation of physical-thermal properties of Nanan granite is mainly associated with the initiation, development and coalescence of microcracks. The values of thermal conductivity and diffusivity both increase linearly with bulk density and P-wave velocity of Nanan granite under air-cooling condition, while they deviated from a straight linear relation of Nanan granite under water-cooling condition. The correlation coefficients of fitting equations presented are all larger than 0.93. This research contributes by providing a theoretical basis for geothermal simulation and engineering projects on deep geothermal heat extraction.