Evolution mechanism of the physical properties and thermal conductivity of thermal shock granite under chemical immersion

To exploit the geothermal energy from low penetration rocks at a depth of 3–10 kilometers below the ground surface, an artificial geothermal system usually must be built. Thermal and chemical stimulation can be used as auxiliary means of hydraulic fracturing for artificial geothermal reservoir reconstruction, which is conducive to reducing the risk of earthquakes. However, thermal shock and chemical corrosion can also cause changes in physical parameters such as density, porosity, longitudinal wave velocity, and the thermal conductivity of high-temperature rock mass, which brings great uncertainty to the service life of a geothermal system. To study the evolution of the physical properties and thermal conductivity of thermal shock granite under chemical modification, long-term acid and neutral solution immersion tests were performed on granite specimens subjected to thermal shock at temperatures ranging from 25 ℃ to 600 ℃. Using ultrasonic testing, nuclear magnetic resonance, thermal constant testing, and scanning electron microscopy, the evolution of the physical parameters of thermal−chemical modified specimens with thermal shock temperature was quantitatively characterized, the internal correlation among physical parameters was established, and the microscopic mechanism of the change in physical properties was revealed. The results show that with increasing thermal shock temperature, the volume of thermal–chemical− modified specimens increases gradually, the mass and density decrease gradually, the longitudinal wave velocity decreases linearly, the porosity increases by a power function, and the thermal conductivity and thermal diffusivity decrease exponentially and linearly, respectively. At the same thermal shock temperature, the volume growth fraction, longitudinal wave velocity, and thermal conductivity of the modified specimens are in the order of non-immersion > water immersion > acid immersion, while the mass loss fraction and porosity are in the order of acid immersion > water immersion > non-immersion. The increase in porosity and the deterioration of thermal conductivity are accompanied by a decrease in longitudinal wave velocity, so the porosity and thermal conductivity can be estimated by measuring the longitudinal wave velocity. The pore structure of the modified specimens is more sensitive to temperatures in the range of 150–450 ℃, while the solid particle skeleton is more sensitive to temperatures above 450 ℃, and the deterioration of the particle skeleton will further cause the transformation of the pore structure. The thermal-chemical modification results in the development of pore structure and phase transformation, which are the fundamental reasons for the changes in the physical properties of granite. High-temperature thermal shock plays a leading role in the process of thermal-chemical modification, while chemical corrosion plays an auxiliary role. At the selected test temperature levels, 300 ℃ can be considered the temperature threshold for severe thermal shock.