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Laboratory tests were conducted to study the physical and mechanical properties of granite after heating and water-cooling treatment for 1 and 30 cycles from room temperature to 500 °C. The change mechanisms for the water-cooling treatment were analysed via scanning electron microscope observation. At 500 °C, the volume of granite increases by 1.73% and 2.55%, the mass decreases by 0.16% and 0.31%, and the density decreases by 1.86% and 2.78% after 1 and 30 thermal cycles, respectively. The average values of UCS and E after 1 and 30 cycles both decrease as the temperature rises, while the peak strain exhibits the reverse trend. A yield platform is observed in the yield stage of the stress–strain curve above 300 °C, and the ductility of granite gradually increases with temperature. The normalized P-wave is linear with respect to the normalized UCS and E at 1 thermal cycle, whereas it shows exponential relationships with the normalized UCS and E at 30 thermal cycles. The degradation of the physical and mechanical properties of granite after 1 and 30 cycles is mainly caused by the generation and development of microcracks inside the rock. Compared to 1 thermal cycle, more microcracks are observed at 30 thermal cycles. Therefore, the thermal cyclic treatment can further deteriorate and weaken the physical and mechanical properties of granite. [ABSTRACT FROM AUTHOR]

Geothermal energy is recommended as a clean and renewable energy resource. During the exploitation process, hot rocks in a geothermal reservoir will be subjected to cyclic water cooling. Laboratory tests were conducted to investigate the physical and mechanical behaviors of water-cooled granite specimens subjected to various thermal cycles with temperatures ranging from 20 °C to 500 °C. Meanwhile, scanning electron microscope (SEM) images were used to capture the development of the microcracks within the specimens. It was found that volume increases with thermal cycle and mass, that density and P wave velocity decrease with thermal cycle, especially after the first thermal cycling, and their change rates diminish with increasing thermal cycle. Both uniaxial compressive strength (UCS) and elastic modulus (E) of granite decrease with thermal cycle. The decreasing extents gradually tend to constant with thermal cycle, and they remain almost unchanged once the thermal cycle reaches 20 iterations. The deterioration mechanisms of the physical and mechanical behaviors of granite are mainly caused by the generation and propagation of microcracks. Microcrack density increases slowly with the thermal cycle rising from 10 to 30, corresponding to the decrease of the physical and mechanical properties. The results are expected to provide an important contribution to the stability of well boreholes during the exploitation of deep geothermal energy. [ABSTRACT FROM AUTHOR]

During the construction of nuclear waste storage facilities, deep drilling, and geothermal energy development, high-temperature rocks are inevitably subjected to thermal shock. The physical and mechanical behaviors of granite treated with different thermal shocks were analyzed by non-destructive (P-wave velocity test) and destructive tests (uniaxial compression test and Brazil splitting test). The results show that the P-wave velocity (VP), uniaxial compressive strength (UCS), elastic modulus (E), and tensile strength (st) of specimens all decrease with the treatment temperature. Compared with air cooling, water cooling causes greater damage to the mechanical properties of granite. Thermal shock induces thermal stress inside the rock due to inhomogeneous expansion of mineral particles and further causes the initiation and propagation of microcracks which alter the mechanical behaviors of granite. Rapid cooling aggravates the damage degree of specimens. The failure pattern gradually transforms from longitudinal fracture to shear failure with temperature. In addition, there is a good fitting relationship between P-wave velocity and mechanical parameters of granite after different temperature treatments, which indicates P-wave velocity can be used to evaluate rock damage and predict rock mechanical parameters. The research results can provide guidance for high-temperature rock engineering. [ABSTRACT FROM AUTHOR]

Water has been used as a working fluid injected into the hot reservoirs during the exploitation of deep geothermal energy, therefore, understanding the thermomechanical properties of reservoir rocks after water cooling is essential. For that reason, we have conducted a series of laboratory tests on air and water cooled granites from normal temperature to 600 °C, to reveal the changes in their thermomechanical properties. At 600 °C, the average values of uniaxial compressive strength, elastic modulus and P -wave velocity of water cooled granite decrease by 84.9%, 73.1% and 66.2%, which are 11.0%, 17.0% and 17.7% larger than those of air cooled granite. Through optical microscopic analysis, the microcrack density and average width of water cooled granite increase with thermal temperature and are 4.18 mm/mm2 and 54.62 μm at 600 °C, while the values of air cooled granite are only 1.97 mm/mm2 and 25.16 μm. We thus combined the deterioration of the macroscopic mechanical characteristics of air and water cooled granites with the propagation and development of microcracks. Supported by data from international literature, the changes in the thermomechanical characteristics of granite has been systematically compared to international literature, which is hoped to provide technical support for the geothermal energy exploitation. • Heating and cooling induce degradation of the mechanical properties of granite. • Rapid cooling in water induces more serious thermal damage to the granite samples. • Thermal shock and water intruding cause the alterations in microstructure of granite. • Degradation mechanism of mechanical properties is revel by microscopic observation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Understanding the mechanical responses of granites after various thermal shocks is of utmost significance for heat extraction through from enhanced geothermal systems (EGSs). In this research, the changes in the mechanical characteristics of granites after various thermal shocks are analyzed and determined following according to a comprehensive review of research. The change mechanisms of the mechanical responses of granites after various thermal shocks are revealed by microstructural observations. The normalized values of mechanical parameters decrease linearly with heating temperature, while the confining stress enhances the mechanical parameters. The thermal cycle markedly reduces the mechanical parameters of granites only in the first few thermal cycles. With the rise of heating temperature and confining stress, the failure pattern of various granites transfers from axial splitting failure to shear failure and multiple shear failure. The macroscale deterioration of the mechanical parameters of various granites after thermal shocks is closely associated with the initiation, development and coalescence of microcracks. It is hoped that the comprehensive data review of the mechanical responses of granites in this study will provide reliable parameter values for wellbore stability and reservoir stimulation in EGSs. • Mechanical parameters of granites after thermal shocks decrease linearly with temperature. • Unloading path decreases the strength of granite due to superimposing a lateral tensile stress. • Temperature above 400 °C has a greater effect on mechanical properties than unloading treatment. • Mechanical deterioration of granite is closely associated with the development of microcracks. [ABSTRACT FROM AUTHOR]