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A deep understanding of the physico-mechanical characteristics of rocks undergoing thermal treatments has gained a great interest in deep rock projects. In this research, the size and mass of cylindrical granite samples were measured before, under and after a temperature up to 1000°C, and their bulk densities at different temperature levels were calculated. Under and after high temperature, the granite density decreased with temperature due to the increase of the granite volume and decrease of the granite mass, with the rock volume increase being the prominent factor driving this behaviour. Additionally, the rate at which density increased under high temperature was always larger than the rate after high temperature. The rock mass loss after thermal treatment was attributed to the evaporation of different types of water, thermal reactions of rock-forming minerals and generation of rock fragments, while the rock volume expansion was mainly caused by thermal expansion and reactions of the rock-forming minerals, which was in accordance with the optical microscope observation results. 400°C is considered as the threshold temperature for the relationship between the decrease rate of rock density and temperature. Two fitting equations were proposed as the two boundaries of the rock density decrease rates. [ABSTRACT FROM AUTHOR]

Understanding the mechanical behaviors of granite after high temperature exposure and under confining stress conditions is an important issue in deep rock engineering projects such as the mining of deep underground solid mineral resources, deep geothermal energy exploitation and deep nuclear waste repositories. In this research, conventional triaxial compression experiments were conducted on Nanan granite after thermal treatment from 200 °C to 600 °C. Based on the experimental results, the influences of pressure and temperature on the deformation and strength characteristics were analysed. The physico-mechanical change mechanisms of the heat-treated granite were revealed by optical microscopy. The test results show that under 600 °C, granite volume increases by 4.11%, whereas the mass and density decrease by 0.28% and 4.21%, respectively. Average values of triaxial compressive strength and elastic modulus, cohesion and internal friction angle all reduce with temperature, decreasing rapidly by 54.99%, 39.81%, 49.39% and 27.51% from 500 to 600 °C, respectively. Granite specimens are less brittle and have higher ductility and plasticity as the temperature increases. However, confining pressure improves the mechanical properties of granite. Optical microscope images show that microcracks in granite specimens are generated and extend gradually with temperature, causing the deterioration of the physico-mechanical behaviors of heat-treated granite. [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]

• High-temperature and water-cooling cycles influence the micro-structural and mechanical behaviours of granite. • The crack rate and main crack aperture are used to quantify the microcrack evolution. • The coupling of inhomogeneous expansion, cyclic thermal shock and water-induced weakening leads to the damage of granite. • Different cyclic cooling methods are compared and analysed. • Relations between V p , UCS and E after different high-temperature and water-cooling cycles are found. During drilling, reservoir fracturing and hot dry rock development, high-temperature rock is subjected to cyclic water cooling. The mechanical properties and microscopic characteristics of granite exposed to high-temperature and water-cooling cycles were investigated experimentally. The results show that the uniaxial compressive strength and elasticity modulus decrease with increasing temperature and cycles, especially above 400 °C and after 1 cycle. In addition, the P-wave velocity decreases continuously and rapidly with temperature and it drops dramatically after 1 cycle and then more slowly with increasing cycles. The inhomogeneous expansion of minerals and cyclic thermal shock are the essential reasons for rock deterioration. The decay of elasticity modulus and enlargement of void space stops the damage to granite from being aggravated after a certain number of cycles. There is a good correlation between the P-wave velocity and mechanical parameters, and the damage factor based on the uniaxial compressive strength and elastic modulus has a positive correlation with the damage factor obtained by the ultrasonic method, illustrating that the ultrasonic method can be utilized to reflect the changes in mechanical characteristics. [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]

The bedrocks of deep geothermal reservoirs are exposed to cyclic water cooling during the exploitation of deep geothermal energy. Therefore, it is important to understand the physico-mechanical parameters of geothermal reservoir rocks. This paper reports on the P-wave velocity (V p), uniaxial compressive strength (UCS) and elastic modulus (E) of granite specimens after exposure to different cyclic heating and water cooling treatments based on laboratory tests, and the relationships between V p , UCS and E established through regression analysis. The physico-mechanical parameters of granite specimens all decrease remarkably in the first few thermal cycles, and their rates of decrease gradually diminish with thermal cycles, which is beneficial for the long-term exploitation of deep geothermal resources. Both UCS and E show a logarithmic correlation with V p of granite under different high temperatures. There is a transformation from a linear relation (1 and 5 cycles) to an exponential relation (from 10 to 30 cycles) between V p , UCS and E with thermal cycles. Such a correlation can provide a good estimation and avoid the costly, time-consuming and tedious mechanical tests. SEM observation reveals the change mechanism of the deterioration of physico-mechanical parameters, which can guide the well borehole stability during the deep geothermal energy exploitation. • Physico-mechanical properties of granite decrease rapidly in the first few thermal cycles. • Cyclic heating and water cooling have different effects on physico-mechanical properties. • Degradation mechanism of mechanical properties is revealed by microscopic observation. • Relations between V p , UCS and E under different high temperature and cycles were found. • There correlations can avoid the costly, time-consuming and tedious mechanical test. [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]

• Temperature treatment cause obvious decrease in the P-wave velocity, density, thermal conductivity, and thermal diffusivity of the granite specimens. • Degradation of granite thermal properties is caused by thermal microcracks. • There is an obvious correlation between physical properties and the thermal conductivity or thermal diffusivity of the granite specimens, while the volumetric specific heat capacity of the specimens is opposite. In this work, thermal properties (thermal conductivity (k), thermal diffusivity (λ) and volumetric specific heat capacity (C)) tests and physical properties (P-wave velocity (V P) and density (ρ)) tests were performed to assess the impact of cyclic thermal treatment on the thermal characteristics of granite. The key findings are as follows: (1) the k, λ, V P and ρ all deteriorate due to cyclic thermal treatment. Cyclic thermal treatment has a negligible effect on the C of granite, and there is no obvious relationship between cycles and C. (2) There is a good fitting relationship between the k or λ and the physical properties, which indicate physical attributes can be utilized to assess the thermal properties of granite. (3) The loss of various water, as well as the initiation and propagation of microcracks, are responsible for the deterioration of physical and thermal characteristics. [ABSTRACT FROM AUTHOR]