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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]

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]