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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]

The fluid flow route in an enhanced geothermal system (EGS) is jointly determined by well layout and fracture network, which affects the fluid flow and heat transmission process. To obtain an efficient EGS, it is critical to explore the best well layout and fracture parameter. In this study, combination effects of well layout scheme and fracturing networks are researched. A thermal-hydraulic-mechanical coupling model, accounting for the thermal shock impact of cool water on high temperature rock, is built to evaluate the heat production performance, which takes production temperature, production thermal power and reservoir recovery rate as indicators. Results indicate that addition of symmetrical injection wells could lengthen EGS's operating life and boost production thermal power. Connection between injection-production wells should be as close to the main direction of the fracture as possible to improve fluid flow rate, accelerate heat transfer, and increase heat recovery rate. Furthermore, Morris based sensitivity analyses are performed to identify the key variables influencing EGS production performance. It reveals that the most crucial parameters are injection pressure and fracture opening, which should be raised in practice as much as allowed on the assumption that the production temperature meets the threshold. • There is a strong coupling among reservoir temperature, thermal stress and permeability. • Thermal shock of water cooling affects the rock fracture opening and permeability. • Increasing injection wells symmetrically distributed can improve the EGS service life. • Key elements of production performance are injection pressure and fracture opening. [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]

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]