## 4 Results  
The decrease in rock's mechanical properties due to the thermal impact of the working fluid in the horizontal section of the loop storage cavity is caused by rapid temperature changes on the rock surface, creating temperature gradients. This process leads to the destruction of rock mineral bonding, the formation of microcracks within the rock, and potential penetration. High-temperature rocks may experience rapid fracturing at rock interfaces due to cooling by cold fluids, primarily resulting from the destructive force of thermal shock. When using water as a heat transfer medium in contact with granite in dry hot rock formations, significant thermal shock velocities are generated. For instance, the cooling rate of a 400 \({}^{\circ}\)C hot dry rock in 20 \({}^{\circ}\)C water within the first 20 s is 11.407 \({}^{\circ}\)C/s (the expression for thermal shock velocity is \(v_{T}=\Delta T/\Delta t\), \(\Delta T=T_{1}-T_{2}\), \(\Delta t=t_{1}-t_{2}\), \(t_{1}\) and \(t_{2}\) represent time, while \(T_{1}\) and \(T_{2}\) represent the temperature corresponding to \(t_{1}\) and \(t_{2}\)). The cavity in the horizontal section of the loop storage technology proposed in this paper has a metallic lining. By avoiding direct contact between the working fluid and the rock, the U-shaped well horizontal section loop storage technology effectively reduces thermal shock damage to the rock. In hot dry rock formations, the main rock is granite, with a density of about 2.65-2.67 g/cm\({}^{3}\), a thermal expansion coefficient of 9.0 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C, a compressive strength of 193.5 MPa for a sample size of \(\phi\)50 mm \(\times\) 100 mm at room temperature, and an elastic modulus of 14.79 GPa [23]. However, after cooling at different temperatures, the compressive strength decreases. For instance, in hot dry rock formations, when the temperature exchanges heat with water at 100 \({}^{\circ}\)C within the range of 250-600 \({}^{\circ}\)C, the compressive strength decreases from 105.59 MPa to 50.87 MPa, and the elastic modulus decreases from 11.30 GPa to 2.80 GPa [23]. The rock's tensile strength decreases from 7.28 MPa to 2.31 MPa. This finding indicates that the decreased temperature of the surrounding rock mass in the horizontal section of the hot dry rock formation leads to significant changes in the compressive strength, tensile strength, and elastic modulus of the rock material. Therefore, calculating the thermal stress of hot dry rock materials in the heat conduction process, considering changes in parameters under different temperatures and times, is quite challenging. The calculation formula for thermal stress includes the temperature gradient \(\frac{\mathrm{dT}}{\mathrm{dx}}\) of the rock material, which represents the rate of temperature change inside the rock under a fixed temperature difference. According to test data [24], \(\frac{\mathrm{dT}}{\mathrm{dx}}\) decreases exponentially with an increase in heat conduction time. Thermal stress reaches its maximum only under high-temperature gradient conditions, and failure may occur only when thermal stress is at its peak. However, when maximum strain is used as the criterion for failure, considering variations in temperature gradient throughout the entire heat conduction process and calculating thermal strain in the direction of heat conduction is necessary. During stability analysis of the cavity interface, the criterion for the tensile failure of hot dry rock is determined based on the relationship between total stress and critical stress value. Therefore, this study only focuses on parameters with the highest temperature gradient, allowing for a fixed temperature gradient value. The maximum temperature gradients for 600 \({}^{\circ}\)C high-temperature granites under 20 \({}^{\circ}\)C water thermal shock and natural cooling are 57,204 \({}^{\circ}\)C/m and 3022 \({}^{\circ}\)C/m, respectively.