## 2 Technology Description  
This article proposes a volume storage technology for the horizontal section of U-shaped wells after hydraulic fracturing and reservoir acidification, forming an efficient heat exchange loop for efficient heat exchange, as shown in Figure 1.  
Figure 1 shows that a network of fractures is formed near the wellbore in the ultra-deep dry hot reservoir after hydraulic fracturing. After small-scale fracturing in the U-shaped well, the formed fracture network consists of main fractures and microfractures, making it difficult to achieve closure of the horizontal section. Therefore, acid is injected into the formation through continuous tubing via surface acid injection equipment to acidify the dry hot rock formation, creating spatial volume post-hydraulic fracturing. Products after acidification can be returned through fluid flow to form a cavity with a diameter much larger than the wellbore diameter, as shown in Figure 1a. After hydraulic fracturing and acidification, the horizontal section closes the channel of the formed horizontal section cavity. Before closing the cavity, water injection is required for pressure testing. If the pressure drops sharply, the leakage points of the horizontal section cavity must be plugged. While ensuring that the fluid medium does not leak, circulating air is injected underground to ensure a dry state. A liquid alloy with a melting point higher than the formation temperature is heated until a phase change occurs to form liquid metal. The high-temperature liquid metal is injected from one side of the U-shaped well until the liquid alloy appears on the other side, and then the injection is stopped. The liquid alloy with a melting point at the formation temperature is heated and undergoes a phase change to form a liquid metal. Then, it is injected into the closed loop to displace the liquid alloy with a melting point higher than the formation temperature, thereby achieving closure of the horizontal section cavity. Figure 1: A schematic diagram of a U-shaped well filled with low-melting-point liquid metal forming a closed loop. (**a**) Schematic diagram of channel formed after hydraulic fracturing and acidizing of U-shaped well. (**b**) Schematic diagram of passage containing lining in horizontal section of U-shaped well. 1. Pressure acidizing device, 2. Casing, 3. Cement ring, 4. Continuous tubing, 5. Spatial volume after hydraulic fracturing acidification, 6. Formation, 7. Injection pump, 8. Solidified alloy, 9. Liquid alloy heat exchange medium, 10. Surface heat exchange device, 11. Suction pump. The horizontal section of the U-shaped well has significant advantages over volumetric storage technology by achieving heat conduction between hot dry rock at a high temperature and pressure and the flow channel of the thermal working fluid. Unlike EGS technology, the fluid within the hot dry rock formation flows through the wellbore to the surface for efficient heat exchange. The thermal working fluid circulating in the horizontal section of the U-shaped well is liquid alloy metal, which has high density, low melting point, and high boiling point characteristics. It is a more advantageous medium-to-low temperature phase change heat storage material and heat transfer material than water as a heat exchange medium. Utilizing liquid alloys as the heat exchange medium in the U-shaped well's horizontal section cavity exploits this metal material's excellent heat transfer properties. The low-melting-point liquid alloy Sn\({}_{17.8}\)Cd\({}_{13.5}\)Pb\({}^{31.1}\)Bi\({}_{37.6}\) containing the metal elements Cd, Sn, Pb, and Bi has a thermal diffusion rate of approximately 13.72 mm\({}^{2}\)/s at 25 \({}^{\circ}\)C [22], whereas the thermal diffusion rate of water is 1.43 mm\({}^{2}\)/s. The thermal diffusion rate of the low-melting-point liquid alloy is 9.6 times that of water, showing promising prospects when applied to a closed-loop heat exchange system.