## 1 Introduction
However, in terms of large-scale heat extraction, the contact heat exchange area is the primary limitation hindering the widespread application of DHE technology. The ground source heat exchanger system can serve as a feasible alternative for wells with very low geothermal flow rates [13]. [PERSON] collected existing literature on deep downhole heat exchangers to analyze their advantages and disadvantages. The analysis included heat transfer in simulated geothermal reservoirs, heat exchange between formations and wells, modeling of deep downhole heat exchanger design, and heat exchange in double-pipe systems, as well as performance analysis of deep downhole heat exchangers in producing thermal and/or electrical energy. It is evident that deep borehole heat exchangers can extract geothermal energy from deep wells, but the heat exchange flow is limited [14]. The SLGHP utilizes closed heat transfer technology based on phase change-free convection, differing from underground heat transfer techniques in principle [15]. This technology operates by transferring heat from the high-temperature end at a lower position to the low-temperature end at a higher position through the phase change of the heat transfer medium and the thermal convection of steam under the influence of gravity, without needing pump power [16; 17; 18; 19]. The heat in the strata is transferred to the heat transfer medium, stored in the form of latent heat, and carried to the surface. The heat transfer medium does not experience a significant temperature increase with the amount of heat absorbed. The technical advantages of SLGHP are low energy consumption and low operational maintenance costs. However, applying this technology directly to heat extraction from ultra-deep hot dry rock formations poses challenges. Both domestic and international efforts have been made to utilize SLGHP technology for geothermal energy development. Experimental tests have been conducted on pipelines with lengths of 150 m and 3000 m by [PERSON] [16] and [PERSON] [20,21], respectively, outputting heat at steam temperatures of 80 \({}^{\circ}\)C and 34.3 \({}^{\circ}\)C, generating 90 kW and 174 kW of heat. Based on the analysis of EGS, DHE, and SLGHP technologies, DHE has certain limitations in the scale of downhole heat exchange. SLGPH technology cannot be widely applied for heat extraction in ultra-deep hot dry rock formations. EGS technology involves the physical and chemical interactions between the heat exchange working fluid and rock during the fracture seepage process. The physical interaction includes rock fracturing due to thermal stress in hot dry rock formations, while the chemical interaction involves collapse caused by the dissolution of high-temperature heat exchange working fluid with the rock, with scaling and corrosion potential. To efficiently exchange heat in the horizontal section of new U-shaped wells while maintaining the overall stability of horizontal wells, using new efficient heat exchange working fluids and ensuring the rocks' stability is necessary. The new U-shaped well differs from traditional U-shaped wells of equal diameter and does not have a single-well U-shaped heat exchange pipeline. Instead, it involves expanding the horizontal section of the U-shaped well to increase the heat exchange area. Therefore, improving existing horizontal wells and hydraulic fracturing technologies to enhance heat transfer between reservoirs and efficient heat exchange working fluids within the reservoir is essential. On the other hand, the application of new heat exchange working fluids is necessary to enhance heat exchange between high-temperature rocks and improve rock stability. Based on the above analysis, one of the key technologies in geothermal resource development is the effective sealing and storage of the fracture volume generated by hydraulic fracturing and acidizing in the horizontal section of U-shaped wells, achieving efficient heat conduction at the interface of high-temperature rock layers.