## 5 Discussion  
The stability of the interface rock stored in the horizontal section of the dry hot rock formation is related to factors such as temperature, temperature gradient, depth of the horizontal section, the density of the heat transfer medium, and surface injection pressure of the dry hot rock. Experimental studies indicate that higher temperatures in dry hot rock formations may have higher temperature gradients. To ensure stable interface rock stored in the horizontal section of a dry hot rock formation, the temperature must be considered comprehensively. To ensure the stability of interface rock stored in dry hot rock formations under high-temperature gradients, constructing horizontal section storage cavities in deeper formations or increasing surface injection pressure is considerable. To investigate the impact of formation depth on the stability of the interface rock stored in the horizontal section of dry hot rock formations, assuming that the formation temperature gradient and injection pressure are determined, the depth of the horizontal section formation must be designed so that the interface rock stored in the horizontal section formation cavity rock is stable. The critical value of the horizontal section depth of the U-shaped well under  
Figure 3: Relationship curve between temperature gradient and stress.  
different temperature gradients at an injection pressure of 20 MPa can be determined using Formula (4) based on the criterion of dry hot rock tensile failure, as shown in Figure 4.  
Under a pump injection pressure of 20 MPa, different temperature gradients affected the critical depth of the horizontal section of the U-shaped well, as shown in Figure 4. To maintain the interface stability of the horizontal section's annulus cavity, the horizontal depth section must be above the critical line; otherwise, the rock may fracture. This finding indicates that the criterion Formula (4) for the tensile failure of dry hot rock provides an important basis for designing the depth of the horizontal section of the U-shaped well.  
As shown in Figure 5, during the long-term thermal production process of the dry hot rock U-shaped well, cyclic thermal shocks can gradually reduce the tensile strength of the rock at the interface of the horizontal section storage cavity. To maintain the stability of the interface rock, increasing the injection well's pump pressure in a timely manner is necessary. For example, with a 0.5 MPa decrease in tensile strength every 3 years, after 18 years of continuous heat extraction, the rock's tensile strength will decrease from its initial value to 14.35 MPa. This process requires adjusting the pump pressure from 44.65 MPa to 49.65 MPa. This change demonstrates that by appropriately adjusting the pump pressure, even during continuous extraction of dry hot rock, the rock interface stability of the horizontal section storage cavity can be effectively maintained without significant pressure adjustments. In deep reservoirs within hot dry rock formations, the higher the depth, the higher the temperature, and the lower the critical strength, higher surface pressure is required. Therefore, current surface pumping equipment may have limitations for deeper hot dry rock reservoirs. This technology is not suitable for sandstone-type hot dry rock formations.  
Figure 4: Critical depth of horizontal section of U-shaped well at different temperature gradients (pump injection pressure 20 MPa).