## 2 Progress in Enhanced Geothermal System Based on Excavating
### Physical Components of EGS-E  
The structure of this extensive EGS-E scheme consists of a shaft, tunnel, lined piping system, a powered drive system, and other supporting accessories, as shown in Figures 1-3. Figure 1: Three-dimensional schematic illustration for an extensive version of EGS-E. In the above diagrams:  
1--Shaft; 2--Connecting tunnel; 3--Maintenance tunnel; 4--Heat exchange tunnel; 5--Near-field tunnel being lined for the secondary stage of heat extraction; 6--Pre-introducing (to fail together for primary stage of heat exchange) and post-connecting (to be retained for facilitating heat transport) sections of far-field roadway; 7--Annulus formed due to liner installation; 8--Caved/collapsed area; 9--Gate of Maintenance tunnel; 10--Gate of heat extraction tunnel and heat exchange tunnel; 11--1# injection pipe; 12--pump; 13--2# injection pipe; 14--Production pipe; 15--Heat exchange pipe;16--Lifting equipment. The dashed arrow represents the flow direction of the 2# working fluid, and the solid arrow represents the flow direction of the 1# working fluid. At least one shaft with a large diameter is constructed from the surface to the targeted depth by excavating implementation. It will perform the main access from or existing to the ground. Various functional facilities are installed inside in a separate and safety-guaranteed way. The hoisting, ventilating, conveying, and communicating devices are partitioned there and protected well. The network of roadways consists of multiple tunnels for connecting, heat exchange and extraction, local maintenance, etc. The connecting tunnels play as the main passages which represent the frameworks of development corresponding to horizons. The tunnels of the EGS-E function as a framework for the horizontal stratification of the geothermal reservoirs. Multiple layers of connection tunnels can be arranged longitudinally along the shaft as required. The heat extraction tunnels are the core structure of the first-stage  
Figure 3: Longitudinal section of a horizon illustrating near and far-field heat extracting stages. Figure 2: Plain view of the structural framework corresponding to one horizon. heat exchange of \"fluid-rock,\" which forms the near-field heat extraction zone and the far-field heat extraction zone. A near-field heat extraction zone consists of rigorously regular tunnels constructed by boring machines as well as installed pipelines, whose operation will be precisely predictable and controllable. Notably, during heat extraction, a low-temperature zone is formed in the center of the tunnel, which reduces the average production temperature, and then affects the heat exchange efficiency. Therefore, an annular structure is introduced to reduce the influence of this phenomenon, as shown in Figure 4. Inner concrete tubes are installed in the tunnel, thus creating an annulus between the tunnel walls and inner tubes. The concrete inner pipe is fixed in the tunnel through the support. The near-field heat extraction zone can provide a stable, predictable, and high-quality heat supply to the system. As for the far-field heat extraction zone, blasting or caving is carried out in an orderly manner, causing the rocks around the tunnel to collapse and break, forming a rough but large-volume compensatory reservoir. The far-field heat extraction zone provides sufficient heat for the system to compensate for the small amount of heat in the near-field heat extraction zone. The near-field and far-field heat extraction zone should be far apart from each other. Where there are multiple tunnel system levels in geothermal reservoirs, the locations of the heat extraction tunnels at the upper and lower levels are staggered to avoid mutual interference.