## 5 Conclusions  
This paper proposes a novel geothermal energy extraction method of Excavation based Enhanced Geothermal System (EGS-E). The EGS-E uses mining technology to complete the construction of a geothermal reservoir instead of using drilling and fracking methods. Accordingly, a huge but customized scale of the project could be designed as a structural system, which mainly consists of two zones: near-field heat extraction zone and far-field heat extraction zone. A near-field zone consists of rigorously regular tunnels whose operation will be precisely predictable and controllable. As for the far-field heat extraction zone, blasting or saving is carried out in an orderly manner, causing the rocks around the tunnel to collapse and break, forming a rough but scalable compensatory reservoir. The EGS-E uses a two-stage heat exchange system of \"fluid-rock\" and \"fluid-fluid.\" As for the first-stage heat exchange of \"fluid-rock,\" the 1# working fluid flows in the annulus of the tunnel and SRV zone, and absorbs heat from the hot tunnel walls and gravel. As for the second-stage heat exchange of \"fluid-fluid,\" the 2# working fluid is injected into the spiral heat exchange pipes to exchange heat with the 1# working fluid filled in the heat exchange tunnels. To demonstrate the heat extraction performance of EGS-E, we have established a representative and applicable 3D transient thermal-hydraulic numerical calculation model. The 1D non-isothermal pipe flow model and equivalent porous medium model were adopted to efficiently represent the tunnels and SRV, respectively. The temperature field results show that there are obvious low-temperature zones around the tunnel and SRV and that the zones gradually expand with time. The maximum thermal influence area radius of the near- and far-field models are 156.60 m and 172.92 m, respectively, which provide an important reference for the optimal spacing design of the structure. The production temperature of the near-field zone decreased rapidly in the first 0.2 years and then gradually remained stable from the fifth year. In contrast, the temperature change in the far field area can be divided into three stages: stable region (0-0.8 years), decline region (0.8-3 years), and stable region (3-20 years). The cumulative heat extraction of the near-field zone and the far-field zone is 1.017 \(\times\) 10\({}^{15}\) J and 8.421 \(\times\) 10\({}^{15}\) J, respectively. In an orderly regulatory manner, the latter will provide the former with sufficient compensatory heat transport, and then this system can provide sufficient energy for utilization. By combining the characteristics of these two zones, the EGS-E can realize scalable and controllable geothermal energy extraction.  
Conceptualization, H.L., K.J., Y.T. and C.T.; Formal analysis, H.L. and K.J.; Funding acquisition, H.L.; Investigation, H.L. and K.J.; Methodology, H.L. and K.J.; Software, Y.T.; Supervision, H.L. and C.T.; Validation, K.J.; Visualization, K.J. and Y.T.; Writing--original draft, H.L., K.J. and Y.T.; Writing--review & editing, H.L. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.  
Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Grant NO. SKLGME021021.  
The authors declare no conflict of interest.