Heat extraction efficiency in deep geothermal energy mining and implications for EGS-E

Geothermal energy has recently attracted substantial attention due to its abundant reserve, cleanness, and sustainability. Geothermal reservoirs can be stimulated via different approaches/techniques that lead to different heat extraction efficiencies and production through heat transfer between the working fluid and the reservoir network. Typical reservoir stimulation strategies include hydraulic fracturing, which is employed in conventional geothermal systems based on drilling, namely, EGS-D; indirect heat exchange using U-shaped pipes, namely, EGS-P; and block caving, which is based on the well-developed mining excavation framework, namely, EGS-E. Although the above three reservoir stimulation modes have been made available, their heat extraction performances for a certain reservoir over the operation lifespan have been unexplored. Selecting the appropriate reservoir stimulation approach and assessing the corresponding heat extraction performance are crucial for the design and subsequent operation of geothermal systems. Here, we systematically compared the heat extraction efficiencies of different stimulated reservoir networks under four typical stimulation modes, including a high-permeability reservoir (representing a reservoir stimulated by EGS-E), a connected fracture (representing a reservoir stimulated by EGS-P) reservoir, a reservoir with randomly distributed fractures (representing a reservoir simulated by EGS-D), and a reservoir with randomly distributed fractures and connected fractures (representing a reservoir simulated by the combination of EGS-D and EGS-P). The mechanical, hydraulic, and thermal coupling among the rock matrix, fracture network, and working fluid was realized in COMSOL Multiphysics. We found that the heat extraction efficiency of the high-permeability reservoir was the highest and that of the reservoir with randomly distributed fractures and connected fractures was the lowest. Crack aperture evolution was modulated by the competition between matrix contraction and hydraulic enhancement. The total crack aperture can be increased by increasing the matrix contraction and the hydraulic pressure of the working flow. Injection capability improved when the matrix contraction (thermal effect) prevailed but decreased when the working flow pressure (hydraulic effect) dominated. We also found that the smaller the matrix spacing, the larger the thermal effect-induced crack aperture and thus the total aperture. When the matrix spacing was reduced to 50 m, the thermal effect-induced crack aperture was nearly five times the hydraulic effect-induced crack aperture. The above findings have the following implications for EGS-E: first, the reservoir should be caved into fractured blocks that are as small as possible to increase permeability. Heat extraction efficiency and heat production can thus be highly promoted. Second, for the EGS-E with multiple reservoir slices, the slice spacing should be appropriately optimized to ensure high crack apertures and thus commensurate heat extraction efficiency.