Research status and development trend of key technologies for enhanced geothermal systems

Enhanced geothermal system (EGS) is a primary method to develop geothermal resources stored in hot dry rock (HDR), but it faces several key problems, such as unreasonable hydraulic fracture networks at high reservoir temperature, unclear multi-scale and multi-field coupling regularity, low heat extraction efficiency caused by the flashing flow in geothermal wells, and low thermoelectric conversion efficiency of geothermal fluid, which restricts the large-scale commercial development of geothermal resources. To resolve these major bottleneck problems, systematically reviews and analysis of the research progress and development trend of EGS are conducted in this paper. Particular attentions are devoted to four key technologies involved in the development of HDR geothermal resource by EGS: (1) the hydraulic fracturing technology for HDR reservoirs, including reservoir reconstruction methods, hydraulic fracturing network forming mechanisms and fracture propagation prediction models is illustrated in detail; (2) the fracture characterization methods, mathematical models and solution methods are described from three aspects including pore-scale multi-field coupled models, reservoir-scale multi-field coupled models and upscaling methods; (3) the efficient extraction technology of wellbore thermal fluid, involving the mechanism of flashing flows in geothermal wells and the experimental and numerical methods for investigating the characteristics offlashing flows are discussed; (4) the HDR geothermal power generation technologies, considering the principles of geothermal power generation, the types of power generation systems and the main application markets are introduced. In conclusion, EGS is a technology-intensive system, however, due to the complex working conditions of the underground reservoirs and the instability of the ground equipment, theoretical research tends to be separated from the practice. For the purpose of promoting the applicability of EGS, intimate combination and mutual guidance with the pilot tests are necessary to develop a production-research combined mode, and to raise the awareness and break through the key points in a constant back-and-forth.