Challenges and opportunities of enhanced geothermal systems: A review

Exploiting geothermal resources, especially hot dry rock (HDR), is essential to reduce carbon emissions to build an acceptable energy structure. The enhanced geothermal system (EGS) for mining HDR has experienced more than 50 years since it was proposed in 1970, obtaining rich research results and construction experience. It is of great significance to review the EGS history, which includes discussing the project site selection and thermal storage stimulations, summarizing the reasons for the shutdown of demonstration projects, and indicating the key factors restricting EGS development. Based on this, the future development direction of EGS is clarified, which can help explore deep geothermal energy and construct associated demonstration projects in China. The overall development of EGS is divided into two stages, namely, the research and development stage before 2000 (a total of 14 EGS projects) and the demonstration and quasi-commercialization stage since 2000 with a rapid development speed (a total of 27 EGS projects). By the end of 2021, the cumulative number of EGS worldwide has increased to 41. However, the cumulative installed capacity of power generation only reaches 37.41 MW. EGS is still on the learning curve, resulting in a long way to go to realize the large-scale commercialization of HDR geothermal energy. The factors restricting the commercialization of EGS are the lack of policy support and capital investment, the limitations of technical difficulty, and the unpredictability of the geological condition of the thermal reservoir, which weakens EGS development and even causes its suspension or termination. Because of the complex geological environment of thermal reservoirs, the fracture network and associated reservoir quality induced by hydraulic stimulations are uncontrollable, causing the fractured quality of the thermal reservoir to be lower than its critical value. It results in numerous adverse problems in most EGS projects, including insufficient thermal reservoir volume, an unstable fracture network, associated heat exchange area, severe fluid loss, and induced unacceptable earthquakes. Thus, the fundamental reason for EGS’s inability to commercialize is that it is challenging to form a reproducible thermal reservoir stimulation model induced by the difference in thermal reservoir geological conditions and the dependence of the existing stimulation technologies on the in situ reservoir geological environment. Establishing the database of HDR and EGS plays an urgent role in EGS development by forming an accurate quantitative system of reservoir geological conditions to explore the relationship between geological conditions and reservoir reconstruction and then build a replicable thermal reservoir reconstruction technology. Focusing on new and demonstration stimulations for the thermal reservoir, such as the enhanced geothermal system based on caving technology (EGS-E), FORGE, and DEEPEGS projects, may provide an acceptable way to break through the dependence of thermal reservoir stimulation on in-situ geological conditions and form the “reproducible” deep-geothermal resource mining system to realize the large-scale commercialization of deep-geothermal resources.