Major challenges of deep geothermal systems and an innovative development mode of REGS integrated with energy storage

China has long been a global leader in the direct utilization of moderate- to low-temperature geothermal energy, in contrast to its sluggish progress in power generation using deep geothermal energy. Rocks in deep reservoirs exhibit decreased permeability under high-temperature and high-pressure conditions, necessitating the establishment of engineered geothermal system (EGS) for the exploitation of deep geothermal energy. In an EGS, hydraulic fracturing is employed for reservoir stimulation to create artificially enhanced geothermal reservoirs with higher permeability. The current techniques for deep geothermal reservoir stimulation are predominantly borrowed from hydraulic fracturing processes employed in the oil and gas sector, placing limitations on the stimulation performance, earthquake risk control, and efficient heat extraction of geothermal reservoirs. This study summarized the features of hydraulic fracturing for deep geothermal energy: (1) fracturing-induced damage is dominated by the shear mechanism. (2) The tensile stress generated by cold water injection-induced differential temperature encourages fractures to propagate further. (3) Continuous water injection keeps the wellbore pressure higher than the formation pressure, creating favorable conditions for fractures to maintain open. Therefore, no proppant is required for hydraulic fracturing in an EGS. This is totally different from the hydraulic fracturing of oil and gas wells for production growth, which relies heavily on proppants. Furthermore, this study systematically analyzed four major challenges to EGS: low power generation capacity, poor connectivity between injection and production wells, risks inducing damaging earthquakes, and difficult profit making with no subsidy. From the aspects of innovative fracturing and energy recycling, this study proposed an innovative enhanced development mode integrated with energy storage, termed regenerative engineered geothermal system (REGS). This study investigated the advantages of the REGS through numerical simulation. The results indicate that multistage fracturing using horizontal wells and unequal spacings, areas, and volumes of injected water can enhance the connectivity between injection and production wells. The fracturing process is optimized in the REGS. Specifically, multistage fracturing is adopted. In each fracturing, the water injection rate is rapidly increased in the early stage and gradually decreased in the late stage. This can avoid the abrupt fluctuations in the wellbore pressure, thus governing the magnitude of induced earthquakes and preventing damaging earthquakes. The REGS integrates large-scale underground storage of renewable energy, achieving multi-energy complementation and enhancing REGS projects’ production lifespan and profitability. The results of this study will lay the foundation for the pilot projects and standardization promotion of the technology for combined heat and power generation integrated with energy storage of deep geothermal energy in China.