Review of Techniques to Mitigate Thermal Breakthrough in Enhanced Geothermal Systems

Enhanced geothermal systems (EGSs) offer potential for geothermal energy in areas lacking natural resources, but their economic success depends on optimal heat extraction, which is impeded by thermal breakthrough caused by fluid short-circuiting. This paper systematically reviews techniques to mitigate thermal breakthrough and improve heat production of EGSs. It aims to provide an overview of current techniques, identify challenges, and suggest future research directions. This review stands out by integrating findings from diverse fields, providing a comprehensive view of EGS thermal breakthrough mitigation. It discusses and evaluates five major techniques: working fluid flow management, reservoir stimulation, intermittent thermal extraction, use of carbon dioxide (CO2) as working fluid, and fracture conductivity management. It is shown that while fluid circulation rate adjustment and real-time monitoring and control can improve thermal output from EGSs noticeably, they bring significant risks of induced seismicity and increased cost and complexity of EGS design and operation. Multi-stage fracturing can create evenly distributed fractures in the reservoir, utilizing complex fracture networks to increase the heat sweep volume and postpone cold-water advancement. Through dissolving minerals in fluid flow paths, chemical stimulation can generate more extensive and interconnected flow channels, thus helping to delay thermal breakthrough; however, it needs careful environmental consideration. Intermittent thermal extraction, by alternating periods of active heat extraction with shut-in phases, enables temperature restoration in surrounding rock that has been cooled due to heat production, thus effectively delaying the decline of fluid outlet temperature. The unique thermodynamic and transport properties of CO2 improves the sweep of a reservoir and fosters a more comprehensive heat exchange within fracture networks, leading to more efficient heat extraction and thus higher thermal output. Temperature-sensitive proppants can considerably enhance long-thermal heat production of EGSs by automatically tuning fluid flow distribution across fractures. Future promising research directions include reactive tracer-based diagnostics, temperature-sensitive viscosity fluids, and nanofluid engineering.