## 1 Introduction  
Geothermal energy is increasingly recognized as a key renewable energy resource with the potential to make significant contributions to global decarbonization efforts. As of 2021, global geothermal electricity generation has reached an installed capacity of 15.96 gigawatts-electric (GWe), with an average annual growth rate of 3.5% [1]. While these advancements are notable, the growth trajectory of geothermal energy remains modest compared to other renewable energy technologies, suggesting that innovative strategies are needed to accelerate its adoption. Enhanced Geothermal Systems (EGS), which aim to expand geothermal resource utilization by engineering reservoirs in low-permeability rocks, represent a promising avenue for such advancements. Geothermal energy comes from Earth's heat, including primordial and radioactive sources produced by the long-term decay of isotopes such as \({}^{40}\)K, \({}^{232}\)Th, \({}^{235}\)U, and \({}^{238}\)U, the primary contributors to terrestrial heat flow over geologic timescales [2]. Heat is transported from the inner core to the geothermal reservoirs through conduction, convection, and advection, allowing for the drilling of wells to harvest heat for electricity generation, as well as heating and cooling systems (see Figure 1a). EGS generates electricity through a process analogous to the Rankine cycle, as illustrated in Figure 1. In this thermodynamic system, isentropic compression (+Win) by a pump pressurizes the cold working fluid, which is then injected into subsurface fractures. The fluid absorbs heat from the hot dry rock (HDR) reservoir through heat addition (+QH), followed by isentropic expansion (-W) in the turbine to produce mechanical work. The heated fluid brought to the surface will either vaporize directly or transfer heat to a secondary working fluid in binary systems, producing vapor that drives a turbine-generator set [3]. Finally, the fluid undergoes isobaric heat rejection (-QL) in the condenser before the cycle repeats. Unlike conventional geothermal systems that rely on naturally permeable reservoirs, EGS utilizes engineered fracture networks to extract heat from low-permeability hot rock formations via circulation of a working fluid. The creation and maintenance of a stable fracture network is therefore critical to ensuring sustained heat extraction and long-term energy production from the Earth's crust. A critical limitation in geothermal systems is the low to ultra-low permeability between injection and production wells, which severely restricts heat recovery. Nevertheless, a significant portion of geothermal heat can be extracted from HDR by enhancing conductivity of the fracture network between injection and production wells. This limitation can be addressed by developing enhanced or engineered geothermal systems, which are artificially created reservoirs designed to improve connectivity between the wellbore and the low-permeability geothermal formations. Within this context, the development and deployment of high-performance, thermo-chemically enhanced proppant materials specifically tailored for EGS applications could play a transformative role in improving long-term reservoir performance and fracture conductivity. In the oil and gas industry, proppants have revolutionized hydraulic fracturing by maintaining fracture conductivity under considerable pressure and temperature conditions [4]. However, their direct transfer to EGS applications requires careful evaluation due to the significantly different operational environments, including elevated temperatures and complex geochemical interactions in geothermal reservoirs. The thermal and chemical stability of proppants has been enhanced through innovative coating technologies, such as resin-based, polymer-based, and nanomaterial-enhanced coatings, which improve critical performance metrics like crush resistance, thermal stability, and wettability [5; 6; 7; 8].