## 1 Introduction
These enhancements can reduce the impact of common challenges such as proppant crushing, embedment, fracture conductivity loss, and flowback issues. Proppants play a central role in maintaining fracture conductivity across subsurface energy domains, ranging from shale gas and CO2 fracturing to hydrate-bearing sediments and EGS reservoirs. Ref. [9]showed that kaolinite proppant behavior in CO\({}_{2}\)-rich fluids is strongly influenced by particle size, roughness, and the opposing effects of temperature and pressure on sedimentation, with CO\({}_{2}\) thickener adsorption and fluid microstructure playing key roles. Similarly, ref. [10] demonstrated that fracture conductivity in hydrate-bearing sediments increases with proppant size, concentration, and hydrate saturation, but declines under higher closure pressure--directly impacting gas recovery potential. While conventional hydrocarbon reservoirs typically operate within a temperature range of 60-120 \({}^{\circ}\)C (\(\pm\)2 \({}^{\circ}\)C) [11], with unconventional reservoirs going just above 120 \({}^{\circ}\)C, as shown in Table 1, geothermal systems typically exceed these thresholds. According to [12], geothermal power generation typically requires a minimum reservoir temperature of approximately 100 \({}^{\circ}\)C, with moderate-temperature systems operating between 230-300 \({}^{\circ}\)C, and ultra-high-temperature systems exceeding 300 \({}^{\circ}\)C. Given the elevated thermal conditions in EGS reservoirs, there is a critical need for proppants with high thermo-chemical resistance to ensure long-term mechanical integrity and chemical stability under those subsurface environments. Table 1 presents a comparative overview of subsurface conditions across representative unconventional oil and gas reservoirs, namely the Lower Eagle Ford Group and the Permian Basin, and two EGS sites: Utah FORGE and the Blue Mountain geothermal field. These environments differ markedly in terms of thermal, mechanical, and geochemical regimes, all of which critically influence proppant performance and long-term stability. EGS reservoirs such as Utah FORGE and Blue Mountain are characterized by elevated formation temperatures exceeding 175 \({}^{\circ}\)C, significantly surpassing the operational tolerance of many conventional proppants. In particular, sand, resin-coated sands (RCS), and polymeric proppants, commonly used in oil and gas stimulation, are prone to thermal degradation under such conditions, manifesting as resin softening, fines release, and eventual structural collapse [5; 11]. While the effective closure stresses in EGS may be comparable to those in unconventional oil and gas formations, the mechanical toughness of the host rocks, including granite,  
Figure 1: Schematic illustration of (**a**) heat extraction in Enhanced Geothermal Systems (EGS) and (**b**) a conventional Rankine cycle power generation system. Both systems share fundamental thermodynamic principles, where heat is extracted from a source and transported to a turbine-generator to produce electricity. In EGS, engineered fracture networks enable the transfer of heat from hot rock formations to a working fluid, which is circulated in a loop system. quartzite, diorite, and other crystalline lithologies, results in increased abrasive wear and potential crushing of proppants during closure. EGS environments typically expose proppants to geothermal brines with low pH and elevated salinity, conditions that significantly enhance silica leaching and destabilization, especially for sand-based proppants [13]. Thus, in geothermal applications, it is not only mechanical stress but also temperature, fluid, and proppant chemistry that become dominant controls on long-term proppant stability. Additionally, fault-controlled flow regimes, such as those at Blue Mountain, introduce complex fracture geometries where proppants are susceptible to transport inefficiencies, stranding, or displacement, further complicating the maintenance of fracture conductivity.