## 5 Discussion  
A notable limitation of this study was the failure of some reactor caps during the initial 24 h at 180 \({}^{\circ}\)C, which resulted in fluid loss. This was remedied by resealing the vessels and replenishing them with a 75% DI water and 25% FORGE fluid mixture, leading to a dilution of the original geothermal brine composition. Consequently, the experimental temperature was lowered and maintained at 130 \({}^{\circ}\)C. Although this deviated from the original design, the revised regime remains representative of low- to moderate-temperature Enhanced Geothermal System (EGS) conditions. While the reaction kinetics at 130 \({}^{\circ}\)C are slower, the fundamental degradation pathways such as surface precipitation, ion exchange, and internal pore modification are still expected to occur and were clearly identified in SEM-EDS, XRD, and micro-CT analyses. However, extrapolation to in-situ reservoir behavior must consider that dissolution-precipitation reactions scale nonlinearly with both temperature and ionic strength. The dilution likely reduced brine reactivity, moderating precipitation rates and possibly slowing certain mineral transformation processes. As a result, the chemical interpretations, particularly regarding ion exchange and secondary mineral growth, should be considered semi-quantitative. To improve accuracy in future studies, we recommend the use of pressure-rated reactors capable of maintaining high temperature and undiluted brine conditions to better simulate reservoir-scale behavior. The findings of this study demonstrate that chemical degradation is a critical mechanism influencing proppant stability, particularly during prolonged exposure to geothermal fluids subjected to repeated thermal cycling. Unlike previous studies that primarily assessed mechanical behavior, such as proppant crushing, embedment, or conductivity loss under closure stress, this work isolates static thermo-hydro-chemical (THC) effects, thereby providing a more precise evaluation of fluid-proppant chemical compatibility. Significant internal and surface changes were observed in the ultra-low-density (ULD) ceramic proppant, including a 30.4% reduction in intragranular porosity and extensive transformations in XRD peak profiles. These changes are consistent with the resin and binder degradation reported in prior studies [30; 34], where elevated temperatures (above 120 \({}^{\circ}\)C) triggered resin melting, blistering, and proppant adhesion. The emergence of new crystalline peaks in ULD and quartz-based sand proppants further supports the occurrence of mineral precipitation and silica transformation, corroborating earlier observations of dissolution-precipitation cycles and high-temperature diagenetic reactions in geothermal systems [28; 31]. The RCS proppant exhibited limited internal porosity change, yet pronounced intergranular porosity increase (+17.2%) due to resin softening and redistribution. SEM-EDS revealed carbon-, chlorine-, and calcium-enriched regions within agglomerated grains, suggesting resin melting and ion precipitation. These findings are consistent with resin layer liquefaction effects at >120 \({}^{\circ}\)C, as reported by [34], where coating degradation led to grain clustering and potential flow restriction. Such resin reflow and precipitation-driven agglomeration may compromise proppant pack uniformity, potentially reducing fracture conductivity over time, particularly under low-viscosity fluid conditions typical of EGS reservoirs. In contrast, the uncoated white and brown sand proppants, composed primarily of quartz, exhibited minimal morphological change via SEM, in line with previous findings suggesting quartz-based proppants are chemically stable under geothermal conditions [38]. However, micro-CT analysis revealed increased intergranular porosity in both (7.6% for white sand and 4.2% for brown sand), and EDS spectra identified surface deposition of Na, Mg, and Cl.