## 5 Discussion
This suggests localized reactivity, particularly at grain edges and microfractures, even in otherwise inert silica-rich proppants. Surface-phase contributions followed the order: 0.5% (ULD), 4.1% (RCS), 7.1% (white sand), and 15.2% (brown sand),indicating that proppant reactivity increases from coated to uncoated materials. These trends align with [13], which documented enhanced silica leaching and instability in sand-based proppants under geothermal conditions. In this study, we investigated several degradation pathways, chemical, morphological, and mineralogical, that can ultimately compromise proppant integrity under geothermal conditions. To meaningfully compare materials and assess their relative tolerance to low-temperature EGS environments, it is essential to limit comparisons to materials with similar structural and compositional properties, as comparing coated and uncoated proppants would be inappropriate due to their fundamentally different degradation mechanisms. We encourage future studies to adopt a structured approach, such as multi-criteria decision analysis (MCDA), using methods like the Analytic Hierarchy Process (AHP), to guide proppant selection based on both degradation indicators and cost. For illustrative purposes, we compared two coated proppants; ULD Ceramic and resin-coated sand (RCS) across six criteria: fine generation (from SEM, Figure 9), intragranular porosity change (Table 4), surface precipitation area (Figure 10), carbon content loss (from SEM-EDS), number of new XRD peaks (Figure 5), and market price. Assuming cost ranges of $350-$400 per ton for ULD Ceramic and $50-$200 per ton for RCS, the AHP pairwise matrix to emphasize the dominant importance of minimizing fines and structural degradation while accounting for price as a practical constraint was estimated. The resulting criterion weights were: fine generation (0.520), intragranular porosity change (0.190), surface precipitation area (0.080), carbon content loss (0.060), XRD peak changes (0.050), and price (0.100). After normalization using experimental data and assumed pricing, ULD Ceramic achieved a composite AHP score of 0.880, compared to 0.410 for RCS. This example illustrates how future studies can incorporate cost, degradation metrics, and performance priorities into a unified framework, which can be further expanded to include mechanical performance under fully coupled THMC conditions. This study advances the understanding of proppant degradation mechanisms in geothermal systems by isolating chemical effects from mechanical loading. The findings indicate that chemical degradation alone, manifested through coating redistribution, ion exchange, internal pore modification, and secondary mineral precipitation, has the potential to significantly compromise proppant integrity. These microstructural changes may directly impact long-term fracture conductivity and the overall sustainability of EGS reservoirs. To translate these laboratory-scale observations to field-scale performance, future work should incorporate flow-through and mechanical crushing tests. Such experiments are essential for quantifying how mineral precipitation and surface alteration influence proppant strength, pack permeability, and hydraulic conductivity under realistic subsurface conditions. Additionally, proppant behavior should be assessed under fully coupled THMC stresses. For example, the application of confining stress may expose further degradation mechanisms, such as enhanced fines generation in sand proppants, which can severely reduce fracture conductivity through pore throat blockage and increase geochemical reactivity due to expanded reactive surface area. Future research should therefore prioritize dynamic flow-through experiments under controlled stress conditions to better simulate the complex, high-temperature environment of geothermal reservoirs and comprehensively evaluate proppant durability.