## 4 Results
### SEM-EDS Analysis  
Figure 7 presents representative SEM images of proppant materials before and after static fluid exposure: (a) ULD ceramic proppant, (b) resin-coated sand (RCS), (c) white sand, and (d) brown sand. Prior to exposure, the ULD ceramic proppant exhibited a smooth, dark surface characteristic of a continuous resin coating. Post-exposure, notable surface degradation was observed, including whitening and the development of localized surface disruptions. Regions of clustering indicated by arrows revealed elevated concentrations of silicon and aluminum in EDS scans (elements associated with the ceramic core), suggesting rupture of the coating and exposure of the underlying structure. The RCS proppant (Figure 7b) initially displayed a uniform resin coating, which, after exposure, showed evidence of delamination and scaling. Scale-like textures and small surface voids were apparent, accompanied by whitening and particle clustering. SEM-EDS analysis of the agglomerated regions (shown by arrows) indicated enrichment in carbon, chlorine, and calcium, consistent with resin softening, migration, and ion precipitation from the geothermal fluid. In the white sand proppant (Figure 7c), only minor morphological changes were visible. However, bright surface deposit features emerged after fluid exposure, possibly indicating incipient surface precipitation. The brown sand proppant (Figure 7d) displayed more visible alteration than the white sand. SEM images revealed the formation of surface deposits or secondary mineral phases, particularly along grain boundaries, along with similar white features as seen in the white sand. These are interpreted as precipitation products resulting from dissolution-precipitation processes at the solid-fluid interface. SEM observations collectively highlight distinct chemical and morphological responses among the tested proppants. The coated proppants (ULD and RCS) exhibited significant surface degradation, further evidenced in Figure 8. In the ULD proppant (Figure 8a), white, extrusion-like deposits were observed on the surface. SEM-EDS point analysis of these features revealed a composition primarily of oxygen (61.55%), calcium (20.29%), and carbon (16.67%), along with minor elemental constituents, suggesting precipitation of fluid-derived species and resin degradation. Figure 8b shows the RCS proppant after exposure, with an  
Figure 7: SEM images of proppants before and after exposure to geothermal fluids: (**a**) ULD ceramic, (**b**) resin-coated sand (RCS), (**c**) white sand, and (**d**) brown sand. Arrows indicate regions of particle agglomeration and surface modification. Coated proppants exhibit resin degradation, whitening, and clustering due to thermal and chemical alteration. EDS analysis of marked areas revealed core material exposure in the ULD proppant and carbon-, calcium-, and chlorine-rich deposits on the RCS proppant, indicating resin redistribution and ion precipitation. Sand proppants show white surface deposits and minor morphological changes, consistent with secondary phase formation at the fluid–grain interface. exposed quartz-rich substrate and an unevenly redistributed coating layer, indicative of thermal softening and partial resin delamination. Although the uncoated sand proppants appeared morphologically stable, they may have experienced chemical alterations that are not readily detectable by SEM imaging alone. Despite their intact appearance, these materials may exhibit higher chemical reactivity than visually suggested. In contrast, while the coated proppants showed clear physical degradation, they may offer greater resistance to fluid-induced chemical alteration due to the protective function of their resin or ceramic coatings. To further investigate internal compositional characteristics, Figure 9a,b present SEM images of mechanically fractured, unreacted ULD ceramic and RCS proppants, respectively.