## 4 Results
### X-Ray Diffraction Analysis  
Pre- and post-reaction XRD assessment focused on identifying the emergence of new diffraction peaks, shifts in peak positions, and changes in relative peak intensities, all indicative of possible phase transformations, dissolution-precipitation dynamics, or  
Figure 4: (a) Qualitative visual comparison of fluid clarity before and after reaction, highlighting color changes and particulate presence across different proppant types. (b) Visual and morphological changes in proppants before and after fluid exposure: ULD ceramic shows signs of coating degradation and chemical alteration; RCS proppant exhibits darkening, resin redistribution, and potential grain clustering; white and brown sand proppants show no significant visual or morphological changes post-reaction. structural modifications of the proppant powder material. While these metrics provide useful insight into mineralogical evolution, it is recognized that reaction-induced products may include amorphous or poorly crystalline phases, which cannot be reliably detected by XRD, particularly when present in quantities below the \(\sim\)2 wt.% detection limit. Given the limitations, no specific mineral phases were assigned to newly observed peaks in order to avoid speculative interpretations. Instead, proppant reactivity was qualitatively assessed based on observable peak modifications, including the appearance of new reflections, peak intensity fluctuations, and lattice shifts, which collectively signal geochemical or structural changes resulting from the static interaction with diluted geothermal fluids. Figures 5 and 6 present the XRD diffractograms of coated and sand-based proppant powders before and after the experiment. Figure 5a presents the XRD diffractograms for the ULD ceramic proppant, which is primarily composed of mullite along with broad amorphous peaks likely associated with binders, resins, dispersants, and foaming agents. Notable reductions in peak intensities were observed at 12.2\(\lx@math@degree\), 16.4\(\lx@math@degree\), and 21.6\(\lx@math@degree\) 2\(\theta\), corresponding to known epoxy resin signatures, as indicated by the comparison between pre- and post-reaction spectra (ULD before vs. ULD after). Additional major declines were noted at 26.3\(\lx@math@degree\) and 35.2\(\varnothing\) 2\(\theta\) (refer to \(\Delta\) peak), suggesting both resin degradation and possible breakdown of binder components. The difference in peak intensity was also quantified and is represented by the purple graph (\(\Delta\) Peak) in Figure 5a, highlighting regions of structural and chemical transformation. To isolate newly formed peaks, the following threshold condition was applied: \"If Intensity\({}_{\text{before}}\) < 1 and Intensity\({}_{\text{after}}\) > 50, then classify as a 'New Peak'.\" Following exposure, new major peaks (New peak) emerged at 29.0\(\lx@math@degree\), 31.7\(\lx@math@degree\), and 45.4\(\lx@math@degree\) 2\(\theta\), indicating the formation of secondary mineral phases likely due to geochemical reactions between the ceramic matrix and geothermal fluid. Figure 5: XRD patterns of coated proppants before and after exposure to working fluid. **(a)** ULD ceramic shows peak reductions (e.g., 12.2\(\lx@math@degree\), 21.6\(\lx@math@degree\) 2\(\theta\)) and emergence of new phases (e.g., 29\(\lx@math@degree\), 31.7\(\lx@math@degree\) 2\(\theta\)), indicating resin degradation and mineral formation. **(b)** RCS exhibits minor peak changes and new peaks at 31.7\(\lx@math@degree\) 2\(\theta\). Collectively, the disappearance of primary peaks and emergence of new reflections confirms significant thermo-chemical reactivity of the ULD ceramic proppant under simulated low-temperature ECS conditions. Figure 5b presents XRD data for the RCS proppant, which was mainly quartz. Compared to ULD ceramic, the RCS proppant exhibited more subtle changes in diffractograms.