## 4 Results
### SEM-EDS Analysis
These images were obtained to examine the baseline elemental distribution between the outer coating layers and the internal core structures prior to fluid exposure. Internal EDS analysis was conducted only for the ULD ceramic, as the RCS proppant's surface alterations closely resembled those of uncoated sand, suggesting that similar geochemical interactions likely occurred within the core, making additional internal characterization unnecessary. Two key factors influencing proppant integrity were identified from the SEM analysis of fractured proppants:  
* Fine Particle Generation: In conventional hydraulic fracturing, once fluid injection ceases, the reservoir transitions to closure stress conditions normally dominated by the minimum horizontal principal stress (Sh\({}_{\text{min}}\)). Under such metamorphic or igneous formation-induced stress, proppant crushing may occur, resulting in the formation of fines, which can significantly impair fracture conductivity and clog pore throats within the formation. As shown in Figure 9a, the ULD ceramic proppant exhibits less fine generation, indicating strong resistance to internal fragmentation. In contrast, the RCS proppant, Figure 9b, displays substantial fine particle generation highlighted by the white circle, suggesting reduced mechanical robustness and a greater susceptibility to degradation under confinement. * Coating Thickness: The ULD proppant is characterized by a thicker resin coating compared to the thinner coating observed on the RCS proppant. In the context of geothermal environments, where high temperatures and chemically aggressive fluids can degrade coating integrity, a thicker coating may offer enhanced chemical shielding, thereby extending proppant durability. This increased barrier may also help preserve  
Figure 8: SEM images highlighting surface degradation features on coated proppants post experiment: (a) ULD ceramic proppant showing surface discoloration and white extrusion-like deposits; EDS point analysis of these features indicates enrichment in oxygen, calcium, and carbon, (b) RCS proppant displaying exposed quartz substrate and unevenly redistributed coating. important to the development of the ULD program. Figure 10a,b presents EDS data for the ULD ceramic proppant and RCS proppant, respectively. For the ULD ceramic proppant, the pre-reaction resin coating was primarily composed of Carbon (C) (74.97%) and Oxygen (O) (22.69%), with trace amounts of Sodium (Na) (0.14%), Aluminum (Al) (0.49%), Silica (Si) (0.51%), and Phosphorus (P) (1.21%). Post-exposure analysis revealed a decrease in C content to 66.80%, indicating thermal and chemical degradation of the organic resin. Concurrently, Calcium (Ca) emerged in the elemental profile, and concentrations of Na (0.29%), Al (1.22%), Si (2.54%), and P (1.15%) increased, suggesting partial erosion of the coating and exposure of the underlying ceramic matrix. Analysis of the inner core of the ULD proppant revealed it was originally dominated by Si (20.86%), Al (17.54%), and O (55.45%). Following liquid-solid interactions, both Si (17.75%) and Al (16.56%) decreased, while C increased, implying resin softening and diffusion into the internal structure. This carbon infiltration may reduce intraparticle porosity, potentially compromising the initial structural integrity of the proppant. The appearance of additional elements in the post-reaction spectrum further supports chemical exchange between the degraded coating and fluid components. Figure 10: EDS spectra of coated proppants before and after THC coupled effects: (**a**) ULD ceramic and (**b**) resin-coated sand (RCS). Both proppants show significant reductions in carbon content and increases in elements such as Si, Na, Ca, and Cl. Figure 9: SEM images of fractured proppants showing inner and outer layers: (**a**) ULD ceramic and (**b**) RCS. The ULD proppant exhibits a thicker coating and intact core with less fine generation, while the RCS shows a thinner coating and significant fine particle formation shown by a circle.