## 6 Conclusions  
This study systematically evaluated the geochemical reactivity of four commercially available proppants, ultra-low-density (ULD) ceramic, resin-coated sand (RCS), white sand, and brown sand with diluted Utah FORGE working fluids under controlled Enhanced Geothermal System (EGS) conditions at 130 \({}^{\circ}\)C with thermal cycling over a 25-day period. Using a multi-modal analytical approach (XRD, SEM-EDS, and micro-CT), the research decoupled chemical degradation from mechanical effects to isolate fluid-proppant interactions that occur in low-temperature geothermal environments. The results demonstrated that proppant degradation is governed not only by temperature and pressure, as shown in prior studies, but also by fluid chemistry and material-specific composition. The ULD ceramic proppant exhibited significant internal pore filling (\(-\)30.4%) and secondary mineral precipitation. RCS proppants showed substantial surface-level resin redistribution, leading to a 17.2% increase in intergranular porosity and exhibited a substantial decrease in surface C concentration, dropping from 72.98% to 53.05%. Uncoated sand proppants, despite minimal visual morphological change, exhibited measurable increases in porosity and surface precipitation, highlighting their interactions with geothermal fluids. These findings reinforce the importance of fluid-proppant chemical compatibility in the long-term viability of EGS systems. The study also revealed that coating thickness, elemental composition, and proppant morphology collectively influence degradation mechanisms, such as resin leaching, grain clustering, and ion exchange reactions. By decoupling chemical and mechanical degradation and introducing a comprehensive fluid-proppant framework, this work advances the mechanistic understanding of proppant behavior in chemically evolving geothermal reservoirs. These insights support the need for continued development of high-temperature-stable proppants and coatings, and for integrating realistic reservoir fluid pathways, including evolved rock-fluid interactions, into future testing protocols. Conceptualization, M.R. ; Data curation, B.M. ; Formal analysis, B.M. and B.D.D. ; Funding acquisition, M.R. ; Investigation, B.M. ; Methodology, B.M. and M.R. ; Project administration, M.R. ; Resources, T.P. and M.R. ; Software, B.M. and B.D.D. ; Supervision, A.E. and M.R. ; Validation, B.M., A.E., B.D.D. and M.R. ; Visualization, A.E., B.D.D. and M.R. ; Writing--original draft, B.M. ; Writing--review & editing, A.E., B.D.D., T.P. and M.R. All authors have read and agreed to the published version of the manuscript. This material is based upon work supported by the U.S. Department of Energy, Geothermal Technologies Office, under Award Number DE-EE0007080 for the Enhanced Geothermal System Concept Testing and Development at the Milford City, Utah FORGE Site, and under Contract UEI Number NNYDFK5 FTSX9, Grant Award Number 9-3706. The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author(s). Parts of this work were carried out in the Microscopy Laboratory at Oklahoma State University, which received funding for equipment from the NSF MRI program. Part of this work was carried out in the ATRC Imaging Suite, which is funded by the College of Engineering, Architecture, and Technology at Oklahoma State University. We are grateful to the Utah FORGE team for providing proppants and fluid samples. We also thank CARBO Ceramics for supplying the proppant materials used in this study. Special thanks to [PERSON] for granting access to his laboratory for sample preparation and to the OSU Geomimicry Group for their valuable support throughout this work. Their generous support and collaboration were invaluable to the success of this research. Conflicts of Interest: Author [PERSON] is employed by the CARBO Ceramics Inc.