## 3 Methodology  
While the batch reactors were initially charged with site-specific geothermal fluids sourced from the Utah FORGE Harpoon storage tank, several vessels, particularly those containing uncrushed proppants, experienced cap failures within the first 24 h of exposure at 180 \({}^{\circ}\)C, likely due to pressure build-up and thermal expansion. This resulted in partial fluid loss through evaporation. To prevent further vessel damage and ensure safe experimental conditions, the tests were paused. The compromised caps were replaced with high-temperature-rated (200 \({}^{\circ}\)C) alternatives, and approximately 75% deionized (DI) water was added to each vessel to restore the original fluid volume. Consequently, the working fluids in the affected reactors became a mixture of diluted FORGE geothermal fluid and deionized (DI) water. This unintended adjustment effectively created an intermediate fluid composition that bridges the two end-member systems commonly reported in previous proppant stability studies. On one end of the spectrum, several studies have used DI water exclusively as the working fluid to assess baseline proppant reactivity under simplified geochemical conditions [26; 28; 29; 31; 35]. On the other end, researchers have tested proppants in synthetic brines formulated from DI water and specific chemical additives (e.g., NaCl, CaCl\({}_{2}\), NaOH, KCl, or silica) to simulate particular subsurface chemistries [26; 28; 30; 34]. Given this context, the diluted FORGE fluid employed in this study provides a valuable intermediate case, capturing aspects of both natural reservoir water composition and controlled laboratory formulations, thereby enhancing the relevance of the results for both field and laboratory-scale geothermal applications. Following these modifications, the remainder of the experiment was conducted at a revised temperature of 130 \({}^{\circ}\)C. All subsequent analyses and interpretations presented in this study reflect the impact of this altered fluid composition and adjusted temperature regime. The experimental framework, illustrated in Figure 3, was designed to evaluate the thermo-hydro-chemical (THC) interactions between selected proppant materials (1 ultra-low density ceramic (ULD), 1 coated proppant and 2 sand proppants), both in their as-received granular (uncrushed) and mechanically milled forms (using Spex Ball Mill from SPEX Industries, Inc., Metuchen, NJ, USA) and working fluids under simulated low-temperature EGS conditions. Figure 3: Experimental workflow showing baseline characterization (**a**) of proppants and fluids, static exposure (**b**) to working fluids at 130 \({}^{\circ}\)C with thermal cycling, and post-reaction analysis (**a**) to assess proppant chemical interactions and morphological changes. The investigation was conducted in three sequential stages:  
Stage 1 (Pre-exposure Characterization): Baseline characterization of the proppant materials was conducted to evaluate their initial morphological, mineralogical, and elemental properties prior to fluid exposure. As shown in Figure 3, both uncrushed and powdered proppant samples were analyzed to investigate the influence of particle size and surface area on subsequent reactivity. Uncrushed proppants were examined using SEM-EDS and micro-CT, while powdered samples were analyzed via XRD to determine mineralogical composition. Stage 2 (Static THC Exposure Tests): Proppant-fluid static exposure experiments were conducted to simulate geochemical interactions under low-temperature geothermal conditions. In Step (i), proppant materials were mixed with working fluids and sealed in PYREX round media storage bottles equipped with high-temperature caps. In Step (ii), the bottles were placed in water-filled nest containers at ambient conditions. In Step (iii), the sealed systems were exposed to static batch reactions at 130 \({}^{\circ}\)C for 25 days to simulate subsurface fluid-solid interactions. Step (iv) introduced thermal cycling to replicate the injection-production fluctuations characteristic of EGS.