## 3 Methodology
### Stage 2--Batch Reactor Experiments  
In Stage 2, as received and powdered proppants were subjected to static batch reactions. The experiments incorporated daily thermal cycling to emulate subsurface temperature fluctuations associated with cyclic injection-production operations typical of EGS. This setup was designed to simulate the coupled THC processes that govern in situ proppant performance. Although mechanical stresses were not applied in this study, the experimental framework was designed to capture the dominant chemical interactions, including resin degradation, dissolution-precipitation processes, and elemental leaching at the proppant-fluid interface. The proppant materials tested include,  
* White Sand (20/40 mesh) and Brown Sand (40/70 mesh): Representative of standard silica-based proppants widely used in hydraulic fracturing due to their cost-effectiveness and abundant availability.
* RCS (20/40 mesh): Selected for its enhanced resistance to pronounced degradation compared to sand proppants.
* ULD Ceramic Proppant (40/70 mesh): This ultra-low-density ceramic proppant was engineered primarily to enhance production rates and estimated ultimate recovery (EUR) in slickwater fracturing operations. Its design incorporates significant intragraular porosity, yielding an apparent density of approximately 2.0 g/cm\({}^{3}\), substantially lower than that of conventional quartz sand (-2.65 g/cm\({}^{3}\)) and standard low-density ceramic proppants. This reduced density improves proppant transport and suspension in low-viscosity carrier fluids. It is important to note that, although ceramic-based, this material is intentionally not a fully sintered ceramic, distinguishing it from conventional low, intermediate, and high-strength ceramic proppants commonly used in oil and gas applications.  
The working geothermal fluid was sourced from the Harpoon storage tank, which contains fluids to be injected into the Utah FORGE 16A(78)-32 injection well. The fluid's initial composition (before adding DI), including major cations, anions, and trace elements, is detailed in Table 3. This site-specific fluid was chosen to accurately represent the in-situ geochemical environment of the Utah FORGE reservoir, thereby ensuring that laboratory conditions closely mimic the reactive interactions between proppants and reservoir fluids under low-temperature geothermal settings.  
Following static exposure, all reacted proppant samples were thoroughly rinsed with deionized water to remove residual fluid and loosely bound surface precipitates, then oven-dried at 40 \({}^{\circ}\)C for 24 h. The dried samples were subsequently subjected to a comprehensive suite of post-reaction characterization techniques as part of Stage 3, aimed at evaluating morphological, mineralogical, and elemental changes resulting from thermo-hydro-chemical interactions.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ion** & **Concentration [ppm]** & **Ion** & **Concentration [ppm]** \\ \hline Na\({}^{+}\) & 1530 & SiO\({}_{2\text{ (aq)}}\) & 94 \\ \hline K\({}^{+}\) & 181 & Cl\({}^{-}\) & 2770 \\ \hline Ca\({}^{2+}\) & 209 & SO\({}_{4}\)\({}^{2-}\) & 74 \\ \hline Mg\({}^{2+}\) & 17.3 & HCO\({}_{3}\)\({}^{-}\) & 339 \\ \hline B(OH)\({}_{4}\)\({}^{-}\) & 25.2 & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Chemical composition of Utah FORGE working fluid [37].