## 4 Results
### Micro-CT Analysis  
To evaluate microstructural evolution, high-resolution micro-CT imaging was used to quantify both intra- and intergranular porosity in proppant samples before and after the THC static experiment. Changes in intragranular porosity were interpreted as indicators of internal reactivity:  
* An increase in intraporosity suggests dominant internal dissolution or matrix degradation. * A decrease indicates dominant pore-filling precipitation or mineral deposition within the proppant structure. Variations in intergranular (interparticle) porosity reflect morphological changes influenced by particle sphericity, roundness, and potential coating deformation, often resulting from precipitation-deposition reactions. An increase in intergranular porosity for coated proppants may indicate resin redistribution or particle agglomeration, leading to reduced  
Figure 11: EDS spectra of uncoated sand proppants following geothermal fluid exposure: (**a**) white sand and (**b**) brown sand. Both proppants show decreased Si content (\(-\)10% and \(-\)27.5%, respectively) and the appearance of Mg, Na, and Cl. packing efficiency. Conversely, if no fluid-solid interactions occur, intergranular porosity should remain largely unchanged. Image segmentation was performed using the Otsu thresholding algorithm. Figure 12a,b shows the segmentation of proppant and air space, respectively, using the Otsu method. Closed pores within the proppants (intragranular pores) were isolated by removing the external air space from the region of interest through the \"process island, remove by largest\" process, as determined by the 6-connected method (Figure 12c). The segmented intragranular pores were then subtracted from the air space (Figure 12b) to isolate the external air space (Figure 12d). Figure 12e displays the segmented constituents in proppant samples. This multistep segmentation approach was applied to all samples to separate pores and proppants. Quantification was conducted only on the isolated internal pores and the external air space, both before and after the exposure to working fluids, to determine the compositional changes in the proppants. To quantitatively assess microstructural changes, intragranular porosity was calculated for each proppant type. Figure 13 presents pore space segmentation, illustrating the distinction between solid proppant grains while highlighting internal proppant void spaces for all tested materials. The corresponding porosity values, measured before and after fluid exposure, are reported in Table 4, as determined by multi-ROI analysis, providing insight into changes in proppant packing structure, grain morphology, and res  
Figure 12: Image segmentation by Otsu thresholding: (**a**) proppants in yellow, (**b**) air space in blue, (**c**) intragranular pores in blue, (**d**) intragranular in pink, (**e**) segmented 2D image. Figure 13: 3D distribution of pores in proppants (**a**–**d**) before and (**e**–**h**) after exposure to working fluids at 130 \({}^{\circ}\)C for 25 days, showing intra-granular (blue) changes with huge changes being for the ULD proppant. The ULD ceramic proppant exhibited the most notable reduction in both intra- and intergranular porosity, with a 30.4% decrease in intra pore volume and a 4.8% decrease in inter pore space. This pattern suggests dominant pore-filling deposition processes, likely due to resin redistribution and secondary mineral precipitation, consistent with observations from SEM-EDS. In contrast, the RCS proppant exhibited a substantial increase in intergranular porosity (+17.2%) while maintaining constant intragranular porosity, indicating notable resin softening and redistribution that led to grain agglomeration and reduced packing efficiency. These changes are clearly illustrated in Figure 14, which highlights alterations in packing structure driven by resin migration. This interpretation is further supported by SEM observations (Figure 7b) showing evidence of coating degradation and localized particle clustering.