## 3 Results and Discussion  
### Measurement of the Surface Roughness Value for the Sandstone Slabs  
Three-dimensional SPM surface images of the sandstone slab before and after the reaction confirmed that the serious surface roughness change occurred rapidly by the scCO\({}_{2}\)-sandstone-groundwater reaction (Figure 5). As seen by comparison with the initial surface, the polished surface of three major minerals (quartz, plagioclase, and biotite) transformed into a bumpy surface at 120 reaction days, suggesting that the weathering processes such as dissolution and precipitation actively occurred on the mineral surfaces of the S1 sandstone, directly increasing the surface roughness. SPM analysis of the calcite surfaces for the sandstone slabs was impossible because the calcites were perforated or seriously damaged by dissolution within 30 reaction days. Results of the SPM image analyses confirmed that the scCO\({}_{2}\)-groundwater-sandstone reaction directly resulted in a surface roughness change of the sandstones. The \(SR_{\text{RMS}}\) values for three different locations of three minerals for the three sandstone slabs were used to estimate the extent of the surface weathering process of the sandstone at different reaction times. A total of nine \(SR_{\text{RMS}}\) values for each sandstone slab at a certain reaction time and their average value for each sandstone (S1, S2, and S3) are shown in Table 3 and all the \(SR_{\text{RMS}}\) values are plotted in Figure 6. In the case of the S1 slab, the average \(SR_{\text{RMS}}\) of quartz increased from 0.573 to 1.293 \(\upmu\)m within 30 reaction days, and the rate of the change became gradually stable after 60 days (Table 3). The average \(SR_{\text{RMS}}\) of plagioclase and biotite also increased from 0.344 and 0.772 \(\upmu\)m to 1.905 and 1.485 \(\upmu\)m, respectively, within 30 days, and their \(SR_{\text{RMS}}\) values gently increased after 60 days. The trends of the \(SR_{\text{RMS}}\) change for the S2 and the S3 slabs were very similar to that for the S1 slab. The results suggest that the surface roughness increased actively at the early reaction stage, but the change in roughness stabilized after a few months of reaction. Figure 4: Schematic of the sandstone core experiment for physical property changes. Figure 5: Scanning Probe Microscope (SPM) images of three mineral surfaces for the S1 slab before (**left**) and after 120 reaction days (**right**). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Slab Type**} & \multirow{2}{*}{**Mineral Type**} & \multicolumn{6}{c}{**Reaction Time (day)**} \\ \cline{3-8}  & & **0** & **10** & **20** & **30** & **60** & **90** \\ \hline \multirow{8}{*}{S1} & Quartz-1 & 0.878 & 1.057 & 1.176 & 1.136 & 1.223 & 1.261 \\  & Quartz-2 & 0.322 & 1.164 & 1.438 & 1.541 & 1.441 & 1.446 \\  & Quartz-3 & 0.518 & 0.945 & 0.976 & 1.201 & 1.242 & 1.041 \\  & Plagioclase-1 & 0.164 & 1.007 & 1.012 & 1.711 & 1.143 & 2.017 \\  & Plagioclase-2 & 0.442 & 1.598 & 1.133 & 1.748 & 2.760 & 3.068 \\  & Plagioclase-3 & 0.427 & 1.130 & 1.294 & 1.905 & 2.160 & 3.391 \\  & Biotite-1 & 0.908 & 1.173 & 1.752 & 2.052 & 1.815 & 2.091 \\  & Biotite-2 & 0.749 & 0.994 & 1.259 & 1.078 & 1.612 & 2.071 \\  & Biotite-3 & 0.658 & 1.250 & 1.435 & 1.326 & 1.273 & 1.297 \\ S1 average \(\pm\) std & & \(0.563\pm 0.25\) & \(1.146\pm 0.20\) & \(1.275\pm 0.24\) & \(1.522\pm 0.35\) & \(1.630\pm 0.54\) & \(1.965\pm 0.82\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: \(SR_{\text{RMS}}\) (unit: \(\upmu\)m) vs. reaction time for the three sandstone slabs. Figure 6 shows the average \(SR_{\text{RMS}}\) for the three sandstones at different reaction days, comparing \(SR_{\text{RMS}}\) with the control slab (S1 slab reacted with only groundwater). The increase in the average \(SR_{\text{RMS}}\) for the S1 slab with only groundwater was negligible for 120 reaction days (only a 2.8% increase).