## 3 Results and Discussion
### Measurement of the Surface Roughness Value for the Sandstone Slabs
However, the average \(SR_{\text{RMS}}\) for the S1 slab with scCO\({}_{2}\) and groundwater increased by 2.5 times during 120 reaction days and 4.4 and 3.5 times for the S2 and the S3 slab, respectively. For all three sandstones with the scCO\({}_{2}\) and groundwater reaction, the average \(SR_{\text{RMS}}\) increased precipitously for 30 days of the reaction. After 60 days of the reaction, the \(SR_{\text{RMS}}\) increase became less severe, suggesting that no more serious increase in surface roughness occurred. For quantitative analysis of the surface roughness change for the three sandstones (S1, S2, and S3), the curve fitting results for their \(SR_{\text{RMS}}\) values using the logarithmic Equation (2) are delineated in Figure 6:  
\[SR_{\text{RMS}}\left(t\right)=a\ln(t)+b \tag{2}\]  
where \(t\) is the reaction time (day); \(a\) is the rate constant of the surface roughness change; and \(b\) is the intercept constant [54]. The \(r\)-square values for all fitted curves are higher than 0.83 (average of 0.891 in Figure 6), indicating that the \(SR_{\text{RMS}}\) change follows a well-defined logarithmic pattern. The pH of the groundwater decreased to 5.3 due to the dissolved scCO\({}_{2}\), which led to the dissolution of the sandstone slab in groundwater, but it became stable after 60 reaction days, suggesting that the amount of hydrogen ions in the groundwater increased due to the dissolution of CO\({}_{2}\) at an early stage, but the pH levels remained stable due to the buffering of dissolved ions with the increase of the reaction time (within 90-120 days). The trend of the pH decrease in the groundwater vs. the reaction time was also very similar to that of the \(SR_{\text{RMS}}\) increase of the slabs (the logarithmic decrease (or increase) pattern in Figure 6), indicating that the geochemical weathering process of the sandstone reservoir was very active in the early CO\({}_{2}\) injection time but stabilized within a few months. If the trend of the scCO\({}_{2}\) involved geochemical reaction is similar to that of the \(SR_{\text{RMS}}\), the surface roughness change could be used to quantify the extent of the scCO\({}_{2}\)-rock-groundwater reaction. In this study, the trend of the dissolved ion concentration change in the groundwater during the scCO\({}_{2}\)-involved reaction was also compared to the trend of the \(SR_{\text{RMS}}\) and pH change. The concentration of five cations in the groundwater during the reaction is shown in Figure 7.  
The average K\({}^{+}\) and Si\({}^{4+}\) concentrations in the groundwater increased from 23.3 and 12.4 mg/L to 30.0 and 26.0 mg/L after 30 reaction days, respectively, and were then stabilized by gradually decreasing after 90 days. The results suggest that the concentration of K\({}^{+}\) and Si\({}^{4+}\) mainly increased by the dissolution of silicate minerals such as K-feldspar and then decreased by the precipitation of secondary minerals, including K\({}^{+}\) ions (this is also supported by SEM/EDS analyses in Figures 8  
Figure 6: The \(SR_{\text{RMS}}\) and pH change by the scCO\({}_{2}\)-sandstone-groundwater reaction. Figure 7: Concentration change of five cations in the groundwater during the scCO\({}_{2}\) involved reaction (trend lines calculated from the data after 30 days). and 9). The average concentration of Ca\({}^{2+}\) and Na\({}^{+}\) increased from 547.1 and 633.2 mg/L to 663.8 and 692.0 mg/L after 30 days, respectively. However, after 90 days, they gradually decreased to 579.4 and 627.0 mg/L. The results showed that most minerals in the sandstone slab had a tendency to dissolve in the groundwater during the early reaction days (about 30-60 days), but the dissolution became stable after 90 days of reaction, resulting from the secondary precipitations overcoming the continuous dissolution during the geochemical reaction in the cell.