## 3 Results and Discussion
### Measurement of the Surface Roughness Value for the Sandstone Slabs
The trend of cation concentration change in the groundwater after 30 days was also similar to those of the \(SR_{\text{RMS}}\) and pH change during the scCO\({}_{2}\)-sandstone-groundwater reaction, showing an interesting possibility for the use of \(SR_{\text{RMS}}\) change of the rock surface to estimate the process of the geochemical reaction (Figure 7). From these comparisons of trends, it was concluded that the dissolution process of the CO\({}_{2}\) reservoir sandstones was dominant at the early CO\({}_{2}\) injection time (less than 60 days), but precipitation also occurred sequentially to decrease the concentrations of dissolved ions and to stabilize the unbalanced geochemical system within a few months after CO\({}_{2}\) injection. To identify the weathering process of the reservoir rock by the scCO\({}_{2}\)-reservoir rock-groundwater reaction, the surfaces of the S1-1 sandstone slab were analyzed using SEM/EDS before the reaction and after 120 reaction days and their SEM images are shown in Figure 8.  
The SEM images show that the slab surface was heavily deformed by the geochemical reaction and the weathering process actively occurred along the existing fractures or mineral boundaries. The crack that existed at the mineral boundaries before the reaction was clearly expanded, and the calcites were perforated by the dissolution during the reaction, suggesting that these geochemical weathering processes can increase the porosity or the permeability of the reservoir rocks (this is discussed in detail in Section 3.2). In contrast, some pre-existing pores on the mineral surface were filled with secondary precipitates after the reaction, and additional precipitation on mineral surfaces such as quartz and biotite also occurred (Figure 8). The typical precipitates created on the S2-1 and S3-1 slab surfaces were analyzed using SEM/EDS after 120 reaction days, where they were determined to be amorphous silicate minerals as well as calcite, halite, and kaolinite, mostly consisting of Si\({}^{4+}\), Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Al\({}^{3+}\), and Fe\({}^{2+}\) (Figure 9). These SEM/EDS analyses also indicated that both dissolution and precipitation occur on the sandstone surface due to the weathering process during the scCO\({}_{2}\)-rock-groundwater reaction, which directly caused the roughness change of the rock surface and which represented the extent of the geochemical reaction over the reaction time. If these experimental results confirm that the \(SR_{\text{RMS}}\) change is directly proportional to the extent of the geochemical reaction, the trend of the \(SR_{\text{RMS}}\) change can be used to estimate the progress of the reaction, which mostly results from dissolution and precipitation. If the active dissolution and precipitation of sandstones occur over a short period of  
Figure 8: Scanning Electron Microscope (SEM) images of the S1-1 slab surface (**a**) before and (**b**) after 120 reaction days (Q: quartz, Pl: plagioclase, Pe: perhtite, B: biotite, C: calcite, and \(\square\): secondary precipitates). reaction time, accompanying the \(SR_{\text{RMS}}\) and physical property changes when CO\({}_{2}\) is injected into the sandstone reservoir, the activity of the geochemical reaction is likely to be an important parameter in determining the optimal CO\({}_{2}\) injection site, and further studies will be necessary to conclusively link the extent and influence with leakage safety of CO\({}_{2}\) sequestration.