## 3 Results and Discussion
### Measurement of Physical Property Changes for the Sandstone Cores  
Table 4 shows the porosity changes for the three sandstones (total nine cores) vs. the reaction time (total of 150 days). The initial average porosity of the S1 sandstone cores was 8.097%, and it increased to 8.642% after 150 reaction days (6.7% increase from the initial value). The average porosity of the S2 and S3 sandstones also increased to 8.2% and 9.9% compared with the initial values, respectively. Even though the porosity increase of the rock cores occurred at the early stage of the reaction because of crack extension and perforation dominated by the dissolution, this was moderated within a few months, showing a logarithmic increase vs. reaction time (Figure 10a). The average dry density of the sandstone cores decreased from 2.390 to 2.384 g/cm\({}^{3}\) for 120 reaction days (0.25% decrease from the initial value) (Table 5). The average seismic wave velocities of the sandstones also decreased in a logarithmic manner to 5.67% for P-wave and to 5.75% for S-wave from the initial values for 150 reaction days (Figure 10b and Table 5). The decrease in the dry density and seismic wave velocity for the sandstones was associated with an increase in porosity due to the geochemical weathering process during the reaction, which considers their interrelation characteristics. For a more quantitative analysis of the physical property changes for the nine sandstone cores, the curve fitting results for their property values using the logarithmic Equation (2) are delineated in Figure 10. The \(r\)-squared values for all fitted curves are higher than 0.88 (average of 0.921), indicating that the rock property changes follow a well-defined logarithmic increase (or decrease) pattern, which were very similar to those of the geochemical parameter changes such as pH, cation concentration, and \(SR_{\text{RMS}}\) over the reaction time. Figure 9: SEM images for precipitates created on the slab surface after 120 reaction days (\(\square\): halite, \(\square\): calcite, and \(\square\): amorphous silicate minerals). These similarities are evidence demonstrating that the rock property change directly originated from the geochemical reaction with the injected scCO\({}_{2}\), which further resulted in the rock surface roughness change and ion concentration change in the groundwater, and also that the surface roughness measurement provides a lot of quantitative information regarding the extent of the geochemical reaction related to the scCO\({}_{2}\) in the subsurface. The results showed that the physical properties of the sandstone changed rapidly within a few months due to the geochemical reaction with the injected CO\({}_{2}\), but the rate decreased in a relatively short time (a few months), verifying that the physical property change of the CO\({}_{2}\) reservoir rocks occurred faster than expected from previous research [56; 57; 58]. Because the effect of the geochemical reaction on the reservoir rock property change is moderated after a short time, attention to the geochemical reaction at the early CO\({}_{2}\) injection stage is very important in preventing leakage and in understanding CO\({}_{2}\) sequestration site characteristics from the CO\({}_{2}\) storage capacity point of view.