## 4 Conclusions  
The dissolution of the supercritical CO\({}_{2}\) in groundwater, which is referred to as CO\({}_{2}\) solubility trapping, is an important trapping mechanism of CO\({}_{2}\) geological storage in the deep aquifer. Considering the diffusion and dispersion of the CO\({}_{2}\) migration, an accurate measurement of the dissolution rate of CO\({}_{2}\)-brine at the pore scale is necessary to evaluate the dissolution process in the reservoir. In this study, the pore-scale dissolution rate during brine imbibition was calculated for both supercritical and gaseous CO\({}_{2}\) using X-ray computed microtomography. Also, we verified the different impacts of the porosity and saturation and the Reynolds number on both supercritical and gaseous CO\({}_{2}\). The key novelties of the presented results in this study are summarized as:  
* For flow development under a supercritical CO\({}_{2}\) state, high contact between the brine and CO\({}_{2}\) resulted in a greater CO\({}_{2}\) dissolution rate, while the flow bypassing restrained the dissolution rate with CO\({}_{2}\) confined to the central region. The detached part of the bubbles which happened to snap-off rapidly dissolved. The isolated bubbles reduced the overall CO\({}_{2}\) dissolution rate and prolonged the depletion time due to the slow diffusion. For gaseous CO\({}_{2}\), the flow development was limited due to the greater density difference, implying lower dissolution rates and a lower probability of dissolution events. The isolated gaseous CO\({}_{2}\) existed for much longer and significantly prolonged the duration of the dissolution. * The dissolution rate of supercritical CO\({}_{2}\) was about two or three orders of magnitude greater than that of gaseous CO\({}_{2}\), while the value distributions both spanned about four orders of magnitude. The dissolution rate of gaseous CO\({}_{2}\) decreased more dramatically because of the diverse number of residual CO\({}_{2}\) bubbles. * The dissolution rate of supercritical CO\({}_{2}\) decreased as the porosity increased, and this trend increased with the increment of porosity. The dissolution rate of gaseous CO\({}_{2}\) gradually increased with porosity, which showed no obvious effect on the variation of the dissolution rate. When the porosity is between 0.36 and 0.38, the value ranges of dissolution rates under a supercritical state were greater. * At high CO\({}_{2}\) saturation, the dissolution rate of supercritical CO\({}_{2}\) increased rapidly with the saturation reduction. At low CO\({}_{2}\) saturation, the correlation between the dissolution rate and saturation was inverted at later PVs, and was controlled by the location and quantity of the dissolution events. By contrast, the dissolution rate of gaseous CO\({}_{2}\) remained inversely proportional to the saturation. However, the relationships were close to supercritical CO\({}_{2}\) after saturations were less than 0.5. * During the dissolution expansion, the dissolution rate of supercritical CO\({}_{2}\) increased rapidly as the Re decreased, and the differences between Re were about one order of magnitude. At later PVs, the dissolution rate gradually increased with the Re, and the trend was not correlated with the injected PV. By contrast, the dissolution rate of gaseous CO\({}_{2}\) significantly decreased with the increase of Re. As most CO\({}_{2}\) was dissolved, the dissolution rate decreased slightly at larger Re values, whereas the differences between Re were inconspicuous. Measurements from this study provided the relationship between parameters in the pore-scale flow regime. It is not evident that an upscaling simulation may readily import the dissolution rate results of this study to reservoirs. Also note that the effects of salinity and wettability on the CO\({}_{2}\)-brine dissolution process remain pertinent in porous media, which deserves further investigations. This study has been supported by the National Natural Science Foundation of China (Grant No.