## 3 Results and Discussion
### Effects of Pore-Throat Structures  
Figures 5 and 6 illustrate the impact of pore-throat structures on the space volume increase and reservoir pressure dynamics on the respective limestone and dolostone reservoirs during the CO\({}_{2}\) injection process, respectively. The space volume in this subsection means pore volume for the pore-dominant reservoir and fracture volume for the fracture-dominant reservoir. Figure 5a,b presents a comparative analysis of space volume changes and pressure buildup in both pore-dominant and fracture-dominant limestone reservoirs during the CO\({}_{2}\) injection process. Similarly, Figure 6a,b details the comparison of space volume change and pressure buildup in the dolostone reservoir under the same conditions. It is worth noting that both the pore or fracture radius and the CO\({}_{2}\) injection rate were held constant across all scenarios, set at 5 \(\upmu\)m and 0.1 mol/min, respectively. Figure 4: The comparison of (**a**) pore volume (PV) increase and (**b**) pressure increment in the dolostone reservoir with different pore sizes during CO\({}_{2}\) injection. The analysis shows a notably faster increase in the space volume within the pore-dominant reservoirs compared to their fracture-dominant counterparts. For instance, a limestone reservoir exhibited an 8.5% increase in space volume upon the injection of 4200 mol of CO2 in a pore-dominant structure, as opposed to a 6.8% increase in a fracture-dominant structure under identical injection conditions. This discrepancy is attributed to the differences in the reaction surface area provided by the geometrical shapes of the reservoir structures: cylindrical for fractures and spherical for pores. The spherical shape offers a larger surface-volume ratio, hence providing a larger reaction surface area in pore-dominant reservoirs when the space volume of the two types of reservoirs is the same. It has been stated in Section 3.2 that the larger reaction surface area not only accelerates rock matrix dissolution but also facilitates higher CO2 dissolution in brine, leading to a more significant increase in space volume. Furthermore, the disparity in space volume increase between pore-dominant and fracture-dominant reservoirs is more pronounced in limestone  
Figure 5: The comparison of (**a**) pore volume (PV) increase and (**b**) pressure increment in the limestone reservoirs with pore-dominant and fracture-dominant structures. than in dolostone. This is due to the faster CO\({}_{2}\)-brine-rock matrix interaction in limestone, which amplifies the effects of pore-throat structures on space volume increment. The reservoir pressure change with the injected CO\({}_{2}\) amount follows an opposite trend as the space volume increase vs. injected CO\({}_{2}\) amount data. Fracture-dominant reservoirs display larger and faster reservoir pressure increases than pore-dominant reservoirs, either in limestone or dolostone. This phenomenon is directly associated with the fact that fracture-dominant reservoirs possess smaller and slower space volume enlargement. Since the pressure is inversely related to the space volume based on the real gas EOS stated in Section 3.1, the reservoir pressures in the fracture-dominant reservoirs are larger and increase faster than those in the pore-dominant reservoirs. Similarly, the larger pressure difference between the pore-dominant and fracture-dominant reservoirs observed in limestone in comparison to that in dolostone can be explained by the larger discrepancy in space volume increase between the pore-dominant and fracture-dominant reservoirs in limestone. Figure 6: The comparison of (**a**) pore volume (PV) increase and (**b**) pressure increment in the dolostone reservoirs with pore-dominant and fracture-dominant structures. These findings underscore the dynamic interplay between pore-throat structures and CO\({}_{2}\) injection in carbonate reservoirs.