## 4 Discussion
### Model Diagram of the Influence of Soaking Time on Microstructure  
Based on the experimental results of NMR, CT scanning and salinity measurements, we constructed a model diagram to illustrate the microstructure changes of the rock during immersion. This diagram contains changes in mineral structure and rock structure, as shown in Figure 14. During the initial soaking stage, as shown in Figure 14a, the distribution of minerals was irregular and the grain arrangement between the different mineral components was also different. In addition to carbon dioxide solutions permeating the connected pores, bound water was also present in the molecular structure of the mineral. When we entered the dissolution stage, due to the corrosion of the CO2 solution and the increase in the acidity of the solution, minerals such as calcium carbonate gradually dissolved, resulting in a significant increase in the number of connecting holes. At the same time, the dissolution of certain minerals also opened channels in the unconnected pores, further increasing the porosity [11; 12; 13; 14; 15; 16; 17]. With long-term exposure to a CO2 solution, the crystal structure of calcium carbonate minerals is gradually destroyed and the rock structure changes, indicating the interaction between molecules of CO2 solution and molecules of solute attractor particles, as shown in Figure 14b. During the precipitation phase, the CO2 solution and minerals (such as calcium bicarbonate) produced secondary minerals, including calcium carbonate, which causes some fluid channels to narrow and subsequently reduces the overall porosity of the rock. At this time, CO2 solution molecules are expelled outward due to the formation of precipitates, as shown in Figure 14c. During the secondary dissolution stage, as the amount of secondary minerals (such as calcium carbonate) increased, the CO2 solution began to corrode these secondary minerals again. At this time, the corrosion effect was weaker than previously observed, resulting in a reduction in the amount of dissolved minerals. At the same time, when the porosity of the rock increased, the increase rate decreased. Under certain reaction conditions, the reaction between the mineral and the CO2 solution molecules continued and tended toward the equilibrium. However, this equilibrium position moved inward, as shown in Figure 14d. Most of the current CO2 injection research has been applied to CO2 geological sequestration and EOR projects in brackish water layers [41; 42]. [PERSON] et al. [43] applied the interaction of carbon dioxide, salt water and rock to the long-term storage of minerals in the salt water layer of carbon dioxide, and the analysis showed that both dissolution and chemical precipitation occurred in the long-term storage process of carbon dioxide. Calcite is dissolved in large quantities in low-pH and unsaturated solutions, and as the concentration of calcium ions in the solution increases, the precipitate is regenerated to produce pure secondary calcite. Such precipitation can result in a loss of CO2 injection effectiveness, which depends on the degree to which formation permeability decreases as the brine saturation increases [44]. During long-term CO2 injection and production, CO2, groundwater and reservoir rocks interact with each other, including the dissolution and precipitation processes. The strength of the rock in the dissolution process decreases obviously, and the injection and production efficiency is higher at this time, so the injection and production intensity should be increased. On the contrary, the strength of the rock in the precipitation process increases, and the difficulty of injection and production increases at this time; corresponding measures should be taken to improve the injection and production effect. This study has certain guiding significance for the long-term oil and gas injection and production of carbon dioxide.