## 5 Conclusions and Prospects  
This study employed triaxial compression tests to investigate the effect of reaction time on the compressive strength of sandstone subjected to brine-CO\({}_{2}\) treatment. Utilizing technologies such as nuclear magnetic resonance (NMR), and computed tomography (CT) scanning, pore distribution characteristics, porosity and microstructure of samples with varying reaction times were analyzed. This analysis aimed to elucidate the interaction mechanisms between these factors. The study reached the following conclusions:  
1. In sandstone samples soaked in salt water-CO\({}_{2}\) for an extended period, a significant decrease in compressive strength was observed; however, this decline did not persist with prolonged soaking time. Specifically, the compressive strength decreased from 152.7 MPa to 121.8 MPa during the initial months, rebounded to 148.9 MPa by the third month, and subsequently declined again by 16.8 MPa in the fourth month. Concurrently, similar trends were noted in the evolution of porosity and fractal dimension. As the immersion time increased, the failure mode progressively transitioned from simple shear failure to a combination of tension and shear failure. 2. In addition, based on the mathematical relationship between sandstone mass reduction and soaking time, the researchers established an exponential relationship curve. Through this curve, they defined the reaction rate of Ca\({}^{2+}\) and developed a corresponding formula for salinity. 3. The change curve of sandstone mass reduction can be categorized into three distinct reaction stages: the dissolution stage, the precipitation stage and the secondary dissolution stage. An analysis of the variations in the proportion of microscopic pore structures reveals that the dissolution of calcium carbonate predominantly occurs during the dissolution and secondary dissolution stages, whereas the precipitation of Ca minerals is primarily concentrated between the dissolution stages. Building on these findings, a microstructure model diagram of rocks in a saltwater-CO\({}_{2}\) environment was established. 4. Based on the three-dimensional crack expansion shapes of coarse sandstone observed over different corrosion durations, it was found that as the immersion time increased, the complexity of cracks in the reaction samples exhibited a pattern of first increasing, then decreasing, and subsequently increasing again. This variation is primarily concentrated during the dissolution stage, where fracture complexity increases, while a decreasing trend is noted during the precipitation stage. The long-term oil and gas injection and production process in deep carbon dioxide contains dynamic corrosion, and further experimental and numerical simulation studies  
Figure 14: Microstructure model diagram of the rock immersion process. (**a**) Beginning stage. (**b**) Dissolution stage. (**c**) Precipitation stage. (**d**) Secondary dissolution stage. are needed to explore the dynamic corrosion mechanism of carbon dioxide and salt water on reservoir rocks. Long-term CO\({}_{2}\) injection and production are controlled by many factors, such as reservoir characteristics, interfacial tension between salt water and CO\({}_{2}\), and reservoir pore pressure, so it is necessary to control the CO\({}_{2}\) injection and production rate according to the actual formation conditions of the reservoir. This long-term chemical interaction will promote the expansion of existing fractures and trigger new ones, and further research on the mechanism of fracture growth is necessary. A variety of numerical simulators have been developed to simulate chemical effects; however, current simulators have taken concentration into account relatively little, and laboratory experiments have not fully addressed actual reservoir conditions. Therefore, relevant experiments and numerical simulations are needed in the future to further study the mechanism of the dynamic corrosion of reservoir rocks by carbon dioxide and salt water, and consider the actual reservoir conditions.