## 2 Materials and Methods
### Apparatus and Methodology
#### 2.2.4 Co\({}_{2}\)-Water-Rock Interaction  
Two groups of samples were prepared for the experiments. In order to accelerate the corrosion process, rock samples of the Group CO\({}_{2}\) were interacted with the high pressure of CO\({}_{2}\) (7 MPa). Another group of rock samples was immersed in seawater in a beaker without adding extra CO\({}_{2}\), serving as a reference group. Each group contained 11 specimens, including five cube samples, three cylinder specimens for uniaxial compression test, and three Brazilian disc samples.  
The CO\({}_{2}\)-seawater-rock chemical interaction was simulated by exposing the rock samples in a stainless steel (316 L) hydrothermal reactor, as shown in Figure 5, equipped with a manometer and automatic temperature controller. The cylindrical reactor was fitted with a gas inlet and liquid sampling port, and the dimensions were as follows: inner diameter, 110 mm; and height, 150 mm. The CO\({}_{2}\) gas was injected by the syringe pump. In this study, the reactor was set at 30 \({}^{\circ}\)C, and the pressure was kept at 7 MPa. Because the salinity of seawater of the South China Sea is about 3.32-3.44% [34], the reaction solution salinity used in this study was 3.4% prepared by dissolving 34 g sea salt per liter of ultrapure water. The testing procedures for the specimens of Group CO\({}_{2}\) were as follows:  
1. The solution was prepared, and its concentration of Ca\({}^{2+}\) before interaction was measured by a compact calcium ion meter (HORIBA Ca-11, Kyoto, Japan). The initial Ca\({}^{2+}\) concentration of the solution is 460 ppm.
2. All samples were dried and weighed prior to testing. Then, the volume and permeability of the cylinder specimens were measured,
3. All samples were placed into the hydrothermal reactor, and 800 mL seawater was added. All samples were immersed by the solution. Then, CO\({}_{2}\) was injected into the reactor by the syringe pump. The pressure and temperature were held at 7 MPa and 30 \({}^{\circ}\)C, respectively.
4. The solution was analyzed for dissolved Ca\({}^{2+}\) concentration during the reaction. After 3 days of reaction time, the specimens were taken out and washed repeatedly with distilled water to avoid influence from salt crystals after drying. Then, they were oven-dried to study the changes of mass. The effective volume and permeability of the cylinder specimens were tested by the methods introduced above. One of the cube specimens was taken and prepared for SEM analysis.
5. The specimens (except the sample analyzed by SEM) were placed back into the reactor, and the same amount of seawater was added to continue the next stage of reaction. Then, steps (3)-(4) were repeated.
6. After 15 days, all the samples were taken out from the reactor to investigate the evolution of the dry mass, effective volume, permeability, and the change of surface properties and mechanical behavior.  
Figure 5: Schematic diagram of the hydrothermal reactor system: pressure up to 30 MPa and temperature up to 100 \({}^{\circ}\)C. The inlet in the reaction kettle allowed CO\({}_{2}\) in. The outlet with a filter enabled liquid sampling.  
For the specimens of Group control, the same amount of seawater was used, and the temperature was kept at the constant temperature of 30 \({}^{\circ}\)C by a thermostatic water bath. To account for time-dependent effects, the cube samples of Group control were taken out to investigate the evolution of the dry mass every 3 days. After 15 days, all samples were taken out to investigate the changes of the dry mass, and the effective volume and permeability of the cylinder samples were tested. The last cube sample was observed by SEM. In addition, mechanical tests were conducted.