## 3 Results
### Influence of CO2 Transport  
Consideration of CO2 transport (in liquid and gas phases) in the numerical analysis allowed the CO2 distribution across the barrier to be obtained. CO2 diffusion in the gas phase is a fast process and will govern the transfer of the total CO2. In Case C1, the CO2 initial partial pressure in the bentonite (pCO2 = 10-2 bar) was higher than that imposed by the granite, and therefore, the gas diffusion was oriented towards the bentonite-granite interface. Due to degassing near the heater, the CO2 content increased to some extent and then quickly decreased to the boundary value (pCO2 = 10-4 bar) (Figure 10a). Figure 9: pH profile in the bentonite at t = 1, 5 and 18 y: (**a**) without consideration of mineral dissolution, cation exchange, and surface complexation, Case A; (**b**) with consideration of mineral dissolution, cation exchange, and surface complexation, Case B. In Case C2, the granite imposed a much higher partial CO2 pressure, and thus, the influx of CO2 into the bentonite was observed. Over time, its dissolution decreased with an increasing temperature, and some buildup of CO2 was observed (Figure 10b). This distribution equilibrated quickly across the barrier and the profile was showing the decreasing trend towards the bentonite-granite interface. CO2 distribution is important for mineral calcite: as the pressure of CO2 gas in a system increases then calcite dissolves, and the decrease of CO2 gas pressure results in calcite precipitation. However, if water temperature increases, then the amount of CO2 dissolved in water decreases and leads to calcite precipitation. Due to the decrease in water temperature, the amount of CO2 dissolved in water increases and causes calcite to dissolve. In our modelled system, these processes were not taking place one by one but occurred at the same time. Thus, a combined effect could be expected. Additionally, they were accompanied by other processes, such as dissolution/precipitation of calcite, anhydrite/gypsum, and cation exchange, where Ca2+ ions were involved. The modelling results showed that an imposed higher pCO2 at the bentonite-granite interface led to a more significant calcite precipitation and dissolution of quartz close to the interface but not in the region close to the heater. In the inner part of the bentonite layer (closer to the heater), the anhydrite precipitation took over the other minerals in terms of increased volume fraction (Figure 11) in both cases C1 and C2. Subsequently, the resulting changes in porosity are presented in Figure 12.  
In the case of a low pCO2 at the interface, the calcite dissolution at the bentonite-granite interface led to a slight porosity increase (Figure 12a) near the interface. A slight increase in porosity was also obtained in Case B without consideration of CO2 gas transport. An imposed higher pCO2 would lead to a decreased porosity, which was also more variable farther from the bentonite-granite interface. As the porosity variations were observed near the bentonite-granite interface, it seems that CO2 de-gassing due to temperature-dependent solubility did not have a significant impact. Nevertheless, the observed porosity changes did not impose a significant impact on non-reactive tracer (CI\({}^{-}\)) transport based on the concentration profiles presented in Figure 13.  
Figure 10: Partial pressure of CO2 across the bentonite at different times (after 0.001 years, 1 year, 5 years, 18 years of simulation): (**a**) at boundary pCO2 = 10\({}^{-4}\) bar (Case C1); (**b**) at boundary pCO2 = 10\({}^{-1}\) bar (Case C2). Figure 11: Simulated concentration of minerals by the end of simulation: quartz (1), calcite (2), anhydrite (3) for Case C1 (**a**) and Case C2 (**b**) (not to scale). Figure 12: Impact of pCO\({}_{2}\) of incoming porewater (at bentonite–granite interface) on barrier porosity evolution after 1 y (t1), 5 y (t2), 18 y (t3) of heating in Case C1 (**a**) and Case C2 (**b**) (not to scale).