## 4 Results
### Mass Balance Calculation and Numerical Modelling
According to [17; 53], CO\({}_{2}\) dissolution in the brine plays a crucial role in various storage processes (i.e., solubility trapping and mineral trapping). This dissolution produces carbonic acid (Equation (4)), which, in its turn, reacts with the highly soluble carbonates following the equations described by [54] (Equations (5)-(7)). \[\text{CO}_{2}+\text{H}_{2}\text{O}\rightarrow\text{H}_{2}\text{CO}_{3} \rightarrow\text{H}^{+}+\text{HCO}_{3}{}^{-} \tag{4}\]  
\[\text{CaCO}_{3}+\text{H}^{+}\rightarrow\text{Ca}^{2+}+\text{HCO}_{3}{}^{-} \tag{5}\]  
\[\text{CaCO}_{3}+\text{H}_{2}\text{CO}_{3}\rightarrow\text{Ca}^{2+}+2\text{ HCO}_{3}{}^{-} \tag{6}\]  
\[\text{CaCO}_{3}+\text{H}_{2}\text{O}\rightarrow\text{Ca}^{2+}+\text{HCO}_{3}{}^ {-}+\text{OH}^{-} \tag{7}\]  
Therefore, calcite dissolution is usually expected under these acidic conditions. Nevertheless, the analysis of whole rock composition by XRF (Table 1) did not indicate any dissolution or precipitation reactions in this case. This is probably due to the sample scale of the analysis. The dissolution effects are expected to be rather superficial while for the XRF measurement a 27 cm\({}^{3}\) block was used. The total amount of dissolved minerals is 271.5 mm\({}^{3}\) (Table 4), which is 0.25% of the initial volume of rock (4 blocks of 27 cm\({}^{3}\)). This quantity is very low and validates the results of the XRF acquired on the full rock blocks. Moreover, the high value of CaO in the rock composition also entails a high uncertainty range (\(\sim\)3.5%), which makes it difficult to evince small changes in the calcite composition by this technique. Nevertheless, the dissolution reactions are probably localized at the block surfaces. On the other hand, the changes of water composition clearly indicate dissolution reactions (see Table 2). The obtained data demonstrates the chemical instability of calcite and the relative stability of dolomite. Comparative studies on dissolution rates of different carbonates under similar conditions has shown higher dissolution rates for calcite and higher quantities of dissolved Ca\({}^{2+}\) than Mg\({}^{2+}\) in dolomites [55; 56], which is in agreement with our findings. Additionally, physical changes were observed in the pore network configuration by microscopic techniques including an increase in porosity and changes in pore morphology (Figure 4). Dissolution surfaces and newly formed pores were observed on calcite grains but not on dolomite, although the scarcity of dolomite crystals in the rock makes it difficult to carry out thorough observations. The quantification of pore parameters (optical porosity, aspect, roundness) by means of OIA did not reveal highly significant changes. It is important to point out, however, that while the qualitative observations were conducted both at thin section (OpM) and SEM scales, the quantification was only carried out on thin sections. Evidences of dissolution processes were mostly found by SEM observations so the measurements carried out by OpM clearly provide lower values of porosity and pore morphology changes than the reality. The obtained values should be considered keeping in mind this deviation. The described changes could be illustrated by the following simplified conceptual model (Figure 6a-d). The CO\({}_{2}\)-rich brine reacts with the sample producing chemical and physical changes on the external surface of the carbonate blocks. These effects are a result of partial carbonate dissolution affecting calcite grains and carbonate cement. In order to validate the obtained results and the conceptual model, mass balance calculation and numerical modelling was applied. Using the initial mineral composition of the rock sample determined by the XRF measurement and the water chemistry, we evaluated the quantity of dissolved and precipitated minerals and identified the dissolution and precipitation reactions that occurred during the 970 h of CO\({}_{2}\) rich water exposure. The numerical results are in agreement with the XRF data and water chemistry analysis.