## 2 Background on Mineral Carbonation Experiments and Tests  
During mineral carbonation, the conversion of CO2 to stable minerals starts with the CO2 dissolution in the aqueous phase, a function of the fluid ionic strength, pressure and temperature [16]. The resulting carbonic acid (H2 CO3) decomposition releases hydrogen protons (H+), lowering the pH (Equation (1)). Subsequent consumption of the H+ due to reaction with the Mg-Ca-Fe-rich mineralsreleases the metallic cations and increases the pH of the solution (Equations (2) to (6)). At suitable pH and saturation conditions, cations combine with hydrogen carbonate and form (Ca, Mg, Fe) CO\({}_{3}\). \[\text{CO}_{2}\left(\text{g}\right)+\text{H}_{2}\text{O}\left(\text{aq}\right)= \text{H}_{2}\text{CO}_{3}\left(\text{aq}\right)=\text{H}^{+}\left(\text{aq} \right)+\text{HCO}_{3}\left(\text{aq}\right) \tag{1}\]  
\[\text{(Forsterite)}\text{Mg}_{2}\text{SiO}_{4}\left(\text{s}\right)+4\text{H}^ {+}\left(\text{aq}\right)=2\text{Mg}^{2+}\left(\text{aq}\right)+\text{SiO}_{2} \left(\text{aq}\right)+2\text{H}_{2}\text{O}\left(\text{l}\right) \tag{2}\]  
\[\text{(Ca-plagioclase)}\text{CaAl}_{2}\text{Si}_{2}\text{O}_{8}\left(\text{s} \right)+8\text{H}^{+}\left(\text{aq}\right)=\text{Ca}^{2+}\left(\text{aq} \right)+2\text{Al}^{3+}\left(\text{aq}\right)+2\text{SiO}_{2}\left(\text{aq }\right)+4\text{H}_{2}\text{O}\left(\text{l}\right) \tag{3}\]  
\[\text{(Clinopyroxene)}\text{CaMg}_{2}\text{Si}_{2}\text{O}_{6}\left(\text{s} \right)+4\text{H}^{+}\left(\text{aq}\right)=\text{Ca}^{2+}\left(\text{aq} \right)+\text{Mg}^{2+}\left(\text{aq}\right)+2\text{SiO}_{2}\left(\text{aq }\right)+2\text{H}_{2}\text{O}\left(\text{l}\right) \tag{4}\]  
\[\text{Mg}^{2+}\left(\text{aq}\right)+\text{HCO}_{3}{}^{-}\left(\text{aq} \right)=\text{MgCO}_{3}\left(\text{s}\right)+\text{H}^{+}\left(\text{aq} \right) \tag{5}\]  
\[\text{Mg}^{2+}\left(\text{aq}\right)+\text{Ca}^{2+}\left(\text{aq}\right)+2 \text{HCO}_{3}{}^{-}\left(\text{aq}\right)=\text{(Ca,Mg)CO}_{3}\left(\text{s} \right)+2\text{H}^{+}\left(\text{aq}\right) \tag{6}\]  
Numerous laboratory experiments have been performed to study the reactivity of olivine, serpentine, pyroxene, amphibole and plagioclase mineral groups, and glass basalt lavas with CO\({}_{2}\)-enriched solutions [17, 18, 19, 20]. Most of these experiments were conducted in batch conditions at controlled pressure, temperature and P\({}_{\text{CO}_{2}}\), using crushed rock. Sodium hydrogen carbonate (NaHCO\({}_{3}\)) is often added in the reactor to increase the hydrogen carbonate concentration in solution and buffer the pH up to 7.7 and 8.0 [21]. This pH range, temperatures from 155 \({}^{\circ}\)C to 185 \({}^{\circ}\)C and total pressures from 11.5 MPa to 19 MPa [21, 22, 23] provide the optimal conditions for carbonation enhancement. In general, the parameters that affect the rate of carbonate minerals' precipitation are brine composition, temperature, pressure and, principally, pH [24]. Mineral carbonation is favored over a higher pH--for instance, above 6.5 pH for Ca-Mg carbonates [25] or above 9.0 pH for CaCO\({}_{3}\)[9]. In the CarbFix project, the water in the Hellisheidi carbon injection site has a temperature ranging from 15 to 35 \({}^{\circ}\)C and the in situ pH ranges from 8.4 to 9.8 [26]. The injected water (with dissolved CO\({}_{2}\)) has a temperature of 25 \({}^{\circ}\)C and a pH of 3.7 to 4.0 [27]. According to. [28], during the injection phase, the water with CO\({}_{2}\) will create porosity in the near vicinity of the injection by dissolving primary and secondary minerals. Furthermore, away from the injection well, secondary minerals will precipitate due to the reaction with the Ca-Mg-Fe-rich reservoir rocks. Other authors [14, 29, 30] observed a decrease in the permeability of the carbonated rock. Inter- and intragranular pores (10 \(\upmu\)m pore throats) were filled with magnesite and characterized using \(\upmu\)Raman and SEM-TEM images.