## 3 Methodology
### Applied Techniques and Methods  
A detailed petrographic description was the first step of the study using optical microscopy (OpM) (Leica DM 6000, 20\(\times\), 10\(\times\), Wetzlar, Germany). This study was repeated after the SC CO\({}_{2}\) exposure. In order to obtain more detailed qualitative data about the rock petrographic, Scanning Electron Microscopy was also applied (JEOL 6100 SEM, using W-Filament, acceleration voltage of 20 kV and Inca Energy-200 software (Inca Energy, Abingdon, UK)). By means of optical image analysis (OIA) the main parameters of optical porosity were quantified using ImageJ software on OpM images acquired by a Charge-Coupled Device (CCD) camera (ProgRes 5.0, Jena, Germany). The chemical composition of the brine was determined by the combination of ion chromatography, inductively coupled plasma optical emission spectrometry (ICP-OES), pH and conductivity measurements, both before and after the experiment. Similarly, the whole rock chemical composition was also measured twice (before and after SC CO\({}_{2}\) exposure) by XRF. These studies were developed in the facilities of the IGME and the University of Oviedo (UNIOVI). The mineral composition of the initial rock sample used in this study was determined from the XRF analyses performed at the IGME laboratory. To do so, we used Excel solver in order to determine the optimal mineral composition corresponding to the XRF analyses and the rock description. This mineral composition as well as the injected water solution were used as input for the mass balance calculation and the numerical modelling. The geochemical modelling was performed using the CrunchFlow code [39]. The CrunchFlow code solves numerically the mass balance of solutes solving the reactive transport equation. This code has been validated by several studies focused on CO\({}_{2}\) storage [40; 41; 42; 43]. The total reaction rate \(R_{i}\) is given by (1):  
\[R_{i}=-\sum_{m}v_{im}R_{m} \tag{1}\]  
where \(R_{m}\) is the rate of precipitation (\(R_{m}>0\)) or dissolution (\(R_{m}<0\)) of mineral \(m\) in mol m\({}^{-3}\) rock s\({}^{-1}\), and \(v_{im}\) is the number of the moles of \(i\) in mineral \(m\). The reaction rate laws used in the calculations are of the form (2):  
\[R_{m}=A_{m}\sum_{terms}k_{m,T}a_{H+}^{n_{H+}}\left(\prod_{i}a_{i}{}^{n_{i}} \right)\left(\left(\frac{IAP}{K_{eq}}\right)^{m_{2}}-1\right)^{m_{1}} \tag{2}\]  
where \(A_{m}\) is the mineral surface area in m\({}^{2}\) m\({}^{-3}\) rock, \(k_{m,T}\) is the kinetic rate in mol m\({}^{-2}\) s\({}^{-1}\), \(a_{H+}^{n_{i}}\) is the term describing the effect of pH on the rate, \(a_{i}^{ni}\) is the term describing a catalytic/inhibitory effect on the rate by other species, \(\Delta G_{r}\) is the Gibbs free energy of the reaction (J mol\({}^{-1}\)), \(IAP\) is the ionic activity product of the solution with respect to the mineral, \(K_{eq}\) is the equilibrium constant for that mineral reaction (ionic activity product at equilibrium) and \(m_{1}\) and \(m_{2}\) exponents allow for nonlinear dependencies on the affinity term and are normally taken from experimental studies. The summation term indicates that several parallel rate laws may be used to describe the dependence of the rate on pH or on other species. The kinetic rate at temperature \(T\) (K) is calculated from (3):  
\[k_{m,T}=k_{m,25}\exp\biggl{(}\frac{Ea}{R}\biggl{(}\frac{1}{T_{25}}-\frac{1}{T} \biggr{)}\biggr{)} \tag{3}\]  
where \(k_{m,25}\) is the kinetic rate at 25 \({}^{\circ}\)C, \(E_{a}\) is the apparent activation energy of the overall reaction (J mol\({}^{-1}\)) and \(R\) is the gas constant (J mol\({}^{-1}\) K\({}^{-1}\)). Change in mineral surface area \(A_{m}\) (m\({}^{2}\) mineral m\({}^{-3}\) rock) is calculated assuming that area is proportional to the volume fraction of the mineral to the power 2/3.