## 6 Conclusions  
Experimental CO\({}_{2}\) injection into carbonates with real CO\({}_{2}\) storage potential was used to simulate possible interactions in the rock-gas system in dry and in water saturated conditions. Both of these conditions can evolve in the vicinity of the injection well during the first phases of injection. The experiments carried out in a hyperbaric chamber provide valuable insight about the processes although they were limited to specific cases. The tests presented here (approximately 1000 h) demonstrate that in the absence of available pore water chemical reactions do not take place in the SC CO\({}_{2}\)--carbonate system. On the other hand, under water saturated conditions, significant mineralogical and physical changes were observed in the rock at thin section scale (OpM, OIA and SEM) and in the brine, even though they were not confirmed by whole rock chemical analysis. In particular, the exposure of carbonate rocks to SC CO\({}_{2}\) rich brine under static conditions resulted in:  
1. The dissolution of CO\({}_{2}\) in the brine, which created an acidified environment (HCO\({}^{3-}\) increase in brine: 1400%; pH decrease).
2. Calcite dissolution in the low pH brine (Ca\({}^{2+}\) increase in brine: 290%), producing dissolution phenomena observable by microscopic techniques (OpM and SEM).
3. Increased porosity (absolute porosity variation: 0.32%) and modified pore morphology due to the calcite dissolution (Ro decrease: 2.40%; As increase: 2.26%; and increase of pore number: 12.26%).
4. Dolomite crystals also present in the rock were found to be less reactive than the calcite.
5. Illite and gypsum precipitation was evaluated by both mass balance calculation from the water chemistry and numerical modelling.  
The laboratory experiment and numerical modelling performed in this study indicate that calcite dissolution is the main reaction. One of the most important results is that equilibrium between the fluid and calcite is reached in around 50 h. This dissolution reaction induces a slight increase of porosity which favours CO\({}_{2}\) storage capacity. Nevertheless, in this typical rock composition, numerical modelling performed in this study indicates precipitation of clay. At the end of the simulation (almost 1000 h) the saturation index for illite is still positive which indicates that illite precipitation may continue after this period. This precipitation reaction can be an important issue in term of CO\({}_{2}\) injectivity as previous studies have demonstrated that clay precipitation usually reduces the sample permeability [15; 65].  
The authors would like to thank the funding provided through the ALGECO\({}_{2}\)-IRMC Project (Instituto Geologico y Minero de Espana: 2294-2013), CO\({}_{2}\)-Pore Project (Plan Nacional de Espana: 2009-10934, FEDER-UE). Thanks are due to [PERSON], [PERSON] and [PERSON] for providing help in sample collection and application of SEM techniques. We also would like to thank the editorial office for the editorial handling and two anonymous reviewers for their constructive comments and corrections.  
[PERSON] collected the rock samples, studied hand samples and completed the geological setting of area studied; [PERSON] and [PERSON] conceived, designed and performed the experiments by Optical microscopy, SEM, Optical image analysis and XRF analysis; [PERSON] contributed analysis tools, analysed the data of XRF measures and performed numerical modelling; all authors wrote the paper and participated in discussion and conclusion of this research article.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
SC Supercritical Conditions  
OpM Optical Microscopy  
SEM Scanning Electron Microscopy  
OIA Optical Image Analysis  
XRF X-ray fluorescence  
IGME Geological Survey of Spain  
UNIOVI University of Oviedo  
ICP-EOS Inductively coupled plasma optical emission spectrometry  
Cal Calcite  
Dolomite