The development of carbon capture and storage (CCS) technique aims to reduce the atmospheric concentration of greenhouse gases emitted by industrial activities [1] . The carbon dioxide (CO₂) captured from large stationary sources can be safely injected and stored in appropriate geological formations, such as deep saline formations and depleted oil and gas reservoirs [2] . These geological formations are considered as the most stable in the CO₂ storage process on long term scale [3] . Four trapping and storing mechanisms are widely discussed in the CCS literature: residual, structural, mineral trapping and hydrodynamic [4] . After CO₂ injection, the cover rocks constitute the first barrier preventing the migration of CO₂ outer the geological reservoir. Generally, the most effective caprocks are siliciclastics (clay), evaporites (gypsum, anhydrites, halites) and organic-rich rocks [5] . The long-term confinement of CO₂ injected in the deep reservoir will be crucially dependent on cap rock and CO₂ interaction. The reaction between CO₂ and two-caprock samples of carbonate and clay-types has been studied in a laboratory reactor under the conditions of geological storage [6] . It has been shown a change in mineralogical compositions for the two samples. Using gravimetric method, the sorption capacity and kinetics of CO₂ have been measured among the clay minerals (montmorillonite, illite, and sepiolite) [7] . A thermodynamic study of CO₂ adsorption has been performed on different adsorbents (Clay, Jurassic evaporates and Triassic sandstone) [8] . This study evaluated the best material able to absorb the maximum of CO₂ and therefore to optimize the choice of the storage site. The CO₂-brine-rock interaction can also generate some new mineral precipitation so as to change the properties of the reservoir. The properties change can influence the physical and chemical retention mechanisms of CO₂ (drainage and imbibitions) [9] . Pressure and temperature effects on the reactivity of the host rock minerals with supercritical CO₂ have been studied by Regnault et al. [10] . The authors have discussed CO₂ storage capacity, mechanical reservoir behavior and chemical alteration. Other experimental studies and theoretical methods have been interested in the forsterite dissolution and magnesite precipitation at geological storage conditions [11] . Their experiments offer insights into the effects of relevant temperature and CO₂ pressure levels on mineral dissolution and carbonate precipitation. The chemical modification of the solid phase has been observed by scanning electron microscopy (SEM), infrared spectroscopy (IR), and X-ray diffraction techniques.

The clay cover rock was used to determine the change in electrical and capillary forces between clay, CO₂ and water [12] . This change leads to chemo-hydro-mechanical phenomena that could facilitate CO₂ break and advection through porosity cap rocks. Computational models [13] offer a means of comparing and selecting storage reservoirs (storage capacity, escape potential, risk analysis escape routes and storage). These models require an understanding of minerals clay effects on scales variety. Therefore, it is crucial to understand the cover rocks nature in order to assess the reactivity of these minerals with respect to CO₂.

The main objective of the present experimental study is to examine the characteristics of some geological cap rocks from real site located in southern region of Tunisia. It aims to identify common features that may impact long-term CO₂ storage. Four different simples of clay-type will be chosen for the experiments. Different techniques will be used to characterize the physical and chemical properties at different observation scales.

The present study deals with chemico-mineralogical characterization and technological properties of clay minerals, raw material collected from real site located in the city of Gabes in southern Tunisia. The site from which the samples are taken is drawn in Figure 1 [14] .

X-ray diffraction and infrared spectra allow us to describe the mineralogical compositions of the simples. The samples structure will be investigated using scanning electron microscope. While measurement of the mass change will be delineated by thermal analysis. However, the fluorescence measurement of the samples will be achieved by photoluminescence.

X-ray diffraction analysis was carried out by a “Philips MPD1880-PW1710” diffractometer using radiation, in the 2˚ - 80˚ interval with a step size of 0.02˚ and counting time of 20 s/step. The quantification phase was performed on one sample by the Rietveld method (R-QPA), using a PANalytical X’Pert High-Score Plus program. The chemical analyses and composition of the rock samples and clay minerals were examined using a JEOL JSM 5600LV scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS) (Bruker AXS Microanalysis). Infrared spectra were obtained using a Vertex 70-RAM II Bruker spectrometer (Bruker Analytical, Madison, WI). Differential and thermo gravimetric analyses were obtained using

an (ATG-DSC) STA 449C Netzsch instrument operating in helium atmosphere and heated at a rate of 20˚C from room temperature to 1500˚C. Photoluminescence (PL) measurements were collected on a Jobin-Yvon Fluorolog 3 spectrometer using a Xenon lamp (500 W) at room temperature.

The present experimental research aimed to examine the chemical characteristics of cap rock formations considered for CO₂ storage process. Different characterization techniques have been used to characterize the cover rock. Experimental results obtained with DRX demonstrated the presence of quartz, illite, Calcite, and Dolomite for the different selected samples of clay-type. The presence of these minerals was also confirmed by IR analysis.

EDX analyses justified that these clays were rich in Si, Cl, Na and O accompanied by a significant number of iron oxides. The DTA curves of clay samples revealed that three endothermic peaks were mainly due to the loss of H₂O from clay minerals and from the carbonates decomposition. Florescence results indicated that the spectrum was broad in the case of sample S4. Moreover, the obtained experimental results offered us a means of evaluating, comparing, and selecting storage reservoirs on criteria such as ease of injection, storage capacity, migration, and escape of CO₂ from a potential reservoir.

The authors would like to express their gratitude to the members of the Physics Complex Systems Laboratory at the University Picardy Jule Verne in France for their kind help in the characterization process of selected samples with fluorescence spectra and thermal analysis.

One of the authors (Mr. Hedi Jedli) is grateful to the Tunisian Ministry of Higher Education and Scientific Research for the grant in the framework of “Bourse d’alternance”.

HediJedli,HachemHedfi,AbdessalemJbara,SouhailBouzgarrou,KhalifaSlimi, (2015) Mineralogical and Geochemical Characteristics of Caprock Formations Used for Storage and Sequestration of Carbon Dioxide. Journal of Minerals and Materials Characterization and Engineering,03,409-419. doi: 10.4236/jmmce.2015.35043