CHARACTERIZATION OF CO2 AND CH4 GAS HYDRATES: A RAMAN OBSERVATION OF CO2 SEQUESTRATION VIA GAS HYDRATES

1. Introduction Gas hydrates (GHs) or clathrates, are crystalline structure where water cages host gas molecules. The cage is formed by water molecules keep together by hydrogen bond interaction, while host molecule is free to rotate inside the structure. GHs structure possesses some similarities to the ice structure of water since it is composed by 85% of water. Natural GHs (NGHs) represents an energy resource for CH4 that is distributed evenly around the world and near the cost, making recovery action easier and less expensive. The most important and promising strategies to collect CH4 are related to the CO2 replacement that allows permanent storage of CO2, as reducing greenhouse gas without additional energy. In the contest of the PRIN Project “Methane recovery and carbon dioxide disposal in natural gas hydrate reservoirs”, some marine sediment samples that contained NGHs, were chemically analyzed to obtain indications for the synthetic reproduction of GHs in laboratory. Different CH4, CO2 and CO2/CH4 GHs, prepared with two different apparatus, were ex situ analyzed by using Raman-spectroscopy and morphological characterized by low-temperature SEM (LTSEM) measurements. 2. Results and Discussion The chemical composition in seawater of marine sediments, as well as the physical properties and chemical composition of soils, influence the phase behavior of NGHs by disturbing the H-bond network in the water-rich phase before GH formation. The salts influence the formation of NGHs and their nucleation occurs preferentially in a region with low ions concentration; for these reasons, the investigation about the chemical composition of seawater containing NGHs is very important to obtain useful data for the comprehension of their formation mechanisms in the natural environment. To this purpose, water inside some marine sediment samples that contained natural GHs, sampled in the Antarctic Peninsula during the summer of 2003–2004 by Italian National Antarctic Research Program, were chemically analyzed (by ICP-MS and ionic chromatography) to obtain the indications for the synthetic reproduction of GHs. The obtained results show that 1 kg of water contained about 4.7% of salts; these results show an enrichment of ions with respect to the seawater because the sediments, sampled at different depths, can accumulate different quantities of salts during the formation. The application of the replacement strategies in NGHs reservoirs, always leads to the formation of “mixed” hydrates, whose mechanical and chemical properties are different from those of pure CH4 and CO2 hydrates. In this study, the process of the replacement of CH4 with CO2 molecules into hydrates was described at a molecular level by studying with Raman measurements different laboratory reproduced GHs; the obtained data permitted to evaluate properties and behavior of the different GHs through the study of fingerprint of host molecules and of water OHs bands. Specifically, the results showed that the CO2 hydrates showed a less ordered structure in the presence of sand but however, thanks to the interaction between water and silanol groups of sand particles, their stability was maintained with the variation of temperature, favoring the expansion of the cages that was highlighted by the Raman shifts of the CO2 Fermi diad. The observation of the water OHs bands, interpreted by SD indices4 showed that these parameters correlated perfectly with the increase of temperature and also provided information about the characteristics of water inside GHs, showing that the least ordered water structure was present on CO2GHs in the presence of sediments, while the most ordered one was that of (CH4/CO2)GHs. By these measurements, (CH4/CO2)GHs was proved to contain the most quantity of CO2 confirming therefore a favorable replacement of CH4; these results were in accordance with larger surface morphology observed in the LTSEM analysis highlighted clear differences in the micro-shapes between the various GHs samples, displaying changes in the surface morphology related to specific hydrate composition. 3. Conclusions The results related to the composition of seawater clarified the chemical conditions under which NGHs are formed and was used to reproduce synthetic GHs. The collected information by the Raman spectra of synthetic reproduced GHs improves the knowledge on the complex phase behavior, on the specific occupation of the cages of external host molecules and on the characteristics of the mixed gas hydrates. The analysis of water OHs bands of the different GHs permitted to describe the relation between symmetric and asymmetric water OHs bands, but also provided information about the characteristics of water inside the different GHs, showing that the least ordered water structure was that of GHs containing sand, while the most ordered one was present on binary CO2/CH4 GH