Chapter 6. Conclusion and Future Work
6.2. Future Work
6.2.2. Experimental Verification of CH 4 Recovery Induced by Flue Gas Injection into sII CH 4 +
Much research on the CH4 - CO2 exchange in gas hydrates has been conducted, primarily focusing on kinetic and thermodynamic approaches to the CH4 - CO2 replacement that occurs in sI hydrates.
However, only little attention has been paid to the thermal behaviors and physico-chemical properties of the replacement involved in sII hydrates. This study will investigated on the unique replacement behavior occurring in sII hydrates, primarily focusing on the structural characteristics and the guest- exchange behavior during replacement in order to reveal the mechanism of the CH4 + C3H8 - CO2 + N2
replacement. The previous studies on replacement in sI- or sH- isostructural system and sH-sI structure- transitional system reported that the extent of replacement was not significantly changed depending on the pressure of injected CO2. However, it was found that the replacement efficiency in CH4 + C3H8 - CO2 replacement system was increased as the PCO2 was increased because the increase of PCO2
accelerated the structural transition of initial CH4 + C3H8 hydrates to CO2-rich sI hydrates. As shown in Figure 6.2.6., the replacement efficiency in the CH4 + C3H8 - CO2 + N2 replacement system was also increased slightly but lower than that in C3H8 - CO2 replacement system on the basis of partial pressure of injected CO2.
Figure 6.2.6. Comparison of replacement efficiency after CH4 + C3H8 - CO2 replacement and CH4 + C3H8 - CO2 + N2 replacement.
CO2 Partial Pressure (MPa)
1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6
Composition (%)
0 20 40 60 80 100
CH4+C3H8-CO2 CH4+C3H8-CO2+N2
110
This phenomenon could be attributed to the partial structure-transition as occurred in the C3H8 - CO2
replacement system, or the different occupation characteristics of CO2 or N2 molecules corresponding to each cage lattice. In this study, therefore, the CO2 (20%) + N2 (80%) gas mixture will be injected into the sII CH4 (90%) + C3H8 (10%) hydrates to identify the influence of the replacement on the cage- dependent guest distribution as well as on the hydrate structure and dissociation enthalpy (ΔHd).
The three-phase (H-LW-V) equilibrium line before and after replacement will be first examined to identify the influence of flue gas injection into the sII CH4 (90%) + C3H8 (10%) hydrates on the thermodynamic stabilities. The shift in the equilibrium line provides thermodynamic stability conditions after replacement, and an approximate estimation of replacement efficiency. The hydrate structures and cage-dependent distribution of guest molecules are important factors for estimating the amount of natural gas deposited within hydrate reservoirs and the potential capacity of CO2
sequestration through replacement.
Figure 6.2.7. 13C NMR spectra of (a) the pure CH4, pure C3H8, CH4 (90%) + C3H8 (10%) hydrates [86], and (b) CH4 (90%) + C3H8 (10%) hydrates and the replaced hydrates with CO2 (50%) + N2 (50%) at 10.0 MPa [103].
111
To identify the influence of CO2 (20%) + N2 (80%) injection into the sII CH4 (90%) + C3H8 (10%) hydrates on the hydrate crystalline structures, PXRD, Raman and 13C NMR spectroscopy will be adopted. As seen in Figure 6.2.7., for example, the 13C NMR spectra of CH4 molecules will be shifted when the structure-transitional replacement (sII to sI) occurs. These cage-dependent chemical shifts of CH4 molecules enclathrated in hydrate lattices can be effectively used to examine a possible structural transformation after replacement and its influence on the distribution of CH4 and C3H8 molecules.
Dissociation enthalpy (ΔHd) of gas hydrates is useful thermal property for estimating and predicting the heat required for exploiting the CH4 in the natural gas hydrate reservoirs; it can also provide insights into the thermal stability of gas hydrates or the structural transition of gas hydrates before and after replacement. Thus, in this study, the influence of the replacement on ΔHd will be examined using a high- pressure micro-differential scanning calorimeter (HP μ-DSC). As reported in previous studies, the ΔHd
values of replaced hydrates in both structure-transitional and isostructural systems became similar to those of the hydrates formed from the corresponding injecting gases. However, the ΔHd values of replaced hydrate in the structure-transitional system was significantly lower than that of the initial hydrate, whereas the ΔHd values of the replaced hydrates in the isostructural replacement did not change as remarkably as in the structure-transitional replacement. This tendency of ΔHd changes can be useful to predict the occurrence of the structural transition and to estimate the extent of replacement. This research will provide the further insights of the guest exchange behavior and cage occupation characteristics. Also this will expand a field of exploration of the CH4 - CO2 replacement in sII hydrates that have been confirmed to exist in deep sea sediments.
112
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