Chapter 6. Conclusion and Future Work
6.1. Conclusion
In this study, CH4 - CO2 or flue gas replacement in various hydrate reservoirs was investigated in the aspects of thermodynamic stability, physicochemical properties, and thermal behavior to explore a possible structural transition and reveal the replacement mechanism. To verify the influence of CO2/flue gas injection on the thermodynamic stability of various hydrate reservoirs, the phase equilibrium conditions of the sI CH4, sII CH4 + C3H8, sH CH4 + 2,2-dimethylbutane (neohexane, NH) hydrates and those after replaced with CO2/flue gas were determined. The phase equilibria of CO2, CO2 + N2, CO2 + N2 + NH hydrates were also compared as thermodynamic stability conditions of the hydrates formed with those replacing gases. The shift in the phase equilibrium conditions after replacement implied the injection of CO2 or CO2 + N2 gas mixtures into hydrate reservoirs results substantial achievement in replacement.
To examine a possible structural transition and to elucidate the cage-dependent guest distribution, the hydrates involved in the swapping process were analyzed via PXRD, 13C NMR, and Raman spectroscopy. The PXRD patterns clearly demonstrate that the gas mixtures of flue gas compositions (10-20% CO2) form sI hydrates, whereas those form sH hydrates in the presence of NH. In addition, the experimental results from Raman spectroscopy also demonstrated that CO2 molecules could be enclathrated into the cages of sH hydrates as co-guests in the N2-rich systems and the CO2 (40%) + N2
(60%) was a boundary composition for the structural transition.
In the replacement systems, it was clearly demonstrated that the CH4 - CO2 or CH4 - flue gas replacement occurred in the sI-isostructural system, and the CH4 + NH - flue gas replacement occurred in the sH-isostructural system, whereas the CH4 + NH - CO2 replacement reaction accompanied the structural transformation from the sH to sI. Besides, the partial structure-transition (sII → sI) was also observed in the CH4 + C3H8 - CO2 replacement system. The 13C NMR spectroscopic results also revealed that N2 molecules of the flue gas preferentially replaced CH4 molecules residing in the small 512 cages of both sI and sH hydrates, while CO2 molecules more preferentially occupy the medium 435663 cages of the sH hydrate and the large 51262 cages of the sI hydrate. In light of the cage-dependent guest distribution, N2 molecules in the injecting gas could improve the extent of replacement by swapping with residual CH4 molecules in small 512 cages those irreplaceable with CO2 molecules. On the other hand, in the structure-transitional replacement system, the structural transformation during the CH4 - CO2 replacement that occurred in sH hydrates involves the dissociation of the sH hydrate and the reformation of the sI gas hydrate and alters the cage-filling characteristics of guest molecules and guest distributions in each cage; thus, it results in enhanced CH4 recovery via CO2 injection into the sH hydrates. However, in the replacement occurring in sII hydrates, the initial sII CH4 + C3H8 hydrates
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converted into the CO2-rich sI hydrates due to the abundance of CO2 molecules and the depletion of C3H8 molecules, and also changed into CH4 + C3H8 + CO2 hydrate (sII) without a structural transformation as the CO2 replacement reaction continued to proceed. The PXRD patterns of the replaced hydrates indicated that the higher replacement efficiency at the higher PCO2 was attributed to the higher portion of CO2-rich sI hydrate that was converted from initial sII hydrates. Also, the subsequent quantitative investigation on the cage-dependent distribution of guest molecules after the replacement suggested the structural transformation boundary of sII hydrates to sI hydrates. For the replacement occurring in the sII CH4 + C3H8 hydrates, the Raman study demonstrated that CO2
molecules first occupied sII hydrates predominantly, considering the rapid guest-exchange at the surface, and thereby converting the initial sII CH4 + C3H8 hydrate into the CO2-rich sI hydrate from the surface of the initial sII hydrate and then to be extended to its inner side. Furthermore, the rapid growth of enclathration of CO2 in sII hydrates indicates that the sII-isostructural conversion occurred more dominantly compared to the structural transformation rate with sI-isostructural conversion. The enclathration behavior of CO2 molecules and the release behavior of CH4 molecules during the replacement also indicated that the replacement rate increased with the increase in PCO2 as well as the extent of the replacement enhanced by PCO2.
To identify heat generation or absorption during the replacement process and subsequent influence on the thermal properties of the replaced hydrates, changes in heat flow and dissociation enthalpies were investigated using an HP μ-DSC. During the CH4 - CO2 or CH4 - flue gas replacement (sI-isostructural replacement) and the CH4 + NH - flue gas replacement (sH-isostructural replacement), no significant heat flow change from endothermic or exothermic reactions was observed. This changes in heat flow suggested that substantial portions of the hydrate frames were maintained during the replacement in isostructural system despite the possible partial breakup and restoration of some cage structures.
However, a large exothermic peak after the appearance of a significant endothermic peak was observed in the CH4 + NH - CO2 replacement (sH-sI structure-transitional replacement), and this indicates that the initial sH hydrate dissociation was followed by the formation of sI hydrate as the replacement reaction proceeded.
The DHd value of the replaced hydrate in the sH-sI structure-transitional system was significantly lower than that of the initial CH4 + NH hydrate and even slightly lower than that of pure CO2 hydrate. On the other hand, the DHd values of the replaced hydrates in both sI- and sH- isostructural systems did not change as remarkably as in the sH-sI structure-transitional replacement. Besides, in the isostructural replacements, the DHd values of each replaced hydrate was located between those of each initial hydrate and those of each hydrate formed from the injecting gases or injecting gases in the presence of NH. The distinguishable changes in the ΔHd values were observed in CH4 + C3H8 - CO2 replacement system. The replacement accompanying partial structure-transition (CH4 + C3H8 - CO2 replacement, sII → sII + sI)
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showed a significant change in the ΔHd values before and after replacement, depending on PCO2. The ΔHd values of replaced hydrates were decreased as PCO2 was increased, which is attributed to the increased portion of sI in the replaced hydrates at a higher PCO2. However, unlike the complete structure- transitional and isostructural replacement systems, the ΔHd values of the hydrates after CH4 + C3H8 - CO2 replacement were located between those of the initial CH4 + C3H8 hydrates and the pure CO2
hydrates. As the DHd value is generally a function of both hydrate structure and cage occupation of guest molecules, the DHd values before and after replacement can be effectively used to predict the structural transition and to estimate the extent of replacement. In Figure 6.1.1., the schematic illustration of sI-isostructural replacements and a structure-transitional replacement process was described. The overall experimental results provide further insights into the cage-specific occupation of external gas molecules and thermodynamic stability for the real replacement occurring in NGH reservoirs as a dual function of CH4 recovery and CO2 sequestration.
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Figure 6.1.1. The schematic illustration of (a) sI-isostructural CH4 - flue gas replacement, (b) sH- isostructural CH4 + NH - flue gas replacement, (c) sH-sI structure-transitional CH4 + NH - CO2
replacement, and (d) sII-sI partial structure-transitional CH4 + C3H8 - CO2 replacement.
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