5.3. Hydrate Crystalline Structure and Cage-dependent Guest Distribution
5.3.1. Structural Identification and Cage-filling Characteristics as Revealed by PXRD
The hydrate crystalline structure and the cage-dependent distribution of guest molecules can play an important role in estimating the amount of natural gas deposited in the reservoirs and the potential capacity of CO2 sequestration through replacement. In this study, to identify the crystalline structures and quantify the cage-dependent guest distribution of the initial hydrates and replaced hydrates, collected PXRD patterns were analysed with the structure refinement using rigid-body constraints for guest molecules.
As shown Figure 5.3.1., the PXRD patterns of the initial CH4 + C3H8 hydrate and the replaced hydrate with CO2 at 2.4 and 3.9 MPa were first investigated to identify the hydrate crystalline structures. Figure 5.3.1. (a) presents the PXRD patterns of the initial CH4 + C3H8 hydrate and the patterns of the initial CH4 + C3H8 hydrate were well-refined on a cubic 3 structure, converging into reliability factors of Rwp = 9.7 % (background subtracted) with a lattice parameter of 17.1343 (5) Å, which is a reasonable value for sII hydrates [99-101]. The XRD patterns of the gas hydrates replaced at two different PCO2
(2.4 and 3.9 MPa) were also shown in Figures 5.3.1. (b) and (c). The diffraction patterns of the replaced hydrates clearly demonstrated the coexistence of sI hydrates (cubic 3) and sII hydrates (cubic
3), which indicates that a certain portion of the initial sII CH4 + C3H8 hydrates was partially converted into sI hydrates.
2 theta (o)
5 10 15 20 25 30 35 40 45 50
Intensity (a.u.)
0 100000 200000 300000 400000 500000
observed data calculated data
sII hydrate hexagonal ice difference
2 theta (o)
5 10 15 20 25 30 35 40 45 50
Intensity (a.u.)
0 50000 100000 150000 200000
observed data calculated data
difference sII hydrate hexagonal ice sI hydrate
2 theta (o)
5 10 15 20 25 30 35 40 45 50
Intensity (a.u.)
0 100000 200000 300000 400000
observed data calculated data
difference sII hydrate hexagonal ice sI hydrate
(a) (b) (c)
Figure 5.3.1. PXRD patterns of (a) the initial CH4 + C3H8 hydrate, (b) the replaced hydrate at 2.4 MPa of PCO2, and (c) the replaced hydrate at 3.9 MPa of PCO2. The vertical tick marks represent the calculated positions of diffraction peaks for sI, sII hydrates and hexagonal ice.
81
For the replaced hydrates, multiphase refinement was performed to quantify the ratio of two hydrate structures. The patterns of the replaced hydrate at 2.4 MPa of PCO2 converged into reliability factors of Rwp = 14.3 % (background subtracted) with a lattice parameter of 11.8436(1) Å for the sI hydrate phase and 17.1368(1) Å for the sII hydrate phase. For the replaced hydrate at 3.9 MPa of PCO2, the lattice parameters were indexed to 11.8398(1) Å for the sI hydrate phase and 17.1356(1) Å for the sII hydrate phase with reliability factors of Rwp = 13.6 % (background subtracted).
In the 2θ range of 28.5 to 31.5⁰ as shown in Figure 5.3.2. (a), the intensity of Bragg positions corresponding to each structure were distinguishable for the replaced hydrates at two difference PCO2
(2.4 and 3.9 MPa). The Bragg intensities corresponding to sII hydrates decreased with increase in PCO2
whereas those corresponding to sI hydrates increased. This PCO2-dependent intensity difference implied the portion of the sII hydrate after replacement decreased as PCO2 increased because the intensity of the X-ray diffraction profile is proportional to the hydrate crystal volume. For the precise estimation of phase ratio of the replaced hydrates, multiphase refinement was performed to quantify the ratio of two hydrate structures. As presented in Figure 5.3.2. (b) and Table 5.3.1., the initial CH4 + C3H8 hydrate was composed of 99.5 % of sII hydrates with 0.5 % of ice content. After being replaced with 2.2 MPa of PCO2, the converted hydrates contained 24.9 % of sI hydrates and 70.5 % of sII hydrates with ice contents of 4.6 %. On the other hand, the portion of sI hydrates increased to 71.6 % (28.3 % for sI hydrates, 0.1 % for ice) after being replaced at 3.9 MPa of PCO2. The coexistence of sI and sII hydrates and the changes in the portion of sI hydrate depending on PCO2 as revealed by crystallographic analyses indicated that the possible replacement mechanism is that the initial sII CH4 + C3H8 hydrate undergoes a structural transformation into sI hydrate as well as sII-isostructural conversion. Besides, they also support the fact that the amount of the structural transformation in the hydrate phase was dependent on PCO2.
82
Figure 5.3.2. (a) Comparison of PXRD profiles of the replaced hydrate in the the 2θ range of 28.5 to 31.5⁰, and (b) phase ratio of initial CH4 + C3H8 hydrate, and replaced hydrates at 2.4 and 3.9 MPa of CO2.
Phase ratio (%)
0 20 40 60 80 100
sI sII ice
initial CH4 + C3H8 hydrate
replaced hydrate with 2.4 MPa of CO2
replaced hydrate with 3.9 MPa of CO2
(b)
2 theta (o)
28.5 29.0 29.5 30.0 30.5 31.0 31.5
initial
replaced with 2.4 MPa of CO2 replaced with 3.9 MPa of CO2
sII hydrate sI hydrate
sII (4 4 0) sI (4 0 0) sII (5 3 1) sI (4 1 0) sII (4 4 2)
(a)
83
To examine the guest-exchange behavior depending on the phase ratio, the cage-dependent distribution of guest molecules in each replaced hydrate phase was quantified and presented in Figure 5.3.3. As shown in Figure 5.3.3. and Table 5.3.1., the CO2 compositions in the hydrate phase were 47.6 % and 71.2 % for the replaced hydrate at 2.4 MPa and 3.9 MPa, respectively. The total amount of CO2
molecules contained in the sI hydrates after being replaced with 2.4 MPa of CO2 was 11.9 % and that with 3.9 MPa of CO2 was 46.4 %. However, although the replacement efficiency was higher at the high PCO2, the amount of CO2 molecules contained in the sII hydrates replaced with 3.9 MPa of CO2 was smaller than that in the sII hydrates replaced with 2.4 MPa of CO2 (24.9 % of CO2 in sII hydrate at 3.9 MPa, 35.7 % of CO2 in sII hydrate at 2.4 MPa). This smaller CO2 content in the sII hydrate at the high PCO2 replacement condition could be attributed to the fact that the absolute amount of sII hydrate phase was much smaller than that of sI hydrate phase.
Figure 5.3.3. Cage-dependent CO2 composition in the hydrate phase with a consideration of phase ratio.
CO2 Composition in Hydrate Phase (%)
0 20 40 60 80 100
sI-SC sI-LC sII-SC sII-LC
84
Figure 5.3.4. presents the cage occupancy of guest molecules corresponding to each hydrate phase after being replaced with both 2.4 MPa and 3.9 MPa of CO2. The enclathrated CO2 in the large 51262 cages of sI hydrates showed almost full cage occupancies for the hydrates replaced at both 2.4 and 3.9 MPa of PCO2 as shown in Figure 5.3.4. (a) and Table 5.3.1. Also, the enclathration of CH4 molecules in the large 51262 cages of sI hydrates was negligible considering the almost full occupancy of CO2 molecules in the large 51262 cages of transformed sI hydrates after being replaced at both PCO2 conditions. On the other hand, the enclathrated CO2 in the small 512 cages of sI hydrates at 2.4 MPa showed slightly higher cage occupancy than that at 3.9 MPa (θS, CO2, sI = 0.7320 at 2.4 MPa and 0.6673 at 3.9MPa), and accordingly slightly lower cage occupancy of CH4 in the small 512 cages of sI hydrates was observed for the replaced hydrate at 2.4 MPa of CO2 (θS, CH4, sI = 0.0751 at 2.4 MPa and 0.0974 at 3.9MPa). It should be noted that more abundance of CO2 molecules than CH4 molecules in the vapor phase due to the less amount of recovered CH4 from the sII hydrate phase could be favorable circumstance for CO2
molecules to be enclathrated into the newly formed sI hydrates considering the chemical potential difference between CO2 and CH4 molecules.
Cage Occupancy
0.0 0.2 0.4 0.6 0.8 1.0
small cage
large cage
small cage
large cage
Cage Occupancy
0.0 0.2 0.4 0.6 0.8 1.0
small cage
large cage
CH4 in small cage
C3H8 in large cage
(a) 2.4 MPa3.9 MPa 2.4 MPa3.9 MPa (b) 2.4 MPa3.9 MPa 2.4 MPa3.9 MPa
CO2 in sI hydrate CO2 in sII hydrate CH4 in sI hydrate CH4 and C3H8 in sII hydrate
Figure 5.3.4. Cage occupancies of (a) enclathrated CO2 in sI hydrate (left) and sII hydrate (right); and (b) enclathrated CH4 in sI hydrate (left), and CH4 and C3H8 in sII hydrate (right).
sII sII
sII-isostructural replacement
sII sII
sII-isostructural replacement
sII sI
sII-sI structure- transitional replacement
sII sI
sII-sI structure- transitional replacement
85
Therefore, it could be reasonably expected that the higher CO2 occupancy in the small 512 cages of the newly transformed sI hydrates at 2.4 MPa was attributed to the lower driving force at low PCO2, which led to low replacement efficiency. However, as described above, the total amount of CO2 molecules contained in the sI hydrates after being replaced with 3.9 MPa of CO2 was larger even though the cage occupancy of CO2 in the small 512 cages of sI hydrates was lower than the case in the replacement occurring with lower PCO2 condition (2.4 MPa).
For the enclathrated CO2 in the small 512 cages of sII hydrates, the cage occupancy of CO2 molecules after being replaced with 2.4 MPa was lower than that after being replaced with 3.9 MPa (θS, CO2, sII = 0.3341 at 2.4 MPa and 0.4499 at 3.9MPa), whereas corresponding cage occupancies of CH4 molecules were θS, CH4, sII = 0.4152 at 2.4 MPa and 0.3759 at 3.9MPa. On the contrary to the distribution of the CO2
molecules in the small 512 cages of the sI hydrates, less CO2 molecules occupied the small 512 cages of the replaced sII hydrate after being replaced with 2.4 MPa of CO2 while more CH4 molecules resided in the small 512 cages of the replaced sII hydrate as seen in Figure 5.3.4. (a) and (b). This implies that the mobility of CO2 molecules was limited due to the low driving force at low PCO2 replacement condition, and thereby the diffusion length decreased. The distinct guest distribution behavior could be attributed to the fact that the destruction of the initial sII hydrate followed by the re-formation of sI hydrate in the sII-sI structure-transitional replacement system whereas permeated CO2 molecules forced out the CH4 and C3H8 molecules from the sII hydrates in the sII-isostructure replacement system.
Therefore, the CO2-rich sI hydrate could newly form after the destruction of the initial sII hydrate with CO2-rich gas mixture containing small amount of released CH4 molecules from sII hydrates depending on PCO2, along with an isostructural conversion into the CH4 + C3H8 + CO2 hydrate (sII).
In the large 51264 cages of sII hydrates, the CO2 and C3H8 enclathration behavior showed almost same cage occupancy between the replaced hydrates after being replaced with 2.4 MPa and 3.9 MPa. These cage occupation characteristics irrelevant to the PCO2 indicates that the structural transformation could occur when the CO2 occupation in large 51264 cages of sII hydrates exceeds about 34 % or C3H8
occupancy decreased below about 61 %. From the crystallographic analyses on the initial and end-state of the hydrate structures, it was clearly demonstrated that the initial sII CH4 + C3H8 hydrate partially converted into the CO2-rich sI hydrate owing to the abundance of CO2 molecules and the depletion of C3H8 molecules as accelerated with increase in PCO2, as well as an isostructural conversion into the CH4
+ C3H8 + CO2 hydrate (sII). Furthermore, the quantitative investigation on the cage-dependent distribution of guest molecules after the replacement suggested the structural transformation boundary of sII hydrates to sI hydrates.
86
Table 5.3.1. Crystallographic information of initial CH4 + C3H8 hydrate, and replaced hydrates at 2.4 and 3.9 MPa of CO2.
initial CH4 + C3H8 hydrate replaced hydrate with 2.4 MPa of CO2
replaced hydrate with 3.9 MPa of CO2
structure sII, cubic 3 sI, cubic 3
sII, cubic 3
sI, cubic 3
sII, cubic 3
unit cell parameter a = 17.1343(5) Å (sII) a = 11.8436(1) Å (sI) a = 17.1368(1) Å (sII)
a = 11.8398(1) Å (sI) a = 17.1356(1) Å (sII)
phase ratio
sI - 24.9 % 71.6 %
sII 99.5 % 70.5 % 28.3 %
ice 0.5 % 4.6 % 0.1 %
cage occupancy (sI)
CH4 in SC - 0.0751 0.0974
CO2 in SC - 0.7320 0.6673
CO2 in LC - 0.9974 0.9956
cage occupancy (sII)
CH4 in SC 0.8606 0.4152 0.3759
CO2 in SC - 0.3341 0.4499
CH4 in LC 0.1214 - -
CO2 in LC - 0.3197 0.3347
C3H8 in LC 0.8651 0.6120 0.6152
guest composition
CH4 68.1 % 30.3 % 16.4 %
CO2 - 47.6 % 71.2 %
C3H8 31.9 % 22.1 % 12.4 %
Rwp 9.7 % 14.3 % 13.6 %
87
5.3.2. 13C NMR Study for Structural Identification and Cage-dependent Distribution of CH4
Molecules
In this study, cage-dependent distribution of CH4 molecules was also determined using 13C NMR spectroscopy to examine the guest exchange behavior after CO2 injection into the initial CH4 + C3H8
hydrate. The cage-dependent chemical shifts of CH4 molecules can provide not only structural information, but also relative distribution of the CH4 molecules over the different types of cages. Figure 5.3.5. (a) shows a comparison of the cage-dependent chemical shifts of CH4 molecules enclathrated in pure CH4 hydrate, initial CH4 + C3H8 hydrate, and the replaced hydrates with CO2 at two different PCO2. The pure CH4 hydrate exhibited two resonance signals at -4.3 and -6.6 ppm that correspond to CH4
molecules in the small 512 cages and large 51262 cages of sI hydrate, respectively. Before replacement, the resonance peaks of the initial CH4 + C3H8 hydrate were observed at -4.5 and -8.2 ppm that can be assigned to CH4 molecules enclathrated in the small 512 and large 51264 cages of sII hydrate, which are in good agreement with the literature values [86].
Figure 5.3.5. (a) Cage-dependent chemical shifts of CH4 molecules in the pure CH4 hydrate, the CH4 + C3H8 hydrate, and the replaced hydrates at 2.2 and 3.8 MPa of PCO2. (b) The 13C NMR spectrum of the replaced hydrate at 3.8 MPa of PCO2 deconvoluted by pseudo-Voigt model curves.
Chemical Shift (ppm)
-10 -8
-6 -4
-2 0
-4.3
replaced with CO2, 3.8 MPa replaced with CO2, 2.2 MPa
CH4 + C3H8 hydrate pure CH4 hydrate
-8.2
-6.6
-4.5-4.5-4.5
Chemical Shift (ppm)
-6 -5
-4 -3
-2
observed data calculated data CH4 in sI small cage CH4 in sII small cage
-4.5
-4.3
(a)
(b)
88
After replacement, the resonance signal for CH4 molecules from replaced hydrates appeared only at around -4.5 ppm and was slightly broader and asymmetrical as compared to pure CH4 hydrate, indicating that CH4 molecules are captured only in the small 512 cages of replaced hydrates. The resonance peak at around -4.5 ppm was not clearly split because the difference in the chemical shifts of CH4 molecules enclathrated in the small 512 cages of both sI and sII was too small for clear separation.
The asymmetric resonance peak from the replaced hydrates at around -4.5 ppm was deconvoluted into two separated peaks by pseudo-Voigt model curves, so that the two separated peaks could be assigned to the small 512 cages of sI and those of sII. In Figure 5.3.5. (b), the two separated peaks indicate that CH4 molecules after replacement remain more predominantly in the small 512 cages of sII hydrates (at -4.5 ppm) than in the small 512 cages of sI hydrates (at -4.3 ppm), even though the portion of sII hydrate became smaller after replacement at higher PCO2, as observed from PXRD patterns.
The 13C NMR spectra also demonstrated the coexistence of sI and sII hydrates after the CH4 + C3H8 - CO2 replacement. PXRD and NMR results strongly support that the initial sII CH4 + C3H8 hydrate converted into the CO2-rich sI hydrate owing 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 conversion of the initial sII CH4 + C3H8 hydrate into the CO2-rich sI hydrate is expected to occur first at the surface of the initial sII hydrate and then to be extended to its inner side. The gradual conversion of the CH4 + C3H8 + CO2
hydrates (sII) to the CO2-rich sI hydrates could be also inferred from the changes in the CH4/C3H8 ratio of the vapor phase during replacement, as presented in Figure 5.3.6. The CH4/C3H8 ratio of the vapor phase during replacement at both 2.6 and 4.0 MPa of PCO2 showed a rapid increase at the beginning.
The CH4/C3H8 ratio at 2.6 MPa of PCO2 continued to increase gradually as the replacement proceeded, whereas the ratio at 4.0 MPa of PCO2 subsequently decreased after the initial dramatic increase and then almost remained constant. The rapid increase in the CH4/C3H8 ratio at the beginning indicates that CO2
molecules first attack CH4 molecules primarily compared to C3H8 molecules in the sII hydrate phase, and this type of CH4-CO2 exchange behavior is more pronounced at a higher PCO2. However, the slightly decreasing behavior after the rapid increase in the CH4/C3H8 ratio at a higher PCO2 (4.0 MPa) was attributable to the increased release of C3H8 molecules at that time, and could account for the fact that the gradual conversion of sII hydrate to sI hydrate over time is more significant at a higher PCO2.
89
Figure 5.3.6. CH4/C3H8 ratio changes in the vapor phase during replacement
In addition, as shown in Figure 5.3.7., the CH4/C3H8 ratios in the final hydrate compositions after replacement were almost constant for all PCO2 conditions, whereas the ratios in the final vapor phase gradually decreased as the PCO2 increased. This indicates that the CH4 + C3H8 - CO2 replacement occurs with an almost constant CH4/C3H8 ratio in the sII hydrate phase even at different PCO2 although the portion of the sII hydrate decreased as PCO2 increased. Therefore, it is reasonably expected from compositional and structural analyses that the appearance of the CO2-rich sI hydrate and the resulting coexistence of sI and sII hydrates after replacement were attributed to the destruction of the initial sII hydrate followed by the formation of sI hydrate as a result of the increase in the CO2 concentration in the hydrate phase. Furthermore, the increase in PCO2 facilitated the structural transformation from the initial sII hydrate to the CO2-rich sI hydrate, and thereby enhanced the extent of replacement.
Time (h)
0 5 10 15 20 25
in-situ CH4/C3H8 Ratio in Vapor Phase
0 1 2 3 4 5 6
replaced with 2.6 MPa of CO2 replaced with 4.0 MPa of CO2
90
Figure 5.3.7. Final CH4/C3H8 ratios in the vapor and hydrate phases after replacement according to PCO2.
Pressure (MPa)
2.0 2.5 3.0 3.5 4.0 4.5
CH4/C3H8 Ratio in Final Composition
0 1 2 3 4 5 6 7 8 9 10
vapor phase, GC hydrate phase, GC hydrate phase, NMR
91