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Structure-II Clathrate Hydrates in the Daini–Atsumi Knoll of the Nankai Trough, Japan
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Structure-II Clathrate Hydrates in the Daini–Atsumi Knoll of the Nankai Trough, Japan
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  • Yusuke Jin*
    Yusuke Jin
    Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan
    *Email: [email protected]
    More by Yusuke Jin
    Orcidhttps://orcid.org/0000-0002-6256-7278
  • Jun Yoneda
    Jun Yoneda
    Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan
    More by Jun Yoneda
    Orcidhttps://orcid.org/0000-0002-4569-9922
  • Kiyofumi Suzuki
    Kiyofumi Suzuki
    Methane Hydrate Development System Group, Energy Process Research Institute, Department of Energy and Environment, AIST, Onogawa, Tsukuba, Ibaraki 305-8569, Japan
    Methane Hydrate Research and Development Group, Japan Oil, Gas and Metals National Corporation (JOGMEC), Mihama, Chiba 261-0025, Japan
    More by Kiyofumi Suzuki
  • Motoi Oshima
    Motoi Oshima
    Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan
    More by Motoi Oshima
    Orcidhttps://orcid.org/0000-0001-5305-4828
  • Michihiro Muraoka
    Michihiro Muraoka
    Methane Hydrate Development System Group, Energy Process Research Institute, Department of Energy and Environment, AIST, Onogawa, Tsukuba, Ibaraki 305-8569, Japan
    More by Michihiro Muraoka
    Orcidhttps://orcid.org/0000-0002-2397-9835
  • Norio Tenma
    Norio Tenma
    Energy Process Research Institute, Department of Energy and Environment, AIST, Onogawa, Tsukuba, Ibaraki 305-8569, Japan
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  • Jiro Nagao
    Jiro Nagao
    Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan
    More by Jiro Nagao
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Energy & Fuels

Cite this: Energy Fuels 2024, 38, 6, 5218–5225
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https://doi.org/10.1021/acs.energyfuels.4c00343
Published March 7, 2024

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Abstract

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We examined the crystallographic properties of natural gas hydrates (GHs) in hydrate-bearing sandy sediment sampled from new wells near the second offshore gas-production wells in the Daini–Atsumi knoll region of the eastern Nankai Trough (NT) area, Japan. The sediment layers in the Daini–Astumi knoll area include a silt-dominant (thin turbidite) unit, a sand–mud alternation sequence (upper side), and a thick sandy turbidite sequence (deeper side). GHs are concentrated in the sand–mud alternation and thick sandy turbidite sequences. In the literature, all GH crystals in the eastern NT are structure I (sI) methane (CH4) hydrates. In the sand layers of the sand–mud alternation sequences, we similarly observed sI CH4 hydrate crystals, but in the thick sandy turbidite sequence, Raman spectroscopy revealed sI hydrates enclosing both CH4 and ethane (C2H6). In this sequence, we also observed the C–C vibrations of C2H6 in structure II (sII) large (51264) cages and the C–H vibrations of CH4 in sII small (512) cages. The sII hydrates enclosing CH4 and C2H6 were discovered in the deeper, thick sandy turbidite sequence near the bottom surface reflection. As the C2H6–to-CH4 composition ratio increased, the hydrate structure changed from sI to sII. Our new discovery of sII hydrates in the deeper layers of the eastern NT area is consistent with our previous study, which showed that the C2H6 composition ratio increases at deeper sampling depths.

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This paper was published ASAP on March 7, 2024 with a spelling error in the title of the paper. The corrected version was reposted on March 7, 2024.

Introduction

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Gas hydrates (GHs or simply hydrates) are crystalline clathrates composed of H2O and guest gas molecules under a certain pressure–temperature condition. (1,2) The guest gas molecules are enclosed in the hydrogen-bonded framework of the hydrate structure. The crystal structures of hydrates typically exist in three forms: structure I (sI), structure II (sII), and structure H (sH), which mainly depend on the size of the guest molecules. (2) Each crystal structure is composed of two or three cage structures among the following five: dodecahedral (512, D), irregular dodecahedral (435663, ID), tetrakaidecahedral (51262, T), hexakaidecahedral (51264, H), and icosahedral (51268, I). (2) For example, sI and sII hydrate structures are composed of D and T cages and D and H cages, respectively.
In nature, GHs are found under high-pressure and low-temperature conditions and occur in the sediments of permafrost regions, seabed surfaces, and oceanic sediments. As methane (CH4) is the main guest molecule in naturally occurring GHs, natural GHs are considered as a potential new conventional energy resource in countries such as Japan, which lack abundant energy resources. Natural GHs have been confirmed in the oceanic sediments around Japan and pore-filling type GH layers for promising gas production have been found in Nankai Trough (NT). (3−5) To develop these resources, the Japanese government has launched The Research Consortium for Methane Hydrate Resources in Japan (MH21, now termed MH21-S). Thus far, MH21 has conducted two offshore tests of natural gas production from natural GH reservoirs in the Daini–Astumi knoll area of the eastern NT. The first and second tests, conducted in 2013 and 2018, respectively, showed a potential for commercial gas production. (6−8)
The knowledge obtained from natural GH sediments is crucial to understanding and modeling natural GH reservoirs. To acquire this knowledge, MH21 sampled the natural core with a pressure coring tool before the first offshore natural gas production test (2012). (9,10) The sediment layers of the Daini–Astumi knoll area include a silt-dominant (thin turbidite) unit, a sand–mud alternation sequence, and a thick sandy turbidite sequence. The GHs are concentrated in the sand–mud alternation and thick sandy turbidite sequences with approximate lengths of 28 and 32 m, respectively. (9,10) Previously, we obtained the crystallographic properties of GHs in one sand layer of the sand–mud alternation sequence in the Daini–Astumi knoll area, (11) whereas MH21 sampled the full sand–mud alternation sequence and the upper half of the thick sandy turbidite sequences during their coring campaign. Previous studies of the eastern NT (5,11,12) identified the GH crystal as sI CH4 hydrate composed of D and T cages and the enclosed guest molecules were almost exclusively CH4 (>99.99%).
In 2018, MH21 recovered natural GH-bearing sediment from new wells (AT-CW1 and −CW2) close to the second gas-production well (AT1-P3) in the Daini–Astumi knoll area of NT, Japan. (13,14) The natural GH-bearing sediment was newly recovered from almost the entire GH concentrated zone (GHCZ), that is, from the top of the sand–mud alternation sequence to near the bottom surface reflection (BSR) in the thick sandy turbidite sequences. The recovered sample length of the thick sandy turbidite sequence was 30 m longer in 2018 than in the previous coring campaign. In our previous study on the Daini–Astumi knoll area, (11) we reported that the ethane (C2H6) concentration in the gas released from sediment samples increased with depth in the thick sandy turbidite sequence, but was hardly detectable in the sand layer of the sand–mud alternation sequence. With increasing C2H6/(CH4 + C2H6) ratio (hereafter called the C2H6 ratio), the formed hydrate changed from sI CH4 hydrate to sI hydrate enclosing both CH4 and C2H6 (hereafter called the sI CH4–C2H6 hydrate). At higher C2H6 ratios, sII CH4–C2H6 hydrate composed of D and H cages was formed. (15) The C2H6 concentration is expected to further increase in the deep part of the thick sandy turbidite sequence, which was newly recovered in a recent campaign. In the present study, we report the crystal structures and guest composition of GHs sampled from AT1-CW2. Using Raman spectroscopy, we reveal the existence of two hydrates, the expected sI CH4–C2H6 hydrate and the newly discovered sII CH4–C2H6 hydrate, in the eastern NT area of Japan.

Experimental Section

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Natural Hydrate-Bearing Sediments

In 2018 (one year after the second offshore gas-production test in the NT), MH21 performed a core-sampling operation to update the geological/physical information on the Daini–Atsumi knoll region (including the production wells). (14) The sampling location is shown in Figure 1. The core sediments from the new bore hole (AT1-CW2) were recovered by deep-sea drilling vessel CHIKYU (the Japan Agency for Marine Earth Science and Technology, JAMSTEC). The AT1-CW2 is positioned 20 m west of the second gas-production well (AT1-P3). (13,14) In this coring campaign, cores were recovered from the intervals 1286.5–1343.7 and 1356.6–1362.7 mBRT (meter below the rotary table including the water depth (994.4 m) and drill floor elevation (28.5 m)) using the newly updated High Pressure Temperature Corer III (HPTC-III) The upper and deeper intervals correspond to the GHCZ (sand–mud alternation and thick sandy turbidite sequences) above and below the rapid resistivity reduction zone, respectively. The total lengths of the drilled interval and recovered pressure-core samples were 63.3 and 50.3 m (25 runs), respectively. The recovered pressure-core samples were screened by an onboard pressure-core analysis and transfer system (PCATS), chopped, and then stored in pressure chambers for laboratory analysis by onboard PCATS and pressure-core nondestructive analysis tools (PNATs) systems. (19−21) The stored pressure-core samples were transferred to AIST Sapporo, Hokkaido, Japan. Before the laboratory analysis, we scanned the sediment properties using nondestructive methods (X-ray imaging, P-wave velocity, and bulk density). For the analyses, the pressure-core samples were recut under pressurized conditions in the PNATs system (19,20) in our laboratory. The study samples are given in Table 1. Their nomenclature is based on the section from which they were recovered and the number of their pressure coring run. For example, sample 01P-1b was recovered from the second subsubsection (b) of the first subsection (−1) during the first run of pressure coring (01P). AT1–CW2 composite log with coring run numbers and sample position used in this study.

Figure 1

Figure 1. Sampling locations in the Daini–Astumi knoll area of the Nankai Trough, Japan. Land maps were constructed from the ETOPO1 Global Relief Model developed by the National Oceanic and Atmospheric Administration (NOAA) (16) using Generic Mapping Tools (GMT) (17,18) software.

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Table 1. AT1-CW2 Natural Hydrate-Bearing Sediments in the Present Study
sampledepth (mBRT)aP-wave velocity (km·s–1)bulk density (kg·m–3)sediment section
01P-2b1287.361.71869top mud layer in sand–mud alternation sq.
02P-1b1290.332.81872sand layer in sand–mud alternation sq.
09P-3a1308.002.71781sand layer in sand–mud alternation sq.
15P-41322.002.11981thick sandy turbidite sq.
17P-1a1327.202.01954thick sandy turbidite sq.
20P-2a1335.312.72063thick sandy turbidite sq.
22P-31340.823.22005thick sandy turbidite sq.
23P-2b1342.493.51997thick sandy turbidite sq.
23P-3b1344.083.21922thick sandy turbidite sq.
24P-2b1358.592.0b2135bthick sandy turbidite sq. and above rapid resistivity reduction zone
25P-1a1360.701.6b2147bthick sandy turbidite sq. and below rapid resistivity reduction zone
a

Depth data including the water depth (994.4 m) and drill floor elevation (28.5 m).

b

Data were measured by onboard analysis in the PCATS system.

Crystallographic Analysis

The crystalline structures and gas components of the hydrates in the natural sediment were determined using powder X-ray diffraction (PXRD) and Raman spectroscopy. The crystalline structures were determined from the PXRD profiles obtained by an X-ray diffractometer (SmartLab; Rigaku Co., Japan) with a Cu Kα radiation source and a high-speed one-dimensional detector (D/teX Ultra). The voltage and current of the X-ray source were 45 kV and 200 mA, respectively, and 2θ was measured over the 5–42° range with a scan step of 0.01° and a scan speed of 1.0–4.0°/min. The PXRD profiles of the samples were collected in a low-temperature chamber (83 K; TTK 450; Anton Paar GmbH, Germany).
The Raman spectra for determining the crystalline structure and gas components were collected using a Raman spectrometer (LabRAM HR-800; Horiba Ltd., Japan) with a 2400-grooves/mm grating, a thermoelectrically cooled charge-coupled device detector (2048 × 512 pixels), and a 532 nm laser source (torus 532; Laser Quantum, U.K.). The output laser power was 180 mW controlled by a power unit (mpc 3000, Laser Quantum, U.K.). The laser was reduced to 1/100 by using a neutral density filter. The Raman spectra of the samples were collected at 83 K on a low-temperature stage (HFS600E-P; Linkam Scientific Instruments, U.K.), and the Raman shifts were calibrated by using neon emission lines. In our setup, the estimated uncertainty in the calibrated Raman shift was within ±0.05 cm–1. (22) Raman exposure time and spectra accumulations were 10 s and 10–200 times, respectively. The cage occupancy ratios θT/θD in each sI hydrate sample were estimated from the relative peak area ratio of decomposed Raman peaks corresponding to the C–H vibration in CH4 molecules, AT/3AD. The peaks were fitted to a mixed Gaussian–Lorentzian function using the commercial multiple peak-fitting program PeakFit v4.12.

Sample Preparation for Analysis

The pressured sediment samples were first treated by CH4 purge LN2 treatment (for details, see Figure 1 in our previous paper). (19) Next, the LN2-quenched samples were broken and ground into small fragments in a liquid nitrogen atmosphere. The smallest fragments were sieved in a liquid nitrogen atmosphere and reserved for the PXRD measurements, whereas the unsieved small fragments were used in the Raman measurements.

Results and Discussion

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Figure 2 shows representative PXRD profiles of the samples. All samples presented the diffraction peaks of a typical sI hydrate structure with a cubic crystal system (space group Pm3n). In a previous semiquantitative XRD analysis, (23) the major bulk minerals in AT1-C sediments were identified as quartz, Illite-smectite, kaolinite, mica, chlorite hornblende, plagioclase, orthoclase, pyrite, and calcium carbonate (silt-dominant unit). The unit-cell dimensions of the hydrates were refined in the PXRD analysis software PDXL (Rigaku Corp.), assuming quartz, smectite, Illite-smectite, kaolinite, mica, chlorite hornblende, plagioclase, orthoclase, and pyrite particles in the sediments. Calcium carbonate was removed from the PXRD refinements because it dominates the upper muddy sediment (silt-dominant unit) of the AT1-C well. (23) The lattice constants of each sample are given in Table 2. Even if hydrate crystals existed in the 01P-2b and 25P-1a samples, a hydrate crystal volume in the PXRD analysis was unexpected as an insufficient hydrate volume was inferred from P-wave velocity data. (20,24)

Figure 2

Figure 2. Examples of PXRD profiles of measured natural hydrate samples: (a) 09P-3a; (b) 23P-3b. Red, blue, and gray lines indicate sI hydrate, ice Ih, and sediment particles, respectively. All PXRD profiles were collected at 83 K.

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Table 2. Crystal Structures and Lattice Constants at 83 K (Determined by PXRD)
samplecrystal structurelattice constant in a-axis at 83 K (nm)
01P-2ba  
02P-1bsI1.18453(16)
09P-3asI1.18364(9)
15P-4sI1.18405(15)
17P-1asI1.1841(4)
20P-2asI1.18414(11)
22P-3sI1.18411(9)
23P-2bsI1.1834(2)
23P-3bsI1.18448(16)
24P-2bsI1.18528(8)
25P-1aa  
a

Crystal content (P-wave velocity) was too low to measure in the PXRD profile.

The lattice constants of the hydrates were identified from the artificial crystal surroundings and guest combinations. (25,26) The C2H6 concentration increased with increasing well depth at the AT1 site, (11) but the lattice constant of sI CH4–C2H6 hydrate was independent of CH4/C2H6 ratio. (26) The a-axis lattice constant in the artificial CH4 hydrate was 1.1857 nm at 83 K. (26,27) In our previous study, (11) the a-axis lattice constant in the sand layer of the sand–mud alternation sequence (sample AT1-C-13P) was 1.1841(2) nm, versus 1.18364(9) nm in sample 09P-3a at the equivalent horizon in the present study. Most of the samples were ca. 1.1834–1.1845 nm long along their a-axis; the exception was 24P-2b (a axis = 1.1853 nm, similar to that of artificial CH4 hydrate). The grain-size distributions of each sample are shown in Figure S2 in the Supporting Information (SI). The median (D50) sizes of the grains in each sample ranged from 10 to 165 μm. Small grains, clay (<3.9 μm) and small silts (<20 μm) were hardly seen in the 24P-2b sample. Thermodynamic conditions and interactions with the surfaces of clay minerals affect the H2O clustering/hydrate formation process. (28,29) Except for 24P-2b, the lattices of the crystal samples may have been slightly shrunken by clay mineral influences.
Figures 3 and 4 show optical observations of the hydrate crystals and the Raman spectra in each sample section, respectively. All Raman spectra show two peaks in the C–H vibration region, one at approximately 2902, and the other at approximately 2913 cm–1. Considering the PXRD results in Table 2, the peaks at 2902 and 2913 cm–1 were assigned to CH4 molecules in the T and D cages, respectively. (30) In a previous NT study, (11) the GHs in the sand layer of the sand–mud alternation sequence enclathrated only CH4 molecules. The sand–mud alternation sequences in the present study (02P-1b and 09P-3a samples) exhibited the same tendency. The cage occupancy ratios θT/θD, CH4 occupancies in the T and D cages were 1.21 and 1.20 in 02P-1b and 09P-3a, respectively, close to the Raman result in the previous NT study (θT/θD = 1.18). (11)

Figure 3

Figure 3. Examples of the observed hydrate crystals: (a) 01P-2b (a silt-dominant unit above the sand–mud alternation sq); (b) 02P-1b (sand layer in the sand–mud alternation sq); (c) 17P-1a (thick sandy turbidite sq.); (d) 22P-3 (thick sandy turbidite sq.); (e) 24P-2b (thick sandy turbidite sq. above the rapid resistivity reduction zone); (f) 25P-1a (thick sandy turbidite sq. below the rapid resistivity reduction zone). Arrows and squares indicate hydrate particles and small hydrate particles agglomerated or adjacent to each crystal, respectively. Each crystal was identified as a hydrate by a Raman measurement. Scale bars are 50 μm. Photographs were taken at 83 K under a nitrogen atmosphere.

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Figure 4

Figure 4. Comparative Raman spectra of the natural hydrate samples in (a) the C–H vibration region of CH4 and (b) the C–C vibration region of C2H6. The spectra in columns (a, b) were approximately normalized by the peak intensity at 2902 cm–1 and the signal-to-noise ratio of the baseline, respectively. Each spectrum was collected at 83 K.

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In the C–C vibration region of several samples (Figure 4b), we confirmed C–C vibrations at approximately 1000 cm–1, characterizing C2H6 in T cages. (15) In this study, some hydrate crystals were found in the silt-dominant unit above the sand–mud alternation sequence (Figure 3a). Sample 01P-2b was identified as an sI hydrate from the intensity ratio of the two C–H vibration peaks. In addition, the spectrum of 01P-2b presented the C–C vibration peak of C2H6 at 999.5 cm–1, suggesting C2H6 in T cages. (15) The θT/θD (1.14) of CH4 was lower in 01P-2b than in 02P-1b and 09P-3a, indicating that the hydrate crystals in 01P-2b were sI CH4–C2H6 hydrates. The average guest composition (mol %) of 01P-2b was roughly estimated as CH4/C2H6 = 99.5 (±0.3):0.5 (±0.3) based on the band area ratios of two C–H vibrations of CH4 and C–C vibration of C2H6. Here, band area ratio was calibrated by using the effective Raman cross-section factors of CH4 and C2H6. (31) No sI CH4–C2H6 hydrates were confirmed in the sand layer of the sand–mud alternation sequence. The source of the C2H6 molecules is unclear at present. Because the gas permeability is low in the mud layer, the guest flux from deeper regions should also be low. If the initial gas composition is C2H6/(CH4 + C2H6) > 0.001 in the 01P-2b layer (ca. 280 K), (32) it is possible to form sI CH4–C2H6 hydrates (CH4/C2H6 = 99.5:0.5) in the sea-salinity condition. The cubic crystal habit in Figure 3a may have formed over a long time by a low-mass-transfer crystal formation process.
In the upper region of the thick sandy turbidite sequence (samples 15P-4, 17P-1a, and 20P-2a), the gas hydrate crystals enclosed only CH4 molecules (no C2H6 molecules were observed). In the deeper region of the thick sandy turbidite sequence (samples 22P-3, 23P-2b, and 23P-3b), both sI CH4 and sI CH4–C2H6 hydrates were found. The previous study (11) reported that the C2H6 concentration in the sediment of the AT1-C well increased with sediment depth from 5 to 10 m below the top of the thick sandy turbidite sequence (Figure 5b). Therefore, the existence of C2H6–containing hydrate crystals in the deeper region of the thick sandy turbidite sequence is reasonable. The θT/θD values of sI CH4 hydrates in 15P-4, 17P-1a, 20P-2a, 22P-3, 23P-2b, and 23P-3b ranged from 1.20 to 1.31. In the C2H6–containing hydrates (22P-3, 23P-2b, and 23P-3b), the θT/θD values of CH4 were 1.14–1.20 (Figure 5a and Table 3). Because the C2H6 molecules of the sI CH4–C2H6 hydrates were enclosed in T cages, the θT/θD values of CH4 were lower in these hydrates than those in the sI CH4 hydrates. The average guest compositions (mol %) of the 22P-3, 23P-2b, and 23P-3b samples were roughly estimated as CH4/C2H6 = 99.5 (±0.3):0.5 (±0.3), 99.8 (±0.1):0.2 (±0.1), and 99.8 (±0.2):0.2 (±0.2), respectively, using Raman spectra. In the deepest sampling point of the thick sandy turbidite sequence (sample 24P-2b), all hydrate crystals were sI CH4–C2H6 hydrates with a θT/θD of CH4 equal to 1.18 and an estimated CH4/C2H6 of 99.7 (±0.2):0.3 (±0.2) using Raman spectra.

Figure 5

Figure 5. Depth tendencies of (a) cage occupancy ratio θT/θD of CH4 in sI hydrate structure in this study, and (b) methane and ethane mol % of decomposed gas determined in the 2012 coring campaign. (11) Dashed lines are rapid resistivity reduced lines. In (a), the red and blue circles indicate the θT/θD values of CH4 in the sI CH4 and sI CH4–C2H6 hydrates, respectively.

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Table 3. Crystal Structures and Enclosed Guest Molecules, Determined from PXRD, Raman Investigations, and Guest Analysis
 C–H stretching of CH4 (cm–1)C–C stretching of C2H6 (cm–1)θT/θD of CH4 in sI structure
sIsIIsIsIIincluding C2H6
sample512512625125126251264noyes
01P-2b2912.92901.2 999.5  1.136
02P-1b2913.12901.2   1.212 
09P-3a2913.02901.1   1.204 
15P-42912.52900.7   1.221 
17P-1a2913.22901.4   1.215 
20P-2a2913.62901.7   1.212 
22P-32913.32901.6 1000.4 1.2041.137
23P-2b2913.72901.7 1000.1 1.2281.188
23P-3b2913.72901.8 1000.4 1.3051.199
24P-2b2913.32901.7 1000.4  1.179
25P-1a2913.32901.32912.51000.7991.01.2031.194
The 25P-1a sample was recovered below the rapid resistivity reduction zone. Resistivity is an indicator of hydrate occurrence in sediment. (33) As the hydrate content in the 25P-1a sample was extremely low (low P-wave velocity in Table 1), the rapid resistivity reduction line above this sample was assumed as the bottom of the GHCZ in the AT-1 area of the NT. In this study, hydrate crystals were found below the rapid resistivity reduction zone. Figure 3f shows examples of hydrate crystals in the 25P-1a sample. The crystals were approximately 30–70 μm in size. In the optical observations, the hydrate particles in the sand–mud alternation and thick sandy turbidite sequences showed a tendency to agglomerate (Figures 3b,c and S3 in the SI). In contrast, the particles in the 25P-1a and 01P-2b samples showed no agglomeration tendency. From the C–H vibration profile of 25P-1a (Figure 4a), the hydrate crystals in that section were identified as sI hydrates. Most of the sI hydrate crystals in the 25P-1a sample enclosed C2H6 molecules in T cages. The θT/θD ratios of CH4 in the sI CH4 and sI CH4–C2H6 hydrates were approximately 1.20 and 1.19, respectively. From the Ramen spectra, the CH4/C2H6 ratios of the sI CH4–C2H6 hydrates were roughly estimated as 99.8 (±0.1):0.2 (±0.1).
In the Raman spectrum of 25P-1a, we observed a slight Raman signal at approximately 991 cm–1 in some cases (Figure 4). The clear signal at 991 cm–1 was confirmed in the spectra using smoothing filters (Figure S4 in the SI). The signal at 991–992 cm–1 is attributable to C–C vibrations of C2H6 in the 51264 (H) cages of the sII hydrate structure. (30,34) Figure 6 enlarges the C–H vibration regions of the three 25P-1a samples in Figure 4a. In Figure 6a with no Raman signal at 991 cm–1, the Raman peak at 2913.4 cm–1 was fitted well to a mixed Gaussian–Lorentzian function. In contrast, the samples with a slight Raman signal at 991 cm–1 exhibited a C–H vibration band with a shoulder in the lower-wavenumber region (Figure 6b,c). The sizes of the D cages in the sI and sII hydrate structures (hereafter called sI D and sII D, respectively) were almost identical. (2) Nevertheless, the different guest compositions in the hydrate structures caused different guest–host interactions, which manifested as a Raman shift between the two guest molecules. (35) Considering the Raman signal at 991 cm–1, the C–H vibration band with a shoulder was attributed to sII hydrate in the 25P-1a section. To our knowledge, we provide the first spectroscopic evidence of sII hydrate in the NT area.

Figure 6

Figure 6. Enlargements of the Raman spectra (Figure 4a) of the three 25P-1a samples, focusing on the peaks in the C–H vibration region of CH4. The higher-wavenumber peaks in two 25P-1a samples were fitted to a mixed Gaussian–Lorentzian function (dashed lines) using the commercial multiple peak-fitting program PeakFit v4.12.

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In the peak-fitting analysis, the Raman bands with a shoulder decomposed into two peaks: a low-intensity peak at 2912.5–2912.7 cm–1 and a high-intensity peak at 2913.3–2913.5 cm–1. In general, the C–H vibration signal of sII D appeared at lower wavenumbers than that of sI D. (15,35,36) The C–H vibration in the H cage of the sII structure is expected to shift to a lower Raman region (around 2901 cm–1), (30,35) but no band shoulder appeared around 2901 cm–1 in the C–H vibration region of sII. Assuming that the peaks at 2901 cm–1 and the higher-intensity peak (2913.3–2913.5 cm–1) arose from CH4 in T and the sI D cages of the sI hydrate structure, respectively, θT/θD of CH4 was calculated as 1.36. Conversely, if the lower-intensity peak (2912.5–2912.7 cm–1) was contributed by the sI D cage, the θT/θD of CH4 would be 11.0. As mentioned above, the sI CH4–C2H6 hydrate in 25P-1a presented a high CH4 content (99.8 mol %), implying that the CH4 content in the sII CH4–C2H6 hydrate was also high. Consequently, the higher-intensity peak at 2913.3 and 2913.5 cm–1 hydrate can be attributed to C–H vibrations of the sI D cage. In the sII hydrate structure, the ratio of D to H cages was D/H = 2, versus D/T = 1:3 in the sI hydrate structure. Because the C–H vibrations of the sII D cage presented as low-intensity signals, no C–H vibration band with a shoulder related to H cages was considered to appear around 2901 cm–1. From the peak area ratio of sI D and sII D C–H vibration bands in Figure 6b,c, an sII hydrate composition ratio was considered as about 1.5–3%.
In the 25P-1a section, sII CH4–C2H6 hydrates coexisted with sI CH4–C2H6 hydrates. sII CH4–C2H6 hydrates stably form in a certain range of C2H6 concentrations; for example, initial fluid composition C2H6/(CH4 + C2H6) > 0.02 at approximately 14 MPa and 288 K in the sea-salinity condition. (32) The existence of sII CH4–C2H6 hydrate would be reasonable considering our previous study, which showed that the C2H6 composition ratio increases at deeper sampling depths in the NT area. (11) In sediment, the pressure condition induced by a sea-level change decreases by 0.1 MPa per 10 m depth. From continuous records of the sea-level changes, (37) the sea level is known to have lowered 12 000 years ago and has since risen. (38) sII CH4–C2H6 hydrates have more stable crystal phases than sI CH4 hydrate. (2) If the sea level lowers, the stability zone of the sI CH4 hydrate is narrowed and uplifted; consequently, the sI CH4 hydrates dissociate in the bottom region. The zone above the rapid resistivity reduction is maintained under hydrate-stable pressure–temperature conditions over the long term, regardless of sea-level change. The several sea-level changes might have dissociated the sI CH4 hydrates and more stable sII hydrates enclosing C2H6 might have remained in the non-GHCZ region.

Conclusions

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We analyzed natural GHs recovered from the Daini–Atsumi knoll region of the eastern Nankai Trough (NT) area of Japan, where two offshore gas-production tests were performed. In PXRD and Ramen analyses, most of the GHs presented an sI hydrate crystal structure with a cubic crystal system. The lattice constant of the sI hydrate was determined as a = 1.184 nm. In the 24P-2b sample from the deeper thick sandy turbidite section, the lattice constant was a = 1.185 nm, similar to that of the artificial sI CH4 hydrate. The lattice constants of natural GHs can differ by clay mineral influences; unlike the other samples, the 24P-2b sample presented very few small grains, clay (<3.9 μm) and small silt (<20 μm). In our previous study, the C2H6 composition ratio increased with the sampling depth. Here, we detected not only sI CH4 hydrates but also sI CH4–C2H6 hydrates in the Raman spectra (C–H vibrations of CH4 and C–C vibration of C2H6) at depths below 1340 m. In sample 25P-1a (from the deepest sampling zone of the thick sandy turbidite sequence near the BSR), we observed the first signs of sII hydrates enclosing CH4 and C2H6 in the eastern NT area. The 25P-1a section is below the rapid resistivity reduction line, where there is no GHCZ. The sII CH4–C2H6 hydrates are more stable than the sI CH4 hydrate. The records of several sea-level changes in the area suggest that sI CH4 hydrates dissociated and that sII hydrates remained stable in the hydrate nonconcentrated zone.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.energyfuels.4c00343.

  • AT1-CW2 composite log data; grain-size distribution of natural sediment used in this study; example of agglomerated hydrate particles in the 22P-3 section; and peak finding by using smoothing filers (PDF)

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Author Information

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  • Corresponding Author
    • Yusuke Jin - Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan;  Orcidhttps://orcid.org/0000-0002-6256-7278;  Email: [email protected]
  • Authors
    • Jun Yoneda - Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan;  Orcidhttps://orcid.org/0000-0002-4569-9922
    • Kiyofumi Suzuki - Methane Hydrate Development System Group, Energy Process Research Institute, Department of Energy and Environment, AIST, Onogawa, Tsukuba, Ibaraki 305-8569, Japan;  Methane Hydrate Research and Development Group, Japan Oil, Gas and Metals National Corporation (JOGMEC), Mihama, Chiba 261-0025, Japan
    • Motoi Oshima - Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan;  Orcidhttps://orcid.org/0000-0001-5305-4828
    • Michihiro Muraoka - Methane Hydrate Development System Group, Energy Process Research Institute, Department of Energy and Environment, AIST, Onogawa, Tsukuba, Ibaraki 305-8569, Japan;  Orcidhttps://orcid.org/0000-0002-2397-9835
    • Norio Tenma - Energy Process Research Institute, Department of Energy and Environment, AIST, Onogawa, Tsukuba, Ibaraki 305-8569, Japan
    • Jiro Nagao - Methane Hydrate Production Technology Research Group, Energy Process Research Institute, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This study was supported by funding from the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium, now MH21-S), planned by the Ministry of Economy, Trade and Industry (METI), Japan. The authors thank Dr. H. Haneda, Dr. H. Minagawa, Dr. M. Morita, K. Shinjo, T. Uchiumi, T. Yamada of AIST, and Dr. K. Yamamoto, Dr. Y. Nakatsuka, Dr. T. Aung, and Dr T. Imai of JOGMEC for coring operations and experimental supports. They also express gratitude to all members of the shipboard team of the coring cruise in 2018.

References

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This article references 38 other publications.

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    RSC Advances (2019), 9 (45), 25987-26013CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Following the first attempt at producing gas from a naturally occurring methane hydrate (MH) deposit in the Daini-Atsumi Knoll in the eastern Nankai Trough area off Honshu Island, Japan in 2013, a second attempt was made in Apr. to June of 2017 at a nearby location using two producer wells sequentially and applying the depressurization method. The operation in the first borehole (AT1-P3) continued for 12 days with a stable drawdown of around 7.5 MPa and 41 000 m3 of methane gas being produced despite intermittent sand-prodn. events. The operation of the other borehole (AT1-P2) followed, with a total of 24 days of flow and 222 500 m3 of methane gas being produced without sand problems. However, the degree of drawdown was limited to 5 MPa because of a higher water prodn. rate than expected in the second hole. The pressure and temp. sensors deployed in the two producers, along with the two monitoring holes drilled nearby, gathered reservoir response data and information about the long-term MH dissocn. processes in the vicinity of the prodn. holes in the temporal and spatial domains. Although the ratio of energy return to the input was considerably larger than that for the depressurization operation, some observations (e.g., the high contrast in the prodn. rates between the two holes and the almost const. or slightly reduced gas prodn. rates) were not predicted by the numerical models. This failure in prediction raises questions about the veracity of the reservoir characteristics modeled in the numerical simulations. This paper presents the operation summaries and data obtained with thought-expt. based-anticipated prodn. behaviors and preliminary anal. of the obtained data as the comparison with expected behaviors. Detailed observations of gas and water prodn., as well as the pressure and temp. data recorded during the gas flow tests, indicate that the heterogeneous MH distribution within the reservoir was mainly responsible for the discrepancies obsd. between the anticipated and actual behaviors. Furthermore, the motion of the water that does not originate from MH dissocn. introduces complexity, such as the occurrence of concd. water-producing intervals and unexpected gas prodn. responses to decreases in pressure, into the prodn. behavior. The influence of heterogeneity should be clearly understood for the accurate prediction of gas prodn. behavior based on MH reservoirs.
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    Yamamoto, K.; Suzuki, K.; Wang, X.; Matsunaga, T.; Nishioka, I.; Nakatsuka, Y.; Yoneda, J. The Second Offsure Production Test of Methane Hydrates in the Earstern Nankai Trough and Site Characterization Efforts. Fire Ice 2019, 9– 15
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    Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D. Evidence of structure II hydrate formation from methane + ethane mixtures. Chem. Eng. Sci. 2000, 55, 1981– 1999,  DOI: 10.1016/S0009-2509(99)00389-9
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    Evidence of structure II hydrate formation from methane + ethane mixtures
    Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr.
    Chemical Engineering Science (2000), 55 (11), 1981-1999CODEN: CESCAC; ISSN:0009-2509. (Elsevier Science Ltd.)
    Methane and ethane are each known to form structure I (sI) hydrates. However, Raman and NMR spectroscopic measurements made on clathrate hydrates formed from binary gas mixts. of methane (CH4) and ethane (C2H6) indicated that structure II (sII) hydrate forms in this system for certain compns. Raman band frequencies for ethane and 13C-NMR chem. shifts for ethane were used to identify the hydrate structure in these studies. Raman spectra obtained from six different CH4 + C2H6 hydrates at Lw-H-V equil. conditions and 274.2 K indicate a change in hydrate structure from sI to sII between 72.2 and 75 mol% of methane in the vapor. Areas under peaks in the Raman spectra suggested that this change in hydrate structure is assocd. with a 20% change in hydrate guest compn. Abs. cage occupancies and hydration nos. were obtained from the Raman data. Anal. of the sI to sII transition and a phase diagram for the CH4 + C2H6 hydrate system were discussed. High-resoln., solid-state 13C-NMR measurements at 253 K confirmed that CH4 + C2H6 gas mixts. can form either sI or sII hydrates, depending on the vapor compn.
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    Wessel, P.; Smith, W. H. F.; Scharroo, R.; Luis, J.; Wobbe, F. Generic Mapping Tools: Improved Version Released. Eos, Trans. Am. Geophys. Union 2013, 94, 409– 410,  DOI: 10.1002/2013EO450001
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    Wessel, P.; Smith, W. H. F. Free software helps map and display data. Eos, Trans. Am. Geophys. Union 1991, 72, 441– 446,  DOI: 10.1029/90EO00319
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    Yoneda, J.; Masui, A.; Konno, Y.; Jin, Y.; Egawa, K.; Kida, M.; Ito, T.; Nagao, J.; Tenma, N. Mechanical properties of hydrate-bearing turbidite reservoir in the first gas production test site of the Eastern Nankai Trough. Mar. Pet. Geol. 2015, 66, 471– 486,  DOI: 10.1016/j.marpetgeo.2015.02.029
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    Jin, Y.; Konno, Y.; Yoneda, J.; Kida, M.; Nagao, J. In Situ Methane Hydrate Morphology Investigation: Natural Gas Hydrate-Bearing Sediment Recovered from the Eastern Nankai Trough Area. Energy Fuels 2016, 30, 5547– 5554,  DOI: 10.1021/acs.energyfuels.6b00762
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    In Situ Methane Hydrate Morphology Investigation: Natural Gas Hydrate-Bearing Sediment Recovered from the Eastern Nankai Trough Area
    Jin, Yusuke; Konno, Yoshihiro; Yoneda, Jun; Kida, Masato; Nagao, Jiro
    Energy & Fuels (2016), 30 (7), 5547-5554CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    The hydrate morphol. of natural gas hydrate-bearing (GH) sediments recovered from the eastern Nankai trough area was studied under hydrostatic pressurized conditions that prevent dissocn. of gas hydrates in a sediment. The authors developed a novel x-ray computed tomog. system and an attenuated total reflection IR (ATR-IR) probe for use in the Instrumented Pressure Testing Chamber for the set of Pressure-Core Nondestructive Anal. Tools (PNATs), which can measure the sediment structure, primary wave velocity (PWV), d., and shear strength under pressurized conditions. The hydrate satn. values estd. using the ATR-IR absorption bands of H2O mols. strongly correlate with PWV. Assuming homogeneity of hydrate distribution in the planes perpendicular to the sample depth direction, the hydrate morphol. of natural GH sediments in the eastern Nankai trough area demonstrated a load-bearing morphol. type. The predicted hydrate morphol. results are in good agreement with data reported in the literature. The combination of PNATs including ATR-IR spectroscopy can be used to est. the properties of GH sediments without the release of pressure to atm. conditions to model gas hydrate reservoirs for natural gas prodn.
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    Schultheiss, P.; Holland, M.; Roberts, J.; Durce, M.; Fox, P. In PCATS: Pressure Core Analysis and Transfer System ; 7th International Conference on Gas Hydrates (ICGH2011), 2011.
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    Jin, Y.; Kida, M.; Nagao, J. Structure H Clathrate Hydrates in Methane-Halogenic Large Molecule Substance-Water Systems. J. Phys. Chem. C 2019, 123, 17170– 17175,  DOI: 10.1021/acs.jpcc.9b04691
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    Structure H Clathrate Hydrates in Methane-Halogenic Large Molecule Substance-Water Systems
    Jin, Yusuke; Kida, Masato; Nagao, Jiro
    Journal of Physical Chemistry C (2019), 123 (28), 17170-17175CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We investigated crystallog. variations in structure H (sH) hydrates hosting CH4 and halogenic large-mol. guest substances [halogenic large-mol. guest substance (LMGS)]. The three halogenic LMGSs, namely, chlorocyclohexane (ClCH), bromocyclohexane (BrCH), and iodocyclohexane (ICH), share a common mol. structure (X-cyclohexane). The lattice consts. along a and c axes of sH hydrates hosting X-cyclohexane increased with increasing mol. size: ClCH < BrCH < ICH. The c lattice const. was esp. dependent on the mol. size, possibly because X-cyclohexanes align along the longitudinal direction of the 51268 cage, which coincides with the c-axis. Raman spectroscopy revealed that LMGSs altered the surroundings of CH4 mols. in the 512 and 435663 cages. As the crystal size increases, CH4 mols. encounter more attractive (less repulsive) guest-host interactions. The wavenumber shifts of the C-H vibrations of CH4 in the 512 and 435663 cages increased with temp. and were slightly greater in the 435663 cages than in the 512 cages. Different thermal responses between the 512 and 435663 cages may be caused by anisotropic lattice expansion of the sH hydrates. Finally, the phase stabilities of the sH (CH4 and X-cyclohexane) hydrates were evaluated by an isochoric method. The pressure region of equil. pressure-temp. conditions was lower in the sH (CH4 and X-cyclohexane) hydrates than in the pure CH4 hydrate system. Moreover, the temp. region of the equil. pressure-temp. conditions increased in the order ICH < ClCH < BrCH. The dissocn. enthalpies of the sH (CH4 + ClCH) and sH (CH4 + ICH) hydrates were estd. as 380 kJ/mol-1.
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    Egawa, K.; Nishimura, O.; Izumi, S.; Fukami, E.; Jin, Y.; Kida, M.; Konno, Y.; Yoneda, J.; Ito, T.; Suzuki, K. Bulk sediment mineralogy of gas hydrate reservoir at the East Nankai offshore production test site. Mar. Pet. Geol. 2015, 66, 379– 387,  DOI: 10.1016/j.marpetgeo.2015.02.039
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    Bulk sediment mineralogy of gas hydrate reservoir at the East Nankai offshore production test site
    Egawa, Kosuke; Nishimura, Okio; Izumi, Shoko; Fukami, Eiji; Jin, Yusuke; Kida, Masato; Konno, Yoshihiro; Yoneda, Jun; Ito, Takuma; Suzuki, Kiyofumi; Nakatsuka, Yoshihiro; Nagao, Jiro
    Marine and Petroleum Geology (2015), 66 (Part_2), 379-387CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
    Bulk sediment mineralogy was measured from a gas hydrate-bearing Middle Pleistocene deepwater turbidite interval at the offshore prodn. test site of the Eastern Nankai Trough area. We used core samples recovered from a 60-m-long section of the borehole for semi-quant. anal. of mineral and org. contents of the gas hydrate reservoir. Powder X-ray diffraction anal., ignition loss test, and field-emission scanning electronic microscopy revealed the followings: (i) ten bulk minerals vary in concn. and most of them have a good exponential correlation with median grain size; (ii) the upper muddy section is dominated by coccolith-rich hemipelagites, whereas the middle and lower sections are characterized by relatively coccolith-poor, probably humic substance-rich turbiditic sediments; and (iii) the common occurrence of authigenic gypsum, siderite, and framboidal pyrite indicates early diagenesis in anoxic and relatively high salinity conditions probably assocd. with gas hydrate formation in some degree. Such mineralogical data can provide useful information on evaluation of thermal properties, geomech. characteristics, effective permeability, and early diagenetic mechanism of the sediments to characterize and exploit gas hydrate reservoirs.
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    Konno, Y.; Jin, Y.; Yoneda, J.; Kida, M.; Egawa, K.; Ito, T.; Suzuki, K.; Nagao, J. Effect of methane hydrate morphology on compressional wave velocity of sandy sediments: Analysis of pressure cores obtained in the Eastern Nankai Trough. Mar. Pet. Geol. 2015, 66, 425– 433,  DOI: 10.1016/j.marpetgeo.2015.02.021
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    Takeya, S.; Fujihisa, H.; Gotoh, Y.; Istomin, V.; Chuvilin, E.; Sakagami, H.; Hachikubo, A. Methane Clathrate Hydrates Formed within Hydrophilic and Hydrophobic Media: Kinetics of Dissociation and Distortion of Host Structure. J. Phys. Chem. C 2013, 117, 7081– 7085,  DOI: 10.1021/jp312297h
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    Methane Clathrate Hydrates Formed within Hydrophilic and Hydrophobic Media: Kinetics of Dissociation and Distortion of Host Structure
    Takeya, Satoshi; Fujihisa, Hiroshi; Gotoh, Yoshito; Istomin, Vladimir; Chuvilin, Evgeny; Sakagami, Hirotoshi; Hachikubo, Akihiro
    Journal of Physical Chemistry C (2013), 117 (14), 7081-7085CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The crystal structure and dissocn. processes of methane (CH4) hydrates were investigated to better understand their stability in a natural environment. By using powder X-ray diffraction, the authors found that the unit-cell parameters of the hydrates formed with fine hydrophilic and hydrophobic beads were resp. larger and smaller by 0.02 Å than the unit-cell parameters of simple CH4 hydrates. The CH4 hydrates formed with hydrophobic beads dissocd. quickly above 200 K, whereas the CH4 hydrates formed with hydrophilic beads are stable up to about 273 K. The interfacial forces inside the intergranular pores or void spaces of the beads affect the kinetics of dissocn. of CH4 hydrate and are important for both the macroscopic and the crystallog. structures.
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    Takeya, S.; Uchida, T.; Kamata, Y.; Nagao, J.; Kida, M.; Minami, H.; Sakagami, H.; Hachikubo, A.; Takahashi, N.; Shoji, H. Lattice expansion of clathrate hydrates of methane mixtures and natural gas. Angew. Chem., Int. Ed. 2005, 44, 6928– 6931,  DOI: 10.1002/anie.200501845
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    Lattice expansion of clathrate hydrates of methane mixtures and natural gas
    Takeya, Satoshi; Uchida, Tsutomu; Kamata, Yasushi; Nagao, Jiro; Kida, Masato; Minami, Hirotsugu; Sakagami, Hirotoshi; Hachikubo, Akihiro; Takahashi, Nobuo; Shoji, Hitoshi; Khlystov, Oleg; Grachev, Mikhail; Soloviev, Valery
    Angewandte Chemie, International Edition (2005), 44 (42), 6928-6931CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Structural transition from type I to type II is accompanied by a large increase in lattice const. a in CH4+C2H6 clathrate hydrates (see picture). In contrast, the lattice consts. of CH4+CO2 clathrate hydrates are independent of compn. Data for a natural gas hydrate from the sediments of Lake Baikal are also consistent with these trends.
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    Shpakov, V. P.; Tse, J. S.; Tulk, C. A.; Kvamme, B.; Belosludov, V. R. Elastic moduli calculation and instability in structure I methane clathrate hydrate. Chem. Phys. Lett. 1998, 282, 107– 114,  DOI: 10.1016/S0009-2614(97)01241-4
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    Elastic moduli calculation and instability in structure I methane clathrate hydrate
    Shpakov, V. P.; Tse, J. S.; Tulk, C. A.; Kvamme, B.; Belosludov, V. R.
    Chemical Physics Letters (1998), 282 (2), 107-114CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    The thermal expansion of structure I clathrate hydrates with empty cages and with enclathrated methane mols. was calcd. from 5 to 270 K using the lattice dynamics approach within the quasi-harmonic approxn. The temp. dependence of several elastic properties, the dynamical, adiabatic and isothermal elastic moduli, are calcd. The dynamical and thermodynamical stability of the crystals were studied according to the Born stability criteria. Methane hydrate is unstable against small homogeneous deformations near the melting transition at normal pressure. For the empty hydrate lattice, a thermodynamical instability occurs at significantly higher temp. The theor. results are compared with new thermal expansion data on methane hydrate and results from a previous mol. dynamics study.
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    Yan, K. F.; Li, X. S.; Xu, C. G.; Lv, Q. N.; Ruan, X. K. Molecular dynamics simulation of the intercalation behaviors of methane hydrate in montmorillonite. J. Mol. Model. 2014, 20, 2311  DOI: 10.1007/s00894-014-2311-8
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    Molecular dynamics simulation of the intercalation behaviors of methane hydrate in montmorillonite
    Yan KeFeng; Li XiaoSen; Xu ChunGang; Lv QiuNan; Ruan XuKe
    Journal of molecular modeling (2014), 20 (6), 2311 ISSN:.
    The formation and mechanism of CH4 hydrate intercalated in montmorillonite are investigated by molecular dynamics (MD) simulation. The formation process of CH4 hydrate in montmorillonite with 1 ~ 8 H2O layers is observed. In the montmorillonite, the "surface H2O" constructs the network by hydrogen bonds with the surface Si-O ring of clay, forming the surface cage. The "interlayer H2O" constructs the network by hydrogen bonds, forming the interlayer cage. CH4 molecules and their surrounding H2O molecules form clathrate hydrates. The cation of montmorillonite has a steric effect on constructing the network and destroying the balance of hydrogen bonds between the H2O molecules, distorting the cage of hydrate in clay. Therefore, the cages are irregular, which is unlike the ideal CH4 clathrate hydrates cage. The pore size of montmorillonite is another impact factor to the hydrate formation. It is quite easier to form CH4 hydrate nucleation in montmorillonite with large pore size than in montmorillonite with small pore. The MD work provides the constructive information to the investigation of the reservoir formation for natural gas hydrate (NGH) in sediments.
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    Li, Y.; Chen, M.; Song, H. Z.; Yuan, P.; Zhang, B. F.; Liu, D.; Zhou, H. J.; Bu, H. L. Effect of Cations (Na+, K+, and Ca2+) on Methane Hydrate Formation on the External Surface of Montmorillonite: Insights from Molecular Dynamics Simulation. ACS Earth Space Chem. 2020, 4, 572– 582,  DOI: 10.1021/acsearthspacechem.9b00323
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    Effect of Cations (Na+, K+, and Ca2+) on Methane Hydrate Formation on the External Surface of Montmorillonite: Insights from Molecular Dynamics Simulation
    Li, Yun; Chen, Meng; Song, Hongzhe; Yuan, Peng; Zhang, Baifa; Liu, Dong; Zhou, Huijun; Bu, Hongling
    ACS Earth and Space Chemistry (2020), 4 (4), 572-582CODEN: AESCCQ; ISSN:2472-3452. (American Chemical Society)
    In this study, mol. dynamics simulations were performed to investigate the effects of montmorillonite with different surface cations (i.e., Na+, K+, and Ca2+) on CH4 hydrate formation. The results showed that CH4 hydrate cages are mainly formed beyond the montmorillonite surface. The inner-sphere adsorption of K+ and the outer-sphere adsorption of Na+ and Ca2+ occurred on the montmorillonite surface, leading to differences in order parameters and hydrogen bond no. of H2O mols. The no. of structure I cages increased faster than that of structure II cages in different models and were in agreement with the fact that CH4 mols. can only form sI hydrate crystals. The no. of 512 cages increased in the order: Na-Mt < Ca-Mt < K-Mt. The aq. environment dominated by K+ on the external surface of montmorillonite facilitate heterogeneous nucleation of CH4 hydrate rather than that by Ca2+ or Na+. The abovementioned findings suggest that the coordination structure of cations on the external surface of montmorillonite plays an important role in CH4 hydrate formation through altering the occupation of CH4 hydrate.
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    Subramanian, S.; Sloan, E. D. Trends in Vibrational Frequencies of Guests Trapped in Clathrate Hydrate Cages. J. Phys. Chem. B 2002, 106, 4348– 4355,  DOI: 10.1021/jp013644h
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    Trends in Vibrational Frequencies of Guests Trapped in Clathrate Hydrate Cages
    Subramanian, Sivakumar; Sloan, E. Dendy, Jr.
    Journal of Physical Chemistry B (2002), 106 (17), 4348-4355CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Raman spectra of ethane trapped in the small 512 cage of sII hydrate (at ∼70 MPa), isobutane trapped in the large 51264 cage of sII hydrate, and the gauche form of n-butane trapped in the large 51264 cage of sII hydrate were obtained for the first time. These new Raman results are combined with existing Raman and IR results for various guests to infer general trends in vibrational frequencies of guest mols. trapped in clathrate hydrate cages as a function of cage size, guest size, guest vibrational mode, and pressure. The obsd. trend in stretching frequencies of guests with cage size, which can be stated as "the larger the cavity, the lower the frequency", is explained through the qual. "loose cage-tight cage" model of Pimentel and Charles (Pure Appl. Chem. 1963, 7, 111).
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    Raman microspectrometry of fluid inclusions
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    Lithos (2001), 55 (1-4), 139-158CODEN: LITHAN; ISSN:0024-4937. (Elsevier Science B.V.)
    A review with 115 refs. For many kinds of fluid inclusions, the coupling of microthermometry and Raman microspectrometry is still the only viable option to obtain compns. of single fluid inclusions. A review is given on the basis of 16 yr of experience and helped with 115 refs. of the instrumentation, anal. conditions and methodol. of the application of Raman microspectrometry to gaseous, aq. and hydrocarbon inclusions, and their daughter minerals.
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    The Colorado School of Mines Hydrate Prediction Program ″CSMGem″. 2007.
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    Collett, T. S. A Review of Well-Log Analysis Techniques Used to Assess Gas-Hydrate-Bearing Reservoirs. In Natural Gas Hydrates: Occurrence, Distribution, and Detection: Occurrence, Distribution, and Detection; Paull, C. K.; Dillon, W. P., Eds.; American Geophysical Union, 2001; Vol. 124, pp 189– 210.
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    Ohno, H.; Strobel, T. A.; Dec, S. F.; Sloan, E. D.; Koh, C. A. Raman Studies of Methane-Ethane Hydrate Metastability. J. Phys. Chem. A 2009, 113, 1711– 1716,  DOI: 10.1021/jp8010603
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    Raman Studies of Methane-Ethane Hydrate Metastability
    Ohno, Hiroshi; Strobel, Timothy A.; Dec, Steven F.; Sloan, E. Dendy, Jr.; Koh, Carolyn A.
    Journal of Physical Chemistry A (2009), 113 (9), 1711-1716CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The interconversion of methane-ethane hydrate from metastable to stable structures was studied using Raman spectroscopy. SI and sII hydrates were synthesized from methane-ethane gas mixts. of 65% or 93% methane in ethane and water, both with and without the kinetic hydrate inhibitor, poly(N-vinylcaprolactam). The obsd. faster structural conversion rate in the higher methane concn. atm. can be explained in terms of the differences in driving force (difference in chem. potential of water in sI and sII hydrates) and kinetics (mass transfer of gas and water rearrangement). The kinetic hydrate inhibitor increased the conversion rate at 65% methane in ethane (sI is thermodynamically stable) but retards the rate at 93% methane in ethane (sII is thermodynamically stable), implying there is a complex interaction between the polymer, water, and hydrate guests at crystal surfaces.
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    Ohno, H.; Kida, M.; Sakurai, T.; Iizuka, Y.; Hondoh, T.; Narita, H.; Nagao, J. Symmetric Stretching Vibration of CH4 in Clathrate Hydrate Structures. ChemPhysChem 2010, 11, 3070– 3073,  DOI: 10.1002/cphc.201000519
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    Symmetric Stretching Vibration of CH4 in Clathrate Hydrate Structures
    Ohno, Hiroshi; Kida, Masato; Sakurai, Toshimitsu; Iizuka, Yoshinori; Hondoh, Takeo; Narita, Hideo; Nagao, Jiro
    ChemPhysChem (2010), 11 (14), 3070-3073CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The use of high-resoln. Raman measurement as a powerful tool not only for hydrate-phase identification and cage-filling anal., but also for the measure of mol. interactions in clathrate hydrate systems was studied. Distinct Raman bands of CH4 in a sH hydrate phase in the presence of large mol. guest substances was revealed for the first time; methane cage populations were estd. from the Raman intensities. The vibrational states of CH4 were obsd. to differ considerably among different hydrate structures, even for the same host cages (512). These observations indicate that Raman signals of methane mols. trapped in 512 small cavities are particularly sensitive to changes in the mol. environment. Therefore, these measurements can be very useful for understanding the nature of guest-host (perhaps also guest-guest) interactions.
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    Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of Clathrate Hydrates via Raman Spectroscopy. J. Phys. Chem. B 1997, 101, 7371– 7377,  DOI: 10.1021/jp970768e
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    Measurement of Clathrate Hydrates via Raman Spectroscopy
    Sum, Amadeu K.; Burruss, Robert C.; Sloan, E. Dendy, Jr.
    Journal of Physical Chemistry B (1997), 101 (38), 7371-7377CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Raman spectra of clathrate hydrate guest mols. are presented for 3 known structures (I (sI), II (sII), and H (sH)) in the following systems: CH4 (sI), CO2 (sI), C3H8 (sII), CH4 CO2 (sI), CD4 C3H8 (sII), CH4 N2 (sI), CH4 THF-d8 (sII), and CH4 C7D14 (sH). Relative occupancy of CH4 in the large and small cavities of sI were detd. by deconvoluting the ν1 sym. bands, resulting in hydration nos. of 6.04 ± 0.03. The frequency of the ν1 bands for CH4 in structures I, II, and H differ statistically, so that Raman spectroscopy is a potential tool to identify hydrate crystal structure. Hydrate guest compns. were also measured for 2 vapor compns. of the CH4 CO2 system, and they compared favorably with predictions. The large cavities are almost fully occupied by CH4 and CO2, whereas only a small fraction of the small cavities are occupied by CH4. No CO2 was found in the small cavities. Hydration nos. from 7.27 to 7.45 were calcd. for the mixed hydrate.
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    Cutler, K. B.; Edwards, R. L.; Taylor, F. W.; Cheng, H.; Adkins, J.; Gallup, C. D.; Cutler, P. M.; Burr, G. S.; Bloom, A. L. Rapid sea-level fall and deep-ocean temperature change since the last interglacial period. Earth Planet. Sci. Lett. 2003, 206, 253– 271,  DOI: 10.1016/S0012-821X(02)01107-X
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    Rapid sea-level fall and deep-ocean temperature change since the last interglacial period
    Cutler, K. B.; Edwards, R. L.; Taylor, F. W.; Cheng, H.; Adkins, J.; Gallup, C. D.; Cutler, P. M.; Burr, G. S.; Bloom, A. L.
    Earth and Planetary Science Letters (2003), 206 (3-4), 253-271CODEN: EPSLA2; ISSN:0012-821X. (Elsevier Science B.V.)
    We have dated Huon Peninsula, Papua New Guinea and Barbados corals that formed at times since the Last Interglacial Period, applying both 230Th and 231Pa dating techniques as a test of age accuracy. We show that Marine Isotope Stage (MIS) 5e ended prior to 113.1±0.7 kyr, when sea level was -19 m. During MIS 5b sea level was -57 m at 92.6±0.5 kyr, having dropped about 40 m in approx. 10 kyr during the MIS 5c-5b transition. Sea level then rose more than 40 m during the MIS 5b-5a transition, also in about 10 kyr. MIS 5a lasted until at least 76.2±0.4 kyr, at a level of -24 m at that time. Combined with earlier data that places MIS 4 sea level at -81 m at 70.8 kyr, our late MIS 5a data indicate that sea level fell almost 60 m in less than 6 kyr (10.6 m/kyr) during the MIS 5-4 transition. The magnitude of the drop is half that of the glacial-interglacial amplitude and approx. equiv. to the vol. of the present-day Antarctic Ice Sheet. During this interval the min. av. rate of net continental ice accumulation was 18 cm/yr, likely facilitated by efficient moisture transport from lower latitudes. At three specific times (60.6±0.3, 50.8±0.3, and 36.8+0.2 kyr) during MIS 3, sea level was between -85 and -74 m. Sea level then dropped to -107 m at 23.7±0.1 kyr early in MIS 2, before dropping further to Last Glacial Maximum (LGM) values and then rising to present values during the last deglaciation. Times of rapid sea-level drop correspond to times of high winter insolation at low northern latitudes and high winter latitudinal gradients in northern hemisphere insolation, supporting the idea that these factors may have resulted in high water-vapor pressure in moisture sources and efficient moisture transport to high-latitude glaciers, thereby contributing to glacial buildup. We combined our sea-level results with deep-sea δ18O records as a means of estg. the temp. and ice-vol. components in the marine δ18O record. This anal. confirms large deep-ocean temp. shifts following MIS 5e and during Termination I. Deep-ocean temps. changed by much smaller amts. between MIS 5c and 2. Maximum temp. shift in the deep Pacific is about 2°, whereas the shift at a site in the Atlantic is 4°. Under glacial conditions temps. at both sites are near the f.p. The shift in the Atlantic is likely caused by a combination of changing proportions of northern and southern source waters as well as changing temp. at the sites where these deep waters form.
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    Fleming, K.; Johnston, P.; Zwartz, D.; Yokoyama, Y.; Lambeck, K.; Chappell, J. Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites. Earth Planet. Sci. Lett. 1998, 163, 327– 342,  DOI: 10.1016/S0012-821X(98)00198-8
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    Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites
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    Earth and Planetary Science Letters (1998), 163 (1-4), 327-342CODEN: EPSLA2; ISSN:0012-821X. (Elsevier Science B.V.)
    The eustatic component of relative sea-level change provides a measure of the amt. of ice transferred between the continents and oceans during glacial cycles. This has been quantified for the period since the Last Glacial Maximum by correcting obsd. sea-level change for the glacio-hydro-isostatic contributions using realistic ice distribution and earth models. During the Last Glacial Maximum (LGM) the eustatic sea level was 125±5 m lower than the present day, equiv. to a land-based ice vol. of (4.6-4.9)×107 km3. Evidence for a non-uniform rise in eustatic sea level from the LGM to the end of the deglaciation is examd. The initial rate of rise from ca. 21 to 17 ka was relatively slow with an av. rate of ca. 6 m ka-1, followed by an av. rate of ca. 10 m ka-1 for the next 10 ka. Significant departures from these av. rates may have occurred at the time of the Younger Dryas and possibly also around 14 ka. Most of the decay of the large ice sheets was completed by 7 ka, but 3-5 m of water has been added to the oceans since that time.
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  • Abstract

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    Figure 1

    Figure 1. Sampling locations in the Daini–Astumi knoll area of the Nankai Trough, Japan. Land maps were constructed from the ETOPO1 Global Relief Model developed by the National Oceanic and Atmospheric Administration (NOAA) (16) using Generic Mapping Tools (GMT) (17,18) software.

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    Figure 2

    Figure 2. Examples of PXRD profiles of measured natural hydrate samples: (a) 09P-3a; (b) 23P-3b. Red, blue, and gray lines indicate sI hydrate, ice Ih, and sediment particles, respectively. All PXRD profiles were collected at 83 K.

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    Figure 3

    Figure 3. Examples of the observed hydrate crystals: (a) 01P-2b (a silt-dominant unit above the sand–mud alternation sq); (b) 02P-1b (sand layer in the sand–mud alternation sq); (c) 17P-1a (thick sandy turbidite sq.); (d) 22P-3 (thick sandy turbidite sq.); (e) 24P-2b (thick sandy turbidite sq. above the rapid resistivity reduction zone); (f) 25P-1a (thick sandy turbidite sq. below the rapid resistivity reduction zone). Arrows and squares indicate hydrate particles and small hydrate particles agglomerated or adjacent to each crystal, respectively. Each crystal was identified as a hydrate by a Raman measurement. Scale bars are 50 μm. Photographs were taken at 83 K under a nitrogen atmosphere.

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    Figure 4

    Figure 4. Comparative Raman spectra of the natural hydrate samples in (a) the C–H vibration region of CH4 and (b) the C–C vibration region of C2H6. The spectra in columns (a, b) were approximately normalized by the peak intensity at 2902 cm–1 and the signal-to-noise ratio of the baseline, respectively. Each spectrum was collected at 83 K.

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    Figure 5

    Figure 5. Depth tendencies of (a) cage occupancy ratio θT/θD of CH4 in sI hydrate structure in this study, and (b) methane and ethane mol % of decomposed gas determined in the 2012 coring campaign. (11) Dashed lines are rapid resistivity reduced lines. In (a), the red and blue circles indicate the θT/θD values of CH4 in the sI CH4 and sI CH4–C2H6 hydrates, respectively.

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    Figure 6

    Figure 6. Enlargements of the Raman spectra (Figure 4a) of the three 25P-1a samples, focusing on the peaks in the C–H vibration region of CH4. The higher-wavenumber peaks in two 25P-1a samples were fitted to a mixed Gaussian–Lorentzian function (dashed lines) using the commercial multiple peak-fitting program PeakFit v4.12.

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  • References

    This article references 38 other publications.

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      Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough
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      Detailed mapping of a bottom simulating reflector (BSR), which marks the phase transition from the methane hydrate layer above the reflector to free gas below, was conducted in the Nankai accretionary prism off Shikoku and Tokai. BSRs are widely distributed in the prism slope from the toe region to the forearc basin. BSR positions provide information on regional heat flow variations based on pressure-temp. conditions for methane hydrate stability. Estd. heat flows generally show const. values about 50 mW/m2 shallower than the middle slope of the prism, and gradually increase seaward in the lower prism slope. Occurrences of BSRs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field (BGHS) and/or concn. of methane hydrate above the BGHS. These conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concn. is completed by in situ biogenic methanogenesis within sediments including low total org. carbon. Moreover, sedimentation, uplifting, and sediment stacking by thrust faulting cause upward migration of the BGHS and migration of methane from the dissocd. hydrate to new BGHS. Such recycling of methane gas may have actively occurred in accretionary prisms. In contrast, there are five regions of no BSRs: the Nankai Trough floor, prism toe, slope basin, steep slope, and deep-sea canyon. The trough floor, the prism toe and the slope basin are characterized by young sediments with low prodn. of methane gas and sub-horizontal strata unsuitable for migration of gases and fluids. Erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the BGHS. BSR distribution and thermal structure estd. from BSR positions offer information about active processes occurring in accretionary prisms.
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      Colwell, F.; Matsumoto, R.; Reed, D. A review of the gas hydrates, geology, and biology of the Nankai Trough. Chem. Geol. 2004, 205, 391– 404,  DOI: 10.1016/j.chemgeo.2003.12.023
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      A review of the gas hydrates, geology, and biology of the Nankai Trough
      Colwell, Frederick; Matsumoto, Ryo; Reed, David
      Chemical Geology (2004), 205 (3-4), 391-404CODEN: CHGEAD; ISSN:0009-2541. (Elsevier Science B.V.)
      In this paper, we review research conducted during the last 20 yr on the active margin that forms the Nankai Trough off the southwest coast of Japan. The Nankai Trough exists along the convergent plate boundary between the Philippine Sea and Eurasian plates. An accretionary prism and a forearc basin occur landward of the Nankai Trough. Gas hydrates have been acquired through coring in the forearc basin. Bottom simulating reflectors (BSRs), an indicator of free gas which often subtends hydrate-bearing strata, are present throughout the Nankai Trough. Slope sediments are mainly composed of fine-grained materials with low reflectivity. Instability, possibly caused by uplift or tectonic activity, occurs in the Nankai Trough with sliding being a major factor in the structuring of the terrain. Clam colonies (Calyptogena spp.) have been obsd. in numerous locations suggesting fluid flow along major thrust faults. When first reported, these clams represented members of the genus that had not previously been described. Most of the detailed information regarding the methane hydrates of the Nankai Trough comes from the drilling of three test wells 50 km off the Omaezaki Peninsula in water depths of 945 m. The sediments from these boreholes were dominated by calcareous, dark gray clay/silt and siltstone, with a tendency to become sandier toward the bottom of the hole. Cores collected in zones above, within, and below the hydrate-bearing strata in these boreholes show pore water chloride concns. ranging between 100 and 550 mM with a highly variable interval occurring between 150 and 270 m below sea floor (mbsf). Compared with seawater values of ∼550 mM, the freshening of the porewater extd. from the cores strongly suggests the presence of gas hydrates in sediments. Pore satn. of gas hydrate estd. from the chloride anomaly was as high as 80% in sandstones in some of the sediments. Direct counts of microorganisms in the samples show cell d. at 105 cells/g throughout the hydrate strata, consistent with other marine sediments that have been studied. Clone libraries of the 16S, RNA gene created from extd. DNA from the Nankai Trough sediments and isolates from sediment microbial enrichments indicate the presence of methanogenic microorganisms. Sequence anal. suggests that the microbial community in the Nankai Trough sediments is distinct from those in previously characterized methane hydrate-bearing sediments on the Cascadia Margin and in the Gulf of Mexico. The majority of the sequences from the Nankai Trough clone library were either of a novel lineage or most closely related to uncultured clones. Bacterial clones exhibited sequence phylogenetic relatedness to Bacteroidetes, Planctomycetes, Actinobacteria, Proteobacteria and green nonsulfur groups, whereas the archaeal clones could be classified within the Euryarchaeota and Crenarchaeota. Continued investigations of the Nankai Trough sediments will add to our understanding of the biogeochem. characteristics of this active margin that is so important to the islands of Japan.
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      Kida, M.; Suzuki, K.; Kawamura, T.; Oyama, H.; Nagao, J.; Ebinuma, T.; Narita, H.; Suzuki, H.; Sakagami, H.; Takahashi, N. Characteristics of Natural Gas Hydrates Occurring in Pore-Spaces of Marine Sediments Collected from the Eastern Nankai Trough, off Japan. Energy Fuels 2009, 23, 5580– 5586,  DOI: 10.1021/ef900612f
      5
      Characteristics of Natural Gas Hydrates Occurring in Pore-Spaces of Marine Sediments Collected from the Eastern Nankai Trough, off Japan
      Kida, Masato; Suzuki, Kiyofumi; Kawamura, Taro; Oyama, Hiroyuki; Nagao, Jiro; Ebinuma, Takao; Narita, Hideo; Suzuki, Hiroyuki; Sakagami, Hirotoshi; Takahashi, Nobuo
      Energy & Fuels (2009), 23 (11), 5580-5586CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      Pore-space gas hydrates sampled from the eastern Nankai Trough area off of Japan were minutely characterized using several instrumental techniques. Gas chromatog. results indicated that the natural gas in the sediment samples studied comprises mainly CH4. The concns. of minor components varied according to depth. The powder x-ray diffraction patterns showed that the pore-space hydrates were of structure I (sI); the lattice consts. were 1.183-1.207 nm. Both 13C NMR and Raman spectra confirmed that CH4 mols. were encaged in sI hydrate lattice. The av. cage occupancies were calcd., resp., from the Raman data as 0.83 for small cages and 0.97 for large cages. The hydration nos. were detd. as 6.1-6.2.
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      Japan Oil, Gas, and Metals National Corporation (JOGMEC) Gas Produced from Methane Hydrate. http://www.jogmec.go.jp/english/news/release/content/300101080.pdf.
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    7. 7
      Konno, Y.; Fujii, T.; Sato, A.; Akamine, K.; Naiki, M.; Masuda, Y.; Yamamoto, K.; Nagao, J. Key Findings of the World’s First Offshore Methane Hydrate Production Test off the Coast of Japan: Toward Future Commercial Production. Energy Fuels 2017, 31, 2607– 2616,  DOI: 10.1021/acs.energyfuels.6b03143
      7
      Key Findings of the World's First Offshore Methane Hydrate Production Test off the Coast of Japan: Toward Future Commercial Production
      Konno, Yoshihiro; Fujii, Tetsuya; Sato, Akihiko; Akamine, Koya; Naiki, Motoyoshi; Masuda, Yoshihiro; Yamamoto, Koji; Nagao, Jiro
      Energy & Fuels (2017), 31 (3), 2607-2616CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      Marine methane hydrate in sands has huge potential as an unconventional gas resource; however, no field test of their prodn. potential had been conducted. Here, we report the world's first offshore methane hydrate prodn. test conducted at the eastern Nankai Trough and show key findings toward future com. prodn. Geol. anal. indicates that hydrate satn. reaches 80% and permeability in the presence of hydrate ranges from 0.01 to 10 mdarcies. Permeable (1-10 mdarcies) highly hydrate-satd. layers enable depressurization-induced gas prodn. of approx. 20,000 Sm3/D with water of 200 m3/D. Numerical anal. reveals that the dissocn. zone expands laterally 25 m at the front after 6 days. Gas rate is expected to increase with time, owing to the expansion of the dissocn. zone. It is found that permeable highly hydrate-satd. layers increase the gas-water ratio of the prodn. fluid. The identification of such layers is critically important to increase the energy efficiency and the tech. feasibility of depressurization-induced gas prodn. from hydrate reservoirs.
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    8. 8
      Japan Oil, Gas, and Metals National Corporation (JOGMEC) 2nd Gas Produced from Methane Hydrate (in Japanese). http://www.jogmec.go.jp/news/release/content/300335291.pdf.
      There is no corresponding record for this reference.
    9. 9
      Fujii, T.; Suzuki, K.; Takayama, T.; Tamaki, M.; Komatsu, Y.; Konno, Y.; Yoneda, J.; Yamamoto, K.; Nagao, J. Geological setting and characterization of a methane hydrate reservoir distributed at the first offshore production test site on the Daini-Atsumi Knoll in the eastern Nankai Trough, Japan. Mar. Pet. Geol. 2015, 66, 310– 322,  DOI: 10.1016/j.marpetgeo.2015.02.037
      9
      Geological setting and characterization of a methane hydrate reservoir distributed at the first offshore production test site on the Daini-Atsumi Knoll in the eastern Nankai Trough, Japan
      Fujii, Tetsuya; Suzuki, Kiyofumi; Takayama, Tokujiro; Tamaki, Machiko; Komatsu, Yuhei; Konno, Yoshihiro; Yoneda, Jun; Yamamoto, Koji; Nagao, Jiro
      Marine and Petroleum Geology (2015), 66 (Part_2), 310-322CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
      To obtain basic information for methane hydrate (MH) reservoir characterization at the first offshore prodn. test site (AT1) located on the northwestern slope of the Daini-Atsumi Knoll in the eastern Nankai Trough, extensive geophys. logging and pressure coring using a hybrid pressure coring system were conducted in 2012 at a monitoring well (AT1-MC) and a coring well (AT1-C). The MH-concd. zone (MHCZ), which was confirmed by geophys. logging at AT1-MC, has a 60-m-thick turbidite assemblage with sublayers ranging from a few tens to hundreds of centimeters thickness. The turbidite assemblage is composed of lobe/sheet-type sequences in the upper part and relatively thick channel-sand sequences in the lower part. Well-to-well correlations of sandy layers between two monitoring wells within 40 m of one another exhibited fairly good lateral continuity of sand layers in the upper part of the reservoir. This suggests an ideal reservoir for the prodn. test.The validity of MH pore satn. (Sh) evaluated from geophys. logging data were confirmed by comparing with those evaluated by pressure core anal. In the upper part of the MHCZ, Sh values estd. from resistivity logs showed distinct differences between the sand and mud layers, compared with Sh values from NMR (NMR) logs. Resistivity logs have higher vertical resoln. than NMR logs; therefore, they are favorable for these types of thin-bed evaluations. In the upper part, Sh values of 50%-80% were obsd. in sandy layers, which is in fairly good agreement with core-derived Sh values. In the lower part of the MHCZ, Sh values estd. from both resistivity and NMR logs showed higher background values and relatively smoother curves than those for the upper part. In the lower part, Sh values of 50%-80% were also obsd. in sandy layers, and they showed good agreement with the core-derived Sh values.
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    10. 10
      Suzuki, K.; Schultheiss, P.; Nakatsuka, Y.; Ito, T.; Egawa, K.; Holland, M.; Yamamoto, K. Physical properties and sedimentological features of hydrate-bearing samples recovered from the first gas hydrate production test site on Daini-Atsumi Knoll around eastern Nankai Trough. Mar. Pet. Geol. 2015, 66, 346– 357,  DOI: 10.1016/j.marpetgeo.2015.02.025
      10
      Physical properties and sedimentological features of hydrate-bearing samples recovered from the first gas hydrate production test site on Daini-Atsumi Knoll around eastern Nankai Trough
      Suzuki, Kiyofumi; Schultheiss, Peter; Nakatsuka, Yoshihiro; Ito, Takuma; Egawa, Kosuke; Holland, Melanie; Yamamoto, Koji
      Marine and Petroleum Geology (2015), 66 (Part_2), 346-357CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
      Before producing gas from gas hydrate, it is important to clarify the phys. properties of the methane hydrate reservoir and its sediments. During the 2012 pressure coring campaign, pressure core samples were retrieved from the northwest slope of Daini-Atsumi Knoll, one of the outer ridges of fore-arc basins along the northeast the Nankai-Trough. The pressure cores were sampled continuously throughout the turbidite sequences in the Methane Hydrate Concd. Zone (MHCZ); the cores were subjected to onboard nondestructive property analyses, and X-ray Computed Tomog. (X-ray CT) images of the cores were collected. Internal structures of the cores were obsd. in the X-ray images, which were used to judge core quality. Results for P-wave velocities and bulk densities, which were also measured on the pressure cores aboard the ship were compared with data from logging-while-drilling (LWD).P-wave velocities of cores that were retrieved by pressure corer were compared with methane-hydrate saturations calcd. from several methods. In general, P-wave velocities from logging while drilling (LWD) measurements corresponded to gas hydrate satn. calcd. from LWD. After compensating for the different vertical resolns. of LWD tools and pressure core anal., P-wave velocities from the pressure cores corresponded well to methane hydrate satn. calcd. from logging. A unique interval at 290-300 m below seafloor was identified where methane hydrate saturations computed from LWD data did not correspond to P-wave anomalies measured in cores from the same interval. This difference could be due to lateral inhomogeneity in lithol. between the logging and coring wells, with distinct local hydrate crystn./pptn. environments.
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    11. 11
      Kida, M.; Jin, Y.; Watanabe, M.; Konno, Y.; Yoneda, J.; Egawa, K.; Ito, T.; Nakatsuka, Y.; Suzuki, K.; Fujii, T.; Nagao, J. Chemical and crystallographic characterizations of natural gas hydrates recovered from a production test site in the eastern Nankai Trough. Mar. Pet. Geol. 2015, 66, 396– 403,  DOI: 10.1016/j.marpetgeo.2015.02.019
      11
      Chemical and crystallographic characterizations of natural gas hydrates recovered from a production test site in the eastern Nankai Trough
      Kida, Masato; Jin, Yusuke; Watanabe, Mizuho; Konno, Yoshihiro; Yoneda, Jun; Egawa, Kosuke; Ito, Takuma; Nakatsuka, Yoshihiro; Suzuki, Kiyofumi; Fujii, Tetsuya; Nagao, Jiro
      Marine and Petroleum Geology (2015), 66 (Part_2), 396-403CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
      This study describes the chem. and crystallog. properties of natural gas hydrates recovered from a methane prodn. test site in the eastern Nankai Trough. Gases released from the hydrate-bearing sediments contain methane as the main hydrocarbon component. The hydrate-bound gas includes small amts. of ethane and heavier hydrocarbons (less than ∼300 ppm). Concns. of minor hydrocarbon components decrease in sediment cores recovered from shallower subseafloor depths. Mol. and isotopic analyses suggest a microbial origin for the natural gas distributed at this site. The 13C NMR and Raman spectra provide evidence that methane mols. are encaged in two distinct polyhedral cages of the structure I hydrate with a hydration no. of 6.1. The powder X-ray diffraction profile shows that the crystal type of the gas hydrate is structure I (sI), with lattice consts. estd. at 1.1841(2) nm at 83 K. At widely varying temps., the lattice consts. of the pore-space natural gas hydrate crystals agree well with those of massive natural gas hydrate and artificial methane hydrate, suggesting that the mode of hydrate occurrence does not significantly affect the phys. dimensions of the crystal lattice. The small amts. of ethane and heavier hydrocarbons that form sI hydrate have no influence on the lattice expansion of the pore-space hydrate. The d. of the natural gas hydrate crystals in the hydrate-bearing sediment sample is estd. at 0.95 g/cm3 at 83 K.
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    12. 12
      Ijiri, A.; Inagaki, F.; Kubo, Y.; Adhikari, R. R.; Hattori, S.; Hoshino, T.; Imachi, H.; Kawagucci, S.; Morono, Y.; Ohtomo, Y. Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex. Sci. Adv. 2018, 4, eaao4631  DOI: 10.1126/sciadv.aao4631
      There is no corresponding record for this reference.
    13. 13
      Yamamoto, K.; Wang, X. X.; Tamaki, M.; Suzuki, K. The second offshore production of methane hydrate in the Nankai Trough and gas production behavior from a heterogeneous methane hydrate reservoir. Rsc Adv. 2019, 9, 25987– 26013,  DOI: 10.1039/C9RA00755E
      13
      The second offshore production of methane hydrate in the Nankai Trough and gas production behavior from a heterogeneous methane hydrate reservoir
      Yamamoto, K.; Wang, X.-X.; Tamaki, M.; Suzuki, K.
      RSC Advances (2019), 9 (45), 25987-26013CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Following the first attempt at producing gas from a naturally occurring methane hydrate (MH) deposit in the Daini-Atsumi Knoll in the eastern Nankai Trough area off Honshu Island, Japan in 2013, a second attempt was made in Apr. to June of 2017 at a nearby location using two producer wells sequentially and applying the depressurization method. The operation in the first borehole (AT1-P3) continued for 12 days with a stable drawdown of around 7.5 MPa and 41 000 m3 of methane gas being produced despite intermittent sand-prodn. events. The operation of the other borehole (AT1-P2) followed, with a total of 24 days of flow and 222 500 m3 of methane gas being produced without sand problems. However, the degree of drawdown was limited to 5 MPa because of a higher water prodn. rate than expected in the second hole. The pressure and temp. sensors deployed in the two producers, along with the two monitoring holes drilled nearby, gathered reservoir response data and information about the long-term MH dissocn. processes in the vicinity of the prodn. holes in the temporal and spatial domains. Although the ratio of energy return to the input was considerably larger than that for the depressurization operation, some observations (e.g., the high contrast in the prodn. rates between the two holes and the almost const. or slightly reduced gas prodn. rates) were not predicted by the numerical models. This failure in prediction raises questions about the veracity of the reservoir characteristics modeled in the numerical simulations. This paper presents the operation summaries and data obtained with thought-expt. based-anticipated prodn. behaviors and preliminary anal. of the obtained data as the comparison with expected behaviors. Detailed observations of gas and water prodn., as well as the pressure and temp. data recorded during the gas flow tests, indicate that the heterogeneous MH distribution within the reservoir was mainly responsible for the discrepancies obsd. between the anticipated and actual behaviors. Furthermore, the motion of the water that does not originate from MH dissocn. introduces complexity, such as the occurrence of concd. water-producing intervals and unexpected gas prodn. responses to decreases in pressure, into the prodn. behavior. The influence of heterogeneity should be clearly understood for the accurate prediction of gas prodn. behavior based on MH reservoirs.
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    14. 14
      Yamamoto, K.; Suzuki, K.; Wang, X.; Matsunaga, T.; Nishioka, I.; Nakatsuka, Y.; Yoneda, J. The Second Offsure Production Test of Methane Hydrates in the Earstern Nankai Trough and Site Characterization Efforts. Fire Ice 2019, 9– 15
      There is no corresponding record for this reference.
    15. 15
      Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D. Evidence of structure II hydrate formation from methane + ethane mixtures. Chem. Eng. Sci. 2000, 55, 1981– 1999,  DOI: 10.1016/S0009-2509(99)00389-9
      15
      Evidence of structure II hydrate formation from methane + ethane mixtures
      Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr.
      Chemical Engineering Science (2000), 55 (11), 1981-1999CODEN: CESCAC; ISSN:0009-2509. (Elsevier Science Ltd.)
      Methane and ethane are each known to form structure I (sI) hydrates. However, Raman and NMR spectroscopic measurements made on clathrate hydrates formed from binary gas mixts. of methane (CH4) and ethane (C2H6) indicated that structure II (sII) hydrate forms in this system for certain compns. Raman band frequencies for ethane and 13C-NMR chem. shifts for ethane were used to identify the hydrate structure in these studies. Raman spectra obtained from six different CH4 + C2H6 hydrates at Lw-H-V equil. conditions and 274.2 K indicate a change in hydrate structure from sI to sII between 72.2 and 75 mol% of methane in the vapor. Areas under peaks in the Raman spectra suggested that this change in hydrate structure is assocd. with a 20% change in hydrate guest compn. Abs. cage occupancies and hydration nos. were obtained from the Raman data. Anal. of the sI to sII transition and a phase diagram for the CH4 + C2H6 hydrate system were discussed. High-resoln., solid-state 13C-NMR measurements at 253 K confirmed that CH4 + C2H6 gas mixts. can form either sI or sII hydrates, depending on the vapor compn.
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    16. 16
      Amante, C.; Eakins, B. W. ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. (accessed August 8, 2017).
      There is no corresponding record for this reference.
    17. 17
      Wessel, P.; Smith, W. H. F.; Scharroo, R.; Luis, J.; Wobbe, F. Generic Mapping Tools: Improved Version Released. Eos, Trans. Am. Geophys. Union 2013, 94, 409– 410,  DOI: 10.1002/2013EO450001
      There is no corresponding record for this reference.
    18. 18
      Wessel, P.; Smith, W. H. F. Free software helps map and display data. Eos, Trans. Am. Geophys. Union 1991, 72, 441– 446,  DOI: 10.1029/90EO00319
      There is no corresponding record for this reference.
    19. 19
      Yoneda, J.; Masui, A.; Konno, Y.; Jin, Y.; Egawa, K.; Kida, M.; Ito, T.; Nagao, J.; Tenma, N. Mechanical properties of hydrate-bearing turbidite reservoir in the first gas production test site of the Eastern Nankai Trough. Mar. Pet. Geol. 2015, 66, 471– 486,  DOI: 10.1016/j.marpetgeo.2015.02.029
      There is no corresponding record for this reference.
    20. 20
      Jin, Y.; Konno, Y.; Yoneda, J.; Kida, M.; Nagao, J. In Situ Methane Hydrate Morphology Investigation: Natural Gas Hydrate-Bearing Sediment Recovered from the Eastern Nankai Trough Area. Energy Fuels 2016, 30, 5547– 5554,  DOI: 10.1021/acs.energyfuels.6b00762
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      In Situ Methane Hydrate Morphology Investigation: Natural Gas Hydrate-Bearing Sediment Recovered from the Eastern Nankai Trough Area
      Jin, Yusuke; Konno, Yoshihiro; Yoneda, Jun; Kida, Masato; Nagao, Jiro
      Energy & Fuels (2016), 30 (7), 5547-5554CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      The hydrate morphol. of natural gas hydrate-bearing (GH) sediments recovered from the eastern Nankai trough area was studied under hydrostatic pressurized conditions that prevent dissocn. of gas hydrates in a sediment. The authors developed a novel x-ray computed tomog. system and an attenuated total reflection IR (ATR-IR) probe for use in the Instrumented Pressure Testing Chamber for the set of Pressure-Core Nondestructive Anal. Tools (PNATs), which can measure the sediment structure, primary wave velocity (PWV), d., and shear strength under pressurized conditions. The hydrate satn. values estd. using the ATR-IR absorption bands of H2O mols. strongly correlate with PWV. Assuming homogeneity of hydrate distribution in the planes perpendicular to the sample depth direction, the hydrate morphol. of natural GH sediments in the eastern Nankai trough area demonstrated a load-bearing morphol. type. The predicted hydrate morphol. results are in good agreement with data reported in the literature. The combination of PNATs including ATR-IR spectroscopy can be used to est. the properties of GH sediments without the release of pressure to atm. conditions to model gas hydrate reservoirs for natural gas prodn.
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    21. 21
      Schultheiss, P.; Holland, M.; Roberts, J.; Durce, M.; Fox, P. In PCATS: Pressure Core Analysis and Transfer System ; 7th International Conference on Gas Hydrates (ICGH2011), 2011.
      There is no corresponding record for this reference.
    22. 22
      Jin, Y.; Kida, M.; Nagao, J. Structure H Clathrate Hydrates in Methane-Halogenic Large Molecule Substance-Water Systems. J. Phys. Chem. C 2019, 123, 17170– 17175,  DOI: 10.1021/acs.jpcc.9b04691
      22
      Structure H Clathrate Hydrates in Methane-Halogenic Large Molecule Substance-Water Systems
      Jin, Yusuke; Kida, Masato; Nagao, Jiro
      Journal of Physical Chemistry C (2019), 123 (28), 17170-17175CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We investigated crystallog. variations in structure H (sH) hydrates hosting CH4 and halogenic large-mol. guest substances [halogenic large-mol. guest substance (LMGS)]. The three halogenic LMGSs, namely, chlorocyclohexane (ClCH), bromocyclohexane (BrCH), and iodocyclohexane (ICH), share a common mol. structure (X-cyclohexane). The lattice consts. along a and c axes of sH hydrates hosting X-cyclohexane increased with increasing mol. size: ClCH < BrCH < ICH. The c lattice const. was esp. dependent on the mol. size, possibly because X-cyclohexanes align along the longitudinal direction of the 51268 cage, which coincides with the c-axis. Raman spectroscopy revealed that LMGSs altered the surroundings of CH4 mols. in the 512 and 435663 cages. As the crystal size increases, CH4 mols. encounter more attractive (less repulsive) guest-host interactions. The wavenumber shifts of the C-H vibrations of CH4 in the 512 and 435663 cages increased with temp. and were slightly greater in the 435663 cages than in the 512 cages. Different thermal responses between the 512 and 435663 cages may be caused by anisotropic lattice expansion of the sH hydrates. Finally, the phase stabilities of the sH (CH4 and X-cyclohexane) hydrates were evaluated by an isochoric method. The pressure region of equil. pressure-temp. conditions was lower in the sH (CH4 and X-cyclohexane) hydrates than in the pure CH4 hydrate system. Moreover, the temp. region of the equil. pressure-temp. conditions increased in the order ICH < ClCH < BrCH. The dissocn. enthalpies of the sH (CH4 + ClCH) and sH (CH4 + ICH) hydrates were estd. as 380 kJ/mol-1.
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    23. 23
      Egawa, K.; Nishimura, O.; Izumi, S.; Fukami, E.; Jin, Y.; Kida, M.; Konno, Y.; Yoneda, J.; Ito, T.; Suzuki, K. Bulk sediment mineralogy of gas hydrate reservoir at the East Nankai offshore production test site. Mar. Pet. Geol. 2015, 66, 379– 387,  DOI: 10.1016/j.marpetgeo.2015.02.039
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      Bulk sediment mineralogy of gas hydrate reservoir at the East Nankai offshore production test site
      Egawa, Kosuke; Nishimura, Okio; Izumi, Shoko; Fukami, Eiji; Jin, Yusuke; Kida, Masato; Konno, Yoshihiro; Yoneda, Jun; Ito, Takuma; Suzuki, Kiyofumi; Nakatsuka, Yoshihiro; Nagao, Jiro
      Marine and Petroleum Geology (2015), 66 (Part_2), 379-387CODEN: MPEGD8; ISSN:0264-8172. (Elsevier Ltd.)
      Bulk sediment mineralogy was measured from a gas hydrate-bearing Middle Pleistocene deepwater turbidite interval at the offshore prodn. test site of the Eastern Nankai Trough area. We used core samples recovered from a 60-m-long section of the borehole for semi-quant. anal. of mineral and org. contents of the gas hydrate reservoir. Powder X-ray diffraction anal., ignition loss test, and field-emission scanning electronic microscopy revealed the followings: (i) ten bulk minerals vary in concn. and most of them have a good exponential correlation with median grain size; (ii) the upper muddy section is dominated by coccolith-rich hemipelagites, whereas the middle and lower sections are characterized by relatively coccolith-poor, probably humic substance-rich turbiditic sediments; and (iii) the common occurrence of authigenic gypsum, siderite, and framboidal pyrite indicates early diagenesis in anoxic and relatively high salinity conditions probably assocd. with gas hydrate formation in some degree. Such mineralogical data can provide useful information on evaluation of thermal properties, geomech. characteristics, effective permeability, and early diagenetic mechanism of the sediments to characterize and exploit gas hydrate reservoirs.
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    24. 24
      Konno, Y.; Jin, Y.; Yoneda, J.; Kida, M.; Egawa, K.; Ito, T.; Suzuki, K.; Nagao, J. Effect of methane hydrate morphology on compressional wave velocity of sandy sediments: Analysis of pressure cores obtained in the Eastern Nankai Trough. Mar. Pet. Geol. 2015, 66, 425– 433,  DOI: 10.1016/j.marpetgeo.2015.02.021
      There is no corresponding record for this reference.
    25. 25
      Takeya, S.; Fujihisa, H.; Gotoh, Y.; Istomin, V.; Chuvilin, E.; Sakagami, H.; Hachikubo, A. Methane Clathrate Hydrates Formed within Hydrophilic and Hydrophobic Media: Kinetics of Dissociation and Distortion of Host Structure. J. Phys. Chem. C 2013, 117, 7081– 7085,  DOI: 10.1021/jp312297h
      25
      Methane Clathrate Hydrates Formed within Hydrophilic and Hydrophobic Media: Kinetics of Dissociation and Distortion of Host Structure
      Takeya, Satoshi; Fujihisa, Hiroshi; Gotoh, Yoshito; Istomin, Vladimir; Chuvilin, Evgeny; Sakagami, Hirotoshi; Hachikubo, Akihiro
      Journal of Physical Chemistry C (2013), 117 (14), 7081-7085CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The crystal structure and dissocn. processes of methane (CH4) hydrates were investigated to better understand their stability in a natural environment. By using powder X-ray diffraction, the authors found that the unit-cell parameters of the hydrates formed with fine hydrophilic and hydrophobic beads were resp. larger and smaller by 0.02 Å than the unit-cell parameters of simple CH4 hydrates. The CH4 hydrates formed with hydrophobic beads dissocd. quickly above 200 K, whereas the CH4 hydrates formed with hydrophilic beads are stable up to about 273 K. The interfacial forces inside the intergranular pores or void spaces of the beads affect the kinetics of dissocn. of CH4 hydrate and are important for both the macroscopic and the crystallog. structures.
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    26. 26
      Takeya, S.; Uchida, T.; Kamata, Y.; Nagao, J.; Kida, M.; Minami, H.; Sakagami, H.; Hachikubo, A.; Takahashi, N.; Shoji, H. Lattice expansion of clathrate hydrates of methane mixtures and natural gas. Angew. Chem., Int. Ed. 2005, 44, 6928– 6931,  DOI: 10.1002/anie.200501845
      26
      Lattice expansion of clathrate hydrates of methane mixtures and natural gas
      Takeya, Satoshi; Uchida, Tsutomu; Kamata, Yasushi; Nagao, Jiro; Kida, Masato; Minami, Hirotsugu; Sakagami, Hirotoshi; Hachikubo, Akihiro; Takahashi, Nobuo; Shoji, Hitoshi; Khlystov, Oleg; Grachev, Mikhail; Soloviev, Valery
      Angewandte Chemie, International Edition (2005), 44 (42), 6928-6931CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Structural transition from type I to type II is accompanied by a large increase in lattice const. a in CH4+C2H6 clathrate hydrates (see picture). In contrast, the lattice consts. of CH4+CO2 clathrate hydrates are independent of compn. Data for a natural gas hydrate from the sediments of Lake Baikal are also consistent with these trends.
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    27. 27
      Shpakov, V. P.; Tse, J. S.; Tulk, C. A.; Kvamme, B.; Belosludov, V. R. Elastic moduli calculation and instability in structure I methane clathrate hydrate. Chem. Phys. Lett. 1998, 282, 107– 114,  DOI: 10.1016/S0009-2614(97)01241-4
      27
      Elastic moduli calculation and instability in structure I methane clathrate hydrate
      Shpakov, V. P.; Tse, J. S.; Tulk, C. A.; Kvamme, B.; Belosludov, V. R.
      Chemical Physics Letters (1998), 282 (2), 107-114CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
      The thermal expansion of structure I clathrate hydrates with empty cages and with enclathrated methane mols. was calcd. from 5 to 270 K using the lattice dynamics approach within the quasi-harmonic approxn. The temp. dependence of several elastic properties, the dynamical, adiabatic and isothermal elastic moduli, are calcd. The dynamical and thermodynamical stability of the crystals were studied according to the Born stability criteria. Methane hydrate is unstable against small homogeneous deformations near the melting transition at normal pressure. For the empty hydrate lattice, a thermodynamical instability occurs at significantly higher temp. The theor. results are compared with new thermal expansion data on methane hydrate and results from a previous mol. dynamics study.
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    28. 28
      Yan, K. F.; Li, X. S.; Xu, C. G.; Lv, Q. N.; Ruan, X. K. Molecular dynamics simulation of the intercalation behaviors of methane hydrate in montmorillonite. J. Mol. Model. 2014, 20, 2311  DOI: 10.1007/s00894-014-2311-8
      28
      Molecular dynamics simulation of the intercalation behaviors of methane hydrate in montmorillonite
      Yan KeFeng; Li XiaoSen; Xu ChunGang; Lv QiuNan; Ruan XuKe
      Journal of molecular modeling (2014), 20 (6), 2311 ISSN:.
      The formation and mechanism of CH4 hydrate intercalated in montmorillonite are investigated by molecular dynamics (MD) simulation. The formation process of CH4 hydrate in montmorillonite with 1 ~ 8 H2O layers is observed. In the montmorillonite, the "surface H2O" constructs the network by hydrogen bonds with the surface Si-O ring of clay, forming the surface cage. The "interlayer H2O" constructs the network by hydrogen bonds, forming the interlayer cage. CH4 molecules and their surrounding H2O molecules form clathrate hydrates. The cation of montmorillonite has a steric effect on constructing the network and destroying the balance of hydrogen bonds between the H2O molecules, distorting the cage of hydrate in clay. Therefore, the cages are irregular, which is unlike the ideal CH4 clathrate hydrates cage. The pore size of montmorillonite is another impact factor to the hydrate formation. It is quite easier to form CH4 hydrate nucleation in montmorillonite with large pore size than in montmorillonite with small pore. The MD work provides the constructive information to the investigation of the reservoir formation for natural gas hydrate (NGH) in sediments.
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    29. 29
      Li, Y.; Chen, M.; Song, H. Z.; Yuan, P.; Zhang, B. F.; Liu, D.; Zhou, H. J.; Bu, H. L. Effect of Cations (Na+, K+, and Ca2+) on Methane Hydrate Formation on the External Surface of Montmorillonite: Insights from Molecular Dynamics Simulation. ACS Earth Space Chem. 2020, 4, 572– 582,  DOI: 10.1021/acsearthspacechem.9b00323
      29
      Effect of Cations (Na+, K+, and Ca2+) on Methane Hydrate Formation on the External Surface of Montmorillonite: Insights from Molecular Dynamics Simulation
      Li, Yun; Chen, Meng; Song, Hongzhe; Yuan, Peng; Zhang, Baifa; Liu, Dong; Zhou, Huijun; Bu, Hongling
      ACS Earth and Space Chemistry (2020), 4 (4), 572-582CODEN: AESCCQ; ISSN:2472-3452. (American Chemical Society)
      In this study, mol. dynamics simulations were performed to investigate the effects of montmorillonite with different surface cations (i.e., Na+, K+, and Ca2+) on CH4 hydrate formation. The results showed that CH4 hydrate cages are mainly formed beyond the montmorillonite surface. The inner-sphere adsorption of K+ and the outer-sphere adsorption of Na+ and Ca2+ occurred on the montmorillonite surface, leading to differences in order parameters and hydrogen bond no. of H2O mols. The no. of structure I cages increased faster than that of structure II cages in different models and were in agreement with the fact that CH4 mols. can only form sI hydrate crystals. The no. of 512 cages increased in the order: Na-Mt < Ca-Mt < K-Mt. The aq. environment dominated by K+ on the external surface of montmorillonite facilitate heterogeneous nucleation of CH4 hydrate rather than that by Ca2+ or Na+. The abovementioned findings suggest that the coordination structure of cations on the external surface of montmorillonite plays an important role in CH4 hydrate formation through altering the occupation of CH4 hydrate.
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    30. 30
      Subramanian, S.; Sloan, E. D. Trends in Vibrational Frequencies of Guests Trapped in Clathrate Hydrate Cages. J. Phys. Chem. B 2002, 106, 4348– 4355,  DOI: 10.1021/jp013644h
      30
      Trends in Vibrational Frequencies of Guests Trapped in Clathrate Hydrate Cages
      Subramanian, Sivakumar; Sloan, E. Dendy, Jr.
      Journal of Physical Chemistry B (2002), 106 (17), 4348-4355CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      Raman spectra of ethane trapped in the small 512 cage of sII hydrate (at ∼70 MPa), isobutane trapped in the large 51264 cage of sII hydrate, and the gauche form of n-butane trapped in the large 51264 cage of sII hydrate were obtained for the first time. These new Raman results are combined with existing Raman and IR results for various guests to infer general trends in vibrational frequencies of guest mols. trapped in clathrate hydrate cages as a function of cage size, guest size, guest vibrational mode, and pressure. The obsd. trend in stretching frequencies of guests with cage size, which can be stated as "the larger the cavity, the lower the frequency", is explained through the qual. "loose cage-tight cage" model of Pimentel and Charles (Pure Appl. Chem. 1963, 7, 111).
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    31. 31
      Burke, E. A. J. Raman microspectrometry of fluid inclusions. Lithos 2001, 55, 139– 158,  DOI: 10.1016/S0024-4937(00)00043-8
      31
      Raman microspectrometry of fluid inclusions
      Burke, E. A. J.
      Lithos (2001), 55 (1-4), 139-158CODEN: LITHAN; ISSN:0024-4937. (Elsevier Science B.V.)
      A review with 115 refs. For many kinds of fluid inclusions, the coupling of microthermometry and Raman microspectrometry is still the only viable option to obtain compns. of single fluid inclusions. A review is given on the basis of 16 yr of experience and helped with 115 refs. of the instrumentation, anal. conditions and methodol. of the application of Raman microspectrometry to gaseous, aq. and hydrocarbon inclusions, and their daughter minerals.
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    32. 32
      The Colorado School of Mines Hydrate Prediction Program ″CSMGem″. 2007.
      There is no corresponding record for this reference.
    33. 33
      Collett, T. S. A Review of Well-Log Analysis Techniques Used to Assess Gas-Hydrate-Bearing Reservoirs. In Natural Gas Hydrates: Occurrence, Distribution, and Detection: Occurrence, Distribution, and Detection; Paull, C. K.; Dillon, W. P., Eds.; American Geophysical Union, 2001; Vol. 124, pp 189– 210.
      There is no corresponding record for this reference.
    34. 34
      Ohno, H.; Strobel, T. A.; Dec, S. F.; Sloan, E. D.; Koh, C. A. Raman Studies of Methane-Ethane Hydrate Metastability. J. Phys. Chem. A 2009, 113, 1711– 1716,  DOI: 10.1021/jp8010603
      34
      Raman Studies of Methane-Ethane Hydrate Metastability
      Ohno, Hiroshi; Strobel, Timothy A.; Dec, Steven F.; Sloan, E. Dendy, Jr.; Koh, Carolyn A.
      Journal of Physical Chemistry A (2009), 113 (9), 1711-1716CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The interconversion of methane-ethane hydrate from metastable to stable structures was studied using Raman spectroscopy. SI and sII hydrates were synthesized from methane-ethane gas mixts. of 65% or 93% methane in ethane and water, both with and without the kinetic hydrate inhibitor, poly(N-vinylcaprolactam). The obsd. faster structural conversion rate in the higher methane concn. atm. can be explained in terms of the differences in driving force (difference in chem. potential of water in sI and sII hydrates) and kinetics (mass transfer of gas and water rearrangement). The kinetic hydrate inhibitor increased the conversion rate at 65% methane in ethane (sI is thermodynamically stable) but retards the rate at 93% methane in ethane (sII is thermodynamically stable), implying there is a complex interaction between the polymer, water, and hydrate guests at crystal surfaces.
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    35. 35
      Ohno, H.; Kida, M.; Sakurai, T.; Iizuka, Y.; Hondoh, T.; Narita, H.; Nagao, J. Symmetric Stretching Vibration of CH4 in Clathrate Hydrate Structures. ChemPhysChem 2010, 11, 3070– 3073,  DOI: 10.1002/cphc.201000519
      35
      Symmetric Stretching Vibration of CH4 in Clathrate Hydrate Structures
      Ohno, Hiroshi; Kida, Masato; Sakurai, Toshimitsu; Iizuka, Yoshinori; Hondoh, Takeo; Narita, Hideo; Nagao, Jiro
      ChemPhysChem (2010), 11 (14), 3070-3073CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The use of high-resoln. Raman measurement as a powerful tool not only for hydrate-phase identification and cage-filling anal., but also for the measure of mol. interactions in clathrate hydrate systems was studied. Distinct Raman bands of CH4 in a sH hydrate phase in the presence of large mol. guest substances was revealed for the first time; methane cage populations were estd. from the Raman intensities. The vibrational states of CH4 were obsd. to differ considerably among different hydrate structures, even for the same host cages (512). These observations indicate that Raman signals of methane mols. trapped in 512 small cavities are particularly sensitive to changes in the mol. environment. Therefore, these measurements can be very useful for understanding the nature of guest-host (perhaps also guest-guest) interactions.
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    36. 36
      Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of Clathrate Hydrates via Raman Spectroscopy. J. Phys. Chem. B 1997, 101, 7371– 7377,  DOI: 10.1021/jp970768e
      36
      Measurement of Clathrate Hydrates via Raman Spectroscopy
      Sum, Amadeu K.; Burruss, Robert C.; Sloan, E. Dendy, Jr.
      Journal of Physical Chemistry B (1997), 101 (38), 7371-7377CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      Raman spectra of clathrate hydrate guest mols. are presented for 3 known structures (I (sI), II (sII), and H (sH)) in the following systems: CH4 (sI), CO2 (sI), C3H8 (sII), CH4 CO2 (sI), CD4 C3H8 (sII), CH4 N2 (sI), CH4 THF-d8 (sII), and CH4 C7D14 (sH). Relative occupancy of CH4 in the large and small cavities of sI were detd. by deconvoluting the ν1 sym. bands, resulting in hydration nos. of 6.04 ± 0.03. The frequency of the ν1 bands for CH4 in structures I, II, and H differ statistically, so that Raman spectroscopy is a potential tool to identify hydrate crystal structure. Hydrate guest compns. were also measured for 2 vapor compns. of the CH4 CO2 system, and they compared favorably with predictions. The large cavities are almost fully occupied by CH4 and CO2, whereas only a small fraction of the small cavities are occupied by CH4. No CO2 was found in the small cavities. Hydration nos. from 7.27 to 7.45 were calcd. for the mixed hydrate.
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    37. 37
      Cutler, K. B.; Edwards, R. L.; Taylor, F. W.; Cheng, H.; Adkins, J.; Gallup, C. D.; Cutler, P. M.; Burr, G. S.; Bloom, A. L. Rapid sea-level fall and deep-ocean temperature change since the last interglacial period. Earth Planet. Sci. Lett. 2003, 206, 253– 271,  DOI: 10.1016/S0012-821X(02)01107-X
      37
      Rapid sea-level fall and deep-ocean temperature change since the last interglacial period
      Cutler, K. B.; Edwards, R. L.; Taylor, F. W.; Cheng, H.; Adkins, J.; Gallup, C. D.; Cutler, P. M.; Burr, G. S.; Bloom, A. L.
      Earth and Planetary Science Letters (2003), 206 (3-4), 253-271CODEN: EPSLA2; ISSN:0012-821X. (Elsevier Science B.V.)
      We have dated Huon Peninsula, Papua New Guinea and Barbados corals that formed at times since the Last Interglacial Period, applying both 230Th and 231Pa dating techniques as a test of age accuracy. We show that Marine Isotope Stage (MIS) 5e ended prior to 113.1±0.7 kyr, when sea level was -19 m. During MIS 5b sea level was -57 m at 92.6±0.5 kyr, having dropped about 40 m in approx. 10 kyr during the MIS 5c-5b transition. Sea level then rose more than 40 m during the MIS 5b-5a transition, also in about 10 kyr. MIS 5a lasted until at least 76.2±0.4 kyr, at a level of -24 m at that time. Combined with earlier data that places MIS 4 sea level at -81 m at 70.8 kyr, our late MIS 5a data indicate that sea level fell almost 60 m in less than 6 kyr (10.6 m/kyr) during the MIS 5-4 transition. The magnitude of the drop is half that of the glacial-interglacial amplitude and approx. equiv. to the vol. of the present-day Antarctic Ice Sheet. During this interval the min. av. rate of net continental ice accumulation was 18 cm/yr, likely facilitated by efficient moisture transport from lower latitudes. At three specific times (60.6±0.3, 50.8±0.3, and 36.8+0.2 kyr) during MIS 3, sea level was between -85 and -74 m. Sea level then dropped to -107 m at 23.7±0.1 kyr early in MIS 2, before dropping further to Last Glacial Maximum (LGM) values and then rising to present values during the last deglaciation. Times of rapid sea-level drop correspond to times of high winter insolation at low northern latitudes and high winter latitudinal gradients in northern hemisphere insolation, supporting the idea that these factors may have resulted in high water-vapor pressure in moisture sources and efficient moisture transport to high-latitude glaciers, thereby contributing to glacial buildup. We combined our sea-level results with deep-sea δ18O records as a means of estg. the temp. and ice-vol. components in the marine δ18O record. This anal. confirms large deep-ocean temp. shifts following MIS 5e and during Termination I. Deep-ocean temps. changed by much smaller amts. between MIS 5c and 2. Maximum temp. shift in the deep Pacific is about 2°, whereas the shift at a site in the Atlantic is 4°. Under glacial conditions temps. at both sites are near the f.p. The shift in the Atlantic is likely caused by a combination of changing proportions of northern and southern source waters as well as changing temp. at the sites where these deep waters form.
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    38. 38
      Fleming, K.; Johnston, P.; Zwartz, D.; Yokoyama, Y.; Lambeck, K.; Chappell, J. Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites. Earth Planet. Sci. Lett. 1998, 163, 327– 342,  DOI: 10.1016/S0012-821X(98)00198-8
      38
      Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites
      Fleming, Kevin; Johnston, Paul; Zwartz, Dan; Yokoyama, Yusuke; Lambeck, Kurt; Chappell, John
      Earth and Planetary Science Letters (1998), 163 (1-4), 327-342CODEN: EPSLA2; ISSN:0012-821X. (Elsevier Science B.V.)
      The eustatic component of relative sea-level change provides a measure of the amt. of ice transferred between the continents and oceans during glacial cycles. This has been quantified for the period since the Last Glacial Maximum by correcting obsd. sea-level change for the glacio-hydro-isostatic contributions using realistic ice distribution and earth models. During the Last Glacial Maximum (LGM) the eustatic sea level was 125±5 m lower than the present day, equiv. to a land-based ice vol. of (4.6-4.9)×107 km3. Evidence for a non-uniform rise in eustatic sea level from the LGM to the end of the deglaciation is examd. The initial rate of rise from ca. 21 to 17 ka was relatively slow with an av. rate of ca. 6 m ka-1, followed by an av. rate of ca. 10 m ka-1 for the next 10 ka. Significant departures from these av. rates may have occurred at the time of the Younger Dryas and possibly also around 14 ka. Most of the decay of the large ice sheets was completed by 7 ka, but 3-5 m of water has been added to the oceans since that time.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.energyfuels.4c00343.

    • AT1-CW2 composite log data; grain-size distribution of natural sediment used in this study; example of agglomerated hydrate particles in the 22P-3 section; and peak finding by using smoothing filers (PDF)


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