International Conference on Gas Hydrates (ICGH) (6th : 2008)

STRUCTURAL CHARACTERIZATION OF NATURAL GAS HYDRATES IN CORE SAMPLES FROM OFFSHORE INDIA Kumar, Pushpendra; Das, H.C.; Anbazhagen, K.; Lu, Hailong; Ripmeester, John A. Jul 31, 2008

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  STRUCTURAL CHARACTERIZATION OF NATURAL GAS HYDRATES IN CORE SAMPLES FROM OFFSHORE INDIA  Pushpendra Kumar∗ Institute of Engineering & Ocean Technology, Oil and Natural Gas Corporation Ltd,  ONGC Complex Phase-II, Panvel, Navi Mumbai-410221, India  H. C. Das Oil India Ltd, Duliajon, Assam, India  K. Anbazhagan GAIL (India) Ltd, Bhikaji Kama Place, New Delhi, India  Hailong Lu & John A. Ripmeester National Research Council, Ottawa, Canada   ABSTRACT The dedicated gas hydrate coring/drilling program was carried out under National Gas Hydrate Program (NGHP) in four Indian offshore areas (Kerala-Konkan, Krishna- Godavari, Mahanadi and Andman) during 28th April to 19th August, 2006. During NGHP Expedition 01, 2006, total of 39 holes were drilled/cored at 21 sites in these areas. The gas hydrates have been found to be present in large quantities in Indian offshore areas particularly in KG basin. More than 130 confirmed solid gas hydrate samples were recovered during this hydrate coring/drilling program. The laboratory analysis was carried out on the 34 natural gas hydrate samples recovered from offshore India. The gas hydrate characterization was carried out using the microscopic techniques such as Raman, 13C NMR and XRD for its structure, cavity occupancy and hydration number. The gas hydrates occur in grayish green fine sediments, gray medium sands and white volcanic ash as pore-filling hydrate and massive hydrates in fractured shale/clay. The visible massive gas hydrates developed especially at Site NGHP 1-10B, 10C, 10D and 21A in K G area. The structures of the gas hydrates in the studied samples are all sI, with methane as the dominant guest molecule. The occupancy of methane in large cage is almost complete, while it is variable in the small cage (0.75 to 0.99). The hydration number is 6.10 ± 0.15 for most of the hydrates in the samples studied. This paper presents the results of the laboratory analysis on the structural characterization of natural gas hydrates in core samples from offshore India.  Keywords: methane hydrate, gas hydrate structure, cavity occupancy, hydration number   ∗  Corresponding author: Phone: +91 2227486355 Fax +91 2227453692 E-mail: pushpendrakumar_2005@yahoo.com Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. INTRODUCTION  The dedicated gas hydrate coring/ drilling operations were carried out under National Gas Hydrate Program (NGHP) in four Indian offshore areas (Kerala-Konkan, Krishna-Godavari, Mahanadi and Andman) during 28th April to 19th August, 2006 using the research vessel ‘JOIDES Resolution’. During this program, total of 39 holes were drilled/cored at 21 sites in these areas. The gas hydrates have been found to be present in large quantities in Indian offshore areas [1].  A total of 34 samples from Indian offshore were studied under the collaborative efforts of the researchers from the Steacie Institute of Molecular Sciences, National Research Council, Canada and the Indian scientists involved in Indian National Gas Hydrate Program. The gas hydrate characterization was carried out using the microscopic techniques such as Raman, 13C NMR and XRD for its structure, cavity occupancy and hydration number.   THE STRUCTURAL AND COMPOSITIONAL DETERMINATION OF GAS HYDRATES  The gas hydrate samples were studied using X-ray diffraction, Raman and 13C NMR [2], [3] & [4]. The each study along with the results is described below.  XRD analysis  XRD analyses were carried out on a Bruker powder X-ray diffractometer equipped with a low temperature sample chamber in which the temperature of the sample holder can be adjusted by changing the flow rate of liquid nitrogen. The experiments were carried out at 153 K, scanning through a 2Θ interval of 5 to 50 degrees. Figure 1 shows a PXRD pattern of gas hydrate sample HYD87 from the in Indian offshore. The presence of some Ice Ih reflections provides the calibration and the pattern is typical of sI gas hydrate. Gas hydrate in HYD 100 was concentrated in liquid nitrogen, and its PXRD powder pattern is shown in Figure 2, indicating a gas hydrate of sI.  Because the volcanic ash in HYD 111 was so fine that concentrating gas hydrate from it was very difficult, the analysis was carried out directly on the bulk sample. As shown in Figure 3, gas hydrate peaks can be identified although the number of reflections is not great as compared with that of massive gas hydrate like for HYD 87. Similar to HYD 87 the powder pattern can be indexed in terms of sI gas hydrate.  As shown in Figure 4, 5, and 6, the gas hydrate in all of the samples received are sI no matter the site or the depth of recovery.  0 5000 1 104 1.5 104 2 104 2.5 104 10 20 30 40 50 HYD87 2Theta  Figure 1: PXRD powder pattern of gas hydrate from offshore India (HYD87) 0 2000 4000 6000 8000 1 104 10 20 30 40 50 HYD100 2Theta  Figure 2: PXRD spectrum of gas hydrate concentrated from medium sands of HYD100 0500 1000 1500 2000 2500 3000 10 20 30 40 50 HYD111 2Theta (degree)   Figure 3: XRD powder pattern of gas hydrate in the volcanic ash of HYD 111    10 20 30 40 50 HYD9 2Theta HYD18 HYD35 HYD56 In te n s ity  (A . U. ) Figure 4: XRD powder patterns of gas hydrate recovered from NGHP 1-10 (part 1)  10 20 30 40 50 HYD73 2Theta HYD86 HYD87 HYD88  Figure 5: XRD spectra of gas hydrates recovered from Site NGHP 1-10 (part 2) 10 20 30 40 50 HYD100 2Theta HYD111-1 HYD128-1 HYD131-2 HYD133-1  Figure 6: PXRD spectra of gas hydrates recovered from offshore India Raman Spectroscopic Analysis  Raman spectroscopic analysis was carried out with a WITEC confocal Raman spectrometer coupled to a green laser (514.36 nm). Figure 7 is the Raman spectrum of gas hydrate in HYD 9 recovered from offshore India. It can be seen that the gas in the hydrate is methane and the signals of other hydrocarbons, H2S or CO2 are not visible.  The large and small peaks represent methane in the large and small cages respectively. Other samples gave similar spectra so all of the gas hydrate samples studied are sI methane hydrate.  It has been found that confocal Raman spectroscopy is very powerful for gas hydrate analysis when hydrocarbons other than methane are absent. Good signal intensity can be obtained as long as gas hydrate can be seen even if it occurs as a very small spot. For example in HYD 30, gas hydrate appeared as a very thin film in the bedding plane of sediment, but an excellent signal was obtained (Figure 8). We also tried to obtain spectra on a hydrate sample concentrated from sands (HYD 100) and directly on pore- filling gas hydrate in volcanic ash in HYD 111, and signals were obtained from both of these (Figures 9 & 10). However due to the relatively low concentration and the interference from sediment particles, the spectral quality was not comparable with those from hydrate crystals.  2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 40000 80000 120000 In te n s ity Raman shift (cm-1) 2902.2 2914.0 C-H stretching for CH4 O-H stretching for H2O  Figure 7: Raman spectra of sample 9   2880 2890 2900 2910 2920 2930 0 5000 10000 15000 20000 25000 30000 In te n si ty Raman shift (cm-1) HYD30  2880 2890 2900 2910 2920 2930 66000 67000 68000 69000 70000 71000 72000 73000 74000 75000 In te n si ty Raman shift (cm-1) HYD100  Figure 9: Raman spectrum of gas hydrate concentrated from sands (HYD 100)   2880 2890 2900 2910 2920 2930 0 1000 2000 3000 In te n s ity Raman shift (cm-1) HYD111   Figure 10: Raman spectrum of gas hydrate in volcanic ash in sample HYD 111 Figure 8: Raman spectrum of gas hydrate in HYD30  Cage Occupancy and Hydration Number  Efforts were made to obtain the distribution of methane over the hydrate cages and the hydration number from the relative area of the peaks that represent large cage and small cage populations of hydrate in the Raman spectrum.  The results obtained are shown in Table 1. Except for the hydration number of hydrate in HYD 111, all of the other values are around 6.10 ± 0.15.  In consideration of the high quality of the Raman signal and the simple gas composition, the hydration numbers obtained can be taken as quantitative results.  Table 1: Hydration number and cage occupancy in gas hydrate samples from offshore India  Cage occupancy Sample no. ΘL/ ΘS ΘS ΘL Hydration  number HYD1 1.16 0.832(4) 0.965(5) 6.17 HYD1-2 1.12 0.861(8) 0.963(2) 6.13 HYD1-3 1.25 0.775(1) 0.968(7) 6.25 HYD9 1.18 0.820(7) 0.966(2) 6.18 HYD9-2 1.12 0.862(1) 0.963(2) 6.13 HYD9-3 1.25 0.773(7) 0.968(8) 6.25 HYD18 1.18 0.820(1) 0.966(3) 6.18 HYD20 1.26 0.770(0) 0.968(9) 6.26 HYD30 1.10 0.873(5) 0.962(1) 6.12 HYD33 1.18 0.821(8) 0.966(2) 6.18 HYD35 1.22 0.795(3) 0.967(7) 6.21 HYD40 1.20 0.803(4) 0.967(3) 6.21 HYD42 1.12 0.856(8) 0.963(6) 6.14 HYD56 1.17 0.824(5) 0.966(0) 6.18 HYD58 1.16 0.835(2) 0.965(3) 6.16 HYD68 1.18 0.819(7) 0.966(3) 6.19 HYD69 1.17 0.828(1) 0.965(8) 6.17 HYD73 1.17 0.824(3) 0.966(0) 6.18 HYD78 1.17 0.827(7) 0.965(8) 6.17 HYD86 1.18 0.818(3) 0.966(4) 6.19 HYD87 1.20 0.807(8) 0.967(0) 6.20 HYD88 1.19 0.815(5) 0.966(6) 6.19 HYD100 1.04 0.921(3) 0.955(6) 6.07 HYD111* 0.86 0.997(6) 0.858(2) 6.44 HYD128 1.19 0.810(8) 0.966(8) 6.20 HYD131 1.20 0.806(7) 0.967(1) 6.20 HYD133 1.18 0.819(7) 0.966(3) 6.19 * With high noise (Figure 10)  Solid-state 13C NMR analysis  Solid 13C NMR analysis was carried out on a Bruker DSX-400 NMR spectrometer at a frequency of 100.6 MHz. Finely powdered samples were cold-loaded into ZrO2 spinners and spectra were obtained at 173 K in order to avoid hydrate decomposition. The chemical shift region between −40 and +60 ppm was scanned to detect the hydrocarbon hydrate formers.  From the spectra, no hydrocarbons other than CH4 were identified, consistent with what was observed with Raman spectroscopy. Attempts were also made to get hydration numbers from the peak areas of 13C NMR spectra and the results obtained are shown in Table 2. The hydration numbers in Table 2 vary in a large range, from 6.07 to 8.07. Although the high hydration numbers are possibly related to the high spectral noise to a certain degree, additional analyses is needed to give a full explanation. The Hydration number of gas hydrate from offshore India quality samples are consistent with those obtained with Raman, like HYD 9, HYD86, and HYD88. Because the NMR analysis requires a much longer time and much larger sample volume than confocal Raman and considering the high quality of the Raman signals, confocal Raman has some advantages for gas hydrate analysis, especially for gas hydrate with a simple gas composition.  Table 2: The Hydration number of gas hydrate from offshore India  Cage occupancy Sample ΘL/ ΘS ΘS ΘL Hydration number HYD9 0.85 0.998(0) 0.848(7) 6.49 HYD9 1.15 0.840(9) 0.964(9) 6.16 HYD86 0.97 0.966(6) 0.940(9) 6.07 HYD87 0.62 0.999(9) 0.616(5) 8.07 HYD88 0.90  0.994(6) 0.891(0) 6.27 HYD131 0.93 0.987(9) 0.917(1) 6.15 HYD133 0.75 0.999(6) 0.752(5) 7.06   Gas composition of gas hydrates  As discussed previously the composition of gas hydrate can be inferred from the Raman and 13C NMR spectra, both indicating methane as the only gas in the hydrate samples studied. Attempts were also made to study hydrate decomposition with a mass spectrometer that is connected with a temperature controllable cell. The mass of the released gas is 16, associated with methane, and no other gases appearing in significant amounts were identified.  Samples were also analysed by gas chromatography and all of them were determined to contain methane as the dominant component. CONCLUSIONS  From the laboratory analysis carried out and the results obtained, the following conclusions can be reached:  • The structures of the gas hydrates in the studied samples are all sI, with methane as the dominant guest. • The occupancy of methane in large cage is almost complete, while it is variable in the small cage (0.75 to 0.99). • The hydration number is 6.10 ± 0.15 for most of the hydrates in the samples studied. REFERENCES [1] Collett, T., Riedel, M., Cochran, J., Boswell, R., Presley, J., Kumar, P., Sathe, A., Sethi, A., Lall, M., Siball, V., and the NGHP Expedition 01 Scientific Party, Indian National Gas Hydrate Program Expedition 01 Initial Reports: Prepared by the U.S. Geological Survey and Published by the Directorate General of Hydrocarbons, Ministry of Petroleum & Natural Gas (India), 1 DVD, 2008. [2] Ripmeester, J. A. and Ratcliffe, C. I., Low Temperature CP/MAS 13C NMR of solid methane hydrate: structure, cage occupancy and hydration number; Journal of Physical Chemistry B, 1988; v 97: p 337, [3] Sum A. K., Burruss, R. C., and Sloan, E. D., Measurements of clathrate hydrates via Raman Spectroscopy: Journal of Physical Chemistry B, 1997; v 101: p 7371-7377 [4]  Ripmeester, J. A., Lu H., Moudrakovski I. L., Dutrisac R., Wilson L. D., Wright F., and Dallimore S. R., Structure and composition of gas hydrate in sediment recovered from JAPEX/JNOC/GSC et al Mallik 5L-38 gas hydrate production research well, determined by X-ray diffraction and Raman and Solid State nuclear magnetic resonance spectroscopy; in Scientific Results from Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada (ed) S. R. Dallimore and T.S. Collett: Geological Survey of Canada, Bulletin 585 p 2  

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