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

ISOTOPIC FRACTIONATION OF GUEST GAS AT THE FORMATION OF METHANE AND ETHANE HYDRATES Hachikubo, Akihiro; Ozeki, Takahiro; Kosaka, Tomoko; Sakagami, Hirotoshi; Minami, Hirotsugu; Nunokawa, Yutaka; Takahashi, Nobuo; Shoji, Hitoshi; Kida, Masato; Krylov, Alexey 2008-07

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   ISOTOPIC FRACTIONATION OF GUEST GAS AT THE FORMATION OF METHANE AND ETHANE HYDRATES   Akihiro Hachikubo∗, Takahiro Ozeki, Tomoko Kosaka, Hirotoshi Sakagami, Hirotsugu Minami, Yutaka Nunokawa, Nobuo Takahashi and Hitoshi Shoji Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, JAPAN  Masato Kida Advanced Industrial Science and Technology, 2-17-2-1, Tsukisamu-Higashi, Toyohira-ku, Sapporo, Hokkaido 062-8517, JAPAN  Alexey Krylov All-Russia Research Institute for Geology and Mineral Resources of the Ocean, 1 Angliyskiy Prospekt, 190121 St.Petersburg, RUSSIA   ABSTRACT Stable isotope of natural gas hydrates provides useful information of their gas sources. We investigated the isotopic fractionation of gas molecules during the formation of synthetic gas hydrates composed of methane and ethane. The gas hydrate samples were experimentally prepared in a pressure cell and isotopic compositions (δ13C and δD) of both residual and hydrate- bound gases were measured. δD of hydrate-bound molecules of methane and ethane hydrates was several per mil lower than that of residual gas molecules in the formation processes, while there was no difference in the case of δ13C. Effect of temperature on the isotopic fractionation was also investigated and it was found that the fractionation was effective at low temperature.  Keywords: gas hydrate, stable isotope, isotopic fractionation, methane, ethane   ∗ Corresponding author: Phone: +81 157 26 9522 Fax +81 157 26 9534 E-mail: INTRODUCTION Gas hydrates exist in permafrost and submarine /sublacustrine sediments at pressures greater than a few MPa. Their kinetics, phase equilibrium, thermodynamics, etc. obtained from recent studies by researchers were summarized by [1]. The stable isotope ratio of natural gas hydrates seems to be useful in identifying their gas sources. A genetic classification diagram for natural gas using δ13C and δD of methane was proposed [2]. It was expanded and defined as the δ13C–δD diagram for the classification of methane sources [3]. In their diagram, large and small δ13C values of methane indicate thermogenic and microbial origins, respectively, and δD of methane provides information on methyl-type fermentation or CO2 reduction in the microbial region. A global dataset of isotopic compositions of hydrate-bound gases was constructed and Whiticar’s diagram was applied to interpret gas origin [4]. Although hydrate-bound methane was of thermogenic or microbial origin due to CO2 reduction [4], that of microbial origin due to methyl-type fermentation was recently discovered in Lake Baikal [5]. Isotopic fractionation of host molecules (18O and D) at the formation of gas hydrates was reported and it was found that the fractionation factor α is almost the same as that of an ice–liquid water system [6-8]. These results indicate that gas hydrates enrich heavy isotopes of host molecules in their formation process and agree with the field observation that 18O enrichment of pore water in Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. marine sediments corresponds to low salinity, which is caused by dissociation of gas hydrates [9- 11]. However, little attention has been paid to the isotopic fractionation between guest molecules of gas hydrate and gas bubbles/dissolved gas. From the viewpoint of mixed-gas hydrates, gas fractionation can occur between light and heavy guest molecules because the equilibrium pressures of their pure hydrates might differ from each other. We reported isotopic fractionation of guest molecules (13C and D of methane and ethane) [12]. In this paper we checked a temperature effect (from 223K to 278K) on the above isotopic fractionation.  EXPERIMENTAL The details of the experimental apparatus were given in [12-13]. Gas hydrate samples were formed in a cylindrical pressure cell made of stainless steel. Temperature and pressure in the cell were measured. The cell was immersed in a temperature controlled liquid bath or placed in an insulated box in a cold room to maintain temperature with an accuracy of ±0.1 K. A magnetic stirring device agitated liquid water for enhancement of nucleation and mixing of gas and liquid phases. Fine ice powder was added to the cell without any agitation in experiments performed below the freezing point of water [13]. Four kinds of gases were used in this study: gases A and A2 were methane with a purity of 99.999 mol% (Takachiho Chemical Industry Co. Ltd.) of different isotopic composition each other. Gas B was methane with a purity of 99.9 mol% (Air Water Inc.), of which isotopic composition of carbon was relatively small and contained 0.0142% of ethane. Gas C was ethane with a purity of 99.9 mol% again supplied by Takachiho Chemical Industry Co. Ltd. Liquid water was degassed and purified by distillation and deionization, and fine ice powder was prepared by shaving pure ice using a microtome. Liquid water or ice powder was placed in the cell and pressurized by the guest gases above their equilibrium pressures. After the nucleation of gas hydrate, the pressure started to decrease immediately and approached a steady-state value. The temperature was maintained for several days and then both phases of gas and hydrate were sampled to measure their isotopic compositions. For the isotopic analyses of carbon and hydrogen, a continuous flow-isotope ratio mass spectrometer (CF-IRMS, DELTA plus XP; Thermo Finnigan) -180 -175 -170 -165 -160 -155 -42 -41 -40 -39 -38 -37 -36 δ13C (‰VPDB) δD  (‰ VS M O W ) original gas 278.2K gas phase 278.2K hydrate phase 265.2K gas phase 265.2K hydrate phase 255.2K gas phase 255.2K hydrate phase 243.2K gas phase 243.2K hydrate phase 223.2K gas phase 223.2K hydrate phase (b) -185 -180 -175 -170 -42.0 -41.5 -41.0 -40.5 δ13C (‰VPDB) δD  (‰ VS M O W ) original gas 274.2K gas phase 274.2K hydrate phase 265.2K gas phase 265.2K hydrate phase 254.6K gas phase 254.6K hydrate phase 243.2K gas phase 243.2K hydrate phase (a) -188 -183 -178 -173 -67.0 -66.5 -66.0 -65.5 δ13C (‰VPDB) δD  (‰ V S M O W ) original gas 265.2K gas phase 265.2K hydrate phase 254.6K gas phase 254.6K hydrate phase (c) -268 -263 -258 -253 -30.0 -29.5 -29.0 -28.5 δ13C (‰VPDB) δD  (‰ V S M O W ) original gas 274.2K gas phase 274.2K hydrate phase (d) δD  (‰ VS M O W ) δD  (‰ VS M O W ) δD  (‰ V S M O W ) δD  (‰ V S M O W ) Figure 1 δD and δ13C distributions in residual, hydrate-bound, and original gases for (a) methane gas A, (b) methane gas A2, (c) methane gas B, and (d) ethane gas C. was used. Isotopic compositions are reported as δ values (δ13C and δD) given with reference to the VPDB and VSMOW standards, respectively. The analytical precision of δ13C is 0.1‰ and that of δD is 0.6‰. Gas compositions of methane and ethane in Gas B samples were measured using a gas chromatograph (GC-14B; Shimadzu Corp.) equipped with a flame ionization detector with a packed column (Sunpak-S, Shimadzu Corp.).  RESULTS AND DISCUSSIONS Experiments were conducted in the temperature range 223.2 to 278.2 K. We confirmed that the stabilized pressures were higher than the anticipated equilibrium pressures and no liquid water remained in the cell after the complete agitation at 274.2 K and 278.2 K. Results of the isotopic compositions are plotted in Figure 1. Generally, methane and ethane δD values in the hydrate phase were smaller than those in the gas phase, whereas their δ13C values in the hydrate phase were almost the same as those in the gas phase. These results indicate that the heavy molecules, CH3D and C2H5D, are poorly encaged in the hydrate although the behavior of 13CH4 and 13C12CH6 seems to be the same as that of the light molecules (12CH4 and 12C2H6). δD of the residual gas increased compared to that of the original gas because CH3D seemed to remain in the gas phase and light molecules concentrated in the hydrate phase (Figure 1a). Figures 1b, 1c and 1d show δD plotted against δ13C in cases of methane (gas A2), methane (gas B) and ethane (gas C) hydrates, respectively. Even with different isotopic compositions of methane, δD of the hydrate phase was smaller than that of the gas phase as seen in Figure 1a. Although the difference in ethane δD between the two phases was 1.1 ± 0.7‰, which is smaller than that of methane δD, we can say that isotopic fractionation of guest gas also exists in the formation of ethane hydrate, as well as methane hydrate. Gas B contains small amount of ethane. Figure 2 shows a relation for Gas B samples between δD and ethane composition. The amount of ice powder was changed from 1g to 35g at the formation of these samples. δD values of both phases in the case 35g were larger than those in other samples (1, 3, and 10g) according to a Rayleigh process in the batch-type reactor. Both the ethane concentrations of gas and hydrate phases decreased with the increase in initial amount of ice powder due to preferential consumption of ethane into hydrate phase. The difference in δD between the gas and hydrate phases was kept 8.6 ± 0.9‰. 0 2 4 6 8 10 12 14 220 230 240 250 260 270 280 Temperature (K) Δ δD  (‰ V S M O W ) -5 -4 -3 -2 -1 0 1 220 230 240 250 260 270 280 Temperature (K) Δ δ1 3 C  (‰ V P D B ) Δ δD  (‰ V S M O W ) Δ δ1 3 C  (‰ V P D B ) Figure 3 Effect of temperature on isotopic difference between residual and hydrate-bound gases. ΔδD/Δδ13C are defined as the difference in δD/δ13C between the residual and hydrate- bound gases. Red circles: methane hydrate, blue circles: ethane hydrate. -190 -185 -180 -175 -170 0.00 0.02 0.04 0.06 0.08 0.10 C2H6 Composition (%) δD  (‰ V S M O W ) original gas 265.2K gas phase 265.2K hydrate phase 35g 35g 10g 10g 3g 3g 1g 1g δD  (‰ V S M O W ) Figure 2 δD of Gas B samples plotted against ethane composition. The amount of ice powder was changed from 1g to 35g to confirm the Rayleigh process in the batch-type reactor. Figure 3 shows ΔδD and Δδ13C, which are defined as the differences in δD and δ13C between the gas and hydrate phases, respectively, plotted against the formation temperature. ΔδD for methane hydrate was 4.8 ± 0.4‰ at 274.2 K and more than 8‰ below the freezing point of water. ΔδD for ethane hydrate was relatively small as mentioned above. Δδ13C was about 0‰ except for the sample at 223.2 K. We reported in the previous study [12] that there was no difference between both phases in the case of δ13C since we did not confirm that the hydrate phase concentrates heavy methane (13CH4) at 223.2K. We can say that isotopic fractionation for 13C seems negligible in the natural condition, e.g. in permafrost and submarine /sublacustrine sediments. Whiticar’s δ13C–δD diagram showed that the field of microbial origin produced by acetate-type fermentation ranged from −400 to −250‰ in δD and CO2 reduction ranged from −250 to −170‰ in δD [4]. It can be concluded that we can safely apply this diagram to discuss the source types of hydrate-bound gas because the isotopic difference in δD obtained in this study (less than 10‰) seems too small. How methane hydrates concentrate light molecules remains an open question.  SUMMARY The δD value of guest molecules of methane and ethane hydrates was smaller than that of residual gas molecules in their formation processes, whereas δ13C of them was negligibly small. It is confirmed that the isotopic fractionation in δD is enough small for discussing the gas origin of natural gas hydrates because the difference between both phases is small. In addition, an isotopic difference between hydrate-bound and surrounding gases revealed in sediment core analysis can provide information to discuss when the gas hydrate formed. For example, we can infer from the difference in δ13C between hydrate-bound and surrounding gases that a massive gas hydrate was not formed recently. Further studies on the effects of pressure, salinity and formation rate on the process of isotopic fractionation are also needed.  Acknowledgments: This work was supported by funding agencies: Japan Society for the Promotion of Science KAKENHI 17550069, 18206099 and 19550077, the Ministry of Education, Culture, Sports, Science and Technology KAKENHI 15760640 and 19740323, and Kitami Institute of Technology Presidential Grant.  REFERENCES [1] Sloan Jr. ED. Clathrate Hydrates of Natural Gases. 2nd ed., 705 pp., New York: Marcel Dekker, Inc., 1998. [2] Whiticar MJ, Faber E, Schoell M. Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation — isotope evidence. Geochim. Cosmochim. Acta 1986; 50: 693–709. [3] Whiticar MJ. Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chem. Geol. 1999; 161: 291–314. [4] Milkov AV. Molecular and stable isotope compositions of natural gas hydrates: A revised global dataset and basic interpretations in the context of geological settings. Org. Geochem. 2005; 36: 681–702. 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Methane hydrate estimates from the chloride and oxygen isotopic anomalies. Ann. New York Acad. Sci. 2000; 912: 39–50. [11] Matsumoto T, Borowski WS. Gas hydrate estimates from newly determined oxygen isotopic fractionation (αGH-IW) and δ18O anomalies of the interstitial waters: Leg 164, Blake Ridge. Proc. ODP, Sci. Results, College Station, TX (Ocean Drilling Program), 164, 59–66, 2000. [12] Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H. Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys. Res. Lett. 2007; 34: L21502, doi: 10.1029/2007GL030557. [13] Hachikubo A, Yamada K, Miura T, Hyakutake K, Abe K, Shoji H. Formation and dissociation processes of gas hydrate composed of methane and carbon dioxide below freezing. Ocean and Polar Research 2004; 26(3): 515-521.


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