Experimental investigations were conducted to understand the formation process of CO_2 hydrate the chimney structure by using a gas bubble emission technique in water within a pressure cell. The detailed process was video-recorded and analyzed to study the initiation and growth behavior of hydrate chimney while the cell pressure was increased and gas supply rate decreased gradually with time. In the initial stage of chimney growth, a hydrate crystal started to form in a cup shape at the gas nozzle and ascended together with gas bubbles due to mechanical weakness of the hydrate/nozzle contact. Growth of hydrate chimney occurred with supercooling of 3K(overpressure of 0.60MPa) or more, and continued until the top end was closed completely by hydrate

Akihiro Hachikubo

Hitoshi Shoji

Kinji Hyakutake

Kiyoshi Abe

Osamu Kitamura

Satsuki Kataoka

National Institute of Polar Research Repository

Formation experiments of CO, hydrate chimney in a pressure cellSatsuki Kataoka, Osamu Kitamura, Kinji Hyakutake, Kiyoshi Abe,Akihiro Hachikubo and Hitoshi ShojiKitami Institute of Technology, +0/, Koen-cho, Kitami *3*-2/*1Corresponding author. E-mail: dse*/**+@std.kitami-it.ac.jp(Received March -+, ,**/; Accepted August +, ,**/)Abstract: Experimental investigations were conducted to understand the formationprocess of CO, hydrate the chimney structure by using a gas bubble emission techniquein water within a pressure cell. The detailed process was video-recorded and analyzedto study the initiation and growth behavior of hydrate chimney while the cell pressurewas increased and gas supply rate decreased gradually with time. In the initial stageof chimney growth, a hydrate crystal started to form in a cup shape at the gas nozzleand ascended together with gas bubbles due to mechanical weakness of the hydrate/nozzle contact. Growth of hydrate chimney occurred with supercooling of -K(overpressure of *.0*MPa) or more, and continued until the top end was closedcompletely by hydrate.key words: CO, hydrate, gas hydrate chimney, bubble emission, supercooling (over-pressure), gas supply rate+. IntroductionGas hydrates are solid crystalline compounds that consist of gas molecules trappedin a framework of hydrogen-bonded water molecules, and are stable under high pressureand low temperature conditions (Sloan, +332). Natural gas hydrates occur worldwidein submarine sediments in deep oceans and in deep permafrost layers (Kvenvolden,+33-). Submarine gas hydrates in massive, layered, or particle shapes (Suess et al.,+333; Ginsburg, +33-) are thought to be formed by gas di#usion mechanisms in water(Bu#ett and Zatsepina, ,***). In contrast, globular gas hydrate structures are formedfrom gas bubbles as shown by Bohrmann et al. (+332). A CO, hydrate chimneystructure was observed in the Mid-Okinawa Trough by Sakai et al. (+33*). Thischimney was a translucent elongated conical pipe standing on the sea ﬂoor. CO,bubbles were released into seawater through the chimney. Formation of a hydratechimney is connected with gas transfer in the underlying sediment, and the shape of thehydrate is intimately related to the hydrate chimney formation process. However, littleis known about the formation process or stability of the hydrate chimney.This study was conducted to understand the CO, hydrate chimney formationprocess by using a pressure cell with optical windows..,Polar Meteorol. Glaciol., +3, .,.3, ,**/ ,**/ National Institute of Polar Research,. Experimental method,.+. ApparatusA pressure cell with optical windows (pressure cell, A) was especially designed andconstructed by Shoji et al. (,**,). This cell is made of stainless steel (volume capacityof .**ml) and durable to a maximum pressure of /.*MPa (Fig. +). A schematic viewof the experimental apparatus is shown in Fig. ,. Pressure cell A was placed in anethylene glycol cooling bath kept at ,1.K. CO, gas was supplied from anotherpressure cell, B (volume capacity of +,*ml) by decomposition of ‘dry ice’ (solid CO,).A stainless steel tube was connected between cell B and a nozzle (diameter +.*mm) ﬁxedat the bottom of cell A. Internal pressures in both cells (P+, P,) and upper and lowertemperatures in cell A (T+, T,) were measured by pressure gauges (KEYENCE AP-+.,*+*MPa range) and platinum resistance thermometers respectively, and recordedautomatically.,.,. ProcedureCO, gas and -**ml of pure water were kept in pressure cell A for about one day atFig. +. Pressure cell A for hydrate chimney formation.Formation experiments of CO, hydrate chimney 43,1.K and +.**MPa to allow pre-experimental dissolution of CO, gas into water. Thegas supply pressure (P,) was kept between +./* and -.**MPa by using a pressureregulator during each test run. Therefore, the pressure in cell A (P+) increased and therate of gas supply decreased gradually with time after the onset of each experiment.Formation processes of hydrate chimney on the nozzle were continuously recorded by avideo camera through the window.-. Results and discussion-.+. The change of hydrate structure with pressureEighteen experimental runs were conducted with four pressure levels, P,, for gassupply. Each run took between about ,min and one hour.Figure - shows a typical time-change of pressure and temperature during the run.When P, was +./*MPa (P+ was much lower than +./*MPa), no hydrate formation wasobserved. When P, was about ,.**MPa or more, and P+ was about +./*MPa or more,gas bubbles beneath the water surface transformed into globular-shape hydrates (stage+) as shown in Fig. .a. This means that a minimum pressure of about +./*MPa isrequired to form CO, hydrate at ,1.K in pure water. In stage , (P+ ranges from +./0MPa to ,..*MPa) CO, hydrate started to form at the nozzle in a cup shape (Fig. .b).Fig. ,. A schematic view of the experimental apparatus.S. Kataoka et al.44Fig. -. Typical time-changes of temperature (T+ and T,) and pressure (P+) during a test run.Stages +, ,, and - correspond to the hydrate formation periods of globular, cup andchimney structures, respectively.Fig. .. Classiﬁcation of hydrate shapes observedin this study (a: globular, b: cup, c: chimney, d:balloon).Formation experiments of CO, hydrate chimney 45But gas bubbles with cup-shaped hydrates ascended up to the water surface, apart fromthe nozzle without forming chimney structures. Chimney structures (Fig. .c) startedto form in stage - when P+ was higher than about ,.**MPa. When the pressuredi#erence between P+ and P, was lower than *.+*MPa (lower gas supply rate), balloonshaped hydrates sometimes occurred (Fig. .d). When a hydrate chimney was closedby hydrate ﬁlm, cracking and/or breaking of hydrate at/near the top sometimes allowedthe chimney to grow more (Fig. /).-.,. Factors a#ecting formation of hydrate chimneyThe pressure conditions for each type of hydrate structure are summarized in Table+. The formation of a hydrate chimney was observed in pressure cell A when thepressure in pressure cell A was high. However, the important point to notice is that P,,.** and +./*MPa did not form a hydrate chimney when the gas ﬂow to pressure cellA was decreased. So it appears that two factors, fast hydrate growth and strength ofhydrate at the nozzle, are very important for hydrate chimney formation.The supercooling, that is the deﬁciency of the equilibrium temperature under theexperimental pressure, and the overpressure, that is the deﬁciency of the equilibriumpressure under a given temperature, can be calculated from P+, T+ and phase equilibriumdata for CO, hydrate (Larson, +3//). The supercooling and overpressure correspondto the driving force of hydrate growth. All experimental data were plotted in Fig. 0 toFig. /. Sequential photographs of chimney structure growth. A crack appeared at the top of theclosed chimney (a), and a bubble ascended through the top hole (b). The top of the chimneywas closed by hydrate ﬁlm again after some chimney growth (c).S. Kataoka et al.46Fig. 0. Onsets of various types of hydrate formation. Pressure di#erence is a measure of gas sup-ply rate. Note that a chimney structure only appears at supercooling values higher than -K(corresponding to overpressure values higher than *.0*MPa).: Run number. : Formation area of the hydrate chimney.Table +. Initiation pressure values for each type of hydrate structure.Run No. Pressure, P, in pressure cell BIntiation pressure, P+ in pressure cell AGlobular Cup Chimney Balloon+,-./-4**+4.3+40.+403+41/+4/0+42.,4*1+423,4,/,4.*,4.,,43.,403,40.0123+*+++,,4/*+40/+40-+41*+412+41/+401+401+42,+423+42,,4*,+410,4*3+432,4+-,4-*,4,1,4.2,4.3,4.1,4.1,4./+-+.+/+0,4**+41++412+42-+423+420+43,+420+43,+1+2+4/*Formation experiments of CO, hydrate chimney 47examine the relationship between supercooling and overpressure/pressure-di#erence andthe appearance of each type of hydrate structure. Supercooling of more than -K(overpressure of *.0*MPa) is required for hydrate chimney formation. The pressuredi#erence (P,P+) determines the ﬂow rate of CO, gas from the nozzle. When thepressure di#erence is lower, bubbles can stay on the nozzle longer and stronger hydrateﬁlm is formed at the nozzle. However, a much lower pressure di#erence (much lowergas supply rate) may lead to balloon formation and closing of the hydrate chimney.One of the important ﬁndings is that hydrate chimney formation is limited to a higherpressure and lower gas supply rate condition. Gas bubbles emitted from the nozzle willsimply ascend into the water above when supercooling is lower than -K(overpressureis *.0*MPa). The majority of gas in bubbles cannot be ﬁxed in gas hydrate even duringchimney formation.Since this paper reports on experimental results for hydrate chimney formation inpure water the e#ect of the NaCl solution on chimney growth at the sea ﬂoor should beconsidered. The equilibrium pressure of CO, hydrate formed in ..0wt NaCl solutionincreases by about *..MPa compared with pure water (Vlahakis et al., +31,). Asimilar degree of pressure di#erence can be expected for chimney formation at the seaﬂoor.These ﬁndings suggest that the observation of chimney structure may imply acontinuous and relatively constant supply of CO, emitted from the sea ﬂoor in theMid-Okinawa Trough... ConclusionsExperimental investigations were conducted to understand the hydrate chimneyformation process. The CO, hydrate chimney structure was successfully formed in apressure cell. Higher pressure enhances the hydrate crystal growth rate, while a lowerpressure di#erence causes a lower gas supply rate and ensures the growth of stronghydrate contact at the nozzle. More than -K of supercooling (*.0*MPa of over-pressure) is required for CO, gas hydrate formation within the limit of the present studyconditions.ReferencesBohrmann, G., Greinert, J., Suess, E. and Torres, M. (+332): Antigenic carbonates from the Cascadiasubduction zone and their relation to gas hydrate stability. Geology, ,0, 0.10/*.Bu#ett, B.A. and Zatsepina, O.Y. (,***): Formation of gas hydrate from dissolved gas in natural porousmedia. Mar. Geol., +0., 0311.Ginsburg, G.D., Soloviev, V.A., Cranston, R.E., Lorenson, T.D. and Kvenvolden, K.A. (+33-): Gas hydratefrom the continental slope, o#shore Sakhalin Island, Okhotsk Sea. Geo-Mar. Lett., +-, .+.2.Kvenvolden, K.A. (+33-): Gas hydrate - geological perspective and global change. Rev. Geophys., -+, +1-+21.Larson, S.D. (+3//): Phase studies of the twocomponent carbon dioxidewater system, involving the carbondioxide hydrate. Ph.D. thesis, University of Illinois, Doctoral Dissertation Published No. +/,-/,University Microﬁlms.Sakai, H., Gamo, T., Kim, E.S., Tsutsumi, M., Tanaka, T., Ishibashi, J., Wakata, H., Yamamoto, M. andOomori, T. (+33*): Venting of carbon dioxide-rich ﬂuid and formation in Mid-Okinawa troughS. Kataoka et al.48back-arc basin. Science, ,.2, +*3-+*30.Shoji, H., Hachikubo, A., Miyamoto, A., Hyakutake, K., Abe, K., Bohrmann, G. and Kipfstuhl, S. (,**,):Construction of a pressure cell for visual observations on pressures of globular gas hydrate.Yokohama, Proceedings of the Fourth International Conference on Gas Hydrate, ,, 2-32.-.Sloan, E.D. (+332): Clathrate Hydrates of Natural Gases. ,nd ed. New York, Marcel Dekker, 1*/ p.Suess, E., Torres, M., Bohrmann, G., Collier, R.W, Greinert, J., Linke, P., Rehder, G., Trehu, A., Wallmann,K., Winckler, G. and Zuleger, E. (+333): Gas hydrate destabilization: enhanced dewatering, benthicmaterial turnover and large methane plumes at the Cascadia convergent margin. Earth Planet. Sci.Lett., +1*, ++/.Vlahakis, J.G., Chen, H.S., Suwandi, M.S. and Barduhn, A.J. (+31,): The growth rate of ice crystals-properties of carbon dioxide hydrate, a review of properties of /+ gas hydrates. Syracuse U. Researchand Development Report 2-*, prepared for US Department of the Interrior.Formation experiments of CO, hydrate chimney 49

Formation experiments of CO2 hydrate chimney in a pressure cell

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Formation experiments of CO2 hydrate chimney in a pressure cell

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