6th International Conference on Gas Hydrates

CRYOGENIC-SEM INVESTIGATION OF CO2 HYDRATE MORPHOLOGIES Camps, A.P 2008

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  CRYOGENIC-SEM INVESTIGATION OF CO2 HYDRATE MORPHOLOGIES   1,2 A. P. Camps ∗ , 1 A. E. Milodowski, 1 C. A. Rochelle, 2 M. A. Lovell, 1,2 J. F. Williams, 1 P. D. Jackson.  1 British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG, UK 2 Department of Geology, University of Leicester, LE1 7RH, UK 3 British Geological Survey, Edinburgh, EH9 3LA, UK.    ABSTRACT  Gas hydrates occur naturally around the world in the shallow-marine geosphere, and have received diverse attention, crossing many disciplines, ranging from interest as a drilling hazard in the petroleum industry through to their role in the carbon cycle, and their possible contribution in past and present climate change.  Carbon dioxide (CO2) hydrates also occur naturally on Earth in the Okinawa Trough, offshore Japan, and they could exist elsewhere in the solar system. Additionally, CO2 hydrates are being investigated for their potential to store large volumes of CO2 to reduce atmospheric emissions of greenhouse gases as a climate change mitigation strategy.  Although research into hydrates has rapidly gained pace in more recent years their mineralogy and formation processes are still relatively poorly understood.  Various imaging techniques have been used to study gas hydrates, such as Nuclear Magnetic Resonance; Magnetic Resonance Imaging; X-ray Computed Tomography and Scanning Electron Microscopy (SEM). We have investigated CO2 hydrates formed within the BGS laboratories, using a cryogenic-SEM. This investigation has produced various different hydrate morphologies resulting from different formation conditions.  Morphologies range from well-defined euhedral crystals to acicular needles, and more complex, intricate forms.  Cryogenic-SEM of these hydrates has yielded a wealth of information, and with further investigation of hydrate formed within different formation conditions we may begin to comprehend the complex growth mechanisms involved.  Keywords: hydrates, CO2, SEM observations, morphologies    ∗ Corresponding author: Phone: +44 01159363110 ext. 4036 E-mail: apcamps@bgs.ac.uk; apc25@le.ac.uk NOMENCLATURE  ºC = degrees celcius g = grams µm = micron or micrometer mm = millimeter nm = nanometre Torr = pressure in Torres bar = pressure in bars INTRODUCTION  Hydrates are ice-like structures composed of cages of water molecules containing one or more guest molecules, such as methane (CH4) and carbon dioxide (CO2).  Methane hydrates occur naturally around the world in the shallow-marine geosphere, and have received diverse attention, crossing many disciplines, ranging from interest as a drilling Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. hazard in the petroleum industry [1] through to their role in the carbon cycle, and their possible contribution in past and present climate change [2, 3, 4].  CO2 hydrates also occur naturally on Earth in the Okinawa Trough, offshore Japan [5], and they could exist elsewhere in the solar system [6, 7].  Additionally, CO2 hydrates are being investigated for their potential to store large volumes of CO2 to reduce atmospheric emissions of greenhouse gases as a climate change mitigation strategy [8, 9, 10].  Fossil fuel combustion has caused a dramatic increase in atmospheric CO2 concentrations, which is believed to be having a substantial effect on global climate [11].  One method of reducing environmentally damaging CO2 emissions is capturing the CO2 at point sources and storing it underground.  Underground geological storage schemes are already in place, storing CO2 in its supercritical phase under warm, deep conditions. A more novel approach is to store CO2 as a liquid and a solid hydrate in cool sub-seabed aquifers [8, 9, 10].  If liquid CO2 was injected below the hydrate stability zone, a ‘cap’ of solid CO2 hydrate could form immobilizing the stored CO2 aiding containment.  To appreciate the feasibility of this storage approach hydrate characterization becomes imperative.  Although research into hydrates has rapidly gained pace in more recent years, their mineralogy and formation processes are still relatively poorly understood.  Various imaging techniques have been used to study gas hydrates, such as Nuclear Magnetic Resonance (NMR) [12, 13]; Magnetic Resonance Imaging (MRI) [12, 14]; X-ray Computed Tomography (CT) [15, 16], and have been proven to be very useful for non-invasive imaging of hydrate formation, dissociation and distribution.  Scanning Electron Microscopy (SEM) at cryogenic temperatures offers an additional technique to study hydrates, and has been used successfully for investigation of methane and CO2 hydrates [e.g. 17, 18, 19, 20]. SEM imaging has been used to investigate CO2 hydrate samples formed within a controlled laboratory environment, revealing a number of different hydrate morphologies.  These observations are presented and discussed.    METHODS  CO2 hydrate samples were formed within stainless steel pressure vessels in batch experiments (Figure 1); where a known weight of sediment mixed with water (liquid/ice of seawater salinity or deionised liquid water) was placed within a polytetraflouroethylene (PTFE) liner, which in turn was placed within a pressure vessel.  A thermocouple was added to each experiment to monitor temperature changes.  Pressure vessels were sealed with a Viton O-ring and connected to CO2 pressure lines within a cooled incubator to maintain temperature.  An ISCO 260D syringe pump was used to maintain pressure.  At the end of each experiment, vessels were depressurized rapidly in a controlled manner, and the samples were removed and rapidly cooled to liquid nitrogen temperature (c. -180 ºC).  Once cooled, the samples were wrapped in aluminium capsules and stored over liquid nitrogen until subsequent SEM analysis.  For specific experimental details for each sample presented see Appendix.  Prior to SEM analysis hydrate samples were transferred from cryogenic store into a controlled dry nitrogen atmosphere cryogenic sample preparation box.  Within the sample preparation box the samples were cleaved into smaller pieces (typically 5-10 mm across), under liquid nitrogen, and placed in a cooled sample holder to insert into the SEM transfer chamber.  Sub-samples were then analyzed using a LEO 435VP variable pressure SEM fitted with an Oxford Instruments CT1500 cryogenic transfer and cold stage facility and a solid-state backscattered electron detector. All samples were left uncoated.  Backscattered Scanning Electron Microscope (BSEM) image phase contrast was used to differentiate between ice and hydrate, and energy-dispersive X-ray microanalysis (EDXA) was used to identify other phases.  The sample stage was maintained at approximately -160 ºC with typical pressures of ~ 0.45 Torr, a typical working distance of 14 mm, and observations made in variable pressure mode to prevent specimen charging.    Figure 1.  Schematic diagram of a simple batch experiment; with water and sand in a PTFE liner inside a stainless steel pressure vessel, which when sealed was pressurised with CO2.   SEM OBSERVATIONS  Microscale imaging of sub-samples from four CO2 hydrate samples, formed by different experimental conditions (see Appendix) revealing different hydrate morphologies.  Observations of these morphologies are described below.  Euhedral crystalline carbon dioxide hydrate  Well-formed euhedral hydrate crystals appear in two of the samples under investigation.  Polygonal crystal shape varies slightly, with some having sharper crystal edges (Figure 2a) and more easily identifiable faces than others, and some with slight distortion due to compression with surrounding crystals (Figure 2b).  This morphology appeared in Sample 1 formed from artificial seawater ice balls, and Sample 2 with CO2 hydrate formed within quartz-rich sand partially saturated with artificial seawater.  The best-formed, largest crystals appear in larger pore spaces, reaching up to 100 µm.  The euhedral crystals seen in sample 1 (Figure 2a) have grown within spherical hydrate ‘shells’ which have been seen in previous SEM investigations [17], and have been explained by initial hydrate formation around the exterior of the ice ball and liquid water draining from the grain interior during the melt cycle [17]: therefore specific to the experimental technique used.  An interesting microporosity is also apparent in the euhedral crystals formed from seawater ice (Figure 2a), similar to textures observed in previous studies [21].     Figure 2(a) BSEM image of euhedral CO2 hydrate crystals formed within a void inside a CO2 hydrate spherical ‘shell’ representing the ice-ball formation precursor (Sample 1).  (b) BSEM image of euhedral CO2 hydrate crystals formed from a water meniscus surrounding quartz sand grains, with a larger crystal growing into a large pore space (Sample 2).   Acicular carbon dioxide hydrate  This morphology was only seen in one sample – Sample 3.  Images appear to show acicular hydrate needles growing downwards from overlying artificial seawater into underlying fully saturated sediment (Figure 3a).  Each acicular hydrate needle appears to be separated by interstitial precipitated salt and sub-micron holes (Figure 3b). It is assumed this form resulted from very rapid growth after initial nucleation on liquid seawater- CO2 interface, similar to other well known mineralogical acicular crystal formation [22].     Figure 3(a) BSEM image of Acicular hydrate crystals growing from liquid seawater-CO2 interface into underlying quartz rich sand (sample 3).  (b) Higher magnification BSEM image of acicular hydrate separated by sub-micron holes and precipitated salt.   Granoblastic carbon dioxide hydrate  Within samples formed from using ice balls (Sample 2) as a precursor for hydrate formation, there appears to be an additional morphology possibly resulting from warming and cooling cycles to convert the ice balls into hydrate. Reaction rims can also be seen where parts of the ice balls have converted to hydrate.  Around some euhedral hydrate crystals more irregular shaped hydrate grains can be seen with a thin layer appearing to separate each grain, expressing a granoblastic-polygonal texture (Figure 4a). Granoblastic hydrate was also seen within sample 4 which was subjected to partial dissociation and reformation on depressurisation (Figure 4b).  This morphology may represent a melt structure, or a secondary hydrate form, created by the formation processes.      Figure 4(a) BSEM image of granoblastic CO2 hydrate surrounding larger hydrate crystals in the upper part of the image (Sample 1).  A reaction rim can be seen to separate the two morphologies. (b) BSEM image of granoblastic polyhedral CO2 hydrate formed around a sand grain (Sample 4). (c) BSEM image of gas-rich CO2 hydrate formed as a lens on depressurization.   Gas rich carbon dioxide hydrate  This morphology appeared in sample 4, which contained CO2 hydrate formed as a lens within sediment on depressurisation of the sample. Figure 4c shows one central gas bubble, with other trapped gas bubbles to the left hand side of the image.  Hydrate crystals appear to have grown into and around these gas bubbles, in some areas forming densely packed hydrate, and in others a crystalline ‘mush’ with each crystal separated by a distinct gas filled zone.  This morphology seems to Granoblastic hydrate Sand Acicular hydrate D ir ec ti o n  o f G ro w th  Granoblastic hydrate Gas-rich hydrate Sand Euhedral hydrate represent a secondary hydrate phase forming in a gas rich system after partial dissociation on depressurisation.   DISCUSSION  Four different hydrate morphologies appear to be present in the samples presented: euhedral, acicular, granoblastic and gas-rich.  The euhedral hydrate morphology appears where open porosity remains in the sample; forming within sediment partially saturated with liquid seawater and within a sediment/seawater ice matrix.  Interestingly, generating hydrate from seawater ice balls creates a fragile sediment/hydrate matrix (Figure 5) dissimilar to other samples, with hydrate ‘shells’ remnant from the ice ball precursor (also seen in [17]).  This may indicate that hydrate formation from seawater ice balls is unrepresentative. Acicular CO2 hydrate appears to result from rapid nucleation on the surface of liquid water.  Salt can be seen included within these crystals indicating growth at salt saturation due to rapid growth in a static system.  Granoblastic CO2 hydrate appears in samples which were subjected to melting and cooling cycles, possibly representing a secondary hydrate form.  The existence of different morphologies resulting from different formation conditions indicates hydrate formation processes are complex, and further research is required to obtain a full understanding of growth mechanisms.    Figure 5.  BSEM image of euhedral CO2 hydrate crystals within a hydrate shell expressing the resulting sediment/hydrate matrix from hydrate formation using ice balls as a precursor.   A considerable challenge in hydrate characterization by imaging is to distinguish between ice and hydrate phases.  EDXA is used to recognize a carbon peak (in CO2 and CH4) and to identify elemental composition; however, as carbon is a light element only weak carbon peaks can be seen, and accuracy is difficult in these micro-pore-scale investigations.  In this study we have used backscattered image phase contrast to distinguish between phases, but these results are not conclusive, and raise the importance of the development of new techniques for distinguishing between ices in high resolution pore-scale investigations.  Rapidly forming CO2 hydrate in these static experimental conditions also appears to have grown at salt saturation, including precipitated salt within its acicular form.  In an artificial CO2 hydrate storage system conditions may be similar to those in the laboratory, forming hydrate very rapidly; therefore the inclusion of salt during rapid formation, as well as the nature and form of the hydrate, would need to be considered for conceptual development.   CONCLUSIONS  This cryogenic-SEM investigation has observed different hydrate morphologies resulting from different formation conditions, ranging from well- defined euhedral crystals to acicular needles. Cryogenic-SEM of these hydrates has yielded a wealth of information, and with further investigation of hydrate formed from different formation conditions we may begin to comprehend the complex growth mechanisms involved.   ACKNOWLEDGEMENTS  APC acknowledges funding from the Natural Environment Research Council (under grant NER/S/A/2003/11923) and the BGS University Funding Initiative (BUFI).  This paper is published with the permission of Executive Director of the British Geological Survey.      REFERENCES  [1] Collett, T.  S, Lewis, R & Uchida, T.  2000. Oilfield Review: Growing Interest in Gas Hydrates.  [2] Kvenvolden, K. A.  1998.  A primer on the geological occurrence of gas hydrate.  In Gas Hydrates: Relevance to World Margin Stability and Climate Change, Geological Society, Special Publications, 137, 9-30.  [3] Kennett, J. P., Cannariato, K. G., Hendy, I. L. & Behl, R. J.  2000.  Carbon Isotopic Evidence for Methane Hydrate Instability During Quaternary Interstadials.  Science, 288.  [4] Kemp, D., Coe, A. & Cohen, A.  2005.  Burps that warmed the world.  Planet Earth, p25.  [5] Sakai, H., Gamo, T., Kim, E-S, Tsutsumi, M., Tanaka, T., Ishibashi, J., Wakita, H., Yamano, M. & Oomori, T.  1990.  Venting of Carbon Dioxide- Rich Fluid and Hydrate Formation in Mid- Okinawa Trough Backarc Basin.  Science, 248, 1093-1095.  [6] Miller, S. L. & Smythe, W. D.  1970.  Carbon Dioxide Clathrate in the Martian Ice Cap.  Science, 170, 531-533.  [7] Prieto-Ballesteros, O., Kargel, J. S., Fernandez- Sampedro, M., Selsis, F., Martinez, E. S. & Hogenboom, D. L.  2005.  Evaluation of the possible presence of clathrate hydrates in Europa’s icy shell or seafloor.  Icarus, 177 (2), 491-505.  [8] Kiode, H, Takahashi, M, Shindo, Y, Tazaki, Y, Ijiima, M, Ito, K, Kimura, N & Omata, K.  1997. Hydrate formation in sediments in the sub-seabed disposal of CO2.  Energy, 22 (273), 279-283.  [9] Rochelle, C. & Camps, A.  2006. Underground storage of CO2 as a Liquid and Solid Hydrate.  Greenhouse Issues, 82, June 2006, 8-9. Published by the IEA Greenhouse Gas R&D Programme.  [10] House, K. Z., Schrag, D. P., Harvey, C. F. & Lackner, K. S.  2006.  Permanent carbon dioxide storage in deep-sea sediments.  Proceedings of the National Academy of Science, USA, 103 (33), 12291-12295. [11] IPCC.  2007.  Climate Change 2007: The Physical Science Basis.  IPCC WGI Fourth Assessment Report.  [12] Gao, S., House, W. & Chapman, W. G.  2005. NMR/MRI Study of Clathrate Hydrate Mechanisms.  Journal of Physical Chemistry B, 109 (41), 19090-19093.  [13] Kleinberg, R. L., Faum, C., Griffin, D. D., Brewer, P. G., Malby, G. E., Peltzer, E. T. & Yesinowski, J. P.  2003.  Deep sea NMR: Methane hydrate growth habit in porous media and its relationship to hydraulic permeability, deposit accumulation, and submarine slope stability. Journal of Geophysical Research, 108 (B10), 2508.  [14] Hirai, S., Tabe, Y., Kuwano, K., Ogawa, K. & Okazaki, K.  2000.  MRI Measurement of Hydrate Growth and an Application to Advanced CO2 Sequestration Technology.  In Gas Hydrates: Challenges for the Future, Annals of the New York Academy of Sciences, 912, 246-253.  [15] Jin, S., Takeya, S., Hayashi, J., Nagao, J., Kamata, Y., Ebinuma, T. & Narita, H.  2004. Structure Analyses of Artificial Methane Hydrate Sediments by Microfocus X-ray Computed Tomography.  Japanese Journal of Applied Physics, 43 (8A), 5673-5675.  [16] Schultheiss, P. J., Francis, T. J. G., Holland, M., Roberts, J. A., Amann, H., Thjunjoto, Parkes, R. J., Martin, D., Rothfuss, M., Tyunder, F. & Jackson, P. D.  2006.  Pressure coring, logging and subsampling with the HYACINTH system. Geological Society, London, Special Publications, 267, 151-163.  [17] Stern, L. A., Kirby, S. H., Circone, S. & Durham, W. B.  2004.  Scanning Electron Microscopy investigations of laboratory-grown gas clathrate hydrates formed from melting ice, and comparison to natural hydrates.  American Mineralogist, 89, 1162-1175.  [18] Genov, G., Kuhs, W. F., Stayakova, D. K., Goreshnik, E. & Salamatin, A. N.  2004. Experimental studies on the formation of porous gas hydrates.  American Mineralogist, 89, 1228- 1239. [19] Bohrmann, G., Kuhs, W. F., Klapp, S. A., Techmer, K. S., Klein, H., Murshed, M. & Abegg, F.  2007.  Appearance and preservation of natural gas hydrate from Hydrate Ridge sampled during ODP Leg 204 drilling.  Marine Geology, 244, 1- 14.  [20] Camps, A. P., Rochelle, C. A., Milodowski, A. E., Sims, M. R., Pullan, D. & Lovell, M. A. 2008.  The potential for applying cryogenic SEM imaging to help understand ices and hydrates under conditions of the Martian cryo-sphere. Submitted to Journal of Astrobiology.  [21] Kuhs, W. F., Klapproth, A., Gotthardt, F., Techmer, K. & Heinrichs, T.  2000.  The formation of meso- and macroporous gas hydrates. Geophysical Research Letters, 27 (18), 2929- 2932.  [22] Spry, A.  1969.  Metamorphic Textures. Pergamon Press, pp350.   APPENDIX  Sample 1.  Quartz rich sand (355 - 600 µm) mixed with seawater spherical ice balls used as a precursor for carbon dioxide hydrate formation (Figure 6).  Using an ISCO 260D pressure pump artificial seawater solution was injected through 1/16 th  inch tubing at continuous flow rate into a vat of ‘bubbling’ liquid nitrogen (due to the injection of nitrogen gas).  This procedure formed fine (sub- millimetre) frozen seawater ice ‘balls’ (Figure 6).  A known quantity of sediment was weighed into Teflon pressure vessel liners, and this sediment was cooled to liquid nitrogen temperatures by placing in an aluminium coated tray of liquid nitrogen.  The previously made seawater ice balls were then sieved from the liquid nitrogen and mixed into the cooled sediment, forming a 50:50 sediment/ice ratio.  This filled liner was placed into a cooled stainless steel pressure vessel, and the pressure vessel lid was attached after defrosting the screw threads and ensuring the CO2 inlet tube was situated in the centre of the experiment.  Samples of the sediment/ice ball mixture were placed in aluminium capsules to store for later analysis.  The pressure vessel containing was then placed into a cooled incubator, and slowly pressurised using an ISCO 260D pressure pump filled with liquid CO2.    Figure 6.  BSEM image of seawater ice ball/quartz rich sediment mix for sample 1.  The ice balls can be seen to be surrounded by a crust of salts excluded during ice formation.   Both pressure (36 bar to 200 bar) and temperature (-8.2 ºC to 1.8 ºC) were varied throughout the experiment (lasting approximately 3 weeks), and the sample was depressurized and placed into cryo-store for subsequent analysis.  Sample 2.  This sample was generated using 44.4g of quartz rich sand (355-600µm) mixed with 13.1g of seawater solution.  After placing the mixed seawater and sand into a Teflon liner, seawater was extracted using a paper towel; absorbing seawater and removing it from the sediment pores. The paper towel was weighed before and after seawater extraction to enable calculation of the quantity of seawater remaining in the sediment. Approximately 2.3g of seawater remained in the sediment; therefore leaving the sample partially saturated, with 4.9% seawater and 95.1% sand. This sample was then placed into a stainless steel pressure vessel, and pressurised using techniques previously described in the methodology.  Pressure remained constant at 200 bar, and temperature at 2 °C (<+0.4 variation).  After approximately 3 weeks the sample was depressurized and placed into cryogenic store until analysis.   Sample 3.  This sample was generated using 35.2g of coarse quartz rich sand (600µm-2mm) mixed and fully saturated with 14.1g of seawater solution.  A layer of seawater remained on the surface of the sand (see Figure 1 for schematic diagram).  Standard pressurisation and depressurisation procedures were followed.  Pressure remained constant at 170 bar, and the temperature was varied between -3.8 ºC and 3.1 ºC over one month.  Sample 4.  Sample 4 was composed of 50.4g of quartz rich sand (355-600µm) mixed and fully saturated with 17.5g of deionised water.  Standard experimental laboratory procedures were followed.  Pressure maintained at 200 bar, and temperature maintained at 2 °C (+/-0.6 °C) for a period of approximately 3 weeks.  The sample was depressurized and placed into cryo-store at the end of the experiment.                                                                    

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