6th International Conference on Gas Hydrates

CRYOGENIC-SEM INVESTIGATION OF CO2 HYDRATE MORPHOLOGIES Camps, A.P; Milodowski, A.E.; Rochelle, C.A.; Lovell, M.A.; Williams, J.F.; Jackson, P.D. 2008

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   CRYOGENIC-SEM INVESTIGATION OF CO2 HYDRATE MORPHOLOGIES   1,2A. P. Campsasteriskmath, 1A. E. Milodowski, 1C. A. Rochelle, 2M. A. Lovell, 1,2J. F. Williams, 1P. D. Jackson.   1British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG, UK 2Department of Geology, University of Leicester, LE1 7RH, UK 3British 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                                                         asteriskmath Corresponding author: Phone: +44 01159363110 ext. 4036 E-mail: apcamps@bgs.ac.uk; apc25@le.ac.uk NOMENCLATURE  ?C = degrees celcius g = grams mu1m = 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  ?C).    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  ?C  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 mu1m.  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  Direction of Growth 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).  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Geophysical  Research  Letters,  27  (18),  2929-2932.  [22]  Spry,  A.    1969.    Metamorphic  Textures.  Pergamon Press, pp350.   APPENDIX  Sample 1.    Quartz  rich  sand  (355  -  600  mu1m)  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  1/16th  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  ?C  to  1.8  ?C)  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  (355-600mu1m)  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  (600mu1m-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 ?C and 3.1 ?C over one month.   Sample 4.    Sample  4  was  composed  of  50.4g  of  quartz  rich sand  (355-600mu1m)  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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Mountain View 2 0
Ashburn 2 0
Brussels 2 2
Redmond 1 0
Ottawa 1 0
Wilmington 1 0
Newark 1 0
Tokyo 1 0
Sunnyvale 1 1

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