6th International Conference on Gas Hydrates


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 PHYSICAL PROPERTIES OF REPRESSURIZED SAMPLES RECOVERED DURING THE 2006 NATIONAL GAS HYDRATE PROGRAM EXPEDITION OFFSHORE INDIA  W.J. Winters?, W.F. Waite, D.H. Mason U.S. Geological Survey 384 Woods Hole Road Woods Hole, MA 02543 USA  P. Kumar Institute of Engineering & Ocean Technology Oil and Natural Gas Corporation Ltd. Panvel 410 221, Navi Mumbai INDIA  ABSTRACT As part of an international cooperative research program, the U.S. Geological Survey (USGS) and researchers from the National Gas Hydrate Program (NGHP) of India are studying the physical properties  of  sediment  recovered  during  the  NGHP-01  cruise  conducted  offshore  India  during 2006.  Here  we  report  on  index  property,  acoustic  velocity,  and  triaxial  shear  test  results  for samples recovered from the Krishna-Godavari Basin. In addition, we discuss the effects of sample storage temperature, handling, and change in structure of fine-grained sediment.  Although complex, sub-vertical planar gas-hydrate structures were observed in the silty clay to clayey silt samples prior to entering the Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI), the samples yielded little gas post test. This suggests most, if not all, gas hydrate dissociated during sample transfer. Mechanical properties of hydrate-bearing marine sediment are best  measured  by  avoiding  sample  depressurization.  By  contrast,  mechanical  properties  of hydrate-free sediments, that are shipped and stored at atmospheric pressure can be approximated by consolidating core material to the original in situ effective stress.   Keywords: acoustic velocity, disturbance, friction angle, pressure cores, shear strength                                                          ? Corresponding author: Phone: 508-457-2358 Fax: 508-457-2310 E-mail: bwinters@usgs.gov NOMENCLATURE A  -  change  in  pore  pressure/change  in  deviator stress [kPa/kPa] c/p - shear strength/consolidation stress [kPa/kPa] Ms - mass of solids [g] Msw - mass of seawater [g] Mt - total mass of sediment sample [g] p?  -  normal  effective  stress  acting  on  a  plane inclined  at  45?  from  the  horizontal,  (?'1  +  ?'3)/2 [kPa] q - shear  stress  acting on  a  plane  inclined at 45? from the horizontal, (?1 - ?3)/2 [kPa] Vp - acoustic P-wave velocity [km/s] ?'max  -  maximum  friction  angle  in  terms  of effective  stresses,  passes  through  the  origin [degrees]  INTRODUCTION As  part  of  an  extensive  international  program  to study gas hydrates offshore India during 2006 [1], pressure  cores  were  recovered  and  preserved  at near  in  situ  conditions.  Three  whole-round, chilled, pressurized sub-samples from the Krishna-Godavari  Basin,  located  along  the  eastern Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  continental  margin  of  India,  were  shipped  to  the USGS  Gas  Hydrate  And  Sediment  Test Laboratory Instrument facility. The sediment was recovered  from  Hole  NGHP-01-21A  (15? 51.8531?N; 81? 50.0827?E; water depth: ~1049 m) which was cored between 48 and 91.5 mbsf. The samples were obtained at a subbottom depth of 58 to 59 mbsf in pressure core 2Y. The 1-m long core had an 84% sediment recovery [1]. Site NGHP-01-21 was the last region occupied during the NGHP-01 cruise, was located near previously drilled sites 10, 12, and 13, and showed strong evidence of gas hydrate  accumulations.  Core  recovery  at  Site NGHP-01-21 was better than in previously drilled nearby holes at the same depth interval [1].   The regional structural setting is interpreted to be a highly  disturbed  or  faulted  sedimentary  sequence overlying  high-amplitude  deep  gas  deposits  [1]. Individual seismic reflectors are lost below a few hundred m subbottom. The recovered sedimentary sequence  is  interpreted  to  be  a  single lithostratigraphic  unit.  The  recovered  sediments were  typically  black-colored  clays  containing terrigenous  organic  matter,  authigenic  carbonate and thin intervals of pyrite, foraminifera, and plant debris  [1].  Gas  hydrates  in  the  Krishna-Godavari Basin often occupied a fractured reservoir network or were present in the pore space of silty clays or sand and silt beds [1].  A  triaxial-test  facility  to  measure  physical properties  of  never-depressurized  pressure-core samples  does  not  exist.  Therefore,  we  measured physical properties of the samples after they were transferred  into  GHASTLI  at  atmospheric pressure, and then repressurized to approximately in  situ  conditions.  Accurate  physical  property measurements  of  natural  gas-hydrate-bearing sediment  are  needed  for  models  that  predict behavior  during  exploration,  drilling,  and production  projects.  However,  this  is  not  easy  to achieve  because  natural  gas  hydrate  dissociates when  removed  from  pressure  and  temperature conditions  in  which  it  is  stable,  and  effective stresses within the sediment change in response to coring and storage conditions.  Natural-hydrate-bearing  coarse-grained  sediment, that was stabilized by freezing during the recovery process,  was  previously  tested  in  GHASTLI  [2]. Freezing  of  the  pore  water  reduced  sediment disturbance  during  transport  of  the  samples  and reduced  gas  hydrate  dissociation  during  transfer into GHASTLI. We contemplated freezing natural hydrate-bearing  NGHP-01  samples  to  facilitate their  transfer  into  GHASTLI,  also.  However,  the effect  of  freezing  on  fine-grained  sedimentary structure  was unknown.  Therefore,  we  performed a preliminary study to determine if freezing could be  used  with  fine-grained  sediment.  Computed axial tomography (CAT) imaging documented the effect  of  different  freezing  rates  on  marine sediment structure.  Triaxial  tests  are  used  for  measuring  acoustic velocity,  determining  shear  strength  and  moduli, and  observing  behavior  under  in  situ  effective stress  conditions  that  are  currently  not  routinely available  by  well  logging  or  borehole  testing. Properties  of  ?undisturbed?  samples  containing natural  gas  hydrate  are  especially  important because  of  the  difficulty in forming complex gas hydrate structures within fine-grained sediment in a laboratory. Unfortunately, all actual samples are disturbed to some degree and an understanding of the  degree  of  disturbance  is  critical  to  data interpretation. These results complement pressure-core studies conducted on the same sediment and physical  property  measurements  made  at  sea  on sediment from nearby holes [1].  FIELD AND LABORATORY EQUIPMENT The  whole-round  sediment  samples  tested  in GHASTLI  were  recovered  at  close  to  in  situ pressure  during  the  NGHP-01  cruise  off  India using  the  Fugro  Pressure  Corer  that  utilizes  a water hammer technique to drive a sample barrel ahead  of  the  drill  bit  into  sediment  with  shear strength  less  than  or  approximately  500  kPa  [1]. The samples were transferred under pressure into storage  chambers  at  sea  and  acoustic  P-wave velocity, gamma ray attenuation, and x-ray image data  were  collected  using  the  Geotek  Pressure Multisensor  Core  Logger  (MSCL-P)  [1].  The pressurized  samples  were  cut,  post  cruise,  into shorter  lengths  in  Singapore,  transferred  under pressure  into  smaller  pressure  vessels  that  were sealed with ball-valve assemblies (Figure 1), then transported  to  Woods  Hole  via  refrigerated  air freight [3], and were stored at +4?C until testing.   The primary system used during testing is the Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI)  [4],  which simulates in situ pressure and  temperature  conditions  on  cylindrical sediment  samples (Figure  2).  A bath circulator  is used  to  control  the  temperature  of  the  sample chamber  and  of  a  heat  exchanger  located immediately above the top specimen end cap. Four     Figure  1.  Pressure  transportation  vessel  (smaller-diameter  section  at  left)  containing  a  NGHP-01 sample, sealed with a ball-valve assembly (larger-diameter section at right).    Figure 2. Close-up view of a test specimen about to be raised into the main pressure vessel (visible at  the  top  of  the  photograph).  The  test  specimen (gray cylinder) is located in the central part of the photo and rests on an interchangeable internal load cell. A heat exchanger that imparts a unidirectional cooling  front  downward  through  the  specimen rests atop the upper end cap, fed through the large diameter, vertical tubes at the front and rear of the specimen [4].  thermocouples  and  four  thermistors  are  placed against  the  outside  perimeter  of  the  specimen  or end  caps  at  different  heights  to  measure temperature variations along the sample surface.  Three  separately  controllable  500-ml-capacity syringe pumps are used to maintain the confining pressure  surrounding  the  specimen  and  internal specimen  pressures.  A  back-pressure  system contains  a  collector  capable  of  separating  and measuring  water  and  gas  volumes  which  are pushed  out  of  the  specimen  by  gas  hydrate dissociation  at  test  pressures  [4].  A  separate, fourth, syringe pump controls the movement of the load  ram  during  the  shear  phase  of  the  test.  The ram  position  is  used  to  determine  the  specimen height.  Load,  pressure,  temperature,  and  acoustic measurements  from  within  the  different subsystems  and  in  close  proximity  to  the  test specimen are logged and displayed by a computer employing custom-designed Labview software.  EXPERIMENTAL METHODS Sample handling methodology Sediment  samples  containing  natural  gas  hydrate cannot be  transferred directly under  pressure  into GHASTLI because it does not have a mechanism that  mates  with  the  transportation  ball-valve assembly. Such a mechanism is more than one m long  [1]  and  cannot  fit  inside  the  main  test chamber.   Hence, pressure-core samples must be depressurized.  However,  depressurizing  natural gas  hydrate  initiates  dissociation  which  can significantly  change  the  overall  sample  physical properties  [5].  Reducing  the  sample  temperature helps stabilize hydrate by bringing it closer to the stability  field,  thereby  mitigating  some  of  the detrimental effects of depressurization.  We investigated methods of freezing sediment that could be used to reduce the dissociation of natural gas  hydrate  within  sediment  during  the  transfer from  ball-valve-sealed  pressure  vessels  into GHASTLI.  But  first  we  needed  to  determine  if freezing had an effect on structure of fine-grained sediment. We froze three marine sediment samples obtained  from  off  the  east  coast  of  the  United States.  Two samples were frozen at different rates in a walk-in freezer and an additional sample was frozen  by  immersing  it  in  liquid  nitrogen  (LN2). CAT  images  were  made  of  the  three  samples before  and  after  freezing.  We  did  not  freeze  any samples recovered from offshore India.  A secondary goal of the program was to determine the effect of side filter drains on the consolidation characteristics of sediment within GHASTLI. Side drains  can  be  used  to  decrease  the  time  required for  consolidation  and  also  to  equilibrate  pore pressure  during  shear.  However,  installing  side drains  is  time  consuming  and  complicates  the handling  process  during  the  transfer  of  samples from pressure vessels into GHASTLI.   The  approach  we  used  to  transfer  a  NGHP-01 sample from a pressurized transportation chamber into  GHASTLI  began  by  venting  the  flammable methane  gas  in  the  ball-valve-sealed  vessel outdoors, next to the building that housed the test facility. Based on what we learned from the study of  freezing  effects  on  fine-grained  sediment,  the test  sample  was  kept  refrigerated  and  was  not frozen.  However,  to  help  minimize  gas  hydrate dissociation,  the  depressurized  transportation pressure  chamber  (without  the  ball-valve assembly)  and  sample  were  quickly  brought  into the laboratory and placed in a top-loading freezer. The  sample,  encased  in  a  clear  plastic  liner,  was removed from the pressure vessel.  A longitudinal slice was made only in the liner with a rotary-bit-cutting tool. The sediment  was not cut.  The liner was expanded and removed from the sample. The ends of the sample were trimmed with a miter box and  wire  saw  or  a  ?chop?  saw.  Side-filter  drains were  applied,  followed  by  a  flexible  membrane that was wired to a top and bottom end  cap. The sample  was quickly placed into GHASTLI  where the  chamber  surrounding  the  test  sample  was flooded with chilled confining fluid. Pressurization of confining and sample-pore fluids completed the process.  Methods used for NGHP-01 samples Index properties Specimens  used  for  index  property  calculations were dried at a temperature of 105?C  for at least 24 hours to determine  the  amount  of fresh  water and solids present. The volume of dried solids was determined  with  an  automatic  gas  pycnometer using helium as the purge and expansion gas [6]. The grain density of the pycnometer specimen was calculated using the mass of solids as determined immediately prior  to insertion of the  sample  into the pycnometer.   As  appropriate,  physical-property  calculations, assuming  pore  water  salinity  of  35  ppt,  were corrected for  the  presence  of  residual salt left on the solid particles after oven drying. In the natural environment, salt and other particles are dissolved in the pore fluid and behave as part of the aqueous phase. The calculations remove the salt precipitate from the solids and add it back to the fluid phase. Previously  published  equations  are  used  to calculate grain density, bulk density, porosity, and water content of the sediment [7].  Grain-size analyses Less than 1 g of wet sediment (grains less than 2 mm  diameter)  was  sonicated  in  a  slurry  and flushed through a model 13320 Beckman Coulter Laser  Diffraction  Particle  Size  Analyzer  to produce a grain-size distribution curve from which statistical parameters were obtained.  Acoustic velocity P-wave  velocity  was  measured  by  pulse transmission through the cylindrical sample using 1.1 MHz (natural frequency) wafer-shaped crystals that are  located on the  back side  (away from the specimen) of each end cap. A pulse as high as 100-volts  was  sent  to  the  transmitting  transducer,  the received signal was amplified, digitized, displayed on  a  digital  oscilloscope,  and  recorded  by  a computer.  Acoustic  P-wave  velocity  (Vp)  was calculated from the specimen length and measured acoustic travel time through the specimen.  Shear strength During  the  shear  strength  phase  of  the  test, specimen  loading  was  produced  by  a  ram contacting the heat exchanger which pushed on the sample.  Samples  were  sheared  at  a  constant  rate (measured using a linear displacement transducer) that  was  slow  enough  to  ensure  equalization  of pore  pressure  throughout  the  test  specimen  [8]. Load, confining pressure, pore pressures at the top and bottom end caps, and sample deformation (to a maximum  of  15  to  20  percent  axial  strain)  were measured and recorded.   RESULTS AND DISCUSSION Sample handling methodology The  structure  of  fine-grained  sediment  at refrigerated  (+4?C),  typical  freezer  (-22?C),  and liquid  nitrogen  (-196?C)  temperatures  are strikingly  different.  Refrigerated  sediment  shows little cracking or disturbance, and appears uniform in  CAT-scan  images  (Figure  3).  Compare  this internal  structure  with  three  frozen  samples (Figure  4)  where  the  degree  of  disturbance  and cracking  is  related  to  the  time  required  for freezing.  The  slower  freezing  rate  allows  pore water  to  migrate  towards  freezing  fronts  thereby developing more extensive ice-lens patterns.    Figure  3.    CAT-scan  image  of  a  refrigerated (+4?C)  POW88-1  (Gulf  of  Maine)  core  section. Notice  the  uniform  sediment  structure  prior  to freezing.  Rapid freezing to -196?C in LN2 does not appear to  disrupt  fine-grained  sediment  structure  as severely as slow freezing to -22?C, however, radial fractures  were  noticed  on  CAT-scan  images (center image, Figure 4). A previous study [9] cast doubt on the use of LN2 for preserving gas hydrate in samples that will be removed from the  LN2 as part of the test procedure.  Upon immersion in water, significant quantities of methane  gas  were  released  from  trimmed sediments  stored  at  subfreezing  temperature  and atmospheric  pressure  for  five  hours.  This  may attest to the benefit of keeping gas hydrate frozen and/or the ability of ice converted from hydrate to trap gas molecules [10]. However, results from the sample  freezing  study  indicate  that  fine-grained sediment, unlike coarse-grained samples from the Mackenzie  Delta, NWT [2], should not be frozen because  of  the  extensive  disturbance  caused  by ice-lens formation.    Figure  4.  CAT-scan  image  of  three  frozen samples:  POW88-1  sample  from  the  Gulf  of Maine stored within a pressure vessel and slowly frozen  at  -22?C  within  a  walk-in  freezer  (left), POW88-1  sample  very  quickly  frozen  in  LN2 (middle),  and  CH-15-00  sample  from  the  Blake Ridge  quickly  frozen  at  -22?C  within  a  walk-in freezer  (right).  Compare  the  extensive  crack pattern  in  the  slowly  frozen  POW88-1  sample (left)  with the  radial cracks in the  quickly frozen POW88-1  sample  frozen  in  LN2  (center),  but otherwise without major noticeable disturbance.  Refrigerated cores maintained at elevated pressure have the potential to preserve existing natural gas hydrate  (Figure  5).  However,  depressurization  of those  cores  typically  imparts  significant detrimental  effects  on  fine-grained  sediment structure  [11].  In  addition,  depressurization  to atmospheric  pressure,  combined  with  even  short times when temperatures are above freezing have the  potential  to  dissociate  most  if  not  all  gas hydrate.  Due  to  increased  strength  and  stiffness [12],  coarse-grained,  hydrate-cemented  or  frozen sediments  are  not  as  significantly  disturbed  by depressurization as are fine-grained sediments.   Figure  5.  NGHP-01  sediment  and  gas-hydrate veins  preserved  within  a  refrigerated,  methane-pressurized chamber.   Index,  acoustic,  and  strength  properties  of NGHP-01 samples Bulk index properties, including water content, are important because they often correlate to sediment behavior  [13,  14]  and  reflect  the  degree  of compaction  and  stress  history  at  various subbottom depths. Initial water contents (based on sediment mass) are uniform and vary from a high of 61% for  GH117 to 55% for  GH115 (Table 1). These  values  are  consistent  with  the  58  mbsf burial depth of the samples.   Porosity values (62% to 59%) are also uniform for these  three  immediately  adjacent  test  samples. Because  porosity  is  a  measure  of  the  relative volume  of  the  pore  space  in  a  sample  and  is independent of any particular pore-filling material, unlike  water  content,  porosity  measurements provide a means for comparing sample attributes. Previous  studies  have  shown  that  for  similar sediment types and test conditions, higher porosity specimens  generate  more  positive  pore  pressure during  shear,  are  weaker,  and  can  have  lower acoustic  velocities  than  a  sample  with  lower porosity [15, 16].  The uniformity of the index properties, which are determined on disturbed sub-samples, suggest that velocity  and  strength  properties  should  also  vary little  between  test  samples.  However,  the  three samples,  were  disturbed  differently  during  the transfer  to  GHASTLI.  Two samples (GH115 and GH117)  had  clean  breaks  through  the  entire sediment  cross-section  caused  by  gas  expansion. The  other  (GH116),  remained  intact,  but  swelled and required significant manipulation to complete the transfer.  P-wave  velocities  varied  from  1.56  to  1.90  km/s (Table 1), and increased with effective stress (0 to 400  kPa)  and  decreasing  porosity  in  agreement with  standard  trends  [17].  However,  disturbance lowered the measured Vp values for GH116.   Shear strength related plots are shown in Figures 6 to  11.  The  adjacent  ball-valve-sealed  samples exhibit  similar  contractive  behavior  during  shear as  evidenced  by  positive-pore-pressure  build  up (Table  1;  Figures  6,  8,  10)  and  stress  paths  that curve  to  the  left  (Figures  6,  8,  10).  The  positive pore-pressure response is in agreement with other fine-grained  samples  tested  in  GHASTLI  [18]. After  consolidation  to  400  kPa,  samples  GH115 and  GH116  also  had  similar  stress  ?  strain properties  (Figures  6  and  8), strength/consolidation  ratio  (0.43)  (Table  1),  and peak  friction  angles  (30  to  31  degrees,  assuming no cohesion intercept) (Table 1; Figures 7 and 9), despite  having  different  degrees  of  initial disturbance. Maximum shear strength of about 175 kPa  developed  between  6  and  10  percent  strain. The  measured  strength  ratio  is  higher  than expected for a typical fine-grained sediment [19]. Evidently,  consolidation  to  identical  stress  states mitigates  differences  in  coring  and  handling disturbance.  Similarities  in  the  mechanical properties  of  GH115  and  GH116  suggests  that effective  stress  exerts  a  primary  control  on behavior  despite  significant  differences  in  initial sample integrity.  GH117  had  a  slickensided  shear  plane  that  may have  contributed  to  a  lower  shear  strength  (119 kPa), strength/consolidation ratio (0.28), and peak friction  angle  (25  degrees)  (Table  1;  Figures  10 and 11).  Cracks  and  higher  initial water  content  Table  1.  Index  properties,  acoustic  velocities,  and  shear  strength  properties  of  sediment  recovered  from National Gas Hydrate Project Expedition 01 (NGHP-01), core 21A, section 2Y and tested in GHASTLI.    Figure 6. Shear strength results for NGHP-01 test sample  GH115.  Individual  plots  are  (A)  shear stress (q) vs. effective normal stress (p') on a plane inclined  at  45?  from  the  horizontal.  Each  data point  represents  the  top  of  a  Mohr?s  circle  and together  they  define  a  ?stress  path?;  (B)  shear stress  (q)  vs.  axial  strain,  and  (C)  change  in relative pore pressures vs. axial strain. At the start of shear, plot A begins in the lower right corner on the horizontal axis and moves to the left, whereas plots B and C start in the lower left corner of the horizontal axis at 0% strain and move to the right.   (61%)  may  have  caused  the  sample?s  relative weakness.  The  slickensided shear  plane  (oriented at 61 degrees from the horizontal) differs markedly from a nearby thin layer of coarse-grained (coarse silt/very  fine  sand)  sediment,  also  in  GH117, inclined  at  32  degrees  from  the  horizontal.  The coarser-grained  layer  may  have  enhanced  the formation of previously observed hydrate veins.  Seating  variances  between  the  sample  end  caps and  load  ram  may  be  responsible  for  changes  in slope noticed on some plots (e.g., Figures 10 and 11) and scatter in secant modulus values below 1% strain (Figures 7, 9, 11).  Figure  7.  Effective  friction  angle  and  secant modulus vs. strain for NGHP-01 sample GH115.  Figure 8. Shear strength results for NGHP-01 test sample  GH116.  Individual  plots  are  (A)  shear stress (q) vs. effective normal stress (p') on a plane inclined  at  45?  from  the  horizontal.  Each  data point  represents  the  top  of  a  Mohr?s  circle  and together  they  define  a  ?stress  path?;  (B)  shear stress  (q)  vs.  axial  strain,  and  (C)  change  in relative pore pressures vs. axial strain.   Figure  9.  Effective  friction  angle  and  secant modulus vs. strain for NGHP-01 sample GH116.  Figure 10. Shear strength results for NGHP-01 test sample  GH117.  Individual  plots  are  (A)  shear stress (q) vs. effective normal stress (p') on a plane inclined  at  45?  from  the  horizontal.  Each  data point  represents  the  top  of  a  Mohr?s  circle  and together  they  define  a  ?stress  path?;  (B)  shear stress  (q)  vs.  axial  strain,  and  (C)  change  in relative pore pressures vs. axial strain.   Figure  11.  Effective  friction  angle  and  secant modulus vs. strain for NGHP-01 sample GH117.   CONCLUSIONS/RECOMMENDATIONS Obtaining  shear  strength  and  other  physical properties  of  hydrate-bearing  fine-grained sediment at near in situ effective stress should be accomplished  without  freezing  the  sample,  and ideally,  without  depressurizing  the  material.  In these  regards,  fine-grained  marine  sediment  is more sensitive to handling and transfer techniques than  coarse-grained,  hydrate-bearing  material. This  is  because  freezing  and  depressurizing coarse-grained,  hydrate-rich  sediment  imparts much  less  disturbance  than  freezing  fine-grained material.   Applying  the  same  effective  stress  states  makes samples  with  different  disturbance  patterns demonstrate  similar  strength  properties.  Effective stress  exerts  a  primary  control  on  sediment behavior.  Different  sediment  lithologies  and possible  pore  pressure  response  over  small linear distances  makes  modeling  in  situ  behavior challenging.  To adequately measure physical properties of gas-hydrate-bearing  pressure  cores,  triaxial  test samples must continually be maintained at close to in situ pressure, demonstrating the need to develop a  pressure-coring  system  that  performs  triaxial testing within the  recovery chamber, or a  cutting and transfer system that moves the sample into a separate test chamber under in situ pressure.  ACKNOWLEDGEMENTS B.  Dugan  and  J.  Germaine  are  thanked  for providing  assistance  and  insights  related  to laboratory  testing.  D.  Twichell  and  K.  Kroeger provided  valuable  reviews  and  discussions.  R. Wilcox-Cline, J. Pohlman, B. Buczkowski, and S. Baldwin assisted with sample set up in GHASTLI. P. Schultheiss and members of  Geotek,  Ltd., and Carlos  Santamarina  and  students  from  Georgia Institute  of  Technology  are  thanked  for transferring  samples  into  transportation  chambers in Singapore. M. Gomes performed the grain-size analyses.  Julie  Arruda  and  Scott  Cramer performed the CAT-scan imaging. This work was supported by the Coastal and Marine Geology and Energy  Programs  of  the  U.S.  Geological  Survey and  funding  was  provided  by  the  National  Gas Hydrate  Program  of  India  and  the  Gas  Hydrate Program of the U.S. Department of Energy.  Any use of trade names is for descriptive purposes only  and  does  not  imply  endorsement  by  the USGS.  REFERENCES [1] Collett, T, Riedel, M, Cochran, J, Boswell, R, Presley, J, Kumar, P, Sathe, A, Sethi, A, Lall, M, Sibal,  V,  the  NGHP  Expedition  01  Scientists. Indian National Gas Hydrate  Program Expedition 01  Initial  Reports.  Directorate  General  of Hydrocarbons, Noida, India, DVD, 2008.  [2]  Winters,  WJ,  Pecher,  IA,  Booth,  JS,  Mason, DH, Relle, MK, Dillon, WP. Properties of samples containing  natural  gas  hydrate  from  the JAPEX/JNOC/GSC  Mallik  2L-38  gas  hydrate research well, determined using Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI). In: Dallimore, SR, Collett, TS, Uchida, T., editors. Geological Survey of Canada, Bulletin 544, 1999. p. 241-250.  [3] Winters, WJ, Waite, WF, Mason, DH, Gilbert, LY. Physical properties of repressurized sediment from  Hydrate  Ridge.  In:  Trehu,  AM,  Bohrmann, G,  Torres,  ME,  and  Colwell,  FS,  editors. Proceedings ODP, Scientific Results 204, 2006.  [4] Winters, WJ, Dillon, WP, Pecher, IA, Mason, DH.  GHASTLI - determining  physical  properties of  sediment  containing  natural  and  laboratory-formed gas hydrate. In: Max, MD, editor. Coastal Systems  and  Continental  Margins  -  Natural  Gas Hydrate in Oceanic and Permafrost Environments. Dordrecht,  Netherlands:  Kluwer  Academic Publishers, 2000. p. 311-322.  [5]  Winters, WJ, Pecher,  IA, Waite,  WF,  Mason, DH. Physical properties and rock physics  models of  sediment  containing  natural  and  laboratory-formed  methane  gas  hydrate.  American Mineralogist 2004;89 (8-9):1221-1227.  [6]  American  Society  for  Testing  and  Materials. Standard test method for specific gravity of solids by gas pycnometer D5550-94. In: Annual Book of ASTM Standards,  v.  04.09,  Soil  and  Rock.  West Conshohocken,  Pennsylvania:  American  Society for Testing and Materials, 1997. p. 380-383.  [7]  Winters,  WJ,  Dallimore,  SR,  Collett,  TS, Katsube,  TJ,  Jenner,  KA,  Cranston,  RE,  Wright, JF, Nixon, FM, Uchida, T. Physical properties of sediments  from  the  JAPEX/JNOC/GSC  Mallik 2L-38  gas  hydrate  research  well.  In:  Dallimore, SR,  Collett,  TS,  Uchida,  T.,  editors.  Geological Survey of Canada, Bulletin 544, 1999. p. 95-100.  [8] Bishop, AW, Henkle, DJ. The Measurement of Soil  Properties  in  the  Triaxial  Test.  London: Edward Arnold (Publishers) LTD,1962.  [9]  Tulk,  CA,  Wright,  JF,  Ratcliffe,  CI, Ripmeester,  JA.  Storage  and  handling  of  natural gas  hydrate.  In:  Dallimore,  SR,  Collett,  TS, Uchida, T., editors. Geological Survey of Canada, Bulletin 544, 1999. p. 263-267.  [10]  Stern,  LA,  Circone,  S,  Kirby,  SH,  Durham, WB.  Temperature,  pressure,  and  compositional effects on anomalous or ?self? preservation of gas hydrates. Canadian Journal of Phys. 2003:81;271-283.   [11]  Yun,  TS,  Narsilio,  GA,  Santamarina,  JC, Ruppel, C. Instrumented pressure testing chamber for  characterizing  sediment  cores  recovered  at  in situ  hydrostatic  pressure.  Marine  Geology 2006;229:285-293.  [12]  Yun,  TS,  Santamarina,  JC,  and  Ruppel,  C. Mechanical  properties  of  sand,  silt,  and  clay containing  tetrahydrofuran  hydrate.  Journal  of Geophysical Research 2007;112; B04106.  [13]  Bryant,  W,  Trabant,  P.  Statistical relationships between geotechnical properties and Gulf  of  Mexico  sediments.  In:  Offshore Technology Conference, 1972.  [14]  Keller,  G.  Marine  geotechnical  properties: Interrelationships  and  relationships  to  depth  of burial.  In:  Inderbitzen,  A,  editor.  Deep  Sea Sediments.  New  York:  Plenum Publishing Corp., 1974. p. 77-100.  [15] Atkinson, J. An Introduction to the Mechanics of  Soils  and  Foundations  Through  Critical  State Soil  Mechanics.  London:  McGraw-Hill  Book Company, 1993.  [16]  Carmichael,  RS.  Handbook  of  Physical Properties of Rocks. Boca Raton, FL: CRC Press, Inc., 1982.  [17] Sharma, PV. Environmental and Engineering Geophysics.  Cambridge,  UK:  Cambridge University Press, 1997.  [18]  Winters,  WJ,  Waite,  WF,  Mason,  DH, Gilbert, LY, and Pecher, IA. Methane gas hydrate effect  on  sediment  acoustic  and  strength properties.  Journal  of  Petroleum  Science  and Engineering  2007;56:127-135;  doi: 10.1016/j.petrol.2006.02.003.  [19]  Bowles,  JE.  Physical  and  Geotechnical Properties of Soils. New York: McGraw-Hill Book Co, 1979.    


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