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


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   VELOCITY ANALYSIS OF LWD AND WIRELINE SONIC DATA IN HYDRATE-BEARING SEDIMENTS ON THE CASCADIA MARGIN  David Goldberg?, Gilles Guerin, Alberto Malinverno, and Ann Cook Lamont-Doherty Earth Observatory Columbia University 61 Rte 9W Palisades, NY 10964  USA   ABSTRACT Downhole acoustic data  were acquired in very low-velocity, hydrate-bearing formations at five sites  drilled  on  the  Cascadia  Margin  during  the  Integrated  Ocean  Drilling  Program  (IODP) Expedition 311. P-wave velocity in marine sediments typically increases with depth as porosity decreases because of compaction. In general, Vp increases from ~1.6 at the seafloor to ~2.0 km/s ~300  m  below  seafloor at  these  sites.    Gas  hydrate-bearing  intervals  appear  as  high-velocity anomalies  over  this  trend  because  solid  hydrates  stiffen  the  sediment.  Logging-while-drilling (LWD) sonic technology, however, is challenged to recover accurate P-wave velocity in shallow sediments where velocities are low and approach the fluid velocity. Low formation Vp make the analysis of LWD sonic data difficult because of the strong effects of leaky-P wave modes, which typically  have  high  amplitudes  and  are  dispersive.  We  examine  the  frequency  dispersion  of borehole leaky-P modes and establish a minimum depth (approx 50-100 m) below the seafloor at each site where Vp can be accurately estimated using LWD data. Below this depth, Vp estimates from  LWD  sonic  data  compare  well  with  wireline  sonic  logs  and  VSP  interval  velocities  in nearby holes, but differ in detail due to local heterogeneity. We derive hydrate saturation using published models and the best estimate of Vp at these sites and compare results with independent resistivity-derived saturations.  Keywords: sonic logging, LWD, marine sediments, CH4 hydrate saturation                                                          ? Corresponding author: Phone: +1 845 365 8674  Fax +1 845 365 3182 E-mail: goldberg@ldeo, INTRODUCTION Sonic velocity logs provide one of the best means to investigate the physical properties and porosity of drilled sequences and to tie logging data with seismic  and  core  measurements.  Increasingly, these measurements are required for geotechnical and  shallow  seismic  exploration  in  shallow marine  sediments  where  P-wave  velocity  is extremely  low,  often  close  to  the  fluid  velocity. Such  low  velocity  values  make  the  analysis  of sonic  logs  from  logging-while-drilling  (LWD) measurements challenging  because  of  the  strong effects of wave modes linked to the presence of a logging  tool  in  the  borehole,  such  as  leaky-P modes  [1].  Leaky-P  (Airy)  modes  are  excited when  trapped  fluid  arrivals  from  a  large-amplitude source in a borehole interfere with the compressional  head  wave  propagating  along  the formation.  The  resulting  leaky-P  modes  are dispersive:  their  phase  velocity  is  near  the formation  velocity  at  low  frequencies  and  tends to the fluid velocity as frequency increases. These dispersion effects must be accurately analyzed to measure  formation  velocity  from  leaky-P waveform data.  In this paper, we present results from LWD and wireline  sonic  tools  deployed  in  shallow  gas-hydrate  bearing  hemipelagic  muds  on  the Cascadia  margin  [2].  Five  sites  were  drilled Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  through  a  relatively  heterogeneous  section  of hemipelagic  sediments  with  generally  high  core recovery (Fig. 1).    Figure 1  Location map and bathymetry for the sites drilled during IODP Expedition 311 [2]   The  Schlumberger  SonicVision  LWD  tool  was deployed at all five sites during IODP Expedition 311,  penetrating  from  the  seafloor  through  the BSR at 200-300 m depth below seafloor (mbsf) at Sites U1325?U1329. (Fig. 2). More than 1400 m of  LWD  sonic  waveform data  were  successfully recorded.  Reidel  et  al.  [2]  describe  preliminary compressional velocity (Vp) logs computed from the leaky-P wave mode in these data. We produce new  Vp  estimates  from  the  LWD  sonic waveforms  using  high-resolution  dispersion analysis  and  compare  the  results  with  both wireline logs and core and VSP data acquired at these sites. Throughout this paper we refer to the P-wave  slowness,  the  inverse  of  Vp,  in  typical units of microseconds per foot (?s/ft).   Figure 2  Multichannel seismic cross-section with site locations and approximate penetration [2]   DATA LWD sonic waveforms The  LWD  sonic  tool  is  17  cm  (6.75  in)  in diameter, and the hole  was drilled  with a 25-cm (9.875  in)  bit.  The  tool  records  four  waveforms over an array of receivers spanning a distance of 3.0-4.2 m from the source at 20-cm intervals [3]. The tool was configured with a wide-band source function,  generating  P-wave  energy  over frequencies from ~1-10 kHz.  Drilling rates were generally maintained between 25-35 m/hr in each hole,  and  waveform  data  were  recorded  within ~20 min of bit penetration [2]. A waveform data set  consisting  of  eight  stacked  waveforms  was measured over every 0.15-m interval. Differential caliper logs measuring the distance from the tool to  the  borehole  wall  show  these  holes  are  in gauge  for  the  most  part,  and  the  quality  of  the waveforms  is  not  significantly  degraded  by borehole  conditions.  The  recorded  waveforms have  relatively  high  signal  amplitude  because drilling noise is strongly attenuated in these high-porosity sediments.  The  LWD  waveform  data  recorded  in  low-velocity sediment contain dispersive energy from leaky-P,  fluid, and tool modes [1]. We  illustrate the dispersion curves for the recorded waveform using Prony?s method, which computes the phase velocity  for  each  mode  at  every  frequency  step [4].  For  example,  Figure  3  shows  the  computed slowness dispersion from multiple leaky-P wave modes  for  representative  depth  points  a  to  f  at Site U1326.     Figure 3, a?f.  Slowness dispersion plots of LWD sonic data at representative depths at Site U1326   These  plots  clearly  illustrate  that  dispersion  is present in the leaky-P modes throughout the hole, illustrated by an overall increase in slowness with increasing frequency, and that slowness tends to the  borehole  fluid  slowness  (red  line)  at  higher frequency.  Dispersion  appears  reduced  at shallower  depths  (e.g.,  Fig.  3a-b)  with  small differences  between  leaky-P  (green  line)  and borehole fluid slowness at low frequency. Higher and lower order modes are indicated by points far above and below these lines. The depth at which the fluid can no longer be distinguished from the leaky-P  slowness  at  low  frequency  ?  where  the red  and  green  lines  converge  to  within  the resolution of the estimate point ? determines the depth at which Vp can be reliably estimated using these data. Above this minimum depth, the leaky-P and fluid modes cannot be separated reliably.  Velocity logs  Figure  4  shows  Vp  logs  computed  from  LWD sonic  data  collected  at  5  sites  during  IODP Expedition 311 [2]. Fluid velocity estimated from LWD  data  (black  curve)  and  an  estimate  of  the base of the gas hydrate stability zone (GHSZ) is shown at each site (black lines). Slowness values were  estimated  from  full  dispersive  analysis  of the waveforms in post processing [5, 6].  Figure 4 LWD and wireline Vp logs, core and VSP velocity estimates at sites U1325-U1329  LWD estimates of formation Vp from dispersion analysis  are  shown;  reliable  values  (red  curve) and unreliable values (green curve) based on the stepwise  slowness  dispersion  analysis.  Vp estimates  from  wireline  logs  (blue  curve)  and data  from  laboratory  measurements  on  core samples (points) are also shown for comparison. Table  1  gives  the  depths  below  which  LWD estimates  are  reliable  for  each  site.  Below  these depths, wireline Vp logs follow closely with the LWD estimates, with some small-scale and local geological differences. Hole locations at each site are offset by ~10 m, on average, which introduces lateral variability over even these short distances between the LWD and wireline holes. LWD and wireline logs may therefore differ significantly in holes where  methane  hydrate or  free gas occurs (e.g., Site U1327, 130-150 mbsf).   Site  Depth (m bsf) U1325  57 U1326  71 U1327  71 U1328  100 U1329  90  Table 1  Minimum depth for reliable Vp estimation from LWD data at IODP Expedition 311 sites   Comparison with VSP In Figure 4, comparison of the LWD and wireline logs  and  the  VSP  estimates  of  Vp    (light  blue curve) indicate that all follow similar trends with depth, but of  course the VSP results have lower vertical  resolution.  Considering  the  lower frequency and greater investigation volume of the measurement,  differences  between  the  logs remain within the  accuracy of  the  VSP  estimate (+/- 0.2 km/s)  at each site and within the lateral variability  between  holes.  These  comparisons corroborate the  reliability of the wireline, LWD, and VSP data below the minimum depth and the local  variability  due  to  formation  properties, methane  hydrate,  and  gas  occurrences  at  these sites.  Comparison with core Core data were acquired shipboard under ambient laboratory  conditions  in  the  20  m  below  the seafloor  at  each  site  [2].  Figure  5  illustrates  Vp estimates from both whole and split core over this interval as well as the LWD-derived Vp and fluid velocity estimates. In general, variations between core and log data can be attributed to one or more of  the  following  effects:  1)  lateral  changes  in properties  between  the  core  and  LWD  holes,  2) sampling bias due to incomplete core recovery, 3) differential porosity rebound of the core samples under ambient laboratory conditions due to local changes in lithology and cementation, or 4) local fracturing  and  structures  that  are  not  sampled through coring.  Figure 5 Comparison of LWD logs and core velocity estimates 20 m below seafloor at sites U1325-U1329   Because  of  these  effects,  comparisons  of laboratory and in situ data are often difficult over short intervals  and most  useful  for  overall trend analysis  with  depth.  Due  to  rebound  expansion when cores are recovered and measured at surface pressure  conditions,  core  Vp  estimates  can  be considered  as  a  lower  bound  on  the  formation velocity.  At these  sites, however,  it  is  important to  observe  that  core  Vp  data  are  systematically greater  than  LWD  measurements  (Fig.  5). Therefore,  the  core  results  provide  strong evidence  that  the  LWD  logs  significantly underestimate  Vp  over  the  interval  immediately below the seafloor and are unreliable.    Estimation of gas hydrate saturation We estimate gas hydrate saturation from porosity and  resistivity  logs  using  Archie's  equation  [7] and  from  acoustic  logs  using  a  pore-scale cementation model [8] in Holes U1326A (LWD) and  U1326D  (wireline).  Gas  hydrate-bearing intervals  correspond  to intervals  of  anomalously high  measured  resistivity  and  high  measured velocity.  Archie's  equation  gives  a  quantitative estimate  of  the  water  saturation  and  these estimates  were  calculated  by  comparing  the measured  electrical  resistivity  to  the  resistivity predicted  from  the  porosity  assuming  that  the formation  is  fully  water  saturated.  The cementation  model  has  been  used  to  predict velocity  log  results  in  other  clay-dominated marine  hydrate  settings  [9]  and  assumes  that hydrate forms on the surface of sediment grains, cementing  the  formation  to  some  extent  [8,  9]. The  cementation  model  requires  computation  of the  bulk  modulus  of  the  formation.  Because  the LWD sonic tool does not measure Vs, we derive a  linear  relationship  between  Vp  and  Vs  using wireline logs in Hole U1326D (i.e., Vs = 0.606Vp - 634.3), and apply this to the  LWD  Vp data  to compute  bulk  modulus  in  Hole  U1326A.  The results  of  both  resisitivity-  and  velocity-based estimation of gas hydrate saturation are shown in Figure 6.   Figure 6 Comparison of gas hydrate saturation estimated from acoustic and resistivity logs in Holes U1326A (LWD) and U1326D (wireline)   Comparing  results  of  the  cementation  model applied  to  the  velocity  logs  and  application  of Archie's relationship to the resistivity logs in the same wells, we observe an overall agreement on the  location  and  on  the  concentrations  of  gas hydrate at Site U1326. Further, the same velocity- and  resisitivity-based  saturation  models  applied to both wireline and LWD data in Hole U1326A and Hole  U1326D,  respectively,  produce  overall agreement  given  the  ~25  m  lateral  distance between  these  LWD  and  wireline  holes. Differences  in  these  profiles  between  Hole U1326A  and  Hole  U1326D  may  in  part  be attributed  to  the  geological  heterogeneity  in  the formations encountered in each hole and the local distribution  of  hydrate  within  sand  layers  and fractures that are not continuous between holes.   Figure  6  also  shows  the  gas  hydrate  saturations obtained  on  core  samples  by  measuring  the freshening  (decrease  in  chlorinity)  of  interstitial pore  waters  due  to  gas  hydrate  dissociation  [7]. The  well  log  and  chlorinity  estimates  of  gas hydrate  saturation  differ  in  detail  because  many of  the  chlorinity  data  were  taken  in  sand  layers that  are  only  a  few  cm  thick  and  are  below  the resolution  of  the  well  logs.    In  addition,  the chlorinity  data  were  obtained  in  holes  U1326C and  U1326D,  and  there  may  be  additional differences  with  well  logs  measured in  different holes due to lateral heterogeneity.   SUMMARY LWD sonic data were recorded during Integrated Ocean  Drilling  Program  (IODP)  Expedition  311 at  five  sites  transecting  the  Cascadia  margin. Dispersion  of  LWD  sonic  data  in  low-velocity formations was observed and we attribute this to the  presence  of  the  tool  in  a  borehole  and  to leaky-P  mode  propagation.  The  extraction  of accurate  velocity  information  in  the  shallowest sediments is difficult because the waveforms are dispersive  and  both  leaky-P  (Airy)  phases  and fluid modes affect the recorded arrivals. Vp logs were estimated from the LWD sonic data at these sites  using  high-resolution  dispersion  analysis and  limiting  Vp  calculation  below  a  minimum depth where the dispersive leaky-wave speed can be distinguished from fluid velocity. Estimates of Vp using this approach compare quite reasonably with wireline logging and VSP results at all sites, with  observed  differences  attributed  to  lateral variability  between  holes  and  the  presence  (or absence)  of  methane  hydrate  and  free  gas. Mismatch  between  the  core  and  LWD  Vp estimates are observed above the minimum depth, and  are  attributed  to  the  very  slow  and inseparable  fluid  and  leaky-P  wave  modes  in shallow  sub-seafloor  sediments.  In  future  uses, care  is  advised  when  interpreting  LWD waveforms  in  shallow  sub-seafloor  formations and  high-resolution  dispersion  analysis  in  the frequency  domain  is  recommended  for  LWD sonic  data  interpretation.  Using  velocity-  and resisitivity  logs  with  established  model assumptions, a comparison of saturation estimates agree  quite  well  through  the  hydrate  occurrence zone  at  one  site,  and  using  both  wireline  and LWD data, considering lateral variability between holes  drilled  on  the  Cascadia  margin.  We attribute  this  agreement  to  the  reasonable,  but independent  assumptions  in  these  models  and critical evaluation of the well log data.   ACKNOWLEDGEMENTS This research used samples and data provided by the  Integrated  Ocean  Drilling  Program  (IODP). We greatly appreciate the efforts of the crew and staff  of  the  JOIDES  Resolution  during  IODP Expedition 311 to collect these data. C. Brenner assisted  with  preparation  of  Figure  3.  The velocity data described here are available directly via  the  ODP  online  logging  database  at   REFERENCES [1]  Paillet,  F.  L.,  and  C.  H.  Cheng,  1986,  A numerical  investigation  of  head  wave  and  leaky modes  in  fluid-filled  boreholes,  Geophysics,  51, 1438-1449.  [2]  Riedel,  M.,  Collett,  T.S.,  Malone,  M.J.,  and the Expedition 311 Scientists, 2006. Proc. IODP v 311. Integrated Ocean Drilling Program, Texas AM  University,  College  Station  TX.,  [3]  Aron,  J.  Chang,  S.,  Dworak,  C.  Hsu,  K., Minerbo,  G.,  and  Yogeswaren,  E.,  1997,  Real-time sonic logging while drilling in hard and soft rocks,  Trans.  Soc.  Petrophys.  and    Well  Log Analysts,  35th  Ann.  Logging  Sympos.,  Houston, TX (USA) paper HH. [4] Ekstrom, M., 1995, Dispersion estimates from borehole acoustic arrays using a modified matrix pencil algorithm, 29th Asilomar Conf on Signals, Systems and Computers, Pacific Grove, CA. [5]  Blanch,  J.,  Cheng,  A.,  Varsamis,  G.  and Araya,  K.,  2003,  Evaluation  of  dispersion estimation  methods  for  borehole  acoustic  data, Trans. of Soc. of Expl. Geophys., 73rd Ann. Int?l Mtg, Dallas, TX (USA) pg. 305-308. [6]  Goldberg,  D.,  A.  Cheng,  S.  Gulick,  J. Blanche, and J. Byun, 2005. Velocity analysis of LWD  sonic  data  in  turbidites  and  hemipelagic sediments  offshore  Japan,  ODP  Sites  1173  and 808, In: Mikada, H. Moore, G. F., G.F. Taira, A., Becker,  K.,  and  Klaus,  A.  (Eds),  Proc.  Ocean Drilling  Program,  Sci.  Results,  190/196,  1-15, Texas AM University, College Station TX. [7]  Malinverno,  A.,  Kastner,  M.,  Torres,  M.E., and  Wortmann,  U.G.,  2008.  Gas  hydrate occurrence  from  pore  water  chlorinity  and downhole logs in a transect across the Northern Cascadia  Margin  (IODP  Expedition  311),  J. Geophys. Res., in review. [8]  Dvorkin, J.,  and  Nur,  A.,  1996,  Elasticity  of high-porosity  sandstones:  Theory  for  two  North Sea data sets, Geophysics, 61, 1363-1370. [9]  Guerin,  G.,  Goldberg,  D.,  and  Meltser,  A., 1999,  Characterization  of  in  situ  elastic properties  of  gas  hydrate-bearing  sediments  on the  Blake  Ridge, J.  Geophys.  Res., 104,  17,781-17,796.      


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