Page 211 - The Geological Interpretation of Well Logs
P. 211

-  IMAGE  LOGS  -


                                                                N      E     s      Ww     N     DIP—>
              m             =                         wn   W    o     30     180    270   360   «=O*   60°

                        |
                                                                           dipping  bed  ~~
                                                                   dip  azimuth
                                                                   =low  point
                                                                       :      ™~  tangent  =  dip  angle   D>
       a          ~-->  oo

                                                                 horizontal  bed


       A.  Borehole                                             B.  Image -  ‘unwrapped’  borehole   C.  Dipmeter
       Figure  13.2  Representation  of  borehole  wall  images  on  a  flat  surface.  The  images  derived  from  the  cylindrical  borehole  (A)  are
       presented  on a  flat  surface  (screen  or  hard  copy  log  plot)  by  ‘unwrapping’  onto  a  vertical  depth  grid  and  horizontal  grid  of  compass
       bearings.  (B)  In  this  format,  horizontal  and  vertical  surfaces  are  vachanged  but  dipping  surfaces  become  represented  by  a  sinusoid.
       (C)  Such  dip  and  azimuth  may  be  represented  on  a  dipmeter  tadpole  plot.


       distortion.  Real  horizontal  features  will  simply  be  seen  on   approximately  8  cm  (3.2”)  wide  and  18  cm  (7")  long;
       the  image  log  format  as  horizontal:  real  vertical  features   flaps  are  8  cm  (3.2")  wide  but  only  6  cm  (2.5")  long.  Both
       as  vertical.  But  real  dipping  surfaces  will  be  represented   pad  and  flap  have  arrays  of  24  button  electrodes
       on  the  plot  (log)  by  a  sine  wave  (Figure  13.25),  the  steep-
       er  the  dip  the  greater  the  wave  amplitude.  The  actual  dip
       of  a  bed  can  be  accurately  measured  from  the  sine  wave.
       The  crest  of  the  curve  is  the  high  point  of  the  surface  as       Digital
                                                                                    telametry
       is  crosses  the  borehole,  the  tangent  to  the  slope  is  the  dip
                                                                                    cartridge
       angle  and  the  trough,  the  low  point  of  the  surface  cross-
       ing  the  borehole,  gives  the  dip  azimuth  (Figure  13.2).               Digital
       Typically  today,  the  sine  signature  of  a  dipping  bed  on  the        telemetry
                                                                                    adapter
       image  can  be  matched  by  an  ideal,  computer  derived  sine
       curve  which  automatically  provides  a  dip  and  azimuth
                                                                                    Tool  for
       (Figure  13.2c).  This  is  described  in  more  detail  below
                                                                                    depth
       (Section  13.3,  The  workstation).                                          correlation
                                                                               Me
       13.2  Electrical  imaging,
                                                                                    cartridge
       the  FMS  and  FMI                                                           Controller
       The  tools
       At  present  (1996)  electrical  imaging  is  dominated  by
       Schlumberger,  although  new  tools  are  being  actively
                                                                                    Flex  joint
       introduced  by  the  other  service  companies.  In  the  mid-
       1980s,  Schiumberger  introduced  their  first  electrical
                                                                                    trsulating  sub
       imaging  tool,  the  Formation  MicroScanner  (FMS),  as  an
       Microlmager  (FMI)  provides  nearly  80%  coverage  in                      inclinometer
       evolution  of  their  SHDT  dipmeter  (Chapter  12).  The  first
       tools  only  provided  an  image  of  20%  of  an  8.5”  borehole,
       using  just  two  pads.  Since  then  there  has  been  steady
       progress  in  borehole  coverage  (Bourke,  1992)  and  tool
                                                                                    Acquisition
       technology.  The  present  tool,  the  Fulfbore  Formation

      “an  8.5"  diameter  borehole  of  high  quality  images                      cartridge
                                                                                    Four-arm
       (Table  13.2).  Since  this  is  the  most  recent  Schlumberger             sonde
       tool  it  will  be  used  for  description  (information  from

       Schlumberger  1994,  unless  otherwise  indicated).
         The  Schlumberger  FMI  consists  of  four  pads  on  two
                                                                           =
       orthogonal  arms  like  the  dipmeter  (Figure  13.3),  but  in

                                                                      flap  [         1   pad
       the  imaging  tool,  the  four  pads  each  have  a  hinged  flap  so
       as  to  extend  the  area  of  electrical  contact  (Figure  13.4).   Figure  13.3  The  FMI  (Fullbore  Formation  Microlmager)  too)
       Pad  faces  are  curved  to  match  borehole  curvature  and  are   of  Schlumberger  (re-drawn  from  Schlumberger,  1994).
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