Well Log Interpretation: Petrophysical Techniques

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					 Well Log Interpretation:
Petrophysical Techniques




 Earth & Environmental Science
 University of Texas at Arlington
                 Interpretation Techniques



Most of the Interpretation techniques have already been
 introduced so this chapter is a summary and in
 introduction to the use of facies maps based on well
 loging.
                Interpretation Techniques:
                                 Lithology
The following logs and cross plots are used and modern
  logging software often computes and displays the
  results automatically:
    Gamma Ray
    Neutron-Density
    Neutron-Sonic
    Density-Sonic
    M-N
    rmaa – Dtmaa
    rmaa-Umaa
                Interpretation Techniques:
                                 Lithology


The lithology cross plots may give conflicting results
  especially when the sonic log is used in carbonates
  with vuggy porosity. The sonic log does not see widely
  spaced holes, so it underestimates porosity and the
  Dtmaa variable, and overestimates the “M” variable
  when vugs are present.
Example          Interpretation   Techniques:
  Lithology Logs                    Lithology
Example          Interpretation   Techniques:
  Lithology Logs                    Lithology
                 Interpretation Techniques
The previous examples show significant disparity in the
  lower interval, so vuggy porosity in carbonates is
  probably present. In those circumstances, the
  neutron-density and rmaa - Umaa data are the most
  reliable.
              Interpretation Techniques:
                Clean Zones (shale free)



Both the SP and Gamma Ray log can be used to map
“clean” sediments, which generally have higher
permeability and porosity.
Interpretation Techniques:
  Clean Zones (shale free)


    The SP response is depressed
    by shale, thin beds and
    hydrocarbons. SP alpha
    mapping can be used to
    determine clean zones,
    assuming a lower response is
    produced only by shale.
Interpretation Techniques:
  Clean Zones (shale free)


       The first step is to compute the
       Static SP (SSP), which is the
       ideal SP response for clean,
       thick rocks.
Interpretation Techniques:
  Clean Zones (shale free)


       Using the shale baseline of the
       SP log, plot a line showing the
       value of SSP. In the example
       SSP=-130: left dashed line on
       log.
Interpretation Techniques:
  Clean Zones (shale free)


       Pick some % (alpha) of the
       SSP to be the shale cutoff and
       draw a line on the log (the
       example has two alpha values,
       75% and 50%, or -98 and -65
       respectively).
Interpretation Techniques:
  Clean Zones (shale free)


       Measure the thickness of the
       “clean” interval where the SP
       curve cuts the alpha line
       (either 27’ for the 75% alpha
       or 45’ for the 50% alpha)>
Interpretation Techniques:
  Clean Zones (shale free)


              Measure the alpha
              thickness in all the
              wells and contour.
              The idea is to look
              for trends to drill
              additional production
              wells.
Interpretation Techniques:
  Clean Zones (shale free)


              Of course thin beds
              and hydrocarbons
              depress the SP log
              so the thick alphas
              could be where
              hydrocarbons are
              absent and thin
              alphas where they
              are present.
Interpretation Techniques:
  Clean Zones (shale free)

 Gamma Ray logs are better than
    SP logs for shale determination.
 First determine GRmax (100%
    shale) and GRmin (0% shale).
 From formulas in Ch. 3 or using
    Figure 3.2, determine IGR for
    Vsh=5% (or whatever Vsh cutoff
    value you want). In this case
    IGR5% = 0.10
Interpretation Techniques:
  Clean Zones (shale free)

    If IGR5% = 0.10, then




   In the example, GR5% = 40 API
   units.
Interpretation Techniques:
  Clean Zones (shale free)



    In the example a line is drawn at
      40 API units and the thickness of
      the interval where the GR log
      reads less than 40 is the
      thickness of the “clean” interval:
      in this case about 44’.
Interpretation Techniques:
  Clean Zones (shale free)


              The “clean”
                thicknesses from a
                number of wells
                can then be
                contoured to
                determine the
                trend of the most
                promising zone.
                Interpretation Techniques:
                Create your own crossplot


If petrographic data is available from cores and
   cuttings, and sedimentary facies can be identified
   in several wells, then the log response to those
   various facies can be identified using various cross
   plots.
Interpretation Techniques:
Create your own crossplot

         In the example, the
           various carbonate
           facies determined
           from petrographic
           data plot in specific
           regions on a Rt-FN
           crossplot.
         Zones in wells without
           petrographic data
           can be assigned a
           facies from the logs.
Facies maps     Interpretation Techniques:
  can be        Create your own crossplot
  made from
  the log
  data to
  determine
  where
  additional
  wells
  should be
  drilled (or
  not).

				
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posted:11/17/2011
language:English
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