Well Log Interpretation

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					Well Log Interpretation




Earth & Environmental Science
University of Texas at Arlington
                                     Interpretation



The primary goal of well log interpretation is to
  determine whether there is petroleum, and if so, how
  much can be recovered and how fast it will flow.
Well log interpretation is used to determine the
  economic viability of the well: How profitable it will be
  and how soon the drilling costs can be recovered.
   Interpretation


Unless the stratigraphy
  and reservoirs are
  well known, the first
  step is to scan the
  well log for likely
  reservoirs.
The well site geologist
  will have information
  about the location of
  petroleum shows
  Interpretation


Which logs would be
 used for each of
 these steps? What
 would you look for in
 those logs?
       Quick-Look
        Methods:
           Rxo/Rt

One of the
 reconnaissance
 methods is the
 relationship between
 the SP curve and the
 resistivity ratio Rxo/Rt
        Quick-Look
         Methods:
            Rxo/Rt

This technique works
  because the SP log is
  based on differences
  in salinity which in
  turn are related to
  differences in
  resistivity:
             Rmf             Rxo
Sp  K * log      K * log
              Rw             Rt
        Quick-Look
         Methods:
            Rxo/Rt
When the rock contains
 only water, Rxo/Rt
 will differ from the SP
 only by a constant. If
 petroleum is present.
 Rt increases so the
 two curves deflect
 away from each
 other.
             Rmf             Rxo
Sp  K * log      K * log
              Rw             Rt
     Quick-Look
      Methods:
           Rwa
Apparent Water
  Resistivity (Rwa)
  compares the deep
  resistivity of various
  zones in the well
  bore. The lowest Rwa
  is assumed to be
  water, so high Rwa
  must contain
  petroleum.
     Quick-Look
      Methods:
           Rwa
This works because
      Ro  F * Rw
      or
           Ro *  m
      Rw 
              a
      and
              Rt *  m
      Rwa   
                 a
Need info about
 lithology and porosity
     Quick-Look
      Methods:
           Rwa
If the lowest Rwa
   reading reflects only
   water in the pores,
   then the apparent
   water saturation
   (Swa) can be
   estimated by:

             Rwa m in
   S wa 
             Rwa zone
    Quick-Look
     Methods:
          Rwa
This Swa assumes that
  the zones being
  compared have the
  same lithology and
  porosity.



            Rwa m in
   S wa 
            Rwa zone
                     Quick-Look Methods:
                      Resistivity porosity

This method calculates a porosity from
                                                              1
                                                  a * Rw    n
  resistivity data using the Archie               R *m 
                                            Sw          
  Equation, and assuming Sw = 1                   t      
                                            or
In zones that are water filled,  is high
  and equal to the true porosity.
                                                              1
                                                 a * Rw     m

In zones that have petroleum, Rt is high         R *Sn 
                                            R          
                                                 t w
  and  is lower than the true value.
                                            When S w  1,
R is plotted with porosity logs and                         1
  knowledge of the lithology is                  a * Rw    m

  assumed.                                       R 
                                            R         
                                                     t  
                      Quick-Look Methods:
                       Wet Resistivity (Ro)

Ro is the actual resistivity of the
  formation and fluids. Rt is the
  measured value.
Ro can be estimated from the formation
  factor (a, m & ), and Rw.                      a * Rw
Assuming a value for Rw and , then Ro       R0 
  is the estimate for the resistivity of a
                                                     m

  water saturated zone.
                     Quick-Look Methods:
                      Wet Resistivity (Ro)


When the calculated Ro is plotted with Rt, the deep
 measurement by the log, the two traces should
 overlay if there is no petroleum. Otherwise, the two
 curves will diverge.


                            a * Rw
                       R0 
                               m
Detailed Log Analysis



   Once prospective hydrocarbon
    zones have been identified,
    then calculations of the
    desired parameters for
    economic evaluation are
    made.
Detailed Log Analysis:
     Water Saturation

   Water saturation in the flushed
    zone and the uninvaded zone
    are calculated using the
    Archie Equation.
                           1
               a * Rw    n

               R * m 
         Sw          
               t      
         and
                               1
                 a * Rmf     n

                 R * m 
         S xo           
                 xo      
Detailed Log Analysis:
     Water Saturation

   Instead of calculating Sw and
     Sxo separately, it is useful to
     calculate their ratio, because
     the lithology factors are
     eliminated.

                  n    Rxo
           Sw     Rt
               
           S  Rmf
           xo 
                    Rw
Detailed Log Analysis:
     Water Saturation
   Sw/Sxo is the Moveable
     Hydrocarbon Index. If
     Sw/Sxo = 1, no
     hydrocarbons were moved.
     If it is less than 0.7 for ss,
     or less then 0.6 for carbs,
     then petroleum will move.
                                1
                    Rxo       2

              Sw        Rt 
                          
              S xo  Rmf    
                        Rw 
                           
Detailed Log Analysis:
     Water Saturation
                            1
                a * Rw    n

                R * m 
          Sw          
                t      
   Instead of calculating Sw using
     the Archie equation where
     lithology parameters must be
     known, water saturation can
     also be estimated using the
     ratio method without knowing
     the lithology parameters.
Detailed Log Analysis:
     Water Saturation
                              1
                  Rxo       2

            Sw        Rt 
                        
            S xo  Rmf    
                      Rw 
                         

   The saturation ratio can be
   determined using only
   resistivity data (above). If
   petroleum is present, then:


         S xo  S w 
                                  1
                                  5
Detailed Log Analysis:
     Water Saturation
                       1
           Rxo       2

                           S xo  S w 
                                       1
     Sw        Rt 
                                    5
     S xo  Rmf    
               Rw 
                  

   Substituting Sxo gives Swr
   (water saturation ratio
   method).
                                 5
                     Rxo       8
                         Rt 
            S wr           
                     Rmf    
                         Rw 
                            
Detailed Log Analysis:
     Water Saturation
   Swr can be used as a check
   on Sw computed using the
   Archie equation
                        5
            Rxo       8                     1
                Rt              a * Rw    n
   S wr                        R * m 
                            Sw          
            Rmf                 t      
                Rw 
                   
              Detailed Log Analysis:
        Irreducible Water Saturation
Water saturation, Sw, includes water that is bound to
   particle surfaces, and water that will not move
   because of capillary pressure. This is called
   irreducible water saturation, Swirr.
If Sw = Swirr, then no water will be produced, which
   is important to know in making an economic
   evaluation of the well.
Detailed Log Analysis:
   Bulk Volume Water
       Bulk water volume
       (BVW) = Sw * .
       Table 7.1 shows estimates
         of BWV at irreducible
         water saturations, so
         calculation of BVW can
         show whether the
         reservoir will produce
         water along with
         petroleum
             Detailed Log Analysis:
                Bulk Volume Water
Buckles plots are a way of determining whether the
  reservoir is at Swirr. (The ordinate should be Sw,
  not Swirr).
             Detailed Log Analysis:
                Bulk Volume Water
Plots of  against Sw will follow the hyperbolic
   curves of BVW if the reservoir is at Swirr (left).
   Otherwise, both petroleum & water production are
   likely.
             Detailed Log Analysis:
                       Assignment
On your spreadsheet from the previous resistivity
   assignment, add columns to calculate water
   saturation using the ratio method (Swr), Moveable
   Hydrocarbon Index (MHI), and Bulk Volume Water
   (BVW).
Make a Buckles plot of Sw and  to determine
   whether the zones are at Swirr.
For each of the zones you have analyzed, describe
   and explain the potential to recover hydrocarbons
   economically.
                 Detailed Log Analysis:
                  Saturation Crossplots

With the advent of computers, graphical solutions to
  the Archie equation aren’t so necessary any more.
  However, there are two that are sometimes used
  to get a visual picture of the productive zone
  saturation.
                   Detailed Log Analysis:
                         Pickett Crossplot

The logarithmic form of the Archie equation can be
  written in a couple of ways:
                   a * Rw 
                    *R 
             S  m
              n
              w               
                           t 

             log Rt  log a * Rw   m log   n log S w
             and , if S w 1,
             log Rt  log a * Rw   m log 
             or

             log   log a * Rw 
                                     1        1
                                         m    log Rt
                                              m
                 Detailed Log Analysis:
                       Pickett Crossplot

The form below is the one traditionally used for the
  Pickett crossplot. (Note equation 7.26 in text and
  the description in Fig. 7.4 is wrong).

             log   log a * Rw 
                                     1        1
                                         m    log Rt
                                              m
                  Detailed Log Analysis:
                        Pickett Crossplot
log   log a * Rw 
                        1        1
                            m    log Rt
                                 m

                                           When  is plotted
                                           with Rt on log-log
                                           graph paper, the
                                           slope of the line is
                                           -1/m and the
                                           intercept, when
                                           Rt=1, is
                                           (a*Rw)1/m.
                  Detailed Log Analysis:
                        Pickett Crossplot
log   log a * Rw 
                        1        1
                            m    log Rt
                                 m         Note that this plot
                                           requires Sw =1.0.
                                           If enough points
                                           can be plotted, a
                                           value of m can be
                                           determined.
                                           “a” can be
                                           calculated if Rw is
                                           known (or vice
                                           versa).
                  Detailed Log Analysis:
                        Pickett Crossplot
log   log a * Rw 
                        1        1
                            m    log Rt
                                 m

                                           This plot also
                                           requires that the
                                           lithology (“a”) and
                                           Rw be the same
                                           in all zones
                                           plotted.
Detailed Log Analysis:
      Pickett Crossplot
         log   log a * Rw 
                                 1        1
                                     m    log Rt
                                          m
       Lines for Sw < 1 can be
       drawn parallel to the Sw=1
       line using the factors in
       table 7.2. Find Rt for Sw=1
       at any arbitrary , and
       multiply that Rt by 1.56 to
       get the Rt at Sw=0.8 for
       that . Draw the line
       parallel to Sw=1.
Detailed Log Analysis:
      Hingle Crossplot

     Hingle crossplots are
     strange and based on this
     form of the Archie
     equation:
             1       1
          a  S   m
                 n
             m
          R    R  
                 w
            t   w
Detailed Log Analysis:
      Hingle Crossplot

     , or any proxy such as
     the density or sonic logs is
     plotted on a linear scale
     at the bottom. The
     ordinate is 1/Rt (or
     conductivity) and has to
     be scaled for particular
     values of “a” and “m”.
               1       1
            a  S   m
                   n
               m
            R    R  
                   w
              t   w
Detailed Log Analysis:
      Hingle Crossplot

     The scaling of the
     ordinate must be
     designed so that values of
     Rt and  plot as a straight
     line for constant Sw.




               1       1
            a  S   m
                   n
               m
            R    R  
                   w
              t   w
Detailed Log Analysis:
      Hingle Crossplot
     While “a” and “m” must
     be assumed to design a
     Hingle plot to get a
     straight line, the data
     plotted on the Sw=1 line
     can be used to calculate
     Rw. Sw<1 can also be
     estimated once the Sw=1
     line is established.
              1       1
           a  S   m
                  n
              m
           R    R  
                  w
             t   w
                Detailed Log Analysis:
                          Permeability
Permeability can be estimated from porosity,
   resistivity, Sw and hydrocarbon density data.
   However, Sw must equal Swirr, the irreducible
   water saturation.
Bulk Volume Water (BVW) must be calculated and
   plotted in advance to made sure the zone of
   interest is at Swirr.
               Detailed Log Analysis:
                         Permeability
There are two simple formulas for medium gravity oil
   and dry gas (i.e. hydrocarbon density is assumed.
For medium gravity oil:
                                          2
                               3
                                      
                 K   250 
                                     
                                      
                            S wirr   

For dry gas:
                                          2
                               3
                                      
                K   79 
                                     
                                      
                          S wirr     
               Detailed Log Analysis:
              
                3
                       2
                         Permeability
K   250 
                  
           S wirr 
                   
                            The equations
                             can be solved
                             graphically.
                             Each
                             hydrocarbon
                             density
                             requires a
                             separate graph.
                  Detailed Log Analysis:
                            Permeability
A more complicated formula that includes variables
   for hydrocarbon density is:

         C  23 465 h  188 h
                               2


                                                2
                       1   Rw            
         W  3.75     log 
                                     2.2 
                                            
                       2   Rt irr        
                                  2
                             
                             
             C 
                   2W
         K                   
             4 R          
            W  w R  
                    t irr  
                Detailed Log Analysis:
                          Permeability
The most reliable permeability comes from well
  testing and direct measurements of discharge and
  hydrocarbon density. If cores are available,
  permeability can be measured in the lab.
                  Detailed Log Analysis:
                   Shale/Clay Analysis
Shale and clay in in rock directly affects resistivity
  and porosity measurements and all of the
  parameters derived from them, especially Sw.
Phyllosilicates do not all affect resistivity the same
  way. It is the cation exchange capacity of the
  layer silicate that is critical; Kaolinite, chlorite
  muscovite and biotite with low capacities do not
  affect the resistivities as much as the smectites.
  Logging tools can not make those distinctions so
  clay content in rocks is a significant problem.
                  Detailed Log Analysis:
                   Shale/Clay Analysis
Some knowledge of the resistivity of the
  phyllosilicate component is important, so the
  usual assumption is that Rt of nearby shale zones
  is the same as Rsh in the reservoir. This is often a
  bad assumption leading to erroneous Sw.
                 Detailed Log Analysis:
                  Shale/Clay Analysis
The usual procedure is:
1. Calculate a volume of shale (Vsh) using the
   gamma ray log the SP log, or a lithology
   crossplot.
2. Use the Vsh to correct porosities calculated by the
   sonic, density and/or neutron logs.
3. Measure a bound water resistivity (Rwb) from
   zones with 100% shale and a free water
   resistivity Rt from a shale free zone.
                  Detailed Log Analysis:
                   Shale/Clay Analysis
4. Calculate a water-bound saturation Swb for the
   100% shale zone.
5. Find the apparent resistivity Rwa of the reservoir
   using the weighted average of Rt and Rwb
   knowing Vsh.
6. Calculate a total, shale corrected, water saturation
   (Swt) for the reservoir.
             Detailed Log Analysis:
              Shale/Clay Analysis

7. Then the effective water saturation for the
   reservoir (Swe) is

                         S wt  S wb
                S we   
                          1  S wb