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					Well Log Interpretation
       Resistivity Logs




Earth & Environmental Science
University of Texas at Arlington
                                Resistivity Logs
                                         Review



Resistivity Logs are used to determine Water saturation,
  Sw and hydrocarbon zones, porosity and permeability.
          Borehole
       Environment

• Dh = Hole Diameter
• R = Resistivity
• Rm = mud
• Rmc = mud cake
• Rmf = mud fluids
  (filtrate)
• Rxo = rock and filtrate
• Rt = rock and
  formation fluids
                                 Resistivity Logs
                                          Review

Sw, water saturation can be estimated from a resistivity
  measurement using the Archie Equation, an empirical
  relationship derived from experiments done by G.E.
  Archie in 1942.
R0 is the resistivity of rock 100% saturated by water, and
  Rt is resistivity of same rock with formation fluids.
                                Resistivity Logs
                                         Review

If,
 Rw = Resistivity of water in the rock pores
    (measured)
 R0 = Resistivity of rock 100% saturated by water of
  Rw.
Then, a Formation Resistivity Factor (F) can be defined:
               F = R0/Rw,
               and
                R0 = F Rw
                                  Resistivity Logs
                                           Review

Based on experiments, Archie found that F could also be
  related to a tortuosity factor (a) the porosity (F) and a
  “cementation exponent” (m) by
                                       Resistivity Logs
                                                Review


Combining these relationships produces the Archie
  Equation:



   N, F, a and m are experimentally determined, and there are
    tables for typical rock types.
   Rw = resistivity of fluids in the rock and must be estimated at
    the well site.
   Rt is the resistivity of the combined rock and fluid measured by
    the logging tool.
Resistivity Logs
         Review



Experimentally
determined formation
factors for various
lithologies
                        Resistivity profiles
                         around Borehole

Formation water is typically saline and normally has
a low Rw
Water used in drilling mud may be saline or fresh
Fresh water has a high Rmf.
The resistivity profile around a borehole depends on
whether the mud uses fresh or saline water or is oil
based.
Resistivity = fcn (Temp, Salinity)
Resistivity profiles
 around Borehole

 Logs typically measure
 three resistivities: shallow,
 intermediate and deep –
 corresponding to the
 three zones around the
 borehole.

 The graph shows
 resistivity profiles in rock
 filled with saline water
Resistivity profiles
 around Borehole

 The graph shows
 resistivity profiles in rock
 filled with oil (or fresh
 water).
                           Resistivity Tools

There are two general types of resistivity tools:
 Electrode: forces a current through the rock and
  measures resistivity
 Induction: Uses a fluctuating electro-magnetic
  field to induce electrical currents in the rock; it
  measures conductivity which is converted to
  restivity.
                          Resistivity Tools

Electrode logs need a conductor in the well bore so
   they don’t work with fresh water and oil based
   mud, or air. Electrode tools over the past 40
   years are able to focus the current to control the
   depth of measurement in the borehole
   environment.
                                 Resistivity Tools
Various electrode logs and depth of measurement:

Flushed Zone         Invaded Zone          Un-invaded Zone
Microlog (ML)        Short Normal (SN)     Long Normal (LN)

Microlaterolog (MLL) Laterolog8 (LL8)      Lateral Log

Proximity Log (PL)   Spherically Focused   Deep Laterolog (LLD)
                     Log (SFL)
Microspherically     Shallow Laterolog     Laterolog 3 (LL3)
Focused Log (MSFL)   (LLs)
                                           Laterolog 7 (LL7)
                         Resistivity Tools

Induction tools do not need a conductor in the bore
   hole and can be used with fresh water and oil
   based mud and air. They are designed to read
   intermediate and deep resistivities and are
   labeled ILm and ILd respectively.
Induction tools do not work well in formations with
   low conductivity (high resistivity) because only
   weak currents will be induced in the rock and
   are hard to measure.
The flushed zone resistivity is normally measured
   with an electrode log.
                    Resistivity Porosity
In water saturated flushed zones, the porosity can
   be determined from the short reading resistivity
   logs. The Archie equation for the flushed zone is:



So solving for F,
                       If there are any hydrocarbons
                       left in the flushed zone, F will
                       be too low.
        High Frequency Dielectric Logs




Induction logs operate at low frequencies (up to 20
   KHz) where the material conductivity is important.
At higher frequencies (GHz) the dielectric properties
   of the material are important. They are a measure
   of the energy loss of an electromagnetic wave
   through the material.
    High Frequency Dielectric Logs



Water’s dielectric constant is an order of magnitude
  higher than rock or hydrocarbons, so these tools
  essentially measure water content.
Depth of penetration depends on tool design and
  usually measures either the flushed zone or
  invaded zone.
         High Frequency Dielectric Logs




The water filled porosity (Fw) from a dielectric log is:
                          0.5  ma
                                  0.5
                    F w  0.5 p 0.5
                          w  ma
P varies with salinity and a value of 1.2 is typical for
  formation water.
        High Frequency Dielectric Logs



If there are hydrocarbons present, Fw will be lower
   than the “true” porosity. Water saturation of the
   flushed or invaded zone can be estimated using
   the neutron-density porosity FND. The water
   saturation, Sxo, will be:
                          Fw
                   S xo 
                          F ND
    High Frequency Dielectric Logs
Electromagnetic Propagation Tools (EPT) measure
   the propagation time of the wave in ns/m. FEPT is:
             tp0 tpma
     F EPT                tpo is derived from the log
             tp fl tpma
    High Frequency Dielectric Logs


The dielectric logs are not effected by water in clay,
  so they measure the amount of free water in the
  rock in contrast to the neutron log.
    Typical
 Resistivity
      Logs


Mud uses fresh
water. What’s in
the rock pores?
         Typical
      Resistivity
           Logs


Mud uses saline
water. What’s in the
rock pores?
   Typical
Resistivity
     Logs


  Mud uses
  fresh
  water.
  What’s in
  the rock
  pores?
   Typical
Resistivity
     Logs


Mud uses
saline water.
What’s in the
rock pores?
            Tornado
             Charts

When invasion of the
mud filtrate is
extensive, the deep
restivity reading (RLLD
or RILD) will not be Rt,
the formation
resistivity. Tornado
Charts are used to
make the correction.
              Tornado
               Charts

Charts are provided by
the logging company,
and the correct chart
must be used for the
particular kind of
resistivity tool. The one
on the left is for an
induction tool.
                                 Assignment


Use the resistivity log in Fig. 5.4 – this log is
                                                    9305
   over the same interval as the neutron and
                                                    9310
   density logs of the previous exercise.
                                                    9332
Using the same spreadsheet as in the last
   exercise, create additional rows for the         9337
   depths at right using the example                9351
   spreadsheet (handout).
                                                    9380
                                                    9384

				
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posted:5/14/2013
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