Well Log Interpretation Resistivity Logs

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Well Log InterpretationResistivity Logs
Earth Environmental Science University of
Texas at Arlington
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Resistivity LogsReview
Resistivity Logs are used to determine Water
saturation, Sw and hydrocarbon zones, porosity
and permeability.
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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

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Resistivity LogsReview
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.
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Resistivity LogsReview

• 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

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Resistivity LogsReview
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
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Resistivity LogsReview
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.

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Resistivity LogsReview
Experimentally determined formation factors for
various lithologies
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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.
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Resistivity fcn (Temp, Salinity)
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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
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Resistivity profiles around Borehole
The graph shows resistivity profiles in rock
filled with oil (or fresh water).
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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.

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Resistivity Tools
Electrode logs need a conductor in the well bore
so they dont 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.
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Resistivity Tools
Various electrode logs and depth of measurement
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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.
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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.
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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.
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High Frequency Dielectric Logs
Waters 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.
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High Frequency Dielectric Logs
The water filled porosity (Fw) from a dielectric
log is
P varies with salinity and a value of 1.2 is
typical for formation water.
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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
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High Frequency Dielectric Logs
Electromagnetic Propagation Tools (EPT) measure
the propagation time of the wave in ns/m. FEPT is
tpo is derived from the log
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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.
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Typical Resistivity Logs
Mud uses fresh water. Whats in the rock pores?
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Typical Resistivity Logs
Mud uses saline water. Whats in the rock pores?
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Typical Resistivity Logs
Mud uses fresh water. Whats in the rock pores?
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Typical Resistivity Logs
Mud uses saline water. Whats in the rock pores?
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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.
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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.
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Assignment
Use the resistivity log in Fig. 5.4 this log is
over the same interval as the neutron and density
logs of the previous exercise. Using the same
spreadsheet as in the last exercise, create
additional rows for the depths at right using the
example spreadsheet (handout).