# Science_ Intelligent Design and Evolution

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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:

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
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?
Charts

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

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