This document discusses resistivity logs and how they are used to analyze borehole formations. Resistivity is measured in ohms per meter and depends on factors like water volume, temperature, and salinity. Resistivity logs can determine hydrocarbon versus water-bearing zones and indicate permeable zones. The Archie equation relates resistivity to water saturation and uses constants determined by rock type. Different resistivity tools like electrode and induction logs measure resistivity at varying depths around the borehole to analyze fluid content and identify zones.

1. 1

2.  Resistivity measures the electric properties of the
formation,
 Resistivity is measured as, R in ohm per m,
 The ability to conduct electric current depends upon:
 The Volume of water,
 The Temperature of the formation,
 The Salinity of the formation
2

3. The Usage:
• Resistivity logs are electric logs
which are used to:
• Determine Hydrocarbon
versus Water-bearing zones,
• Indicate Permeable zones.
3

4. The resistivity of a formation depends on:
• Resistivity of the formation water.
• Amount of water present.
• Pore structure geometry.
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.
4

5. 5
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 resistivity.
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 Log
(SFL)
Deep Laterolog (LLD)
Microspherically Focused
Log (MSFL)
Shallow Laterolog (LLs) Laterolog 3 (LL3)
Laterolog 7 (LL7)

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• In the normal device a current of constant
intensity is passed between two electrodes,
A and B.
• The resultant potential difference is
measured between two other electrodes,
M and N.
• Electrodes A and M are on the sonde. B and
N are, theoretically, located an infinite
distance away.
• The distance AM is called the spacing.

7. 7
• Response of the normal device in beds
more resistive than the surrounding
formations.
• The upper part shows the response in a
thick bed (h= 10 AM). The curve is
symmetrical and a maximum is observed at
the center of the bed, where the reading is
almost equal to Rt (no invasion).
• The apparent bed thickness is less than
actual bed thickness by an amount equal to
the spacing.

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• The lower part shows the response in a bed
with a thickness less than the spacing. The
curve is still symmetrical but is reversed.
• A minimum apparent resistivity is observed
opposite the bed even though bed
resistivity is greater than surrounding bed
resistivity.
• Two spurious peaks appear, one above and
one below the bed; the distance between
the two peaks is equal to bed thickness
plus the spacing of the normal.

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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

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• 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.
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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• Based on experiments, Archie found that F could also be
related to a tortuosity factor (a) the porosity (ɸ) and a
“cementation exponent” (m) by
• 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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• Experimentally determined
formation factors for
various lithologies.

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• 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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Dual laterolog (DLL) Induction Log
• Measure the resistivity between two
electrodes.
• Measure the Conductivity between two
electrode.
1. SFL : Shallow depth zone Rxo 1. MSFL: Shallow Depth Rxo, Rm, Rmc and Rmf.
2. LLS : Shallow to Medium zone Rxo, Rt. 2. ILM: Shallow to Medium Rxo, Rt.
3. LLD : Deep zone Rt. 3. ILD: Deep Rt.

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• Qualitative and Quantitative analysis:
1. Qualitative:
 High Resistivity deflection:
1. Porous Rock ( Fresh water OR Hydrocarbon).
2. Dry Rock ( Anhydrite, Dolomite Or Limestone)
 Low Resistivity:
1. Shale
2. Porous rock bearing Saline water.
2. Quantitative:
Resistivity of
Shale 0.1 : 1 Sand from 2.5

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Fluid type:
1. LLD < LLS and SFL : ( OIL ZONE)
WITH GOOD SEPARATION
2. LLD > LLS and SFL : ( WATER ZONE )

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Shale
Sand
Shale
Shale
Sand
0.2 2000
Ohmm’s
1 10 100 1000
Basic “Archie” resistivity formula:-
Sw n = a x Rw
m x Rt
400
Sw = water saturation
Rw = formation water resistivity
Rt = formation resistivity
= porosity
a, m, n are constants
(default a=1, m=2, n=2)
Rw = .02 (at reservoir temp)
Sw = 100%
Sw = 3%
0.3
Example porosity = .26 (26%)

18. 1. Hydrocarbon, Gas and Water zones.
2. O/W & G/O contacts.
Estimate with coloring