Resistivity log

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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 formation2

The Usage:

• Resistivity logs are electric logs

which are used to:

• Determine Hydrocarbon

versus Water-bearing zones,

• Indicate Permeable zones.

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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.

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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 constantintensity is passed between two electrodes,A and B.

• The resultant potential difference ismeasured between two other electrodes,M and N.

• Electrodes A and M are on the sonde. B andN are, theoretically, located an infinitedistance away.

• The distance AM is called the spacing.

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• Response of the normal device in bedsmore resistive than the surroundingformations.

• 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 thanactual bed thickness by an amount equal tothe 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 observedopposite the bed even though bedresistivity is greater than surrounding bedresistivity.

• Two spurious peaks appear, one above andone below the bed; the distance betweenthe two peaks is equal to bed thicknessplus 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 twoelectrodes.

• 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 2000Ohmm’s

1 10 100 1000Basic “Archie” resistivity formula:-

Sw n = a x Rwm x Rt

400

Sw = water saturationRw = formation water resistivityRt = formation resistivity

= porositya, 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%)

1. Hydrocarbon, Gas and Water zones.

2. O/W & G/O contacts.

Estimate with coloring