resitivity log

7/29/2019 resitivity log

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Well Log Interpretation

Resistivity Logs

Earth & Environmental ScienceUniversity of Texas at Arlington


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Resistivity Logs

Review

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 andformation fluids


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Resistivity Logs

Review

Sw, water saturation can be estimated from a resistivitymeasurement using theArchie 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 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


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Resistivity Logs

Review

Based on experiments, Archie found that F could also berelated to a tortuosity factor (a) the porosity (F) and a

cementation exponent (m) by

R L


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Res st v ty LogsReview

Combining these relationships produces the ArchieEquation:

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 Logs

Review

Experimentallydetermined formation

factors for variouslithologies


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Resistivity profiles

around Borehole

Formation water is typically saline and normally hasa low Rw

Water used in drilling mud may be saline or freshFresh water has a high Rmf.

The resistivity profile around a borehole depends on

whether the mud uses fresh or saline water or is oilbased.


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Resistivity = fcn (Temp, Salinity)


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around Borehole

Logs typically measurethree resistivities: shallow,

intermediate and deepcorresponding to thethree zones around theborehole.

The graph showsresistivity profiles in rock

filled with saline water


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around Borehole

The graph showsresistivity profiles in rock

filled with oil (or freshwater).


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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 torestivity.


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Resistivity Tools

Electrode logs need a conductor in the well bore sothey dont work with fresh water and oil based

mud, or air. Electrode tools over the past 40years are able to focus the current to control thedepth of measurement in the boreholeenvironment.


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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 FocusedLog (SFL)

Deep Laterolog (LLD)

Microspherically

Focused Log (MSFL)

Shallow Laterolog

(LLs)

Laterolog 3 (LL3)

Laterolog 7 (LL7)


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Resistivity Tools

Induction tools do not need a conductor in the borehole and can be used with fresh water and oilbased mud and air. They are designed to read

intermediate and deep resistivities and arelabeled ILm and ILd respectively.

Induction tools do not work well in formations with

low conductivity (high resistivity) because onlyweak currents will be induced in the rock andare 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 canbe determined from the short reading resistivitylogs. The Archie equation for the flushed zone is:

So solving for F,If there are any hydrocarbonsleft 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 20KHz) where the material conductivity is important.

At higher frequencies (GHz) the dielectric properties

of the material are important. They are a measureof the energy loss of an electromagnetic wavethrough the material.


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Waters dielectric constant is an order of magnitudehigher than rock or hydrocarbons, so these toolsessentially measure water content.

Depth of penetration depends on tool design andusually measures either the flushed zone orinvaded zone.


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5.05.0

5.05.0

ma

p

w

maw

F

The water filled porosity (Fw) from a dielectric log is:

P varies with salinity and a value of 1.2 is typical forformation water.

i h i l i


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If there are hydrocarbons present, Fw will be lower

than the true porosity. Water saturation of theflushed or invaded zone can be estimated usingthe neutron-density porosity FND. The watersaturation, Sxo, will be:

ND

wxoS

F

F


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mafl

ma

EPT tptp

tptp

F

0

Electromagnetic Propagation Tools (EPT) measurethe propagation time of the wave in ns/m. FEPT is:

tpo is derived from the log


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

T i l


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Typical

Resistivity

Logs

Mud uses freshwater. Whats inthe rock pores?

T i l


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Typical

Resistivity

Logs

Mud uses salinewater. Whats in therock pores?

T i l


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Typical

Resistivity

Logs

Mud usesfreshwater.Whats inthe rockpores?

T i l


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Typical

Resistivity

Logs

Mud usessaline water.Whats in therock pores?

Tornado


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Tornado

Charts

When invasion of themud filtrate isextensive, the deeprestivity reading (RLLDor RILD) will not be Rt,the formation

resistivity. TornadoCharts are used tomake the correction.

Tornado


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Tornado

Charts

Charts are provided bythe logging company,and the correct chartmust be used for theparticular kind ofresistivity tool. The one

on the left is for aninduction tool.


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Assignment

Use the resistivity log in Fig. 5.4 this log is

over the same interval as the neutron anddensity logs of the previous exercise.

Using the same spreadsheet as in the last

exercise, create additional rows for thedepths at right using the examplespreadsheet (handout).

93059310

9332

9337

9351

9380

9384

resitivity log

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