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Contents Introduction

Spontaneous Potential (SP) Log

Principle

How It Works

Log Presentation

Tools

Applications

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Introduction To Wireline Logging

The continuous recording of

geophysical parameters along a

borehole produces a geophysical log.

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Spontaneous Potential (SP) Log

The spontaneous potential log measures the

natural or spontaneous potential differences

that exists between the borehole and the

surface in the absence of any artificially

applied current.

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Operation

An electrode (usually lead) is lowered down

the well and an electrical potential is

registered at different points in the hole with

respect to surface electrode.

In order to record a potential the hole must

contain conductive mud, as it cannot be

recorded in air or oil-base mud.

Logging rate is approximately 1500m per

hour and recordings are continuous.

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PrincipleCauses of SP

Electrochemical Potential

Electrokinetic Potential

MudcakePotential

Shale wallPotential

Liquid Junction

Membrane Potential

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Electrochemical Potential

Electro chemical potential is sum of

Liquid junction and membrane

potential.

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Diffusion Potential:

The greater the contrast in salinity

between mud filtrate and formation

water, the larger is the potential

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Shale Potential:

Na+ ions penetrates through the shale

from the saline formation water to the

less saline mud column, a potential is set

up.

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Electrokinetic Potential Mud cake Potential

Movement of charged ion through mud cake into permeable formation

Shale wall Potential

Flow of fluids from borehole into shale formation.

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How It Works The spontaneous potential produce in bore hole are actual result of these two

processes combine.

The mud filtrate is less saline then the formation water so it becomes negetivly charge

as a result of diffusion potential.

But above the sand opposite the shale because the shale potential the saline solution

(mud flitrate) becomes positivly charge.

The excess charge is the therefore negetive opposite the sand and positive opposite

the shale.

This couple works in a complementry sense and creates a SP currents flowing

between the mud filtrate the porous formation and shale.

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Log PresentationSP is presented in :

• Track 1

• SP currents measured in milli volts.

• Scale is in +ve or –ve mili volts

• -ve deflection to left and +ve to the

right

• It is usually run with Gamma ray or

Caliper Log

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Log Presentation

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How to read a log In sand A, Rw is less than Rmf; i.e.,

formation water is saltier than the mud

filtrate.

In sand B, the SP deflection is less than in

sand A, indicating a fresher formation

water.

In sand C, the SP is reversed, indicating

formation water that is fresher than the

mud filtrate (Rw > Rmf).

We may guess that, at about 7000 ft, Rmf

and Rw are equal.

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Tools

Electrodes

A galvanometer

Small 1.5 V battery

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Calibration

A small battery and a potentiometer is placed in series

between the two electrodes.

The logging engineer can adjust the potentiometer so that the

SP appears in track 1.

Remove all extraneous potentials to the membrane potential,

the SP needs to be normalised in a computing centre so that

there is no potential (SP=0.0MV) opposite shale beds

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Application

Two principal uses of Sp Logs

QUANTITATIVE USES

Formation Water Resistivity (Rw)determination

Shale Volume Indicator

QUALITATIVE USES

Detecting permeable beds

Correlation from well to well

Facies

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QUANTITATIVE USES

1. Determination of Formation Water Resistivity

The relationship between the SP and the

resistivities of the mud filtrate and the formation

water are determined

SP = -K log (Rmf)e/(Rw)e

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When mud filtrate salinities are lower than

connate water salinities (i.e., Rmf is > Rw),

the SP deflects to the left (the SP potential is

negative). This is called a normal SP.

When the salinities are reversed (i.e., salty

mud and fresh formation water, Rmf < Rw),

the SP deflects to the right. This is called a

reverse SP.

Other things being equal, there is no SP at

all when Rmf = Rw.

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2. Shale Volume Calculation

Shale Base Line

Th definition of s.p zero is made on thick shale intervals where s.p does not move to

the left or right is called shale base line.

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Static sp: (ssp)The theoratical maximum deflection of s.p opposite permeable beds is called static s.por

ssp. It is maximum possible s.p opposite a permeable water bearing formation with no

shale.

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2. Shale Volume Calculation:

V shale = (SPclean – SPlog)/(SPclean-SPshale)

Vshale: shale volume

SPclean: maximum Sp deflection from clean wet zone

SPlog: Sp in the zone of interest (read from the log)

Spshale: SP value at the shale baseline (often considered to be zero)

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QUALITATIVE USES

1. Detection of Permeability and Lithology

If there is even a slight

deflection on SP the bed opposite

the deflection is permeable.

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2. Correlation of formations

Correlation by SP log has been

replace by Gamma Ray log.

Because Gamma Ray Log has

more character and more

repeatable.

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3. Facies

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THANK YOU!

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QUESTIONS?

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