Asst. Lecturer: Amir I. AbdelazizHelwan University

Asst. Lecturer: Amir I. AbdelazizHelwan University

Tools

Electrodes

A galvanometer

Small 1.5 V battery

3

Spontaneous potential

Galvanometer: Record the difference in

voltage between a moving electrode in the

borehole and a reference electrode at the

surface usually located at the mud pit.

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

Facies5

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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Log Presentation

SP 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

7

Factors affect the Sp

1. The Rmf / Rw ratio

2. Fresh mud Rmf < Rw - Ve SP

3. Saline mud Rmf > Rw + Ve

4. If Rmf = Rw No SP deflection

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ESP = -K log (Rmf) /(Rw)

ESP = -K log (Rmf) /(Rw)

• Kc = (61+0.133 T (f) )

• Kc = (65 + 0.24 T (c ) )

• 1 C = 33.8 F

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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shale and clean sand beds alongwith

the idealized response of SP loggingdeflections to the left correspond to

increasingly negative values.In the first sand zone,

there is no SP deflectionsince this case represents equal salinityin the formation water and in the mud filtrate.

The next two zonesshow a development of the SP which is

largest for the largest contrast in mud filtrate and formation water resistivity.

In the last zone,the deflection is seen to be to the right

of the shale baseline and correspondsto the case of a mud filtrate which is saltier than the original formation fluid.

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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.

Static sp: (ssp)

The theoretical maximum deflection of s.p opposite permeable beds is called static s.p or

ssp. It is maximum possible s.p opposite a permeable water bearing formation with

no shale.

Pseudo SP: (PSP)

Any deflection less than (SSP)

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12

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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summary SP curve behavior undera

variety of logging circumstances

Finally, the symmetricresponses of SP logs can be upset by vertical movement of mud filtrate in high permeability sands:upwards in the

presence of heavier saline formation water, and

downwards in the presence of gas and light oil.

Application of SP log

Application of SP log

Fres

h w

ater

san

d

Rw

= 1

0 O

hm

T =

50

c

Fres

h w

ater

Rw

= 1

Oh

mT

= 6

0 c

Salt

wat

er R

w=

0.0

5 O

hm

T =

70

c

Fresh mud Rmf = 1 Ohm Fresh mud Rmf = 1 Ohm

Q2-(B) A number of factors affects the shape and amplitude of SP log as Rmf/Rw

ratio, bed thickness, bed resistivity and Porosity. If there is a succession of shale and sand in

a well where the Rw was = 10 ohm in the sand layer (1) and Rw= 0.5 at sand layer (2) and

Rmf = 1 ohm at Temp. 60 0C and fresh water exists, Calculate

1- Kc and Esp

2- Show the deflection direction of SP log.

-SP+

Shale

Sand (2)

Shale

Sand (1)

Application of SP log

Application of SP log

General Log responses

Lithology DensityGR ResistivityAcousticNeutron

Sandstone 2.65Low(Unless RA min)

high55-4

Salt 2.1Low(Unless K salt)

V.high670

Anhydrite 2.95V.low V.high50-1

Dolomite 2.85Low(higher if U)

high42.5+4

Shale 2.2-2.7(water content)

high low (water content)

50-150(water content)

High(water content)

Limestone 2.71low high47.50

Gas 0.2-0.5(pressure)

0 V.high~100010-50(H2 index)

Oil 0.6-1.0(api)

0 V.high210-240 (api)

70-100 (H2 index)

Water 1-1.1(salt and Temp)

0 0 - infinite(salt and Temp)

180-190100