Spontaneous potential log

1. Asst. Lecturer: Amir I. Abdelaziz
Helwan University

2. Asst. Lecturer: Amir I. Abdelaziz
Helwan University

3. Tools
Electrodes
A galvanometer
Small 1.5 V battery
3

4. 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.

5. 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 5

6. 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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7. 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
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8. 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

9. 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.
9

10. shale and clean sand beds alongwith
the idealized response of SPlogging
deflections to the left correspond to
increasingly negative values.
In the first sand zone,
there is no SP deflection
since this case represents equal salinity
in the formation water and in the mud
filtrate.
The next two zones
show 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 corresponds
to the case of a mud filtrate which is
saltier than the original formation fluid.
10

11. 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. 12

13. 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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14. summary SP curve behavior undera
variety of loggingcircumstances
Finally, the symmetric
responses 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.

15. Application of SP log

16. Application of SP log
Freshwatersand
Rw=10Ohm
T=50c
FreshwaterRw=1Ohm
T=60c
SaltwaterRw=0.05Ohm
T=70c
Fresh mud Rmf = 1 Ohm Fresh mud Rmf = 1 Ohm

17. 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 0
C 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

18. Application of SP log

19. 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