Sp log - Well logging

Spontaneous potential log

1. Asst. Lecturer: Amir I. Abdelaziz
HelwanUniversity

2. Asst. Lecturer: Amir I. Abdelaziz
HelwanUniversity

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 SpLogs :
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.
6

7. Log Presentation
SP is presented in :
•Track 1
•SP currents measured in millivolts.
•Scale is in +veor –vemilivolts
•-vedeflection to left and +veto the right
•It is usually run with Gamma ray or
Caliper Log
7

8. Factors affect the Sp
1.The Rmf/ Rwratio
2.Fresh mud Rmf <Rw -VeSP
3.Saline mud Rmf>Rw + Ve
4.If Rmf= Rw No SP deflection
8
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, Rwis 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 Rware equal.
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10. shale and clean sand beds alongwith
the idealized response of SPlogging
deflections to the left correspondto
increasingly negativevalues.
In the first sandzone,
there is no SPdeflection
since this case represents equalsalinity
in the formation water and in the mud
filtrate.
The next twozones
show a development of the SP which is
largest for the largest contrast in mud
filtrate and formation waterresistivity.
In the lastzone,
the deflection is seen to be to theright
of the shale baseline andcorresponds
to the case of a mud filtrate which is
saltier than the original formationfluid.
10

11. Shale Volume Calculation
Shale Base Line
Th definition of s.pzero is made on thick shale intervals where s.pdoes not move to the
left or right is called shale base line.
Static sp: (ssp)
The theoretical maximum deflection of s.popposite permeable beds is called static s.por
ssp. It is maximum possible s.popposite a permeable water bearing formation with
no shale.
Pseudo SP: (PSP)
Any deflection less than (SSP)
11

12. 12

13. Shale Volume Calculation:
V
shale= (SP
clean–SP
log)/(SP
clean-SP
shale)
V
shale: shale volume
SP
clean: maximum Spdeflection from clean wet zone
SP
log: Spin the zone of interest (read from the log)
Sp
shale: SP value at the shale baseline
(often considered to be zero)
13

14. summary SP curve behavior undera
variety of loggingcircumstances
Finally, thesymmetric
responses of SP logs
can be upset by vertical
movement of mud
filtrate in high
permeabilitysands:
upwards in the
presence of heavier
saline formationwater,
and
downwards in the
presence of gas and
lightoil.

15. Application of SP log

16. Application of SP log
Fresh water sand Rw
=
10
Ohm
T =
50
c
Fresh water
Rw
=
1
Ohm
T =
60
c
Salt water
Rw
=
0.05
Ohm
T =
70
c
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 2.65
Low
(Unless RA min)
high55-4
Salt 2.1
Low
(Unless K salt)
V.high670
Anhydrite 2.95V.low V.high50-1
Dolomite 2.85
Low
(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~1000
10-50
(H
2index)
Oil
0.6-1.0
(api)
0 V.high
210-240
(api)
70-100
(H
2index)
Water
1-1.1
(salt and Temp)
0
0 -infinite
(salt and Temp)
180-190100