Spontaneous Potential Log

1. PETE-303: Well Logging
Spring 2016
Week #02
Eng. Khaled Abdelgawad
Department of Petroleum Engineering
KFUPM ENGINEERING
Well Logging
1

2. Spontaneous
Potential Log

3. Well QATIF-46
Objective
To explain how Spontaneous potential (SP) is
created in the formation & borehole.
How SP is measured.
3

4. Well QATIF-46
Spontaneous Potential Log
The spontaneous potential tool measures natural electrical
potentials occur in boreholes
SP distinguishes porous, permeable sandstones from intervening
shales.
The "natural battery" is caused when the use of drilling mud with a
different salinity from the formation waters, causes two solutions to
be in contact that have different ion concentrations.
4
• Ions diffuse from the more
concentrated solution (typically
formation water) to the more dilute.
• The ion flow constitutes electrical
current, which generates a small
natural potential measured by the SP
tool in millivolts.

5. Well QATIF-46
SP Principals 5
• Current is defined as moves in the same direction as the
positive charge flow.
• The charge carriers (electrons) are negative,
• Current is in the opposite direction as the electrons.
The direction of an electric current ?

6. Well QATIF-46
Spontaneous Potential Log
Spontaneous potential (SP) is used To:
Detect permeable zones
Detect boundaries
Determine formation Rwater.
Determine Vshale in permeable beds
Well to well correlation
6
+
+
+
+
-
-
-
+
+
+
+
-71mV
POROUS,
PERMEABLE
BED
SHALE
SHALE
Property:
Measures formation voltage
Passive measurement

7. Well QATIF-46
SP Principals
Must have water-based mud
Mud--formation water salinity
difference causes battery effect.
Battery effect components
7
Electrochemical Effect
Membrane effect
SAND
SHALE
Flushed
Zone
Less Salty
Water
Virgin
Zone
Salty
Water
Membrane effect
Virgin
Zone
+
+
+
+
+
Electrochemical
1. Liquid Junction Potential (Diffusion), Ej
 In permeable region
 Anions more mobile than cations
2. Membrane Effect, Em
 Shale acts as membrane
 Repels anions / passes cations
Electrokinetic (due to ∆P)
 Usually minor, disregarded

8. Well QATIF-46
Origin of SP
SP is a naturally occurring
potential in the borehole
which Is sum of:
❶The membrane potential
between the shale and the
sand.
❷ The liquid junction potential.
❸ The electro Kinetic effect is
sometimes observed but is
rare and short-lived.
8

9. Well QATIF-46
Electrochemical Potential 9
The electrochemical potential sensed in the
borehole is generated by the sum of two
potentials known as :
• The membrane potential (Em)
• The liquid junction potential Ej.
Ec = Em + Ej

10. Well QATIF-46
Liquid Junction Potential (Ej)
In a permeable sand, if the
formation water is more saline
than the mud filtrate then the
salt ions, (Na+ and Cl-) will diffuse
towards the mud filtrate.
The Cl- ions are more mobile than
the Na+ ions.
The more the saline side will
become positive with respect to
the less saline side.
Ej will always present as long as
there is a difference in salinity
between formation water and
mud filtrate.
10

11. Well QATIF-46
Liquid Junction Potential (Ej)
Where :
aw & amf: the activities of the formation water and the mud filtrate.
𝝁cl : Mobility of Cl- (67.6 x 10-5 cm2/sV)
𝝁Na: Mobility of Na+ (54.6 x 10-5 cm2/sV)
R : Ideal gas constant (8.314 j/oK-mol)
T : absolute temperature (oKelvin)
F : Faraday constant (96489 coulombs/mol)
11
𝑬𝒋 = 𝑲 𝟑 𝐥𝐨𝐠 𝟏𝟎
𝒂 𝒘
𝒂 𝒎𝒇
𝒌 𝟑 = 𝟐. 𝟑
𝝁 𝒄𝒍 − 𝝁 𝑵𝒂
𝝁 𝒄𝒍 + 𝝁 𝑵𝒂
𝑹𝑻
𝑭
K3 = 11.6 mV @ 25oC (77oF)

12. Well QATIF-46
Membrane Potential
The shale acts as an “ion sieve”.
Shale allows the passage of Na+ ions but
not Cl- ions.
Shales are mixtures of various clay
minerals which are composed of Si-4, Al+3
& O-2.
The commonest are Kaolinite,
Montmorillonite and illite.
The silicon and oxygen atoms are
arranged in a tetrahedral.
Si forms a strong bond with O leaving O-2
atoms on the edges of the SiO4 sheet.
The sheets have a –ve charge which
attracts +ve ions (cations) such as K+,
Na+, Ca++, Sr++, Ba++.
12
Layered clay structure

13. Well QATIF-46
Membrane potential
The formation water salinity is
higher than the mud.
The sale allows the only Na+ ions
to move from the formation into
the mud by diffusion.
The mud becomes more +ve than
the formation.
The net effect is a membrane
potential which is 5 times more
larger than the LL-junction.
13

14. Well QATIF-46
Membrane Potential (Em) 14
The membrane potential is larger than
the liquid-junction potential since the
absolute value of the mobilities enters
in, rather than a difference.
The magnitude of the liquid-junction
potential is only about one-fifth the
membrane potential.
𝑬 𝒎 = 𝑲 𝟒 𝐥𝐨𝐠 𝟏𝟎
𝒂 𝒘
𝒂 𝒎𝒇
𝒌 𝟒 = 𝟐. 𝟑
𝝁 𝒄𝒍 − 𝝁 𝑵𝒂
𝝁 𝒄𝒍 + 𝝁 𝑵𝒂
𝑹𝑻
𝑭
K4 = 59.1 mV @ 25oC (77oF)
𝑬 𝒎
𝑬𝒋
=
𝟓𝟗. 𝟏
𝟏𝟏. 𝟔
≈ 𝟓. 𝟎

15. Well QATIF-46
Total electrochemical potential
Sum of the membrane and LL-junction potentials.
K is a constant - depending on T.
15
𝑬 𝒄 = 𝑬𝒋+ 𝑬 𝒎
𝑬 𝒄 = 𝑲 𝐥𝐨𝐠 𝟏𝟎
𝒂 𝒘
𝒂 𝒎𝒇
𝑲 = 𝑲 𝟑+ 𝑲 𝟒
𝐾 = 61 + 0.133𝑇 (𝑇 𝑜 𝐹)
𝐾 = 65 + 0.24𝑇 (𝑇 𝑜 𝐶)
𝐾 = 71 𝑎𝑡 25 𝑜 𝐶 ( 77 𝑜 𝐹)
(courtesy of Schlumberger)
𝑬 𝒄 = −𝑲 𝐥𝐨𝐠 𝟏𝟎
𝑹 𝒎𝒇𝒆
𝑹 𝒘𝒆
Rwe is equal to Rw for NaCl solutions above
0.1 ohm-m.
For fresh mud filtrates, it has been common
practice to set Rmfe = 0.85 Rmf

16. Well QATIF-46
Electrokinetic potential
To be able to control the formation
pressure, there is always a positive
pressure from the mud to the
formation (overbalance).
Saline mud filtrate will penetrate the
sand.
The Movement of saline mud filtrate
create an electrokinetic potential
difference between the formation
and the mud.
16

17. Well QATIF-46
Electrokinetic potential
To be able to control the formation
pressure, there is always a positive
pressure from the mud to the
formation (overbalance).
Saline mud filtrate will penetrate
the sand.
The Movement of saline mud
filtrate create an electrokinetic
potential difference between the
formation and the mud.
Once the mudcake is formed, no
more mud filtrate will form hence
the electrokinetic potential will
disappear.
EKP is short-lived, we will not
normally detect it.
17

18. Well QATIF-46
Abnormal SP 18
SP=-k•log(Rxo/Rt)+Ek
SP=- k•log(Rxo/Rt)

19. Well QATIF-46
Measuring SP in the borehole
The SP measurement is taken as a potential difference between a downhole
electrode and a reference electrode at the surface
The mud column in front of the sandstone behaves as a resistor in the SP loop.
19

20. Well QATIF-46
Static SP (SSP)
SSP is the total
potential drop along a
line of current flow.
The deflections of SP
curve are a measure of
only potential drop in
the borehole resulting
from SP current.
SP is a fraction of the
total potential drop.
20

21. Well QATIF-46
Static SP (SSP)
SSP is the sum of the membrane (Em) and junction potential (Ej)
Where ISP is the SP current.
21
• The SP we measure is the change in potential
from one point in the well bore to another.
• It is developed across the resistance of the
mud column (Rm) from one point to another
due to the SP current (ISP)
𝑺𝑷 = 𝑰 𝒔𝒑 𝒓 𝒎
𝑺𝑺𝑷 = 𝑬 𝒎 + 𝑬 𝒋 = 𝑰 𝒔𝒑 𝒓 𝒎 + 𝒓 𝒕 + 𝒓 𝒔𝒉 + 𝒓 𝒙𝒐
𝒓 =
𝑳
𝑨
𝑹
Guess, when SP will equal SSP using
the fact that:
?

22. Well QATIF-46
Static SP
In front of a thik, permeable
water bearing, clean sand, The
SP will be flat.
This is called static SP or sand
baseline.
Most SP interpretation is
based on the comparison
between sand and shale
baselines.
22

23. Well QATIF-46
SP Circuit
Three voltage sources
• Membrane (80%)
• Electrochemical (15%)
• Streaming (usually <5%)
23
)( mj
mshtxo
m
sp EE
rrrr
r
E 


)( mjsp EEE 
Esp
)( SSPsp
EE 

24. Well QATIF-46
Since EKP is short-lived, we will
not normally detect it.
24
𝑬 𝒔𝒔𝒑 = −𝟎. 𝟏𝟑𝟑(𝑻 + 𝟒𝟔𝟎) 𝐥𝐨𝐠 𝟏𝟎
𝑹 𝒎𝒇𝒆
𝑹 𝒘𝒆
(𝑇 𝑜 𝐹)
𝑬 𝒄 = 𝑬𝒋+ 𝑬 𝒎

25. Well QATIF-46
Cause of fresh mud
The Less saline the mud, the
higher it’s resistance hence
the smaller the current
flowing through mud,
producing a static SP in front
of a relatively thin sand.
25

26. Well QATIF-46
SP Currents 26
Passage of Na+ ions through the adjacent
shale from the more saline formation
water in the bed to the less saline mud.
Reverse SP occurs
when formation
water is fresher
than mud filtrate

27. Well QATIF-46
Low resistivity
suggesting a water
bearing formation
Low GR response and high SP deflection
Determination of SSP 27
❶ Select a 100% Water
Saturated Zone

28. Well QATIF-46
Shale base
line is the SP
response
across a thick
shale or
several shale
intervals
Sandstone
baseline
20
mV
- 110 mV
Determination of SSP 28
❷ Select The Shale
baseline

29. Well QATIF-46
Spontaneous Potential Log 29
Measure SP value is affected by
Bed thickness,
Bed resistivity,
Borehole diameter,
Invasion,
Shale content, and
The ratio of Rmf to Rw

30. Typical SP responses – based on the difference
between RW AND RMF.
REVERSEDSPNORMALSP
1
2
3
4
5
(+)
(-)
30
5. Rmf << Rw - Amplitude large and
positive
4 . Rmf < Rw - Amplitude positive but
not large
3. Rmf = Rw - No SP deflection
2. Rmf > Rw - Amplitude negative but
not large
1. Rmf >> Rw - Amplitude large and
negative

31. Well QATIF-46
Example 31

32. Well QATIF-46
The ratio of Rmf to Rw 32
The upper sandstone, "U", the SP log
shows a deflection to the right, indicating
formation water to be fresher than the
drilling mud,
The lower sandstone, "L", the deflection is
to the left, showing the formation water
to be more saline.
From a well in north-west Kansas

33. Well QATIF-46
Review MCQ
The primary objective of logging the Spontaneous Potential
(SP) is to determine:
A. True resistivity of the uninvaded formation (Rt)
B. A qualitative indicator of formation permeability
C. Mud filtrate resistivity (Rmf)
D. Formation porosity
33

34. Well QATIF-46
Review MCQ
Which of the following statements is true?
A. Electrokinetic processes relate to the movement of fluid at the
borehole/formation interface, while electrochemical processes
relate to ionic imbalances between fluids.
B. Electrokinetic processes relate to ionic imbalances between fluids,
while electrochemical processes relate to the movement of fluid at
the borehole/formation interface.
A majority of the deflection observed on an SP log is the result of
A. Electrokinetic potentials
C. Membrane potential
D. Liquid junction potential
34

35. Well QATIF-46
Review MCQ
Ionic exchange across the boundary of a permeable formation and
an adjacent shale produces an electromotive force in the formation
called the:
A. Membrane potential
B. Liquid junction potential
C. Electrokinetic potential
Ionic exchange across the interface between two fluids of different
salinities produces an electromotive force in the formation called
the:
A. Membrane potential
B. Liquid junction potential
C. Electrokinetic potential
35

36. Well QATIF-46
Review MCQ
The SP measurement is taken as a potential difference between:
A. A downhole electrode and a current return electrode, both located on the tool
B. A downhole electrode and casing
C. A downhole electrode and a reference electrode at the surface
D. A downhole electrode and the wireline
In which of the following conditions is the SP response valid?
A. Water-based mud
B. Cased hole
C. Air-drilled hole
D. Oil-based mud
36

37. Well QATIF-46
Review MCQ
Under what conditions is an electrokinetic potential created as fluid
flows through the mud cake?
A. Borehole drilled with very heavy mud
B. Hydrostatic pressure in the borehole balances formation pressure
C. Borehole drilled with light mud
D. Hydrostatic pressure is less in the formation than in the borehole
Ionic exchange resulting from a pressure differential between the
formation and the borehole produces an electromotive force
called the:
A. Electrochemical potential
B. Electrokinetic potential
C. Membrane potential
D. Liquid junction potential
37

38. Well QATIF-46
Review MCQ
Select all of the potential applications of the SP measurement.
A. Determination of formation water resistivity (Rw)
B. Determination of volume of shale
C. Estimation of diameter of invasion
D. Well-to-well correlation
E. Determination of bed thickness
The spontaneous potential tool is designed to measure a natural –
or spontaneous – current flow in the borehole and formation
resulting from ionic imbalances that exist between different
subsurface fluids and ionic movement near the borehole/formation
interface.
A. True
B. False
38

39. Well QATIF-46
Review MCQ
The exchange in ions (primarily chlorine, Cl-, and sodium, Na+)
between fluids of different salinities creates a _____ drop in a
fluid-filled borehole.
A. Voltage
B. Resistivity
C. Current
D. Power
In which of the following conditions is the SP response not valid?
A. Water-based mud
B. Cased hole
C. Air-drilled hole
D. Oil-based mud
39

40. Well QATIF-46
Review MCQ
Match the letter to the particular type of deflection being
shown on the SP baseline or die.
1. Rmf =Rw ( )
2. Rmf <Rw ( )
3. Rmf >>Rw ( )
4. Rmf >Rw ( )
40