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
Uses of SP log 3
Identify permeable zones
Define bed boundaries
Compute shale content
Depositional Environment from the SP
Determine values of formation water resistivity

4. Well QATIF-46
Uses of SP log 4
1.Identify permeable zones
• The negative abnormal on SP
curve usually indicates the
permeable zone ; the higher
abnormal range , the more
permeable of the formation .
• Since invasion can only occur
in permeable formations,
deflections of SP can be used
to identify permeable
formations.

5. Uses of SP log
1.Identify permeable zones
The negative abnormal on SP curve
usually indicates the permeable
zone ; the higher abnormal range ,
the more permeable of the
formation .
Since invasion can only occur in
permeable formations, deflections
of SP can be used to identify
permeable formations.
5
SP alone can not tell the
whole story

6. Uses of SP log
2. Definition of bed boundaries
Half of abnormal amplitude
point will be boundaries of shale
and sand.
The bed thickness is the interval
between two boundaries .
The vertical resolution of SP is
poor, and often the permeable
bed must be 30 ft or more to
achieve a static (flat baseline) SP
6

7. Uses of SP log
2. Definition of bed boundaries
Half of abnormal amplitude point
will be boundaries of shale and
sand.
The bed thickness is the interval
between two boundaries .
The vertical resolution of SP is
poor, and often the permeable
bed must be 30 ft or more to
achieve a static (flat baseline) SP
7

8. Uses of SP log
3. Compute shale content
The presence of shale in a
permeable formation reduces
the SP deflection.
In water-bearing zones, the
amount of SP reduction is
related to the amount of shale
in the formation.
The SP response of shales is
relatively constant and follows
a straight line called a shale
baseline.
The SP value of the shale
baseline is assumed to be zero,
and SP curve deflections are
measured from this baseline.
8

9. Base Line Drift
Over long intervals (several hundreds to
thousands of feet), the SP baseline can drift,
either in the positive or negative direction.
This characteristic is commonly observed in
shallow sections.
Caused by increases in relative oxidation of
the rocks that are close to the land surface.
This may introduce errors if the SP
magnitude is being calculated over that long
interval, especially by means of a computer.
The baseline drift can be removed (many
programs have such editing routines) so
that the SP baseline retains a constant value
(usually set to zero) over the length of the
logged interval.
9
A shallow section of the Dakota.

10. Shale Content
Presence of shale in the formation will reduce the static SP
Shale lattice will slow the migration of chlorine ions and assist the
flow of sodium ions, decreasing Ej
This reduces SSP to a pseudo-static value, PSP
10
SSP  Max deflection possible for given Rmf/Rw
SP  SP response due to presence of thins beds and/or gas presence
PSP  Pseudostatic SP; SP when shale is present
𝑽 𝒔𝒉 = 𝟏 −
𝑷𝑺𝑷
𝑺𝑺𝑷
The volume of shale can be calculated:

11. Shale Content 11
Example-1
Determine the volume of shale in zone B
𝑽 𝒔𝒉 = 𝟏 −
𝑷𝑺𝑷
𝑺𝑺𝑷
S𝑺𝑷 = 𝟏𝟎𝟎 𝒎𝑽
P𝑺𝑷 = 𝟕𝟔 𝒎𝑽
𝑽 𝒔𝒉 = 𝟏 −
𝟕𝟔
𝟏𝟎𝟎
= 𝟎. 𝟐𝟒
𝑽 𝒔𝒉 = 𝟐𝟒 %
SSP and PSP are measured
from shale baseline

12. Shale Content 12
Example-2
𝑽 𝒔𝒉 = 𝟏 −
𝑷𝑺𝑷
𝑺𝑺𝑷
SSP
S𝑺𝑷 = 𝟔𝟎 𝒎𝑽
P𝑺𝑷 = 𝟓𝟎 𝒎𝑽
𝑽 𝒔𝒉 = 𝟏 −
𝟓𝟎
𝟔𝟎
= 𝟎. 𝟏𝟔
𝑽 𝒔𝒉 = 𝟏𝟔 %
PSP

13. Uses of SP log
4. Determination of Formation Water Resistivity from SP Log
13
• The most direct way of finding water resistivity (Rw) is to obtain
a sample of formation water and measure its resistivity
• Formation water samples, if available, are invariably
contaminated by mud filtrate.
• Rw is therefore usually calculated.

14. Uses of SP log
4. Determination of Formation Water Resistivity from SP Log
14
1.Determine formation
temperature.
2.Find Rmf at formation
temperature
3.Convert Rmf at formation
temperature to Rmfe value
4.Compute Rmfe / Rwe ratio
from the SP
5.Compute the Rwe
6.Convert Rwe at formation
temperature to Rw
Induction electric log with an SP curve
from a Pennsylvanian upper Morrow
sandstone in Beaver County, Oklahoma.

15.  
TD
TBHTFD
TT s
sf


Spontaneous Potential Log: Calculation of Rw
• Resistivities are temperature dependent.
• 1st step Find the formation temperature at the depth at which Rw
is required.
• Using linear interpolation between surface and bottom hole
temperature.
• The petroleum industry usually uses degF, not degC.
15
Data given on well header
• Surface Temp (Ts),
• Formation Depth (FD),
• Bottom Hole Temp (BHT), and
• Total Depth (TD) are usually.

16. Temperature Gradient 16
1 oF/100ft = 1.823 oC/100m
1 oC/100m = 0.5486 oF/100ft
• This chart can be used for
estimating the temperature
in the borehole opposite a
formation at a specific depth.

17. Calculation of Rw 17
The 2nd step: convert Rm & Rmf measured at the surface to Rmf at Tf, the
formation temp. This is NOT a linear interpolation , The constant 6.77 is 0F.
For Celsius it is 21.5
 
0
0
0
0
77.6
77.6



f
mff
mf
T
TR
R
The 3rd step: Find the ideal potential (SSP), correcting for bed thickness.
• Bed thickness is determined from the SP log by measuring the distance
between inflection points of the SP curve
• Ri, from the SN (short reading resistivity) log next to the SP log
• Calculate Ri/Rm (make sure you use the temperature corrected Rm), get
the SP correction factor, then read off
 
0
0
0
0
77.6
77.6



f
mf
m
T
TR
R

18. Well QATIF-46
Find Rm and Rmf at Tf 18

19. Correcting for bed thickness 19
The data necessary to Correct SP to SSP using Chart are:
Bed thickness,
Resistivity from the
shallow-reading
resistivity tool (Ri)
Rm @ BHT

20. The 4th step is to find the equivalent formation water resistivity, Rwe, using
the following equation (remember Rmf is at formation temperature):
The final step is to find the actual water resistivity, Rw, using the following
equation:
Calculation of Rw 20
 
 














8.50log
0426.0
2
9.19log
1
105.0
10131.0
BHT
we
BHT
we
w
R
R
R
n
t
w
w
R
RF
S
1





 

Once Rw is calculated for the reservoir, use
the Archie equation to calculate Sw.
 BHT
SSP
mfwe RR 133.061
10 


21. Well QATIF-46
Equivalent Formation Water Resistivity 21
0.28

22. Well QATIF-46
Actual water resistivity 22

23. Well QATIF-46
Actual water resistivity 23
• Determine Rmf @Tf
• 5.6(11+21.5)/(33+21.5) = 3.3m
• Apply SP equation
– -50 = -
0.24(33+273)log(3.3/Rwe)
– Rwe = 0.68
– Rw = 1.3 ohm-m
(See next page)
10mV
-||+
Rmf = 5.6m @ 11º C
Tf = 33º C
• Determine Essp
– Shale base line
– Maximum deflection line
– Calculate deflection -50mV
Usually use charts, instead

24. Rw or Rmf
RweorRmfe
Rwe=
0.68
Rw = 1.3
24

25. Classical Method Review 25
Rw From the SP-Classic Method
The procedure for using the equations is as following:
1. Determine formation temperature.
2. Find Rmf at formation temperature
3. Convert Rmf at Tf to Rmfe value.
4. Compute The Rmfe/Rwe ratio from the SP.
5. Compute Rwe.
6. Convert Rwe at formation temperature to an Rw value.

26. Example-2
The SP deflection is –60 mV across a thick, water- bearing, clean
zone. The value of Rmf at that temperature of 100 F is 0.5 ohm-m.
Determine Rw at the same temperature (100 F)
Rw from SP: Classical Method
First, Determine the Rmfe (effective Rmf), since the resistivity is not
an accurate determination of the ion activity that produces the SP.
26

27. Rmfe = 0.45 ohm-m
at 100 F.
1. Determine Rmfe
0.5,100F
0.45 ohm-m
Rmf, 0.5 ohm-m
27

28. 2. Determine
Rwe from Rmfe
Rmfe/Rwe = 7. Therefore,
Rwe=0.45 ohm-m/7=0.064 ohm-m at 100 F
60, 100
7
SSP
28

29. (Rwe=0.064 ohm-m at
100F)
3. Finally, determine Rw
•Rw=0.10 ohm-m at
100 F
• Here, Rw<Rmf. This
problem illustrates the
fact that if Rw<Rmf, SP
deflection is negative
(0.1<0.45 ohm-m)
(Normal SP)
0.064 mV
0.064, 100F
29

30. Rw from SP—Silva-Bassiouni Method
The classical method is complicated to the novice.
Simpler method is available and theoretically justified.
The entire process is reduced to a single chart.
30
1.Enter the chart from below with Rmf at
formation temperature.
2.Move up to intersect the temperature
line at point A.
3. Move left to the SP axis, and then move
down by an amount equal to the
negative SP deflection.
4.Move right to the temperature line to
point B.
5.From B proceed down to the resistivity
scale, and read the value of at formation
temperature.

31. Well QATIF-46
Rw from SP—Silva-Bassiouni Method 31

32. Well QATIF-46
For the same problem as
before, ie Rmf=0.5 ohm.m
at 100 F, determine Rw if
the SP deflection is –60
mV.
We see Rw=0.1 ohm-m,
as shown with the
classical method.
145 mV – 60 mV = 85mV
Rw from SP—Silva-Bassiouni Method 32

33. Classical vs. Silva-Bassiouni Method
The classical method requires 3 steps for
the determination of Rw
The Silva Bassiouni method gives you the
same value of Rw. Hence it is easier to use
33

34. Example
Given:
Rm = 2.5 Ω-m at 70°F
Rmf = 2.0 Ω-m at 70°F
Hole diameter = 8 in.
Surface temperature = 60°F
BHT = 164°F at 10,500 ft
From the log:
SP deflection = 70 mV
Bed thickness = 24 ft
Short normal resistivity = 65 Ω-m
34

35. Example
Calculations
35
8114
8138
24ft
Using BHT = 164°F at 10,500 ft and
surface temperature = 60°F:
Tf at 8100 ft=140°F.
Using Rm = 2.5 Ω-m at 70°F and
Rmf = 2.0 Ω-m at 70°F:
Rm = 1.3 Ω-m at 140°F
Rmf = 1.0 Ω-m at 140°F.
SP deflection =–70 mV.
Ri/Rm = RSN/Rm = 65/1.3 = 50.
The SP correction factor is 1.07
Corrected SP of 1.07 × –70 = –75 mV.
From Chart 4, using SP corrected
and Tf: Rmf/Rwe = 9.7.
Rwe = Rmf/(Rmf/Rwe) = 1.0/9.7
= 0.103 Ω-m.
Using Rwe and Tf. Rw = 0.103 Ω-m

36. SP shapes and Depositional Sequence 36
Since shales and clays are generally
finer-grained than sands, a change
in SP suggests a change in grain size.
SP deflections can indicate
depositional sequences, where
either sorting, grain size or
cementation change with depth and
produce characteristic SP shapes.
These shapes are referred to as
bells, funnels, or cylinders .

37. Factors Affecting the SP Response
Hydrocarbons: reduce the SP deflection
Shaliness: reduces the SP deflection
Bed thickness: thin beds do not develop a full SP deflection
Permeability: low permeability zones will have a very high
invasion diameter, so it may be impossible to read the
Junction Potential, hence SP readings may be low
37

38. Passive Log Correlation
GR, SP, and Caliper
• Often correlate
• Different measurements
• Different reasons
Correlation helps
• GR instead of SP in oil base mud
• Easier detection of shales
• Facilitates “zonation”
38

39. Zonation
Zonation - Defines intervals of similar properties
Purpose
• Well-to-well correlation
• Evaluation of specific intervals
Criteria
• Lithology
• Fluids
• Porosity and permeability
 Begin with coarse zonation
oTypically
• Well-to-well correlation 20 - 100 ft
• Detail evaluation 10 ft thick or more
oEasy lithologies first, e.g., shales
Refine
oMore subtle lithology changes
oFluids in porous, perm intervals
oDepends on measurements available
39

40. Well QATIF-46
Zonation and Rw Effect 40

41. Limitations of SP Log
The SP cannot be recorded in air or oil-base muds,
since there is no conductive fluid in the borehole.
Conductive mud is essential for generation of a
spontaneous potential.
In salt-mud, SP tends to be straight line (less salinity
contrast).
If bed is too thin, the full SP will not develop. Chart exist
to correct for this effect, but only significant for bed
thickness < 20ft.
Hydrocarbon and shale in the formation reduce SP
development.
41