The document explains the concept and operation of the spontaneous potential (SP) log in wireline logging, which measures natural potential differences in boreholes. It details how SP is formed through various electrochemical and diffusion processes and discusses its presentation and applications in determining formation water resistivity and shale volume. Additionally, the document outlines the qualitative uses of SP logs for detecting permeability and correlating geological formations.

1. 1

2. 2

3. Contents
 Introduction
 Spontaneous Potential (SP) Log
 Principle
 How It Works
 Log Presentation
 Tools
 Applications
3

4. Introduction To Wireline Logging
The continuous recording of
geophysical parameters along a
borehole produces a geophysical log.
4

5. Spontaneous Potential (SP) Log
The spontaneous potential log measures the
natural or spontaneous potential differences
that exists between the borehole and the
surface in the absence of any artificially
applied current.
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. Principle
Causes of SP
Electrochemical
Potential
Electrokinetic
Potential
Mudcake
Potential
Shale wall
Potential
Liquid Junction
Membrane
Potential
7

8. Electrochemical Potential
Electro chemical potential is sum of
Liquid junction and membrane
potential.
8

9. Diffusion Potential:
The greater the contrast in salinity
between mud filtrate and formation
water, the larger is the potential
9

10. Shale Potential:
Na+ ions penetrates through the shale
from the saline formation water to the
less saline mud column, a potential is set
up.
10

11. Electrokinetic Potential
 Mud cake Potential
 Movement of charged ion through mud cake into permeable formation
 Shale wall Potential
 Flow of fluids from borehole into shale formation.
11

12. How It Works
 The spontaneous potential produce in bore hole are actual result of these two
processes combine.
 The mud filtrate is less saline then the formation water so it becomes negetivly
charge as a result of diffusion potential.
 But above the sand opposite the shale because the shale potential the saline solution
(mud flitrate) becomes positivly charge.
 The excess charge is the therefore negetive opposite the sand and positive opposite
the shale.
 This couple works in a complementry sense and creates a SP currents flowing
between the mud filtrate the porous formation and shale.
12

13. 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
13

14. Log Presentation
14

15. 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. 15

16. Tools
Electrodes
A galvanometer
Small 1.5 V battery
16

17. Calibration
A small battery and a potentiometer is placed in series
between the two electrodes.
The logging engineer can adjust the potentiometer so that the
SP appears in track 1.
Remove all extraneous potentials to the membrane potential,
the SP needs to be normalised in a computing centre so that
there is no potential (SP=0.0MV) opposite shale beds
17

18. 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
18

19. QUANTITATIVE USES
1. Determination of Formation Water Resistivity
The relationship between the SP and the
resistivities of the mud filtrate and the formation
water are determined
SP = -K log (Rmf)e/(Rw)e
19

20.  When mud filtrate salinities are lower
than connate water salinities (i.e., Rmf
is > Rw), the SP deflects to the left
(the SP potential is negative). This is
called a normal SP.
 When the salinities are reversed (i.e.,
salty mud and fresh formation water,
Rmf < Rw), the SP deflects to the
right. This is called a reverse SP.
 Other things being equal, there is no
SP at all when Rmf = Rw.
20

21. 2. 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.
21

22. Static sp: (ssp)
The theoratical maximum deflection of s.p opposite permeable beds is called static s.por
ssp. It is maximum possible s.p opposite a permeable water bearing formation with no
shale.
22

23. 23

24. 2. 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)
24

25. QUALITATIVE USES
1. Detection of Permeability and Lithology
 If there is even a slight
deflection on SP the bed opposite
the deflection is permeable.
25

26. 2. Correlation of formations
 Correlation by SP log has been
replace by Gamma Ray log.
 Because Gamma Ray Log has
more character and more
repeatable.
26

27. 3. Facies
27

28. THANK YOU!
28

29. QUESTIONS?
29