A Spontaneous Potential Measurement
Spontaneous Potential
1
Schlumberger 1999 1
Spontaneous Potential Measurement
SP Theory 1
SP results from electric currents flowing in the
drilling mud.
There are three sources of the currents, two
electrochemical and one electrokinetic.
Membrane potential - largest.
Liquid - junction potential.
Streaming potential - smallest.
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Spontaneous Potential Measurement
SP theory 2
Membrane and Liquid Potential
These two effects are the main components of the SP. They are
caused because the mud flitrate and the formation waters contain
NaCl in different proportions.
Firstly, shales are permeable to the Sodium ions but not
the Chlorine. Hence there is a movement of charged
particles through the shale creating a current and thus a potential.
The ions Na+ and Cl- have different mobilities at the junction of
the invaded and virgin zones. The movement of the ions across
this boundary creates another current and hence a potential.
Streaming Potential
This is generated by flow of the mud filtrate through the mud
cake. As this does not normally occur this effect is small. It will
only become important if there are high differential pressures
across the formations.
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Spontaneous Potential Measurement
SP theory 3
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Spontaneous Potential Measurement
SP scales
The SP is measured in millivolts, mV.
The scale on the log shows a number of mV per
division for example 20mV/division. This gives a
total for the track of 200mV.
The scale across the track is variable and depends
on the conditions in the well.
The scale is set during logging to have the SP
curve in the track over the zone of interest and as
much of the rest of the log as possible.
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Spontaneous Potential Measurement
log-1
The SSP is the
quantity to be
determined.
It is the deflection
seen on the SP
from the Shale
Base Line (zero
point) to the Sand
Line (max.
deflection)
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Spontaneous Potential Measurement
example log 2
The maximum
SP deflection in
this example
occurs at the
same depths as
the resistivity
curves show a
separation.
The minimum
point on the SP
corresponds to
where all the
resistivity
curves overlay,
no invasion, a
shale.
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Spontaneous Potential Measurement
SP uses
Differentiate potentially porous and permeable
reservoir rocks from impermeable clays.
Define bed boundaries.
Give an indication of shaliness (maximum
deflection is clean; minimum is shale).
Determine Rw in both salt and fresh muds.
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Spontaneous Potential Measurement
Rw from the SP
Rw is often known from client information or
local knowledge.
The SP can be used to check the value or compute
it when it is unavailable.
It is especially useful when there are variations
along the borehole.
Rmfe
SSP = k log
Rwe
K is a constant - depending on the temperature.
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Spontaneous Potential Measurement
Rw from the SP
Knowing the SSP (the maximum deflection) from
the log and the temperature, the ratio of
resistivities is obtained from Log Interpretation
Chart SP-1.
Rmfe
output =
Rwe
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Spontaneous Potential Measurement
Rw from SP 2
Rmf is measured, using the mud cell.
Rmfe is computed from Log Interpretation Chart
SP-2.
Rwe is computed, from the ratio from SP1 and
Rmfe.
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Chart SP-2 output is Rw. 11
Spontaneous Potential Measurement
Salinities chart
Grains/gal
10
at 75F
8
ppm
6
5
200
4 10
3 300
15
400
2 20
500
25
600
30
700
800
1 40
NaCl Concentration (ppm or grains/gal)
0
100
50
Resistivity of Solution ( - m)
0.8 2 0 0
1
0
140
0.6
0
170
0.5 0
200
100
0.4
0
300
0.3 150
0
400
200
0.2 50 0 0
0 250
600
70000
0 300
80
400
000
10,
0.1 0 00 500
,
12 0
0.08 14,
0 0
000
17,
0.06 000
20, 1000
0.05
00
30,0
0.04 1500
00
40,0 2000
0.03 00
50,0
00 2500
3000
60,0
00
70,0 00
0.02 80,0 4000
000
100, 5000
000
,000 120,000
300 140, 00
0
170,
0.01 200,0
00 10,000
000
250,000
280, 15,000
50 75 100 125 150 200 250 300 350 400
20,000
10 20 30 40 50 60 70 80 90 100 120 140 160 180 200
Temperature (F or C)
This chart is used to compute salinities from
resistivities of solution e.g. mud, and vice versa.
It is also used to find the resistivities at a given
temperature.
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Spontaneous Potential Measurement
SP borehole Effects - 1
Baseline shifts:
These can occur when there are beds of different
salinities separated by a shale which does not act
as a perfect membrane.
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Spontaneous Potential Measurement
SP Borehole Effects - 2
Resistive formation:
The presence of a resistive bed in a permeable
interval will disrupt the SP deflection. The
current is contained and hence the potential drop
changes with depth. The log takes a sloped
appearance.
The log in this situation can no longer define the
bed boundaries correctly.
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Spontaneous Potential Measurement
SP surface Effects
The SP can be affected by a number of surface
effects as it relies on the fish as its reference
electrode.
Power lines, electric trains, electric welding, close
radio transmitters:
All these create ground currents which disrupt he
"fish" reference causing a poor, sometimes
useless, log.
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