Q922+log+l05 v1

1. Reading A Log
2. Examples of Curve Behavior And Log Display
3. Electrical Properties Of Rocks And Brines

1. Spontaneous Potential
A. membrane potential
B. Application
C. Log Example of The SP

Spontaneous potential
Spontaneous potential main usage:
the identification of permeable zones
The origins of the spontaneous potential in
wellbores involve both
electrochemical potentials and
the cation selectivity of shales.
basis for the spontaneous potential is
the process of diffusion –the self-diffusion of
the dissolved ions in the fluids
• in the borehole and in the formation.
Spring14 H. AlamiNia Well Logging Course (2nd Ed.) 5

The mechanism of generating
the liquid-junction potential
Electrochemical potentials
of interest to the
generation of the
spontaneous potential are
the liquid junction potential
the membrane potential
Figure schematically
illustrates the situation for
the generation of the
liquid-junction potential.
To the left is a saline solution
of low NaCl concentration.
To the right is one of a higher
ionic concentration.

The liquid junction potential
As is often the case,
the resistivity of the drilling mud filtrate (Rmf )
is greater than the resistivity of the formation water
(Rw), so:
Where
Vl−j is The liquid junction potential

A schematic representation of the
development of the SP in a borehole
The cell marked Ed
corresponds to the liquid
junction potential just
discussed
is sketched with the polarity
corresponding to a higher
electrolyte concentration in
the formation water than in
the mud filtrate.
additional source of SP is
associated with the shale
result of the membrane
potential
generated in the
presence of the shale that
contains clay minerals
 which have large negative
surface charge

A representation of a shale
On the left,
consisting of
rock mineral
grains and
small platy clay
particles.
On the right
the
distributions of
ions close to
the face of one
of the clay
minerals is
shown,
 which
illustrates the
so-called
electrical
double-layer.

How does
a cation differ from an anion?
A cation (s)(+)
is a positively (+) charged ion.
It loses one or more negatively charged electrons when
forming ionic compounds.
(are) almost always metals
An anion (s) (-)
is a negatively (-) charged ion.
It gains one or more electrons when forming ionic
compounds.
(are) typically nonmetals
Every ionic compound must contain
both a cation and an anion
so that the compound as a whole has no charge.
A common example: In the ionic compound table salt (NaCl),
sodium (Na+) is the cation, and chloride (Cl-) is the anion.

electrical double layer
We assume that shale is
nearly impermeable to fluid flow,
but still capable of ionic transport, although considerably
altered by the presence of clay minerals.
The shale acts like a cation-selective (+) membrane.
This property is related to the sheet-like structure of the
alumino-silicates that form the basic structure of clay
minerals.
At the surface of the clay minerals there is
a strong negative charge related to unpaired Si and O bonds.
When the clay mineral particles are exposed to an ionic solution,
one containing Na+ and Cl− for example,
• the anions (Cl-) will be repulsed by their surfaces while
the cations (Na+) will be attracted to the surface charge,
forming the so-called electrical double layer.

membrane potential
Close to the clay layers,
the fluid will be dominated by cations since
the anions are excluded by electrostatic repulsion.
In this manner, in a complex mixture of clay minerals and
other small mineral particles, with pore spaces even too small
to permit the hydraulic flow of water,
the cations will be able to diffuse along the charged surfaces, from
high concentration to low concentration while the negative Cl ions
will tend to be excluded.
Such a diffusion process will tend to accumulate a
positive charge on the low ionic concentration side of
the shale barrier, producing an attendant electric field.
In the practical situation,
the cations from the fluid saturating the porous sand zone
diffuse through the shale
to the borehole with the lower cation concentration.

evaluating the membrane potential
In this figure a
semipermeable shale
barrier separates the
solutions of two different
salinities.
A schematic
representation of the
mechanism responsible
for the generation of the
membrane potential.
The diffusion process is
altered by the selective
passage of Na+ through
the shale membrane.

magnitude of the membrane potential
The natural diffusion process is impeded
because of the negative surface charge of the shale.
The Cl ions which otherwise would diffuse more readily
are prevented from traversing the shale membrane,
whereas the less mobile Na ions
can pass through it readily.
The result is that the effective mobility of the chlorine in
this case is reduced to nearly zero.
magnitude of the membrane potential Vm

SP Measurement
In the case of lower NaCl concentration in the mud,
the voltages add, resulting in
a more negative voltage in front of the sand
than in front of the shale zone.
The membrane potential provides
about 4/5 of the SP amplitude,
since the absolute value of mobilities
enters in its potential,
rather than the difference
as in the liquid-junction potential.
The SP is measured,
between an electrode in the borehole and a distant reference.

natural potential vs.
static spontaneous potential
The shale baseline
represents the natural potential
between the two electrodes,
without electrochemical effects, and
is ideally a straight line from top to bottom.
The static spontaneous potential (SSP),
is the ideal SP generated by electrochemical effects
when passing from the shale to a thick porous clean
(shale-free) sand if no current flowed.

potential drop
In practice the electrode can only measure the
potential change in the borehole.
Although the mud is usually less resistive
than the formation,
the area for current flow is much smaller
in the borehole than in the formation,
so that the borehole resistance is usually much higher
than the formation resistance.
Most of the potential drop therefore
takes place in the borehole with the result that
the measured SP amplitude in the center of the bed
is close to the SSP.

The determination of Rw
In the best of cases,
the measurement of the SP allows
the identification of permeable zones and
the determination of formation water resistivity.
Since the mud filtrate resistivity can be measured,
the formation water resistivity
can be calculated using factors
that are well known for NaCl solutions.
A deflection indicates that
a zone is porous and
permeable and
has water with a different ionic concentration
than the mud.

effective water resistivities
vs. actual resistivities
In practice the electrochemical potential is often
written in terms of effective water resistivities (Rmf e)
and (Rwe) rather than actual resistivities.
These are equal to Rmf and Rw
except for concentrated or dilute solutions.
In concentrated solutions,
below about 0.1 ohm- m at 75◦ F,
the conductivity is no longer proportional to
the number density of charge carriers and their mobilities.
• At high concentrations the proximity of the ions to one another is
increased; their mutual attractions begin to compete with the
solvation to reduce their mobilities.
In dilute solutions of most oilfield waters,
other ions than Na+ Cl− become increasingly important.
Numerous charts exist for the determination of Rw from the SP,
knowing Rmf and temperature.

Other applications of SP log
The SP is also used to indicate the amount of clay in a
reservoir.
The presence of clay coating the grains and throats of the
formation will impede the mobility of the Cl anions
because of the negative surface charge, and
thus spoil the development of the liquid-junction potential.
The ideal SP generated opposite a shaley sand when no
current flows is known as the pseudo static potential (PSP).
In addition to these quantitative interpretations,
elaborate connections have been established
between the shape of the SP over depth and
geologically significant events.
Some examples of using the SP curve to determine patterns
of sedimentation are given in Pirson [10].

SP vs. other logging techniques
The measurement of the SP is probably the
antithesis of the high-tech image of many of the
other logging techniques.
The sensor is simply an electrode
(often mounted on an insulated cable,
known as the “bridle,”
some tens of feet above any other measurement
sondes) which is referenced to ground at the surface.
The measurement is essentially a dc voltage
measurement in which it is assumed that unwanted
sources of dc voltage are constant or only slowly varying
with time and depth.

shale and clean sand beds along with
the idealized response of SP logging
deflections to the left correspond to
increasingly negative values.
In the first sand zone,
there is no SP deflection
since this case represents equal salinity
in the formation water and in the mud
filtrate.
The next two zones
show a development of the SP which is
largest for the largest contrast in mud
filtrate and formation water resistivity.
In the last zone,
the deflection is seen to be to the right
of the shale baseline and corresponds
to the case of a mud filtrate which is
saltier than the original formation fluid.

several cases of SP log
for a given contrast in Rmf & Rw
It illustrates several cases,
for a given contrast in mud filtrate
salinity and formation water salinity,
 where the SP deflection
will not attain the full value seen in
a thick, clean sand.
The first point is that
 the deflection will be reduced
if the sand bed is not thick enough
 because not enough of the potential drop
occurs in the borehole.
 The transition at the bed boundary is much
slower for the same reason.
 Depending mainly on the depth of
invasion and the contrast between
invaded zone and mud resistivity,
 the bed thickness needs to be
more than 20 times the borehole diameter
to attain its full value.

several cases of SP log
for a given contrast in Rmf & Rw
The second point is the effect of clay
in reducing the SP.
The third point is the effect of oil or
gas.
In a clean sand the electrochemical
potentials are not affected by oil or gas,
but the formation resistivities are higher
so that the transition at bed boundaries
may be slower and a thicker bed may be
needed for full SP development.
However, the effect of oil or gas is
stronger in a shaley sand.
The electrochemical potentials are
reduced compared to a water-bearing
sand because there is less water in the
pore space, so that the effect of the
surface-charged clay particles is
proportionately higher.

Other effects
which can also upset the SP
electrical noise, and bimetallic currents between
the different metal parts of a logging tool that
can create an unwanted potential at the SP electrode.
the electrokinetic, or streaming, potential
caused by the higher pressure in the borehole moving
cations through a cation-selective membrane.
The membrane may be
a shale that has some very small permeability (Esh 3.8),
or the mudcake which contains a large percentage of
clay particles and
also has some very small permeability (Emc).

Other effects
which can also upset the SP (Cont.)
Normally these effects are small and balance each
other out.
However, when the pressure differential is high,
or the mud and other resistivities are high enough that
even a small current produces a large potential, the
electrokinetic effect can be comparable to the
electrochemical effect.
The baseline often drifts slowly with time and depth.
Sharper shifts occur when the membrane potential at
the top of a sand is different to that at the bottom.
This happens when the top and bottom shales have different
cation selection properties, and also when the formation
water or hydrocarbon saturation changes within the sand.

summary SP curve behavior under a
variety of logging circumstances
Finally, the symmetric
responses of SP logs
can be upset by vertical
movement of mud
filtrate in high
permeability sands:
upwards in the
presence of heavier
saline formation water,
and
downwards in the
presence of gas and
light oil.

1. Ellis, Darwin V., and Julian M. Singer, eds. Well
logging for earth scientists. Springer, 2007.
Chapter 3

1. Early Electric Log Interpretation
2. Formation Factor
3. Water saturation
4. The Porosity Exponent, m
5. The Saturation Exponent, n
6. A Thought Experiment For A Logging
Application

Q922+log+l05 v1

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