4. What is SP ?
Called self or spontaneous potential .
SP is the naturally occuring electrical potential
difference in the Earth .
Spontaneous potentials (SP) are usually caused by
charge separation in clay or other minerals, due to
presence of semi-permeable interface impeding the
diffusion of ions through the pore space of rocks, or by
natural flow of a conducting fluid through the rocks.
5.
A log of the natural difference in electrical
potential, in millivolts, between an electrode in the
borehole and a fixed reference electrode on the
surface.
It is very simple log .
6.
A conductive borehole fluid (i.e., a water based
mud).
A sandwich of a porous and permeable bed
between low porosity and impermeable formations.
A difference in salinity between the borehole fluid
and the formation fluid, which are the mud filtrate
and the formation fluid in most cases.
7.
8. Electrokinetic potential
Arise from the
movement of
electrically charged
ions in the fluid
relative to the the fixed
rock.
Electrochemical potential
Arise from electrical
interactions between
the various chemical
constituents of the
rocks and fluids.
9. Diffusion Potential
The diffusion potential (sometimes called the
liquid-junction potential).
This potential exists at
the junction between the invaded and the
non-invaded zone, and is the direct result of the
difference in salinity between the mud filtrate and
the formation fluid.
10. The membrane potential (sometimes called the
shale potential).
This potential exists at the
junction between the non-invaded zone and the
shale (or other impermeable rock) sandwiching
the permeable bed.
Na+ ions penetrates through the shale from the
saline formation water to the less saline mud
column, a potential is set up.
11.
The shale wall potential. This potential is the
same in origin to the mud cake potential, but
applies to the flow of fluids from the borehole into
shale formations. It is usually very small because
the flow into impermeable shale is small.
The mud cake potential. This potential is produced
by the movement of charged ions through the mud
cake and invaded zone in a permeable formation.
12.
The spontaneous potential produced in bore hole are actual
result of these two processes combine.
The mud filtrate is less saline than 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 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
13.
The tool is extremely simple.
SP log consists of an electrode that is suspended in
the bore hole.
Another electrode that fixed on the surface.
A voltmeter that measure the potential difference
b/w bore hole electrode and ground electrode.
14.
15. The flow of current is focused at the bed junction. It
is only here that there is a change in potential. This
is important since SP measurements are made not of
absolute values, but of changes in values. It is only at
the bed Junctions, then the changes take place and
will be recorded.
If a bed is not permeable, ions will not be able to
move. There will be no current flow and thus no
potential change: that means no SP. However, even
the slightest permeability will permit current low and
an SP change will be recorded.
16.
Good earth is difficult to find .
Sometimes fixed electrode is attatched to rig
legs which is a big mistake.
Because
The operation of sea waves on the rig legs
induces potentials in the rig legs that give
rise to wavy patterns in the recorded SP log
which cannot be removed.
17.
18. SP is shown in millivolts in Track 1.
With negative deflections to the left and positive
ones to the right.
In reading the SP log it is best to first define a shale
base line.
This is the typical SP level for shales
shale base line can be found by comparing the SP
log with the GR log response.
Permeable formation will have then variable intensity
to left or right of this Shale base line .
20.
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.
23.
When mud filtrate salinities are lower than
formation water i.e Rmf>Rw so SP deflection
will be to right called normal or negative
response.
When mud filtrate salinities are more than
connate water i.e Rmf<Rw . SP response
deflect to right this is called positive or
reverse response .
When Rmf=Rw so there is no Sp response .
24.
25.
If layer is thick ,deflection is maximum
If layer is thin ,the curve is recorded as
narrow peak .
28. The SP log is an extremely useful quick-look
indicator of bed permeability.
It is not quantitative
We can not associate very large permeabilities
necessarily with large deflections and vice
versa.
However, the SP log is quite sensitive, and even
a small deflection in the SP log indicates that
the bed has reasonable permeability.
It should be noted that some permeable beds
might give no deflection, such as those where
there is no difference in salinity between the
formation fluids and the mud filtrate.
29.
30.
31.
The SP log is sometimes a useful additional
log to use in correlation, but is rarely used
alone.
If used then, wells should be drilled with the
same mud, and the salinities in the
formations should be constant between wells.
The SP log can be used to follow facies
changes. However, it has been largely
replaced by the GR log which has a higher
resolution and is more reliable
32. 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
33. It is considered that the volume of
shale
in
a
water-wet,
shaley
sandstone can be simply calculated
using the SP as follows:
