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Lecture items
Self Potential Log
* Theory of measurement.
-Shale-base line& Sand line
-SSP, PSP and SP log readings
* Factors affecting on log readings.
* Applications.
- Resistivity Logs
* Definition.
* Types
* Units& Presentation.
* Theories of measurement.
* Factors affecting on log readings.
* Applications.
1- Electrokinetic Potential (can be neglected)
2- Electrochemical Potential
1) Membrane Potential
2) Liquid Junction Potential
Factors affecting SP log measurements
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Rmf/Rw (Salinity effect) Fresh mud: negative SP, Saline
mud: positive SP.
Shale or clay content Shale reduces SP.
Permeability
Presence of hydrocarbon
Bed thickness: SP decreases when bed thickness
decreases.
Invasion: Reduces SP.
Mud filtrate: The magnitude and direction of SP
deflection from the shale baseline depends on relative
resistivities of the mud filtrate and the formation water.
Resistive formations
Clean
Laminated
Structural
Dispersed
Rw
calculation
from SP
Mathematical Calculation of Rw from SSP (modified after Bateman & Konen, 1977)
Rmf at 75oF = Rmf temp* x (temp + 6.77)/81.77
Correction of Rmf to 75o
K = 60 + (0.133 x Tf)
Rmfe / Rwe = 10 – SSP / K**
Rmfe = (146 x Rmf – 5) / (337 x Rmf + 77)
Rmfe formula if Rmf at 75oF < 0.1
Rmfe = 0.85 x Rmf
Rmfe formula if Rmf at 75o > 0.1
Rwe = Rmfe / (Rmfe / Rwe)
Rw at 75oF = (77 x Rwe + 5) / (146 – 377 x Rwe)
Rw at 75o formula if Rwe < 0.12
Rw at 75oF = - [0.58 – 10 (0.69 x Rwe –0.24)]
Rw at 75oF formula if Rwe > 0.12
Rw at formation temperature = Rw at 75o x 81.77 / (Tf + 6.77)
*Rmftemp = Rmf at a temperature other than 75oF
**The e subscript (i.e. Rmfe) stands for equivalent resistivity.
Applications

Differentiation between shaly, clean and shale
zones.
 Differentiation between Permeable and nonpermeable zones.
 Calculation of Rw.
 Determination of the volume of shale.
 For correlation purposes
 For sedimentological analysis and facies
studies.
Notice how the shale baseline shows a
distinctive drift with depth. This
characteristics is commonly caused by an
increases in relative oxidation of the
rocks that are close to the land surface.
The highest sandstone in the well has a
muted deflection on the SP log as
compared with the lower sandstones.
This contrast is an immediate indication
that water in the upper sandstone may be
significantly fresher than waters of the
lower sandstone. In other wells it is not
uncommon to see sandstone units where
the SP deflection goes to the right of the
shale baseline. In these instances, the
drilling mud filtrate is salter than the
formation water.
A
good example of this
phenomenon is shown in the
figure attached. In the upper
sandstone, "U", the SP log
shows a deflection to the
right, indicating formation
water to be fresher than the
drilling mud, while in the
lower sandstone, "L", the
deflection is to the left,
showing the formation water
to be more saline.
Flow chart from oil-industry
log analysis to estimate
formation water resistivity,
Rw, in deep formations from
the SP log. RMF is mud
filtrate resistivity measured at
temperatureTmf and recorded
on the log header; Tf is the
temperature of the formation,
generally
estimated
by
interpolating between the
bottom-hole
temperature
(BHT) at total depth (TD) and
mean annual temperature at
the surface; SSP is the static
self-potential measured on the
log between the "clean line"
and "shale line" in millivolts
(mv) and with associated sign
(positive or negative).