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Asst. Lecturer: Amir I. AbdelazizHelwan University
Asst. Lecturer: Amir I. AbdelazizHelwan University
Tools
Electrodes
A galvanometer
Small 1.5 V battery
3
Spontaneous potential
Galvanometer: Record the difference in
voltage between a moving electrode in the
borehole and a reference electrode at the
surface usually located at the mud pit.
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
Facies5
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
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
7
Factors affect the Sp
1. The Rmf / Rw ratio
2. Fresh mud Rmf < Rw - Ve SP
3. Saline mud Rmf > Rw + Ve
4. If Rmf = Rw No SP deflection
8
ESP = -K log (Rmf) /(Rw)
ESP = -K log (Rmf) /(Rw)
• Kc = (61+0.133 T (f) )
• Kc = (65 + 0.24 T (c ) )
• 1 C = 33.8 F
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.
9
shale and clean sand beds alongwith
the idealized response of SP loggingdeflections to the left correspond to
increasingly negative values.In the first sand zone,
there is no SP deflectionsince this case represents equal salinityin the formation water and in the mud filtrate.
The next two zonesshow 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 correspondsto the case of a mud filtrate which is saltier than the original formation fluid.
10
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.
Static sp: (ssp)
The theoretical maximum deflection of s.p opposite permeable beds is called static s.p or
ssp. It is maximum possible s.p opposite a permeable water bearing formation with
no shale.
Pseudo SP: (PSP)
Any deflection less than (SSP)
11
12
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)
13
summary SP curve behavior undera
variety of logging circumstances
Finally, the symmetricresponses 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.
Application of SP log
Application of SP log
Fres
h w
ater
san
d
Rw
= 1
0 O
hm
T =
50
c
Fres
h w
ater
Rw
= 1
Oh
mT
= 6
0 c
Salt
wat
er R
w=
0.0
5 O
hm
T =
70
c
Fresh mud Rmf = 1 Ohm Fresh mud Rmf = 1 Ohm
Q2-(B) A number of factors affects the shape and amplitude of SP log as Rmf/Rw
ratio, bed thickness, bed resistivity and Porosity. If there is a succession of shale and sand in
a well where the Rw was = 10 ohm in the sand layer (1) and Rw= 0.5 at sand layer (2) and
Rmf = 1 ohm at Temp. 60 0C and fresh water exists, Calculate
1- Kc and Esp
2- Show the deflection direction of SP log.
-SP+
Shale
Sand (2)
Shale
Sand (1)
Application of SP log
Application of SP log
General Log responses
Lithology DensityGR ResistivityAcousticNeutron
Sandstone 2.65Low(Unless RA min)
high55-4
Salt 2.1Low(Unless K salt)
V.high670
Anhydrite 2.95V.low V.high50-1
Dolomite 2.85Low(higher if U)
high42.5+4
Shale 2.2-2.7(water content)
high low (water content)
50-150(water content)
High(water content)
Limestone 2.71low high47.50
Gas 0.2-0.5(pressure)
0 V.high~100010-50(H2 index)
Oil 0.6-1.0(api)
0 V.high210-240 (api)
70-100 (H2 index)
Water 1-1.1(salt and Temp)
0 0 - infinite(salt and Temp)
180-190100