chapter-3 sp caliper
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Chapter-3SP Log – Self Potential
By
Dr. Jorge Salgado Gomes
1Chap-3 Duration of this chapter: 2 classes (90’)9/21/2011
sandstone
shale
Na+
Na+
Na+
Na+ Cl-
Cl-
Cl--
-
-
+
+
+
Cl-
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Educational Outcomes
• What is self potential
• Electrochemical potential
• SP interpretation
• SP applications (sand-shale sequences)
• Factors affecting SP amplitude
• Determination of Rw (water resistivity)
Chap-3 29/21/2011
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Principle of SP (Self Potential)
9/21/2011 Chap-3 3
by Lecturer
The SP is measured mostly simultaneous
with the resistivity log.
The SP is the natural electrical potential –
in our case – in a borehole.
SP is useful particularly for
· sand - shale discrimination, shale content
· Rw calculation - we need it for saturation
calculation
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Electrochemical Potential (Ec)
EC = EM + ELJ
Membrane Potential Liquid Junction Potential
Membrane potential is the migration of cations(Na+) through shale membrane.
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Schematic SP Resistive Circuit
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Schematic SP Current Loop
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SCHEMATIC REPRESENTATION of
POTENTIAL & CURRENT
DISTRIBUTIONin and around a permeable bed
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9/21/2011 Chap-3 8
mf
w
mf
wSPMD
C
C
C
CKEESSP log71log
mf
w
mf
wDD
w
mf
mf
wD
C
C
C
CKE
R
R
uv
uv
C
C
uv
uvE
log6.11log
lnF
RTln
F
RT
mf
w
mf
wMM
w
mf
mf
wM
C
C
C
CKE
R
R
C
CE
log1.59log
lnF
RTln
F
RT
Static SP
R - ideal gas constant
T - absolute Temperature
F - Faraday constant
Cw - formation water concentration
Cmf - mud filtrate concentration
Rw - formation water resistivity
Rmf - mud filtrate resistivity
u - mobility of Cl (67.6 10-5 cm/sV)
v - mobility of Na (45.6 10-5 cm/sV)
Liquid Junction (diffusion) and Membrane Potential
at 77°F (25°C):
at 77°F (25°C):
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SP: Result of Electrochemical Interaction
9/21/2011 Chap-3 9
sandstone
shale
concentration
Na+
Na+
Na+
Na+ Cl-
Cl-
Cl- -
-
-
+
+
+
- SP in mV +
Diffusion-
potential
Membrane-
potential
Cl-
-160 + 40
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Example
9/21/2011 Chap-3 10
Step 2:
draw „sand base line“
Step 1:
draw „shale base line“
Step 3:
select reservoirs,
describe the profile
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TWOEXAMPLES OF
SPBASE-LINE
SHIFT
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SP (Main Observations)
• SP results from conductivity differences between formation water and mud salinity.
• SP-log can be used to:– detect permeable beds
– separate sand and shale
– determine formation water resistivity Rw
• In permeable beds, SP has the following response relative to the shale baseline:– negative deflection where Rmf > Rw
– positive deflection where Rmf < Rw
– no deflection where Rmf = Rw
• SP is suppressed at presence of oil/gas and by thin beds.
9/21/2011 Chap-3 12
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Factors Affecting SP Amplitude
• OBM & AF boreholes
• Clay/shale in the formation
• Hydrocarbon zones
• Bed resistivity
• Formation thickness
• Invasion
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EFFECT OF Rmf/Rw on SP development
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Shape and Amplitude of SP Response
• The shape of the SP curve and the amplitude of the deflection opposite a permeable bed depend on several factors:– Ratio of Rmf/Rw
– Thickness (h) and true resistivity (Rt) of the bed
– Rxo and diameter of the invaded zone (di)
– Resistivity of the adjacent formation
– Resistivity of the mud (Rm) and borehole diameter
• The following slides show examples of these factors
9/21/2011 Chap-3 15
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EFFECT OF SHALINESS ON SP
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EFFECT OF PERMEABLE
BED THICKNESS ON RECORDED
SP
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EFFECT OF VARYING SHALE THICKNESS ON
SP
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EFFECT OF BED RESISTIVITY ON SP
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EFFECT OF INVASION
ON SPwhenRi = Rt
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SP IN RESISTIVE FORMATIONS
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RESPONSE OF SP TO TIGHT ZONES
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Quality Control SP
9/21/2011 Chap-3 23
Should be recorded noise-free as possible
Repeatability: curves should have the same shape and
character as those from previous runs or repeated sections – if
same conditions with respect to mud and no streaming potential.
Cross-check the curve character with other logs from the same
logging run.
Adapted after Krygowski, 2004
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DETERMINATION OF Rw
FROM SP
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SSP Equation & Conditions for RwDetermination
• Clean formation
• Thick formation
• Salinity contrast
we
mfe
SP
mf
wSP
R
RK
C
CKSSP loglog
Where : KSP = 61 + 0.133T (0F)
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Determination of Rweq
INPUT:
SSP = 100 mV @ 250 F
Rmf = 0.7 Ω .m @ 100 F
From Chart Gen-9:
Rmf= 0.33 Ω .m @ 250 F
•If Rmf @ 75 F > 0.1 Ω, then Rmfeq = 0.85 Rmf
•If Rmf @ 75 F < 0.1 Ω , then use Chart SP-2
Output:
Rmfeq = 0.85 * Rmf
Rmfeq @ 250 F = 0.85 * 0.33 = 0.28 Ω . m
Rweq @ 250 F =0.025 Ω .m
9/21/2011 Chap-3 26
weq
mfeq
SPR
RKSSP log
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9/21/2011 Chap-3 27
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Compute Rw from Rweq
If Rweq = 0.025 Ohm.m
@ 250 F
Rw = 0.03 Ohm.m
9/21/2011 Chap-3 28
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9/21/2011 Chap-3 29
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CALIPER LOGS
9/21/2011 Chap-3 30
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Caliper Logs
9/21/2011 Chap-3 31
by Lecturer
Caliper Measurements
2-Arm Calipers
3-Arm Calipers
4-Arm Calipers
6-Arm Calipers
borehole condition (breakouts --> mechanical behaviour)
formation properties (mud cake --> permeable zones,
fractured zones)
borehole volume --> cementation
corrections for quantitative log interpretation
Why ?
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Caliper Log
9/21/2011 Chap-3 32
Source: Baker 2002
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Caliper Measurements
9/21/2011 Chap-3 33
Single axis Three arm
Dual Axis (x,y) Four or six arm (individual radii)
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Interpretation of Caliper Data(Borehole Breakout & Key Seat)
9/21/2011 Chap-3 34
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Geomechanical Information(Relationship between stress fields and
borehole Shape)
9/21/2011 Chap-3 35
Fig 3a Fig 3b
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Quality Control Caliper
9/21/2011 Chap-3 36
Check the caliper value in casing against the casing diameter
Shale values should be similar to those in nearby wells
Repeatability: curves should have the same shape and character as those
from previous runs or repeated sections
Cross-check the curve character with other logs from the same logging run.
Adapted after Krygowski, 2004
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Additional Support Material
9/21/2011 Chap-3 37