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-

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

Principle of SP (Self Potential)

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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

Electrochemical Potential (Ec)

EC = EM + ELJ

Membrane Potential Liquid Junction Potential

Membrane potential is the migration of cations(Na+) through shale membrane.

Schematic SP Resistive Circuit

Schematic SP Current Loop

SCHEMATIC REPRESENTATION of

POTENTIAL & CURRENT

DISTRIBUTIONin and around a permeable bed

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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):

SP: Result of Electrochemical Interaction

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sandstone

shale

concentration

Na+

Na+

Na+

Na+ Cl-

Cl-

Cl- -

-

-

+

+

+

- SP in mV +

Diffusion-

potential

Membrane-

potential

Cl-

-160 + 40

Example

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Step 2:

draw „sand base line“

Step 1:

draw „shale base line“

Step 3:

select reservoirs,

describe the profile

TWOEXAMPLES OF

SPBASE-LINE

SHIFT

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.

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Factors Affecting SP Amplitude

• OBM & AF boreholes

• Clay/shale in the formation

• Hydrocarbon zones

• Bed resistivity

• Formation thickness

• Invasion

EFFECT OF Rmf/Rw on SP development

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

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EFFECT OF SHALINESS ON SP

EFFECT OF PERMEABLE

BED THICKNESS ON RECORDED

SP

EFFECT OF VARYING SHALE THICKNESS ON

SP

EFFECT OF BED RESISTIVITY ON SP

EFFECT OF INVASION

ON SPwhenRi = Rt

SP IN RESISTIVE FORMATIONS

RESPONSE OF SP TO TIGHT ZONES

Quality Control SP

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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

DETERMINATION OF Rw

FROM SP

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)

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

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weq

mfeq

SPR

RKSSP log

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Compute Rw from Rweq

If Rweq = 0.025 Ohm.m

@ 250 F

Rw = 0.03 Ohm.m

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CALIPER LOGS

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Caliper Logs

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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 ?

Caliper Log

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Source: Baker 2002

Caliper Measurements

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Single axis Three arm

Dual Axis (x,y) Four or six arm (individual radii)

Interpretation of Caliper Data(Borehole Breakout & Key Seat)

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Geomechanical Information(Relationship between stress fields and

borehole Shape)

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Fig 3a Fig 3b

Quality Control Caliper

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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

Additional Support Material

9/21/2011 Chap-3 37