Spontaneous Potential

SP and SPDH, why how and what?

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This is probably the first logging system on the worldThis is a picture for the basic SP idea.

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Spontaneous Potential The measurement of the spontaneous or naturally occurring voltage

potential in a formation is classified as a resistivity measurement. The SP log is a recording versus depth of the difference between the

electrical potential of a movable electrode in the borehole and the electrical potential of a fixed surface electrode.

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Spontaneous Potential Applications:

– Correlation – Identify the fresh / salt water interface – Qualitative indication of bed shaliness – Determination of formation water resistivity

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Spontaneous Potential

VGround on Surface

Borehole

Electrode

Isolated Cable

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The world has infinite electrical charges

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What is Spontaneous Potential?Natural occurring voltage or potential is caused when the conductive

drilling mud contacts the formations

Since the voltage occurs naturally, it is spontaneous

The log works by comparing the voltage on the wall of the well bore to a grounded voltage at the surface

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If we drill a hole, we have:If the zone is permeable,

There will be invasion of the mud filtrate into the formation.

Between Shale, Sand and Mud we will have DC voltages being generated.

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SP components

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Summary Three requirements for SP

1- mud or mud filtrate

2-Shale zone

3-permeable zone like sand

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Where that happens

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Curve answerIt depend of the contrast between mud filtrate and formation water.

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Spontaneous Potential

This build up of charge is, in turn, caused by differences in the salt content of the well bore fluid (drilling mud) and the connate water.

Whether the mud contains more or less salt than the connate water will determine which way the SP curve will go

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Spontaneous Potential

Mud Column

Low

Salinity

Na+CL- Shale

Permeable

High Salinity Na+Cl-

Low SP High

Negative Deflection

Low SP High

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Spontaneous Potential

Mud Column

Same

Salinity

Na+CL- Shale

Permeable

Same Salinity Na+Cl-

Low SP High

No Deflection

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Spontaneous Potential

Mud Column

High

Salinity

Na+CL- Shale

PermeableLow Salinity

Na+Cl-

Low SP High

Positive Deflection

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Spontaneous Potential

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SP Response with Gamma Ray

Use SP to define Permeable and Impermeable zone

(salinity)

Use GR to define Clay and no-Clay content

Applications

- Identify fresh / saltwater interface- Indication of bed shaliness- Determination of formation resistivity

- Correlation

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How do we measure the SP

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How we measure it

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Spontaneous Potential

VGround on Surface

Borehole

Electrode

Isolated CableRef 10

SPDH curves are measured between the tool electrode or bridle electrode and the armor of the cable; cond 10.

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End presentation