Spontaneous Potential (SP)

GAMMA RAY AND SPONTANEOUS POTENTIAL

LOGGING CORE

LEARNING OBJECTIVES

By the end of this lesson, you will be able to:

Describe the basic physics of Spontaneous Potential (SP) logs Recognize the scale and units used with the SP log Identify what two liquids most influence direction and

magnitude of the SP log Recognize how the SP curve can be used to estimate potential

reservoir rockOutline petrophysical capabilities and limitations in identifying

producing versus non-producing intervals

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Electrode

V

Isolated cable

Ground on surface

Borehole

NET/GROSS – SPONTANEOUS POTENTIAL

NET/GROSS – SPONTANEOUS POTENTIAL

The SP log is recorded byplacing a movable electrode inthe borehole and measuringthe difference between theelectrical potential of thismovable electrode and theelectrical potential of a fixedsurface electrode. Therecorded curve is scaled inmillivolts (abbreviated as mV).

SP log requires conductive(water-based) mud and cannotbe recorded in oil base mud.

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Key characteristics: SP Scale has no zero Calibrated for sensitivity SP reference is shale

baseline Looking for maximum

deflection

Affected by: Salinity contrast (Rmf & Rw) Bed thickness Tight, resistive formation

SP PRESENTED IN TRACK 1 OF API GRID

Shallow

Deep

SP is recorded not generated!

FACTORS AFFECTING SP: SALINITY CONTRAST

Most significant factor affecting the SP curve is the salinity contrast between the mud filtrate and the formation water.

The lower the salinity, the higher the resistivity of the water.

The Resistivity of mud filtrate, Rmf, and the formation water resistivity, Rw, defines the SP curve.

negative positive

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FACTORS AFFECTING SP: SALINITY CONTRAST

Direction ions move is dependent upon the relative salinities of the fluids involved:

“Negative” SP deflectionwhen Rmf>Rw

“Positive” SP deflectionwhen Rmf<Rw

If Rmf ~= Rw no SP willdevelop

Important to properly establish shale line to distinguish permeable and non-permeable zones

Relevant to Rw calculations(later)

negative positive

INTERPRETATION PROBLEMS WITH SP LOG

Permeable beds defined well in relatively thick, porous sand, shalesequences

Thinly bedded, low permeability formations are poorly resolved SP log measures differences in ionic activities (relative salinity) between

drilling mud and formation waters In salt muds, SP often useless because spontaneous potential at depths of

interest are small (Rmf Rw) Because boundary definition with low resistivity mud and high resistivity

formations is very poor SP curve can reverse under certain circumstances

SP

When Rmf Rw(May indicate highor low permeability)

SP (“Normal”)

When Rmf>Rw(May indicate highpermeability)

SP (“Reversed”)

When Rmf<Rw(May indicate highpermeability)

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SP LITHOLOGY INDICATION

Shale LineSand Line

NET/GROSS – SP AS SAND SHALE INDICATOR

The Net to Gross ratio is calculated by dividing the Net by the Gross interval.

Caution: The gross interval should be selected in consultation with the Geologist and/or Geophysicist!

Watch mud filtrate and formation water resistivities!

Shale LineSand Line

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SP SHAPE FOR DEPOSITIONAL ENVIRONMENT

Fining Upward

Coarsening Upward

SP SUMMARY

Indicator of porous, permeable intervals

Available in most wells with water-based mud

Useful for a number of things: Field-wide correlation Net/Gross estimation Vshale estimation Rw calculation (discussed later)

Number of factors can affect the response and interpretation Hydrocarbon saturation Salinity contrast between mud filtrate and formation water resistivity

“Last resort” method of shale volume estimation, and Rw calculation

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LEARNING OBJECTIVES

Describe the basic physics of Spontaneous Potential (SP) logsRecognize the scale and units used with the SP log Identify what two liquids most influence direction and magnitude

of the SP logRecognize how the SP curve can be used to estimate potential

reservoir rockOutline petrophysical capabilities and limitations in identifying

producing versus non-producing intervals

═════════════════════════════════════════════════════════════════════════Gamma Ray and Spontaneous Potential Logging Core

© PetroSkills, LLC., 2016. All rights reserved.7_____________________________________________________________________________________________

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