Basic well Logging Analysis – The Spontaneous Potential (SP) Log

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BASIC WELL LOGGING ANALYSIS –

THE SPONTANEOUS POTENTIAL (SP) LOG

Hsieh, Bieng-Zih

Fall 2009

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THE SPONTANEOUS POTENTIAL (SP) LOG

The spontaneous potential (SP) log was one of the earliest electric logs used in the petroleum industry, and has continued to play a significant role in well log interpretation.

By far the largest number of wells today have this type of log included in their log suites.

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THE SPONTANEOUS POTENTIAL (SP) LOG (CONT.)

Primarily the spontaneous potential log is used to identify impermeable zones such as shale, and permeable zones such as sand. However, the SP log has several other uses

perhaps equally important.

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THE SPONTANEOUS POTENTIAL (SP) LOG (CONT.)

The spontaneous potential log is a record of direct current (DC) voltage differences between the naturally occurring potential of a moveable electrode in the well bore, and the potential of a fixed electrode located at the surface (Doll, 1948).

It is measured in millivolts.

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THE SPONTANEOUS POTENTIAL (SP) LOG (CONT.)

Electric currents arising primarily from electrochemical factors within the borehole create the SP log response.

These electrochemical factors are brought about by differences in salinities between mud filtrate (Rmf) and formation water resistivity (Rw) within permeable beds.

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THE SPONTANEOUS POTENTIAL (SP) LOG (CONT.)

Because a conductive fluid is needed in the borehole for the SP log to operate, it cannot be used in non-conductive (i.e. oil-based) drilling muds.

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SP DEFLECTION

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FUNCTIONS OF SP LOG

The SP log is recorded on the left hand track of the log in track #1 and is used to:

(1) detect permeable beds, (2) detect boundaries of permeable beds, (3) determine formation water resistivity

(Rw), (4) determine the volume of shale in

permeable beds.

An auxiliary use of the SP curve is in the detection of hydrocarbons by the suppression of the SP response.

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FUNCTIONS OF SP LOG

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STATIC SPONTANEOUS POTENTIAL (SSP)

The concept of static spontaneous potential (SSP) is important because SSP represents the maximum SP that a thick, shale-free, porous and permeable formation can have for a given ratio between Rmf /Rw .

SSP is determined by formula or chart and is a necessary element for determining accurate values of Rw and volume of shale.

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SHALE BASE LINE

The SP response of shales is relatively constant and follows a straight line called a shale baseline.

SP curve deflections are measured from this shale baseline.

Permeable zones are indicated where there is SP deflection from the shale baseline.

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE Step 0: Setup Shale-base-

line read SP log

recording and 16” Normal Resistivity log reading every 2 ft from 7430 to 7460

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE (CONT.) Step 1: determine the

formation temperature (Tf)

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE (CONT.) Step 2: correct the

resistivities of the mud filtrate (Rmf) and drilling mud (Rm) to formation temperature (Tf)

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE (CONT.) Step 3: Determine the SP

reading (in water-bearing zone or wet-formation)

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE (CONT.) Step 4: Correct SP to SSP

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE (CONT.) Step 5: Determine Rmf/Rwe

ratio

Step 6: Determine Rwe

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RESISTIVITY OF FORMATION WATER (RW) CALCULATES FROM THE SP CURVE (CONT.) Step 7: Correct Rwe to Rw

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VOLUME OF SHALE CALCULATION

The SP log can be used to calculate the volume of shale in a permeable zone by the following formula:

The volume of shale in a sand can be used in the evaluation of shaly sand reservoirs.

SSPPSP

0.1Vsh

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CALCULATE VSH FROM THE SP CURVE Use SP log recording

(PSP) every 2 ft from 7430 to 7460

SSP is from step 4

SSPPSP

0.1Vsh

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REVIEW – SP LOG The spontaneous potential log (SP) can be

used to: (1) detect permeable beds; (2) detect boundaries of permeable beds; (3) determine formation water resistivity (Rw); and (4) determine volume of shale (Vsh) in a permeable bed.

The variations in the SP are the result of an electric potential that is present between the well bore and the formation as a result of differences in salinities between Rmf and Rw .

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REVIEW – SP LOG (CONT.) The SP response in shales is relatively

constant and its continuity of amplitude is referred to as the shale baseline.

In permeable beds the SP will do the following relative to the shale baseline: (1) negative deflection to the left of the shale baseline where Rmf ＞ Rw ; (2) positive deflection to the right of the shale baseline where Rmf ＜ Rw ; (3) no deflection where Rmf ＝ Rw .

The SP curve can be suppressed by thin beds, shaliness, and the presence of gas.