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Basic well Logging Analysis

The Spontaneous Potential (SP) Log

Hsieh, Bieng-Zih

Fall 200911The 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.22The 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.33The 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.

44The 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.5

5The 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.6

6SP 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.88Functions of SP log

99Static 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. 1010Shale 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. 11

11Resistivity of Formation Water (Rw) Calculates from the SP Curve1212Resistivity of Formation Water (Rw) Calculates from the SP Curve13Step 0:Setup Shale-base-lineread SP log recording and 16 Normal Resistivity log reading every 2 ft from 7430 to 7460

13Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont.)14Step 1:determine the formation temperature (Tf)

14Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont.)15Step 2:correct the resistivities of the mud filtrate (Rmf) and drilling mud (Rm) to formation temperature (Tf)

15Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont.)16Step 3:Determine the SP reading (in water-bearing zone or wet-formation)

16Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont.)17Step 4:Correct SP to SSP

17Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont.)18Step 5:Determine Rmf/Rwe ratio

Step 6:Determine Rwe

18Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont.)19Step 7:Correct Rwe to Rw

19Volume 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.20

20Calculate Vsh from the SP Curve21Use SP log recording (PSP) every 2 ft from 7430 to 7460

SSP is from step 4

21Review SP LogThe 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 .22Review 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.23