Presentation-Sami ullah 01-161091-082.pptx
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SPONTANOUS
POTENTIAL
LOGS
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TYPES OF ELECTRIC LOGS
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ElectricLogs
ResistivityLogs
InductionLogs
S.P. LogsPorosity
Logs
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INTRODUCTION OF S.P
Natural currents flowing within the earth causea voltage to be produced.
The spontaneous potential log,
commonly called the SELF POTENTIAL log or SP
log, is a passive measurement taken by oil industry
well loggers to characterise rock formation
properties.
The log works by measuring small electricpotentials (in millivolts) between depths in the
borehole and a grounded voltage at the surface.
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BASIC PRINCIPLE
Usually while drilling, drilling fluid invade the formation,changes in the resistivity are measured by the tool in the
invaded zone.
If water based mud is used and oil is displaced,
"deeper" resistivity logs (or those of the "virgin zone")
will show lower conductivity than the invaded zone. If oil based mud is used and water is displaced, deeper
logs will show higher conductivity than the invaded
zone.
This provides not only an indication of the fluids
present, but also, at least quantitatively, whether theformation is permeable or not.
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METHODOLOGY Spontaneous potential is a DC voltage.
Voltage is measured between a ground electrode atthe surface and an electrode lowered down the well.
When formation water is saltier than the drilling mud,shale has a positive SP response and sand a negativeSP response.
Specialized probes are used to connect theinstrumentation
Probes must be non-polarizing
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The SP device measures naturally occurring(spontaneous) potential in the well bore.
This tool utilizes a single moving electrode in the
borehole and a reference electrode at the surface,
usually located in the mud pit.
The SP curve therefore is a record of the potential
difference which exists between the surface electrode
and the moving electrode in the borehole.
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SP TOOLS
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1. The spontaneous potential tool measures
natural electrical potentials that occur in boreholes and generally distinguishes
porous, permeable sandstones from intervening shales.
2. The "natural battery" is formed when the use of drilling
mud with a different salinity from the formation waters, causes two solutions to be in contact that have different ion
concentrations.
3. Ions diffuse from the more concentrated solution
(typically formation water) to the more dilute. The ion flow constitutes electrical current, which generates a
small natural potential measured by the SP tool in millivolts.
S.P TOOL
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Measurement of self-potentials
• Voltage is measured b/w ground electrode andan electrode lowered down the well.
• When formation water is saltier than the drillingmud, shale has a positive SP response andsand a negative SP response.
S.P LOG
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USES OF S.P LOGS
The SP tool is one of the simplest tools and is
generally run as standard when logging a hole,
along with the gamma ray. SP data can be used to
find:
1. Where the permeable formations are
2. The boundaries of these formations
3. Correlation of formations when compared with data from
other analogue wells4. Values for the formation-water resistivity
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Applications:-
SP logs are used to distinguish shale from sand
and to estimate percent sand.
In addition, it can be used to estimate pore water salinity inclean sands.
The SP log is also used in correlation work especially inconjunction with resistivity logs.
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SPONTANEOUS POTENTIAL LOG
S.P logs comes out
like in this form.