spontaneous potential logging

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Spontaneous Potential MeasurementA-02

Spontaneous Potential

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SP Theory 1

SP results from electric currents flowing in the drilling mud.

There are three sources of the currents, two electrochemical and one electrokinetic.

Membrane potential - largest.

Liquid - junction potential.

Streaming potential (electrokinetic)which is the smallest.

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SP theory 2Membrane and Liquid PotentialThese two effects are the main components of the SP. They are caused because the mud flitrate and the formation waters contain NaCl in different proportions.

The ions Na+ and Cl- have different mobilities at the junction of the invaded and virgin zones. The movement of the ions across this boundary creates another current and hence a potential.

Firstly, shales are permeable to the Sodium ions but not the Chlorine. Hence there is a movement of charged particles throughthe shale creating a current and thus a potential.

Electrochemical membrane potential of SP

Electrochemical Liquid-Junction potential of SP

Em = K log ------amf

aw

Ej= K’ Log ------amf

aw

SHALE

SHALE

Sand

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SP theory 3

Electrokinetic or Streaming Potential

This is generated by flow of the mud filtrate through the mud cake. As this does not normally occur this effect is small. It will only become important if there are high differential pressures across the formations.

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SP theory 4Spontaneous Potential (SP)

SP Circuit Path

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SP theory 5

SSP = -K log RmfeRwe

Rmf = Rw Rmf <RwSALINE MUD

Rmf >RwFRESH MUD

The SP Deflection and its Rmf-Rw Dependency

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SP theory 6

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SP scales

The SP is measured in millivolts, mV.

The scale on the log shows a number of mV per division for example 20mV/division. This gives a total for the track of 200mV.

The scale across the track is variable and depends on the conditions in the well.

The scale is set during logging to have the SP curve in the track over the zone of interest and as much of the rest of the log as possible.

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log-1

The SSP is the quantity to be determined.

It is the deflection seen on the SP from the Shale Base Line (zero point) to the Sand Line (max. deflection)

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example log 2

The maximum SP deflection in this example occurs at the same depths as the resistivity curves show a separation.The minimum point on the SP corresponds to where all the resistivity curves overlay, no invasion, a shale.

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SP uses

Differentiate potentially porous and permeable reservoir rocks from impermeable clays.

Define bed boundaries.

Give an indication of shaliness (maximum deflection is clean; minimum is shale).

Determine Rw in both salt and fresh muds.

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Rw from the SP

Rw is often known from client information or local knowledge.

The SP can be used to check the value or compute it when it is unavailable.

It is especially useful when there are variations along the borehole.

K is a constant - depending on the temperature.

we

mfe

RR

kSSP log−=

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Rw from the SP1•Chart SP-1, Outputs Rweq

Rweq

•Enter SSP & FT, you get Rmfeq/Rweq ratio

•Pass through Rmfeq to define the Rweq

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Rweq

RW

Rw from SP 2

Chart SP-2 output is Rw.

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Rw from SP

Rmfeq Determnation

For Nacl muds,

a) If Rmf at 75 degF > 0.1 ohm-m Correct Rmf to FT using Chart Gen-9 and use Rmfeq=0.85 Rmf

b) If Rmf at 75 DegF < 0.1 ohm-m, Use chart SP-2 to derive Rmfeq at Formation temp.

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Salinities chart

This chart is used to compute salinities fromresistivities of solution e.g. mud, and vice versa.It is also used to find the resistivities at a given temperature.

0.1

0.2

0.3

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0.6

0.8

1

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8

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0.08

0.060.05

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0.01

50 75 100 125 150 200 250 300 350 400

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250300

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200025003000

40005000

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15,00020,000

280,000

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250,000 200,000 170,000 140,000 120,000 100,00080,000 70,000 60,000 50,00040,00030,000

300,000

10 20 30 40 50 60 70 140 160 20080 90 100 120 180

ppm

Gra

ins/

gal

at 7

5ÞF

Res

istiv

ity o

f Sol

utio

n (ž

- m

)

Temperature (ÞF or ÞC)

NaC

l Con

cent

ratio

n (p

pm o

r gra

ins/

gal)

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SP borehole Effects - 1

Baseline shifts:

These can occur when there are beds of different salinities separated by a shale which does not act as a perfect membrane.

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SP Borehole Effects - 2

Resistive formation:The presence of a resistive bed in a permeable interval will disrupt the SP deflection. The current is contained and hence the potential drop changes with depth. The log takes a sloped appearance.

The log in this situation can no longer define the bed boundaries correctly.

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SP surface Effects

The SP can be affected by a number of surface effects as it relies on the fish as its reference electrode.Power lines, electric trains, electric welding, close radio transmitters:All these create ground currents which disrupt he "fish" reference causing a poor, sometimes useless, log.

spontaneous potential logging

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