petrophysict 8-SP

Notes

11

Spontaneous Potential Measurement

Spontaneous Potential

© Schlumberger 1999

Notes

22


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 - smallest.

Notes

The Spontaneous Potential (SP) was first noticed as a perturbation on early electrical resistivity logs. It was quickly realised that the curve was reacting to the permeable formations and it was added to the standard presentation. (Very early logs labelled it as “porosity”). The reactions governing this simple effect depend mainly on the difference between the ionic concentrations of the connate water and the invading mud filtrate.

33


SP theory 2

Membrane and Liquid Potential

These two effects are the main components of the SP. They are caused because the mud flitrate and the formation waters containNaCl in different proportions. Firstly, shales are permeable to the Sodium ions but not

the Chlorine. Hence there is a movement of charged particles through the shale creating a current and thus a potential.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.

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.

Notes

In front of a permeable bed the SP currents flow through four media:The borehole and the mud cake.The invaded zone.The virgin zone.The shale.In each there is a potential drop corresponding to the resistivity met.In practice, the drop measured is that of the borehole which is only a part of the total (a major part). In a perfect case with an insulating plug this would be the total e.m.f. (see diagram on previous page).This deflection is the Static SP (SSP). The measurement is close to this theoretical value.

44


SP theory 3

Notes

A typical scale would be -20 mV to +80mV going from the right hand side of track 1 to the left hand side. This is at 10mV per division.It is important to choose the scale so that the curve stays in the track. This is usually done during the repeat section or after monitoring the movement during the run into the well.

55


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.

Notes

The convention is that the SP deflection is negative when the Rmf>Rw and positive when the opposite is true. The Magnitude of the deflection depends on the difference between the two and the temperature. The shale base line may shift over a long interval. Hence, before any computations the base line is adjusted back in agreement for all the shales in the interval.The maximum deflection indicates the cleanest zone. Smaller values of SSP mean increasing shaliness. Note the SSP in this example is -60mV (3 divisions at 20mV per division).

66


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)

Notes

The SP is nearly always run in conjunction with a resistivity log. The example shows an SP deflection opposite the same zones with separation in the resistivity curves. The shale points, where all the resistivity logs overlay, corresponds to the maximum of the SP.

77


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.

Notes

The log depends on invasion, if there is no invasion there is no SP. Hence the SP shows where there is permeability. If there is permeability there must also be porosity.The vertical resolution of the tool is poor, hence while it will show the boundaries they will not be precise.The volume of shale can be computed from the SP using a simple linear equation.

88


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.

Notes

The SP is an excellent method of computing the vital Rw parameter. The Rmf value is usually measured on a sample, if not it can be computed from charts knowing the mud salinity. The constant, K, is a complex figure that is incorporated into the charts.

99


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−=

Notes

The first chart gives the ratio of Rmfe/Rwe knowing the SSP and the temperature.Rmfe and Rwe are used instead of Rmf and Rw as the complete equation relates the chemical Activities of the two solutions. These “equivalent” resistivities take this transform into account.The entire computation must be carried at the relevant borehole temperature.

1010


Rw from the SP

Knowing the SSP (the maximum deflection) from the log and the temperature, the ratio of resistivities is obtained from Log Interpretation Chart SP-1.output = Rmfe

Rwe

Notes

1111


Rw from SP 2

Rmf is measured, using the mud cell.Rmfe is computed from Log Interpretation Chart SP-2.Rwe is computed, from the ratio above and Rmfe.Chart SP-2 is used to determine Rw.

Notes

The chart is given in most Chart Books.

1212


Salinities chart

This chart is used to compute salinities from resistivities 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

0.4

0.5

0.6

0.8

1

2

3

4

56

8

10

0.08

0.060.05

0.04

0.03

0.02

0.01

50 75 100 125 150 200 250 300 350 400

10

15

20

2530

40

50

100

150

200

250300

400

500

1000

1500

200025003000

40005000

10,000

15,00020,000

280,000

200

300

400

500

600

700

800

1000

1200

1400

1700

2000

3000

4000

5000

6000

70008000

10,000

12,000

14,000

17,000

20,000

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)

Notes

In some situations there can be a number of different salinities along the logged interval. In this case the SP deflection will show these beds as distinct from each other. This is the only measurement that will show the changes in salinity. Resistivity tool will simply show more or less resistivity which could be salinity changes or changes in water saturation i.e. hydrocarbon presence.

1313


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.

Notes

Carbonates:Due to the irregular nature of the porosity distribution in most carbonates, the SP is usually poor or non-existent in this type of rock. The current flows are interrupted by resistive rocks and odd pore geometries.

1414


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.

Notes

Unit effects such as magnetized cable drum or measure wheel will superimpose a sine wave on the SP curve.

The source is simple to recognise from the period of the effect.In both these cases, it may be possible to eliminate the problem.

1515


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.

petrophysict 8-SP

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