03 sp theory josan

8/11/2019 03 SP Theory Josan

1/30

CHAPTER

3

SPONTANEOUS POTENTIAL (SP)

Basic Theory, SP Measurement and Problems That Can affect the SP


2/30


3/30

TableofContentsDefinitionoftheSP............................................................................................................................................1

UsesoftheSP.........................................................................................................................................1

SPMeasurementPrinciples...............................................................................................................................1

SourceoftheSP.................................................................................................................................................3

MembranePotential,Em........................................................................................................................3

LiquidJunctionPotential,Ej...................................................................................................................5

SPCharacter.......................................................................................................................................................8

StaticSP(SSP)*......................................................................................................................................8

PseudoStaticSP(PSP)...........................................................................................................................9

FactorsInfluencingAmplitudeandShapeofSPCurve....................................................................... 10

ProblemsThatCanAffectTheSP................................................................................................................... 12

OilBasedMudorAirFilledBoreholes................................................................................................ 12

Magnetism.......................................................................................................................................... 12

LineLeaks............................................................................................................................................ 12

NonStaticUpholeConditions............................................................................................................ 12

StrayGroundCurrents........................................................................................................................ 12

SawToothSP...................................................................................................................................... 12

BiMetallism........................................................................................................................................ 13

TelluricCurrents.................................................................................................................................. 13

RwfromtheSP................................................................................................................................................ 14

SilvaBassiouniMethod.................................................................................................................................. 19

LimitationsonRwEstimations......................................................................................................................... 20

ELECTROKINETICPOTENTIALS........................................................................................................................ 21

DigitalSPforDLL............................................................................................................................................. 24

References...................................................................................................................................................... 26


4/30


5/30

SP THEORY - TRAINING MANUAL

1

DefinitionoftheSP

The SP is a natural occurring potential relative to a surface potential measured in the borehole

mud. The potentials measured are created by chemically induced electric current flow.

UsesoftheSP

1. Determine values of formation water resistivity

2. Define bed boundaries

3. Identify permeable zones

4. Qualitative indication of shale content

5. Well to well correlation

SPMeasurementPrinciples

An electrode is placed on the bridle or is incorporated into the tool string. This electrode is

connected to one terminal of a recording galvanometer. The other terminal of the galvanometer is

connected to a surface reference, which is either a "fish" (usually placed in the mud pit) or a stake

driven into the ground. The potential difference between the downhole electrode and the surface

fish is caused by the spontaneous potential located near the downhole electrode. The potential of

the surface reference must remain constant.

The SP log does not actually display the total potential difference between the surface reference

and the downhole electrode, which may be of the order of several volts. Instead, we measure SP

deflections with respect to the value of the curve in the adjacent shales, and electrically position

this SP shale reading (SP "baseline") at a convenient point on the log grid (usually around 80

divisions of Track 1 if negative SP deflections are expected).


6/30


2

FIG: 1 Orig in of Spontaneous Potential


7/30


3

SourceoftheSP

The electrochemical potential sensed in the borehole is generated by the sum of two potentials

known as the membrane potential Em and the liquid junction potential Ej.*

5-1. Ec = Em +Ej

MembranePotential,Em

The membrane potential is created when shale is introduced between two fluids of different

salinity, typically a concentrated salt solution (formation water) and a diluted salt solution (fresh

mud filtrate).

FIG: 2 Membrane E.M.F.

*NOTE: Actually there are two more potentials possibly contributing to the SP development. Theseare the Electrokinetic potentials discussed in the appendix. They will not be discussed here

because in most cases they sum to a negligible contribution.


8/30


4

The shale on a molecular level is constructed in a lattice arrangement with a negative charge on

the lattice frame. Negative ions are repelled while positive ions are attracted and allowed

through as the concentration of positive ions increases.

The shale lattice acts as an ion filter passing positive ions and blocking negative ions.

In the usual case where the mud is "fresher" (less salt content) than the formation water,

positive ions will migrate to the borehole-shale boundary where they will accumulate on the

negative ion lattice frame. The formation water at the shale-formation boundary will have anaccumulation of negative ions. A potential difference is therefore created across the shale bed.

The membrane potential Em, which is the net potential of the formation relative to the

borehole measured across the shale, can be expressed as:

*NOTE: Rmfe, Rwe are equivalent resistivities and will be discussed in the appendix.


9/30


5

LiquidJunctionPotential,Ej

This potential develops across the boundary between mud filtrate and formation water in

a permeable zone.

- Cl ions

+Na ions

- +Voltage

Difference

DiluteSolution

C2

Mud Filtrate

+

+ Cl

+

+

+

+

+

Cl

+

+

+

ConcentratedSolution

C 1

FormationWater

FIG: 3 Liquid Junct ion E.M.F.

A concentrated salt solution (formation water) is in direct contact with a diluted salt solution

(fresh mud filtrate). Equilibrium will begin to develop through ion transfer. Chlorine ions

quickly migrate to the less concentrated solution while the Sodium ions that tend to migrate muchslower remain in the strongly concentrated solution.

The net effect is that more positive Sodium ions are present in the formation water and more

negative Chlorine ions are present in the mud filtrate. This situation creates a potential difference

across the liquid junction.


10/30


11/30


7

Fortunately, because the borehole presents a much smaller cross-sectional area to current flow

relative to the formations, most of the SP voltage drop does occur in the borehole, provided that

the formation resistivities are low to moderate and beds are moderately thick.

FIG: 4 SP Currents


12/30


8

SPCharacter

StaticSP(SSP)*

If it were possible to prevent SP currents from flowing and measure the potential of mud thiswould provide a value for the SSP. This is shown below.

Fig: 5 Static SP

There are conditions where the SSP is recorded directly.

The SSP can be recorded from the log in zones that satisfy the following criterion:

1. Thick

2. Clean (no shale)

3. Only Water-Bearing

4. Permeable

* We assume no Electokinetic (Electro-filtration) potentials to be present.


13/30


14/30


10

FactorsInfluencingAmplitudeandShapeofSPCurve

A. Bed Thickness - In thick and permeable beds with high Rmf/Rw contrasts, the SP curve

will display a deflection with a large slope where the amplitude will remain steady at an

SSP value. In thin beds the full SP deflection is never developed.

B. Rmf/Rw Contrast (See Figure 7)

1. Rmf >> Rw - the amplitude of the SP curve will be large and negative.

2. Rmf > Rw - the amplitude of the SP curve will be negative but the amplitude will

not be as large as the case above.

FIG: 7 "Normal" And Reversed Sp As A Function Of The Rmf/Rw Contrast

C. Bed Resistivity - As Rt increases, the amplitude of the SP curve decreases andboundary deflections become more rounded and spread making it difficult to determine

bed boundaries.

3. Rmf = Rw - there will be no SP deflection as Em and Ej are both zero.

4. Rmf < Rw - the amplitude of the SP curve will be positive.

5. Rmf


15/30


11

D. Permeability - Weighted drilling mud creates a pressure differential between the

borehole and reservoir rock.

1. High permeability zones and differential pressure will cause a mudcake to build

rapidly effectively sealing the zone. Very little invasion will be present. If the bed

is thick and free of shale the measured SP will be equal to the SSP.

2. Low permeability zones and pressure differential conditions will also build

mudcake, but at a much slower rate. Invasion can become quite deep in low

permeability zones. Deep invasion will reduce the effect of Ej since it will occur

within the invaded zone far from the borehole where SP measurements are taken.

The measured SP across a low permeability zone will be less than the SSP.

E. Hole Diameter - As hole size increases, the SP amplitude will decrease and boundary

slopes will decrease.


16/30


12

ProblemsThatCanAffectTheSP

OilBasedMudorAirFilledBoreholes

Oil emulsions in the mud will cause the SP to read lower or not al all. The SP cannot be logged

in oil based mud or air filled boreholes.

Magnetism

Magnetism will wreck havoc with the SP measurement. This is usually an uphole problem seen

as a cyclic spiking of the SP curve. The three most typical cases are:

Measuring Head Magnetized

Drum Magnetized

Drive Chain Magnetized

EXTREME CARE SHOULD BE EXERCISED WHEN WELDING ON A TRUCK. It is veryhard to degauss cable drums!!

LineLeaks

Line leaks on the wireline conductor carrying the SP signal or loose connections on the

uphole reference cable will cause the SP signal to be smaller than it should be. Poor fish

connections often cause problems because they may not be well maintained and are ignored or

forgotten.

NonStaticUpholeConditions

Running water over the SP fish will cause an unstable reference point for SP measurements.Fish should be in static conditions uphole.

StrayGroundCurrents

Stray ground currents will produce spurious spikes on an SP measurement. Welding, pumps,

generators, or any large current-consuming or generating equipment can introduce stray ground

currents if not properly grounded.

SawToothSP

A Saw Tooth SP can occur adjacent to sands with high vertical permeability. The filtrate, being

less dense than the salt water, will tend to float up toward the upper boundary of the sand andaccumulate below the shale streak. We therefore obtain a horizontal disk shaped cell, consisting

of a shale disk sandwiched between salt water above and mud filtrate below. This anomaly gives

rise to the Saw Tooth SP. See Figure 8.


17/30


13

FIG: 08 Saw Tooth SP

BiMetallism

Bi-metallism occurs when dissimilar metals are near or include the SP electrode. The bi-metallic

interface creates an electric cell producing stray borehole currents usually seen opposite formations

showing high resistivity contrast.

TelluricCurrents

Telluric currents caused by the "Northern Lights."


18/30


14

RwfromtheSP Classical Method

Equation 8 can be solved for the ratio Rmfe/Rwe to give

Since charts are available to convert Rwe to Rw, equation 10 provides an indirect way to

calculate Rw from the SSP. The steps required to accomplish this is given below.

A. Determine Formation Temperature

Use temperature equation or chart Gen-2a, 2b


19/30


20/30


16

B. Find Rmf at Formation Temperature - Use chart Gen-5

FIG: 10 GEN 5


21/30


17

C. Convert Rmf at Formation Temperature to an Rmfe Value

Use chart SP-3, which is the same chart used to convert Rwe to Rw. If Rmf > 0.1 ohm-meter at75 F use Rmfe = 0.85 Rmf.

RW ESTIMATIONFROM RWE

FIG: 11 SP3


22/30


18

D. Compute the Rmfe/Rwe Ratio from the SSP

For thin beds, the bed thickness correction charges SP-1a, 1b should be used to obtain a corrected

SSP. Then use equation 10, or chart SP-2 below.

Fig: 12 SP2

E. Compute Rwe

From the ratio Rmfe/Rwe solve for Rwe.

F. Convert Rwe at formation temperature to an Rw value.

Again use chart SP-3.


23/30


19

SilvaBassiouniMethod

Since the steps outlined above may appear somewhat tedious, an alternate, and theoretically

justified, simpler method has been developed for Rw estimation for the SP. A sample example is

given below using the single chart SP-4.

Applications: Determination of formation water resistivity from static spontaneous potential (an

alternative method)

Nomenclature: T......................formation temperature

SSP..................static spontaneous potential

Rw...................formation water resistivity at formation temperature

Given: T= 220 F

SSP = 80 mV

Rmf = 0.25 ohm-m

Find: Rw

Procedure: Enter the chart at 0.25 ohm-m on the Rmf axis. Project vertically into the chart

and, using the temperature curves as reference, estimate where the projection would intersect a

220 F curve. Project horizontally from that point to the SSP axis, there estimating an SSP value

of approximately -155 mV. Subtract the logged SSP value (-80 mV) to this to obtain an SSP

value of -75 mV. From -75 mV on the SSP axis, project horizontally into the chart and, using

the temperature curves again, estimate where the projection would intersect a 220 F curve.

Project vertically down from that point to the Rw axis, there estimating Rw to be 0.034 ohm-m.

Answer: Rw = 0.034 ohm-m

References: Silva, Pedro, and Bassiouni, Zaki, "One Step Chart for SP Log Interpretation,

"Paper Q. Transactions of the Tenth Formation Evaluation Symposium, Canadian Well Logging

Society, 1985.


24/30


20

Fig: 13 SP4

LimitationsonRwEstimations

The most accurate Rw estimates will be for sandstone formations that are (1) totally clean,

(2) contain only water, and (3) are very thick. In general, as real conditions prevail over the

ideal case, the SSP value will be too low. For a normal SP (one that deflects to the left of the

shale baseline) this will cause the estimated Rw to be too high.


25/30


21

ELECTROKINETICPOTENTIALS


26/30


22


27/30


23


28/30


24

DigitalSPforDLL

The INSITE system is different from the earlier DITS system in that all 6 outer conductors of the 7-

conductor cable are used to transmit both Instrument power and Auxiliary power to the tool string.

The only conductor not participating in power transmission from the surface to the tool string is the

center #7 conductor. The DLL deep and shallow investigation reference voltage is at the surface and

is provided by the DLL fish. It is not possible to connect the DLL fish to conductor #7 and at the

same time take the difference between the voltage on conductor #7 and the SP fish because

connecting the DLL fish to #7 places a low impedance (short) across the input of the surface SP

measurement amplifier. However, number seven conductor can provide a stable reference voltage

for a downhole SP measurement as well as a stable reference for the downhole V0 shallow and deep

measurement.

Shown in figure 1 below is the connection necessary to obtain the downhole digitized SP and at the

same time a downhole conventional DLL V0 shallow and deep measurement.. To obtain a downhole

SP, the SP ring must be run above the cable head and therefore must be a DITS 37 pin type SP sub.

Also, the Analog Interface Board in the GTET must have jumper JP12 open.

U6 and U7 in figure 1 both use the same ground reference which is the same as the tool case and thus

armor. The telemetry system sends both INC board A/D #7 and INC board A/D #6 to the surface.

The surface software computes the difference between these two numbers and displays the digitalSP.

There is a revision level change in the original Analog brd in the GTET required for Digital SP.

Original Sinopec GTET tools didnt have it. The board is 101293966.

In addition, conductor line #7 on the DIMP MUST be jumpered to the DLLT fish until the SP control

and current monitor brd in the DIMP is changed (and tested with firmware) to allow a relay on board

to make this jumper connection. Board is 101322933.


29/30


25

Figure1


30/30


References

1. Ellis, Darwin., Well Logging for Earth Scientist, Elsevier Science Publishing Co., Inc,

New York, N.Y. 1987

2. Sengel, E.W., Handbook on Well Logging, I.E.D. Press, Inc. Oklahoma City, O.K.,1981

3. Bateman, Richard M., Open-Hole Log Analysis and Formation Evaluation, HROL,

Boston, 1985

4. Dewan, John T., Essentials of Modern Open-Hole Log Interpretation, Penn Well

Publishing Company, Tulsa, Oklahoma 1983

5. Sears, Francis W. and Mark W. Zemansky, University Physics, Addison-Wesley

Publishing Company, Reading, Mass., 1977

6. Log Interpretation Charts, Houston Publication 1991

03 sp theory josan

Documents