PASSIVE MEASUREMENTS - SP

FORMATION EVALUATION

PETE 663

Summer 2010

Dr. David Schechter

LOG PRESENTATION - THE HEADING

• Well location• Depth references

• Well depth

• Date of log

• Casing shoe depth

• Bit size

• Mud data

– Type– Properties

– Resistivities• Max. Temperature

DRILLING DISTURBS FORMATION

• Drilling and rock crushing• Damage zone

• Mud systems and invasion• Oil based mud

• Small conductivity mud

• Shallow invasion• Thin cake

• Water based mud• Moderate to very conductive

mud• Shallow to deep invasion• Thin to thick cake

Mudcake

Invading filtrate

Damaged zone

MUD FILTRATE INVASION

Modified from J. Jensen, PETE 321 Lecture Notes

WellboreMud(Rm)

Mud Cake(Rmc)

Uninvaded Zone(Rt)

Invaded Zone (Rxo)

Uninvaded Zone(Rt)

BoreholeRm : Borehole mud resistivityRmc : Mudcake resistivity

Invaded zoneRmf : Mud filtrate resistivityRxo : Invaded zone resistivitySxo : Invaded zone water saturation

Uninvaded zoneRw : Interstitial water resistivityRt : Uninvaded zone resistivitySw : Uninvaded zone water saturation

COMMON TERMINOLOGY

PASSIVE MEASUREMENTS• Caliper• Spontaneous Potential• Gamma Ray

• Natural• Spectral

CALIPERS• Uses

• Hole volume• Mudcake (permeability)• Tool corrections• Crude lithology indicator

• Properties• Two, three, or four arms• Linked or independent

• Calipers may disagree (limitations)• Non-circular hole• Deviated wells

Two-arm caliper

actualapparent

Three-arm caliper

actualapparent

arm

arm

CALIPER INTERPRETATION• Hole volumes

• In general, more arms give better accuracy• Two arms < 100% error• Three arms < 20% error

• Mud cake• If dcaliper < dbit

• hmc = (dbit - dcaliper)/2

• Lithology• Shales may indicate borehole enlargement• Spikey curve may indicate fractures

SP – DEFINITION• SP is a natural occurring electrical potential relative to a surface potential measured in the borehole mud

• Potentials are created by chemically induced electric current

• The potential of the surface reference must remain constant

USES OF SP

1. Determine values of formation water resistivity

2. Identify permeable zones

3. Qualitative indication of shale content

4. Define bed boundaries

5. Well-to-well correlation

SPONTANEOUS POTENTIAL (SP)

• Uses• Correlation• Lithology• Shaliness indicator• Depositional environment

indicator

• Properties• Measures formation voltage• Passive measurement

Ransom, PFE

++++----++++

-12mV

+59mV

-71mV

POROUS,PERMEABLE

BED

SHALE

SHALE

• One electrode

• Insulators on either side

• Surface ground electrode – at a stable potential

THE SP TOOL

SHALE

SHALE

SAND

SP PRINCIPLES• Must have water-based mud• Mud--formation water

salinity difference causes battery effect

• Battery effect components• Electrochemical

• Liquid Junction Potential, Ej

• In permeable region• Anions more mobile than cations• Membrane Effect, Em

• Shale acts as membrane• Repels anions / passes cations

• Electrokinetic (Streaming)• Usually minor, disregarded

Electrochemical Effect

Membrane effect

SAND

SHALE

FlushedZone

Less SaltyWater

VirginZone

SaltyWater

Membrane effect

VirginZone

+++++

ORIGIN OF SPONTANEOUS POTENTIAL

The electrochemical potential sensed in the borehole is generated by the sum of two potentials known as the membrane potential and Em and the liquid junction potential Ej.

Ec = Em + Ej

LIQUID JUNCTION POTENTIAL

A liquid junction potential develops when a concentrated salt solution (formation water ) is in direct contact with a diluted salt solution (fresh mud filtrate)

The net effect of more positive ions in formation water and more negative ions in mud filtrate creates potential difference.

MEMBRANE POTENTIAL, Em

Created when a shale is introduced between a concentrated salt solution(formation water) and a diluted salt solution (fresh mud filtrate)

SP CURRENTSNote:

Reverse SP occurs when formation water is fresher than mud filtrate

TYPICAL SP RESPONSES – BASED ON THE

DIFFERENCE BETWEEN Rw and Rmf.

5. Rmf << Rw - Amplitude large and positive

4 . Rmf < Rw - Amplitude positive but not large

3. Rmf = Rw - No SP deflection

2. Rmf > Rw - Amplitude negative but not large

1. Rmf >> Rw - Amplitude large and negative

REV

ERSE

D S

PN

OR

MA

L SP

1

2

3

4

5

(+)

(-)

STATIC SP (SSP)

If it were possible to prevent SP currents from flowing and measure the potential of mud this would provide a value for the SSP

Conditions where the SSP is recorded directly:

1. Thick zones

2. Clean (no shale) zones

3. Only water – bearing zones

4. Permeable zones

SELECTING A 100% WATER SATURATED ZONE

Low resistivity suggesting a water bearing formation

Low GR response and high SP deflection

SELECTING A SHALE BASE LINE

Shale base line is the SP response across a thick shale or several shale intervals

Sandstone baseline

20mV

- 110 mV

PSEUDO- STATIC SP (PSP)

• Presence of shale in the formation will reduce the static SP

• Shale lattice will slow the migration of chlorine ions and assist the flow of sodium ions, decreasing Ej

• This reduces SSP to a pseudo-static value, PSP

• The volume of shale can be calculated:

Vsh = 1- (PSP)/(SSP)

EXAMPLE PROBLEM

SP RESPONSEIN THINBEDS

USING THE SP EQUATION FORRw DETERMIMATION - CLASSICAL METHOD

1. Determine formation temperature

2. Find Rmf at formation temperature

3. Convert Rmf at formation temperature to Rmfe value

4. Compute Rmfe / Rwe ratio from the SP

5. Compute the Rwe

6. Convert Rwe at formation temperature to Rw

USING THE SP EQUATION FOR Rw

To determine Rw, we must know:1. Formation Temp, Tf

• Actual temp reading or• BHT and geotherm

gradient• Chart GEN-2 (H) GEN-6

(S)2. Rmf at Tf

• Actual measurement or• Correct surface Rmf• Chart GEN-5 (H) GEN-9

(S)• or Arp’s equation• R1(T1 + 7) = R2(T2 + 7) (T ºF)

• R1(T1 + 21.5) = R2(T2 + 21.5) (T ºC)

3. Essp• SP log

A

Original sample: Rw = 0.1 ohm-m@ 150F;What is Rw at formation temperature (Tf),which is 250F?

Rw = 0.058 ohm-m

0.1 ohm-m, 150

0.58 ohm-m, 250

1

2

43

H, GEN-5

THE SP EQUATION - 1

• Define Essp = (Esp)max

• We assume:

• From electrochemical theory:

where Tf = formation temp, deg Faw = formation water activityamf = mud filtrate activityEssp = max SP deflection, mV

)/(log)460(133.0 10 mfwfssp aaTE +−=

)( mlssp EEE +≈

-20mV+

- 80 mV

- 60 mV

Shale

CleanSand

-20 mV

ShalySand

SandyShale

Shale

Essp

Shale Baseline

B

THE SP EQUATION - 2• Difficult to measure activities• Substitute resistivities for activities

• For small salinities, a = 1/R– For fresh mud filtrate, assume– Rmfe = Rmf or– Rmfe = 0.85Rmf (Schlumberger)

• For high salinities– Correction needed– Use Chart SP-2 (Schlumberger)– Use Chart SP-3 (Halliburton)

)/(log)273(24.0

)/(log)460(133.0

10

10

wemfefssp

wemfefssp

RRTE

RRTE

+−=

+−=

Rw or Rmf

Rw

eor

Rm

fe

C

EXAMPLE• Determine Rmf @Tf (Arp’s Eq.)

– 5.6(11+21.5)/(33+21.5) = 3.3Ωm

• Apply SP equation– -50 = -0.24(33+273)log(3.3/Rwe)– Rwe = 0.68– Chart SP-2 gives Rw = 1.3 ohm-m

(See next page)

D

10mV-|↔|+

Rmf = 5.6Ωm @ 11º CTf = 33º C

• Determine Essp– Shale base line– Maximum deflection line

– Calculate deflection -50mV

Rarely known

Usually use charts, instead

Rw or Rmf

Rw

eor

Rm

fe

Rwe=0.68

Rw = 1.3

F

PROBLEMThe SP deflection is –60 mV across a thick, water-

bearing, clean zone. The value of Rmf at that temperature of 100° F is 0.5 ohm-m.

Determine Rw at the same temperature (100° F)

Rw from SP: Classical Method First, we determine the Rmfe (effective Rmf), since

the resistivity is not an accurate determination of the ion activity that produces the SP.

Rw ESTIMATION FROM Rwe

Rmfe = 0.45 ohm-m at 100° F.

Figure 9-13 in the manual.

1. Determine Rmfe

0.5,100F

0.45 ohm-m

Rmf, 0.5 ohm-m

2. Determine Rwe from

Rmfe

Figure 9-14of your manual

Rmfe/Rwe = 7. Therefore,

Rwe=0.45 ohm-m/7=0.064 ohm-m at 100° F

Rw ESTIMATION FROM SSP

60, 1007

SSP

(Rwe=0.064 ohm-m at 100°F)

3. Finally, determine Rw

• Using Figure 9-13 of your text again, we determine Rw=0.10 ohm-m at 100° F

• Here, Rw<Rmf. This problem illustrates the fact that if Rw<Rmf, SP deflection is negative (0.1<0.45 ohm-m)

(Normal SP)

Rw ESTIMATION FROM Rwe

0.064 mV

0.064, 100F

THE SILVA-BASSIOUNI METHOD

Figure 9-16 of your text.

Rw ESTIMATION FROM Rwe

For the same problem as before, ie Rmf=0.5 ohmm at 100° F, determine Rw if the SP deflection is –60 mV.

We see Rw=0.1 ohm-m, as shown with the classical method.

Figure 9-16 of your text

145 mV – 60 mV = 85mV

COMPARISON OF THE CLASSICAL AND SILVA-BASSIOUNI METHOD

• The classical method requires 3 steps for the determination of Rw.

• The Silva Bassiouni method combines Fig 9-13 and 9-14 into one chart ( Fig 9-16 ) and gives you the same value of Rw. Hence it is easier to use.

FACTORS AFFECTING THE SP RESPONSE

• Hydrocarbons: reduce the SP deflection• Shaliness: reduces the SP deflection• Bed thickness: thin beds do not develop a full SP

deflection• Permeability: low permeability zones will have a very

high invasion diameter, so it may be impossible to read the Junction Potential, hence SP readings may be low

OTHER SP ISSUES

• Shaliness

• Environmental– Uses curve shape

• Permeabilityindication– POOR perm predictor

• SP value & polaritydepend on salinity contrast

sspspsh EEV /1−=SSP = -K log Rmfe

Rwe

Rmf = Rw Rmf <RwSALINE MUD

Rmf >RwFRESH MUD

ZONATION• Zonation - Defines intervals of similar properties• Purpose

• Well-to-well correlation• Evaluation of specific intervals

• Criteria• Lithology• Fluids• Porosity and permeability

• Begin with coarse zonation• Typically

• Well-to-well correlation 20 - 100 ft• Detail evaluation 10 ft thick or more

• Easy lithologies first, e.g., shales• Refine

• More subtle lithology changes• Fluids in porous, perm intervals• Depends on measurements available

PASSIVE LOG CORRELATION

• GR, SP, and CAL• Often correlate• Different

measurements• Different reasons

• Correlation helps• GR instead of SP in

oil base mud• Easier detection of

shales• Facilitates “zonation”

SUMMARY• Drilling process affects formation

• Alters rock near wellbore• Invasion

• Passive logs respond to borehole, formation, and fluids• Caliper

• Simple measurement• Care needed when interpreting and

comparing caliper• SP

• Needs water based mud• Estimates Rw

EXTRA SLIDES FOLLOW

MEMBRANE POTENTIAL

LIQUID JUNCTION SP

LIQUID JUNCTION EFFECTS