Pete 663 Lect Pass Sp

8/13/2019 Pete 663 Lect Pass Sp

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PASSIVE MEASUREMENTS - SP

FORMATION EVALUATION

PETE 663

Summer 2010

Dr. David Schechter


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-HEADING

Well location

Depth references

W

ell depth

Date of log

Casing shoe depth

Bit size

Mud data

Type

Properties

Resistivities

Max. Temperature


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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 conductivemud

Shallow to deep invasion Thin to thick cake

Mudcake

Invading filtrate

Damaged zone


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MUD FILTRATE INVASION

Modif ied from J. Jensen, PETE 321 Lecture Notes

WellboreMud

(Rm)

Mud Cake(Rmc)

Uninvaded

Zone

(Rt)

InvadedZone (Rxo)

UninvadedZone

(Rt)


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Borehole

Rm : Borehole mud resistivity

Rmc : Mudcake resistivity

Invaded zone

Rmf : Mud filtrate resistivity

Rxo : Invaded zone resistivity

Sxo : Invaded zone water saturation

Uninvaded zone

Rw : Interstitial water resistivityRt : Uninvaded zone resistivity

Sw : Uninvaded zone water saturation

COMMON TERMINOLOGY


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PASSIVE MEASUREMENTS

Caliper

Spontaneous Potential Gamma Ray

Natural Spectral


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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

actual

apparent

Three-arm caliper

actual

apparent

arm

arm


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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


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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


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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


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One electrode

Insulators on

either side

Surface ground

electrode at a

stable potential

THE SP TOOL

SHALE

SHALE

SAND


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SP PRINCIPLES

Must have water-based mud

Mud--formation watersalinity 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

Flushed

Zone

Less SaltyWater

Virgin

Zone

SaltyWater

Membrane effect

VirginZone

+

++++


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LIQUID JUNCTION POTENTIAL

A liquid junction potential

develops when aconcentrated salt solution

(formation water ) is in

direct contact with adiluted salt solution (fresh

mud filtrate)

The net effect of morepositive ions in formation

water and more negative

ions in mud filtrate createspotential difference.


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MEMBRANE POTENTIAL, Em

Created when a

shale is introduced

between aconcentrated salt

solution(formation

water) and adiluted salt solution

(fresh mud filtrate)


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TYPICAL SP RESPONSES

BASED ON THE

DIFFERENCE BETWEEN

Rw and Rmf.

5. Rmf Rw - Amplitude negative but

not large

1. Rmf >> Rw - Amplitude large andnegative

REVERSE

D

SP

NORMALSP

1

2

3

4

5

(+)

(-)


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STATIC SP (SSP)

If it were possible to prevent SP currents from

flowing and measure the potential of mud thiswould 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


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SELECTING A 100% WATER SATURATED ZONE

Low resistivitysuggesting a

water bearing

formation

Low GR response and high SP deflection


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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)


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EXAMPLE PROBLEM


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SP RESPONSE

IN THINBEDS


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USING THE SP EQUATION FOR

Rw 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


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B


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THE SP EQUATION - 1

Define Essp = (Esp)max

We assume:

From electrochemical theory:

where Tf = formation temp, deg F

aw = formation water activity

amf = mud filtrate activity

Essp = max SP deflection, mV

)/(log)460(133.0 10 mfwfssp aaTE +=

)( mlssp EEE +

-20mV+

- 80 mV

- 60 mV

Shale

Clean

Sand

-20 mV

Shaly

Sand

Sandy

Shale

Shale

Essp

Shale

Baseline

B

THE SP EQUATION 2C


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THE SP EQUATION - 2

Difficult to measure activit ies Substitute resistivities for activit ies

For small salinities, a = 1/R For fresh mud f iltrate, 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

RRTERRTE

+=

+=

Rw or Rmf

Rwe

or

Rmfe

C

EXAMPLED


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EXAMPLE

Determine Rmf @Tf (Arps Eq.) 5.6(11+21.5)/(33+21.5) = 3.3 m

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 = 5.6 m @ 11 CTf = 33 C

Determine Essp Shale base line

Maximum deflection line

Calculate deflection -50mV

Rarely knownUsually use charts, instead

F


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Rw or Rmf

Rw

e

orR

mfe

Rwe=0.68

Rw = 1.3

F


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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 MethodFirst, 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


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Rmfe = 0.45 ohm-mat 100 F.

Figure 9-13 in themanual.

1. Determine Rmfe

0.5,100F

0.45 ohm-m

Rmf, 0.5 ohm-m

R ESTIMATION FROM SSP


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2.

Determine

Rwe fromRmfe

Figure 9-14

of 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


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(Rwe=0.064 ohm-m at 100F)

3. Finally, determine Rw

Using Figure 9-13 of

your text again, wedetermine Rw=0.10

ohm-m at 100 F

Here, Rw


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THE SILVA-BASSIOUNI METHOD

Figure 9-

16 of your

text.



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For the same

problem as

before, ieRmf=0.5 ohmm

at 100 F,

determine Rw ifthe 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


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ZONATION


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ZONATION

Zonation - Defines intervals of similar properties Purpose Well-to-well correlation Evaluation of specific intervals

Criteria Lithology Fluids Porosity and permeabili ty

Begin with coarse zonation Typically Well-to-well correlation 20 - 100 ft Detail evaluation 10 ft thick or more

Easy l ithologies first, e.g., shales Refine More subtle lithology changes Fluids in porous, perm intervals

Depends on measurements available

PASSIVE LOG CORRELATION


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PASSIVE LOG CORRELATION

GR, SP, and CAL Often correlate Different

measurements

Different reasons

Correlation helps

GR instead of SP inoil base mud

Easier detection ofshales Facilitates zonation


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SUMMARY

Drilling process affects formationAlters 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


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MEMBRANE POTENTIAL


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LIQUID JUNCTION EFFECTS

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