04 quicklook interpretation

Schlumberger

(05/96)

Contents

D1.0 BASIC QUICKLOOK INTERPRETATION......................................................................................1D1.1 QUICKLOOK METHODS ........................................................................................................1D1.2 METHOD ONE: OVERLAY TECHNIQUE.................................................................................1D1.3 METHOD TWO: R

WA TECHNIQUE..........................................................................................2

D1.4 METHOD THREE: DIRECT METHOD OF CALCULATING WATER SATURATION FOR CLEAN ZONES ......................................................................................5

D2.0 WORK SESSION.........................................................................................................................9

(05/96)

Introduction to Openhole Logging

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D1.0 Basic Quicklook Interpretation

D1.1 QUICKLOOK METHODS

Quicklook methods of log interpretation canbe classified as those used to identify possibleproducing intervals, usually at the wellsite. Therequirements are to locate permeable beds, cal-culate bed thicknesses, porosities and satura-tions of hydrocarbon zones and predict pro-ducibility. These generally simplifiedtechniques are not intended as a substitute formore comprehensive methods of interpreta-tions.

The methods covered here are1) overlay technique2) R

wa

3) direct method of calculating Sw.

A note of caution, though, because there aresome assumptions that should be consideredwhen using quicklook techniques. The zoneshould have

1) constant Rw

2) thick, homogenous formation3) continuous clean lithology4) clean-water-bearing zone5) moderate invasion and of step profile.

D1.2 METHOD ONE:OVERLAY TECHNIQUE

a. Define the clean zones (no clay) on thelog with the GR and SP.

b. Find a clean, 100%-wet zone on thelog: this should have a good SP de-flection, low GR, good porosity andlow resistivity.

c. In the clean, wet zone found in Step(b), overlay the sonic t on the deepresistivity curve. (If no sonic is avail-able use density porosity.)

d. Keeping the logs parallel and in thesame relative position, trace the deepresistivity curve on the sonic log forthe zones found in Step (a).

e. Any zone where there is high resistiv-ity relative to sonic porosity (t) hashydrocarbon and should be evaluatedfurther.

f. Use the same 100%-wet zone found inStep (b), and overlay the sonic t onthe neutron porosity curve.

g. Trace the neutron porosity curve onthe sonic log for the clean zones de-fined in Step (a). Make sure the neu-tron and sonic log stay parallel and inthe same relative position.

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h. In the hydrocarbon zones defined inStep (e), where the neutron porositydecreases and the sonic t increasesthe zone is gas bearing. All other hy-drocarbon zones contain oil.

i. On the density porosity log define acutoff value of porosity based on testand production experience for the area.

j. When the density porosity is abovethis value, the zone will produce fluid.Below the cutoff value, no productionwill occur.

D1.3 METHOD TWO: Rwa TECHNIQUE

This technique assumes that all zones are100% wet, estimates a value for R

w, and sub-

sequently studies the anomalies to the first as-sumption.

Consider Archie's equation:

aRw

FRw

Sw

2

= = m R

t R

t

Assume: Sw = 100%

FRw

then = 1 R

t

Rt

Rearrange to solve for Rw: R

w =

F

Because we assume that all zones have Sw

=100%, we state

Rt

Rwa

=

F

This value will represent Rw for the forma-

tion if the assumption that all zones are wet iscorrect.

If the zones are not all at Sw = 100%, the

value of Rwa

will vary depending upon the ac-tual S

w of the formation.

If we select the minimum value of Rwa

andcall it R

w, then we can make a comparison of

all calculated Rwa

values against this Rwa

(minimum) value through substitution intoArchie's equation as follows:

FRw

Given Sw

2

=

Rt

If Sw = 100%, then

Rt

Rwa

=

F

or conversely, Rt = FR

wa

Substituting Rwa

(minimum) for Rw, and FR

wa

for Rt yields

FRwa

(minimum)

Sw

2

=

FRwa

Rwa

(minimum)

or Sw

2

=

Rwa

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Hence, we can compare the minimum Rwa

value against all other Rwa

values calculated andcompute S

w.

To work effectively, this technique requiresthat we in fact have a zone at S

w = 100% and

that Rt or vary through the zones to be

evaluated.

Procedure for Rwa

Analysis:Problem: Find: S

w given a resistivity log,

plus either a sonic, neutron or density log.

Solution: This interpretation method isgenerally suited to sands, where porosityplus resistivity logs are available (refer toNomograph in Figure D1).

- Logs must be zoned so that the forma-tions to be evaluated have reasonablyconsistent matrix and R

w values.

- Calculate a series of Rwa

values in perme-able zones. Check the R

wa values (see

later comments).

- When Rwa

3Rw, investigate the zone for

possible hydrocarbon presence, becauseS

w < 58% where R

wa > 3R

w.

- If Rw is known, S

w may be calculated by

Sw

2 = Rw/R

wa.

- If Rw is unknown, choose a minimum R

wa

value Rw. Several points should be ex-

amined to establish a suitable Rw value

(i.e., anomalously low Rw values should

be avoided, because they may be due tocalcareous streaks or other matrix influ-ences, etc.).

- The general rule for indicating zones ofpotential hydrocarbons is when R

wa 3R

w

(approximate Sw = 58%). When R

mf > R

w, such

an Rwa

calculation may be due to the influenceof invasion on the R

t device in a water sand.

To help resolve this problem, an apparent mudfiltrate resistivity value (R

mfa) may be computed

using a shallow investigation resistivity read-ing e.g., Micro-SFL, SFL tool and AT-10.

R(shallow device)R

mfa =

F

Quality Checks on Rwa

Values:Assuming that R

w< R

mf:

1. If Rmfa

Rwa

R

w, invasion is shallow

and Rwa

is correct. The zone is waterbearing.

2. If Rmfa

> Rmf

, there is probably someresidual hydrocarbon saturation in theflushed zone. This would confirm ahydrocarbon indication on the R

wa

curve.3. If R

mfa R

mf and R

w < R

wa < R

mf, deep

invasion may have occurred. Checkfavorable R

wa indications further.

- Having checked Rwa

values andselected an R

w value, proceed

to calculate Sw for all zones

where Rwa

3Rw

(Sw

2 = Rw/R

wa).

LimitationsLimitations of this technique are similar tothose for crossplots. The influence of invasion,shale, gas and matrix changes for each deviceshould be recognized.

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Figure D1

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D1.4 METHOD THREE: DIRECTMETHOD OF CALCULATINGWATER SATURATION FORCLEAN ZONES

All water saturation calculations are based onone form or other of Archie's saturation for-mula, where:

FRw

Sw

n

=

Rt

aRw

=

m Rt

By calculating suitable input parameters wecan solve these equations for water satura-tion in shale-free zones.

Rw - Formation Water ResistivityAn accurate knowledge of R

w is essential but

often difficult to obtain. Rw values can be ob-

tained from:

a. Production Water Samples: samplesshould be collected prior to anychemical treatment; measure resistivityand temperature of the sample.

b. Drillstem Tests (DSTs): if possible,collect three samples, at top, middleand bottom of the tool. Measure allthree resistivities and record tempera-tures. The sample with the lowestvalue should be most representative ofR

w.

c. SP Log: if necessary, bed thicknesscorrections, etc., should be made priorto calculating R

w. (When shale is pres-

ent, the SSP may be estimated byPSP).

PSPSSP

=

1-Vsh

where Vsh

is from the GR.

d. Water Catalog: This is a summary ofDSTs and produced water samples.Some countries have logging societiesthat publish these catalogs.

F - Formation FactorFormation factor may be obtained for R

xo

measurements (e.g., Micro-SFL Focused Log,electromagnetic propagation resistivity [EPR]).

Rxo

F

= Sxo

2

Rmf

- PorosityPorosity may be obtained from neutron,

density, sonic or a combination of these de-vices.

Rt - True Resistivity

True resistivity may be obtained from ILD,IDPH or LLD; any borehole and invasion cor-rections should be applied to the raw readingsto obtain R

t.

Chart Sw-1a (Figure D2) is a convenient

method of solving this formula. However,note that the F versus relationship used is F= 1/2.

If any other relationship is used, F must becalculated before entering the chart.

Remember, knowledge of formation waterresistivity is essential for making an accurateinterpretation.

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Saturation Determination(Clean Formations - Humble Relationship)

This nomograph solves the Archie water saturation equation Sw = R

Rt

0 =

F R

R

r w

t

It should be used in clean (nonshaly) formations only. If R0 (resistivity when 100% water saturated) is known, astraight line from the known R0 value through the measured Rt value gives saturation, Sw. If R0 is known, it maybe determined by connecting the formation water resistivity, Rw, with the formation resistivity factor, FR, orporosity,

Example: Rw = 0.05 .m at formation temperature = 20% (FR = 20)Rt = 10 .mThus, Sw = 31.6%

Chart Sw-1a

Figure D2

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Saturation Determination(Clean Formations - m = 2)

Schlumberger

Rw(ohm-m)

R0(ohm-m)

R0 = FRRw

Rt(ohm-m)

Sw(%)

(%)

FR

2000

1000800600400300200

100806050403020

108654

2.53

4

56789

10

15

20

253035404550

FR =1

2.0

m = 2.0

0.01

0.02

0.03

0.04

0.050.060.070.080.090.1

0.2

0.3

0.4

0.50.60.70.80.91

1.5

2

5

6

7

8

9

10111213141516

1820

25

30

40

50

60

70

80

90

100

10,0008,0006,0005,0004,0003,0002,000

1,000800600500400300200

100806050403020

10865432

1.00.80.60.50.40.3

0.2

0.1

30

20181614121098765

4

3

21.81.61.41.21.00.90.80.70.60.5

0.4

0.3

0.20.180.160.140.120.10

Sw =R0Rt

This nomograph solves the Archie water saturation equation Sw = R

Rt

0 =

F R

R

r w

t

It should be used in clean (nonshaly) formations only. If R0 (resistivity when 100% water saturated) is known, astraight line from the known R0 value through the measured Rt value gives saturation, Sw. If R0 is known, it maybe determined by connecting the formation water resistivity, Rw, with the formation resistivity factor, FR, orporosity,

Example: Rw = 0.05 .m at formation temperature = 20% (FR = 20)Rt = 10 .mThus, Sw = 31.6%

Chart Sw-1b

Figure D3

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Schlumberger

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D2.0 Work Session

1. Using the logs of Figures D4 to D6, follow the overlay technique outlined on pages D1and D2.

2. Given tma

= 182 sec/m tabulate the values and do an Rwa

analysis of the example usingFigures D4 to D6. First find S

w from

s only and then do the calculation again using

T

from the CNT/Litho-Density log to get Sw. Compare the two results.

Depth t S

Rt

Rwa

Sw

N

D

T

= N+

DR

waS

w

2

605

600

595

592.5

590

587.5

585

580

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600

SP

0.0000-150.0000 (MV)

SFLU

2000.00000.2000 (OHMM)

ILD

2000.00000.2000 (OHMM)

ILM

2000.00000.2000 (OHMM)

FILE 2

ILM

DUAL INDUCTION - SP/SFL

Figure D4

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600

GR

150.00000.0000 (GAPI)

CALI

375.0000125.0000 (MM)

BS

375.0000125.0000 (MM)

DT

100.0000500.0000 (US/M)

FILE 2

BS

BOREHOLE COMPENSATED SONIC

Figure D5

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600

GR

150.00000.0000 (GAPI)

CALI

375.0000125.0000 (MM)

BS

375.0000125.0000 (MM)

NPHI

0.0000(V/V)

(K/M3)

0.6000

DPHI

0.6000 0.0000

FILE 2

BS

COMPENSATED NEUTRON LITHODENSITY (NO PEF CURVE)

Figure D6

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3. Use Chart Sw-1 (Figure D2) to calculate S

w for depths 1943 m and 1945 m on Figures D7

and D8. (Rw = 0.06 at formation tempurature.)

Depth RID

N

D

Pe

T

Ro

RT

Sw

(m)_____ __ __ __ __ __ __ __ __

1943

1945

a. What can be said about the lithology from the Pe curve?

b. What can be said about permeability from the caliper?Can the gamma ray curve add anything to this discussion?

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1950

---SP

ILM---

ILD---

SFL---

1925

1/240

1 20-MAY-1992 16:21 INPUT FILE(S) CREATION DATE

CP 32.6 FILE 14 20-MAY-1992 12:06

.20000 2000.0

SFL(OHMM)

.20000 2000.0

ILD(OHMM)

.20000 2000.0

ILM(OHMM)

-80.00 20.000

SP(MV )

DUAL INDUCTION - SFL

Figure D7

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1950

PEF---

NPHI---

DPHI---

DRHO---

---CALI

GR---

---BS

1925

1/240


CP 32.6 FILE 4 20-MAY-1992 11:32

.45000 -.1500

DPHI(V/V )

.45000 -.1500

NPHI(V/V )

0.0 10.000

PEF

125.00 375.00

CALI(MM )

0.0 150.00

GR(GAPI)

125.00 375.00

BS(MM )

PEF---

NPHI---

PEF

-250.0 250.00

DRHO(K/M3)

BS(MM )

125.00 375.00

C2(MM )

COMPENSATED NEUTRON - LITHO DENSITY

SANDSTONE

Figure D8

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4. Interpret the logs in Figures D9 and D10 using the direct method of calculating watersaturation in clean zones. R

mf = 2.35 at formation temperature (24 oC); a = 1; m = 2

a. Zone 303 to 325 m: Rw = at formation temperature

b. Zone 303 to 308 m: Sw = %

c. Zone 309 to 317 m: Sw = %

d. Zone 317 to 325 m: Sw = %

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1/240


CP 32.6 FILE 12 20-MAY-1992 12:00

.20000 2000.0

SFL(OHMM)

.20000 2000.0

-100.0 0.0

SP(MV )

.20000 2000.0

ILD(OHMM)

.20000 2000.0

ILM(OHMM)

325

SP---

---ILM

---ILD

---SFL

300

350

DUAL INDUCTION -SFL

Figure D9

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---BS

GR---

---NPHI

DPHI---

DRHO---

300

---CALI

1/240


CP 32.6 FILE 3 20-MAY-1992 11:23

.60000 0.0

DPHI(V/V )

.60000 0.0

NPHI(V/V )

-250.0 250.00

DRHO(K/M3)

125.00 375.00

CALI(MM )

0.0 150.00

GR(GAPI)

125.00 375.00

BS(MM )

350

325

LITHO - DENSITY

Figure D10

04 quicklook interpretation

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