02_Log Evaluation Theory

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(05/96)

Contents

A1.0 INTRODUCTION TO OPENHOLE LOG INTERPRETATION ...................................1

A.1 USES OF LOGS.......................................................................................................................1

A.2 BASIC PETROLEUM GEOLOGY..............................................................................................2

A.3 BASIC LOG INTERPRETATION CONCEPTS.. ........ ........ ........ ........ ......... ........ ........ ........ ........ 4

A.4 RESISTIVITY AS A BASIS FOR INTERPRETATION—THE ARCHIE EQUATION....... ........ ....... .5

A.5 DEFINITIONS ...........................................................................................................................7

a) Formation Porosity ( φ )...........................................................................................................8

b) Formation Resistivity ( R )........................................................................................................8

c) Formation Factor ( F )..............................................................................................................8

d) Water Saturation: S w ...........................................................................................................8

e) Hydrocarbons Saturation ( S hy )...............................................................................................9

f) Clean Formations ..................................................................................................................9g) Shaly Formations..................................................................................................................9

h) Key Formulas ....................................................................................................................1 1

i) Key Symbols........................................................................................................................1 1

A.6 LOG SCALES AND PRESENTATIONS.. ........ ........ ........ ........ ........ ........ ........ ........ ......... ....... 12



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Introduction to Openhole Logging



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A1.0 Introduction to Openhole LogInterpretation

A.1 USES OF LOGSA set of logs run on a well will usually mean

different things to different people. Let us ex-amine the questions asked—and/or answerssought by a variety of people.

The Geophysicist:• Are the tops where you predicted?• Are the potential zones porous as you have

assumed from seismic data?• What does a synthetic seismic section

show?

The Geologist:• What depths are the formation tops?• Is the environment suitable for accumula-

tion of hydrocarbons?

• Is there evidence of hydrocarbons in thiswell?• What type of hydrocarbons?• Are hydrocarbons present in commercial

quantities?• How good a well is it?• What are the reserves?• Could the formation be commercial in an

offset well?

The Drilling Engineer:• What is the hole volume for cementing?• Are there any keyseats or severe doglegs

in the well?• Where can you get a good packer seat for

testing?

• Where is the best place to set a whipstock?

The Reservoir Engineer:• How thick is the pay zone?• How homogeneous is the section?• What is the volume of hydrocarbons per

cubic meter?• Will the well pay-out?• How long will it take?

The Production Engineer:

• Where should the well be completed (inwhat zone(s))?• What kind of production rate can be ex-

pected?• Will there be any water production?• How should the well be completed?• Is the potential pay zone hydraulically iso-

lated?• Will the well require any stimulation?• What kind of stimulation would be best?



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Log evaluation can be many things to many

people. As the answers are sought each indi-vidual will possibly use the available data in adifferent manner. The common approach willbe in reading the logs and understanding thevarious reactions produced by formation char-acteristics on our logging devices. The factorsinfluencing log reading and the informationthey provide are what we wish to introduce toyou in this course.

A.2 BASIC PETROLEUM GEOLOGYIn order to better understand log responses,

we should first review the types of rocks thatare found in the boreholes.

Common sedimentary rocks aresandstone, siltstone, shale, limestone,dolomite and anhydrite

In general, sedimentary rocks are depositedas either clastic sequences containing sand-stone, siltstones and shales or carbonate se-

quences of limestone, dolomite, anhydrite andshale. (Figure A1).

Clastic DepositionClastic rocks are formed from rock frag-

ments and weathered particles of preexistingrocks. These sediments are transported bywind and water and are usually deposited inrivers, lakes and oceans as relatively flat-lyingbeds. Current and wave action later sorts thesediments such that in high-energy environ-ments coarse-grained sands are deposited andin low energy environments fine-grained siltsand clays are deposited. The nature of the

deposition is such that crossbedding structures,

channel patterns and gradational rock types arecommon. In areas of freshwater depositioncoal beds may occur, indicating non-marineconditions.

After deposition and with deeper burial of the sequence, compaction occurs and the clas-tic grains can become cemented together toform sedimentary rock.

Carbonate DepositionCarbonate deposition occurs in marine con-

ditions by the precipitation of limestone fromorganisms as fine particles, shells or massivegrowths. Limestones are deposited either asflat-lying beds on the ocean floor or asmounds or pinnacle reefs.

Barrier reef chains that grow in this mannermay form restricted ocean basins landward, inwhich dolomite and anhydrite are precipitatedby the evaporation of seawater.

When limestones form near shore, theremay be mixing of limestone and eroded clasticmaterial. In deeper ocean basins, limestoneand shale mixtures are common.

After deposition, later burial may causedolomitization of the limestone in which theactual composition of the rock is changed todolomite.

Because of their brittle nature compared withother sediments, limestones tend to fracturewith deformation, which increases permeabil-ity and helps in the dolomitization process.









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Therefore: Rt α 1/ φ. (2)

Cube E Replace 30% of remaining porosity φ with

hydrocarbons. Resistivity Rt increases with

decreasing water saturation S w

(i.e. as S w de-

creases, Rt increases).

Therefore: Rt α 1/ S

w. (3)

By combining the above observations (1, 2and 3), we can say

1 1 R

tα R

w × ×

φ S w

or

Rw

Rt α (4)

φ S w

To solve for the constants of proportionalitylet us first limit the equation as follows:

Let S w = 100% (i.e. there is no hydrocar-

bon present and the porosity is 100%water filled).

Then, define Ro = R

t (ie: R

o is the wet resis-

tivity of the formation for the condition S w =

100%):

Rw

Ro

α (5)

φ

Now, let φ = 1, then Ro

α Rw

.

Now, let F = constant of proportionalitydefined as the formation factor.

Therefore: Ro = FR

w

Ro

or F = (6) R

w

Returning to Equation 5 and introducing po-rosity as a variable, it is clear that

1F α

φ

This is intuitively obvious as the relationshipbetween R

o and R

w is related to that particular

unit cube of rock and its porosity characteris-tics.

Through empirical measurements, it wasdetermined that

aF = (7)

φm

where a = constant m = cementation factor



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The cementation factor m relates to the po-rosity type and how it will transmit electrical

current to the actual rock (also called tortuos-ity).

Using the above equations

Recall Ro = FR

w(Equation 6)

aRw

Rt

= Ro = when S

w = 100%

φm

if S w ≠ 100%, then

aRw

1 R

t α ×

φm S w

1

or Rt

α Ro

× S

w

Ro

or S w α (8)

Rt

Through laboratory measurements, it wasfound that this relationship (8) is dependent onthe saturation exponent n as

Ro

S w

n

=

Rt

FRw

or S w

n

= R

t

aRw

or S wn = (9)φm R

t

Equation 9 forms the Archie relationship thatis the basis for all conventional log interpreta-tion techniques. Enhancements and refine-ments may be applied for the morecomplicated rock types.

The remainder of this course is dedicated to

measuring, evaluating and using porosity andresistivity to calculate water saturation andhence hydrocarbons reserves using the con-cepts of this equation.

A.5 DEFINITIONS

a) Formation Porosity ( φ)Defined as the fraction of total volume occu-

pied by pores or voids, where

pore volumeφ = × 100%

total volume

When the pore space is intergranular it isknown as primary porosity . When the poros-ity is due to void space created after deposition,(e.g., vugs or fractures in carbonates), the po-rosity is known as secondary porosity . Whenshale is present, the pore space occupied by thewater in the shale is included with the pore

space in the rock to give total porosity (φT ). If only the rock pore space is considered in ashaly formation, the pore space is called effec-tive porosity (φe ).



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b) Formation Resistivity (R)

Defined as the resistance offered by a for-mation to the flow of electrical current. It isexpressed in ohm-meter 2 /meter.

We use several terms to describe formationresistivity under various circumstances of fluidcontent.

Rt : Describes the resistivity of a for-

mation undisturbed by the drillingprocess.

Ro: Describes a special form of R

t . It

is the resistivity of a clean forma-tion when all pore space is filledwith connate water ( R

w).

Rw: Is the symbol for the resistivity of

formation (connate) water.

c) Formation Factor ( F )

For ResistivityAn important relationship exists between the

resistivity of a fully water saturated formationand the resistivity of the contained water. Theratio of these two values is called formationresistivity factor (or more commonly, forma-tion factor) where:

Ro

F = R

w

F is a constant for the formation under con-sideration. The value of F for any particularformation depends on:

- formation porosity- pore distribution- pore size- pore structure.

For Porosity

In a 1942 paper Gus Archie proposed thatthe relationship between formation factor andporosity could be described by the formula

aF =

φm

wherea = empirical constant.m = cementation factor.

Some recommended F and φ relationshipsare

0.62F = (for sands) φ2.15

0.81F = (for sands) φ2

1F = (for carbonates) φ2

Chart Por-1 (figure A4) in the Log Inter- pretation Chart book is based on several dif-ferent F -φ relationships.

d) Water Saturation ( Sw)

Defined as the fraction of pore volume filledwith water where

water filled pore volumes

w = × 100%

total pore volume



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e) Hydrocarbons Saturation ( S h y

)

Defined as the fraction of pore volume filledwith hydrocarbons where:

hydrocarbon-filled pore volumeS

hy = × 100%

total pore volume

or S hy = 1 – S

w.

f) Clean FormationsThe term clean formation refers to those thatare shale free .

g) Shaly Formations

This describes formations where some of theformation void space (porosity) is filled withshale.

Shale distribution is considered to be:- Laminated: The formation is built up

of thin laminae of sand and shale.- Dispersed: The shale particles are dis-

persed in the pore space.- Structural: The shale replaces matrix.



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Formation Resistivity Factor versus Porosity

2.5 5 10 20 50 100 200 500 1000 2000 5000 10,000

2.5 5 10 20 50 100 200 500 1000 2000 5000 10,00050

40

30

25

20

15

109

8

7

6

5

4

3

2

1

FR, formation resistivity factor

φ ,

p o r o s

i t y (

p . u . )

1.4

1.6

1.82.0

2.2

2.5

2.8

FR = 0.81φ2

FR = 1φ2

FR = 0.62φ2.15

FR = 1φm

m

Vugs orspherical pores

Fractures

This chart gives a variety of formation resistivity factor-to-porosity conversions. The proper choice is bestdetermined by laboratory measurement or experience in the area. In the absence of this knowledge,recommended relationships are the following:

0.62 0.81For Soft Formations: Humble Formula: Fr = or Fr =φ 2.15 φ 2

0.62For Hard Formations: Fr = with appropriate cementation factor, m.

φ m

EXAMPLE: φ is 6% in a carbonate in which a cementation factor, m of 2 is appropriateTherefore, from chart, Fr = 280.

Chart Por- 1

Figure A4



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h) Key Formulas

FR w

Archie’s formula: S wn =

R t

where n is usually taken as 2

Formation Factor: Ro

a. From deep resistivity F = Rw

R xo

b. From shallow resistivity F = Rmf

ac. From porosity F =

φm

i) Key SymbolsBHT - bottom hole temperature in degreesCelsius

di - average diameter of invaded zone( Di )

h - bed thickness in meters R IDPH - resistivity from the deep phasor in-

duction R IMPH - resistivity from the medium Phasor

induction RSFL - resistivity from the Spherically Fo-

cused Log Rm - resistivity of the mud

Rmf - resistivity of the mud filtrate Rmc - resistivity of the mudcake Rw - resistivity of the formation water Rwa - apparent resistivity of the formation

water Rt - resistivity of the formation

(uncontaminated zone) Ro - resistivity of the formation when

100% water filled R xo - resistivity of the flushed zone

(close to borehole) Rsh - resistivity of the shales

F - formation resistivity factorφ - porosity in percentS w - water saturation, percent of pore

space occupied by water in uncon-taminated zone

S xo - water saturation, as above, influshed zone

S hc - hydrocarbons saturation as percentof pore space occupied by water

K - coefficient in the sp formulaSSP - static spontaneous potential - the

maximum possible for a particular Rmf / Rw

PSP - pseudostatic spontaneous poten-tial—the SP found in a thick shalysand

k - permeability in millidarcies

pore volumeφ - porosity = ×100%. total volume

φ S - sonic porosityφ D - density porosityφ N - neutron porosity

φ N + φ D

φ T - total porosity 2

φ e - effective porosityφ 2 - secondary porosityV sh - volume of shaleP

e - photoelectric index

A complete list of symbols and subscripts isincluded in Section J (Miscellaneous).



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A.6 LOG SCALES AND PRESENTATIONS

a) Well logs provide a continuous graph of for-mation parameters versus depth.

Normal depth scales are- 1:240 —1 m of log per 240 m of

measured hole depth. Each lineis 1 m, with heavy lines every 5m, and heavier lines every 25 mfor ease of reading. Depths areindicated every 25 m (FiguresA5 and A6).

- 1:600— 1 m of log per 600 m of measured hole depth. Each lineis 5 m, with heavy lines every 25m. Depths are indicated every 25m (Figure A7).

- Other scales are available. Theseinclude 1:1200, 1:120, 1:48 and1:5.

- Log grids may be either loga-rithmic (resistivity logs—FigureA6) or linear (porosity logs—Fig-ure A5).

b) If a caliper device is present or the log being

generated is a type of sonic log, event markersare placed on each side of the depth track inte-grating the quantity of hole volume or transittime recorded.

1. Integrated hole volume—requires caliperdevice (Figure A5)

- placed on the left side of thedepth track

- small marks indicate 0.1 m3

whereas large marks represent1.0 m

3.

2. Integrated cement volume—Requirescaliper device plus future casing size

- placed on the right side of thedepth track when space per-mits— and if sonic not present

- small marks indicate 0.1 m3

while large marks represent1.0 m

3.

3. Integrated transit time—Requires sonictool (Figure A5)- placed on the right side of the

depth track - small marks indicate 1 msec

whereas large marks represent 10msec of time.

If the log is recorded using logging-while-drilling methods, event markers on both sidesof the depth track (Figure A6) represent theconversion from time-based sampling to adepth-based presentation. The markers there-fore indicate the number of data samples perunit depth. In other words, the larger the con-centration of markers over a depth interval, thegreater the number of data samples used tomake the log.

c) Logs also have headings and inserts.- Log headings provide such information as

well depth, casing depth, mud params,maximum temperature and other co m-ments pertinent to the evaluation of logdata (Figures A8 and A9).

- Inserts provide such information as curvescaling, coding, date/time of acquisition,data curve first-reading points and con-stants pertinent to the logging run fol-lowing the insert. Curve coding on thelog data indicates the deepest reading pri-mary measurement (long dashed) to theshallowest reading primary measurement(solid) when two or more measurementsare combined (Figure A10).





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Logarithmic Grid 1/240 Scale Data Sample Event Markers for LWD Curves

Figure A6



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Figure A7: Linear Grid 1/600 Scale





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Figure A9: Log Heading (page 2) and Log Tail



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Figure A10: Log Insert

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02_Log Evaluation Theory