3.introduction to oh log interpretation

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Contents

A1.0 INTRODUCTION TO OPEN HOLE 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 (F) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

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

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

d) Water Saturation: Sw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

e) Hydrocarbon Saturation (Shy

) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

f) Clean Formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

g) Shaly Formations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

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

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

A.6 LOG SCALES AND PRESENTATIONS... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 2



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Introduction to Open Hole Logging



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A 1 . 0 Introduction to Open Hole Log

Interpretation

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 Hydrocarbon in this

well?• What type of Hydrocarbon?• 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 Key-Seats or severe Dog-

legs 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 Hydrocarbon per

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

The Production Engineer:• Where should the well be completed (in

what 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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Thus log evaluation can be many things to

many people. As the answers are sought eachindividual will possibly use the available datain a different manner. The common approachwill be in reading the logs and understandingthe various reactions produced by formationcharacteristics on our logging devices. Thefactors influencing log reading and the infor-mation they provide are what we wish to intro-duce to you in this course.

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

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

Common sedimentary rocks are:Sandstone, Siltstone, Shale, Lime-stone, Dolomite, 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.

Clastic DepositionClastic rocks are formed from rock frag-

ments and weathered particles of pre-existingrocks. 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 silts

and clays are deposited. The nature of the

deposition is such that cross-bedding struc-tures, channel patterns and gradational, rock types are common. In areas of fresh waterdeposition coal beds may occur indicating non-marine conditions.

After deposition and with deeper burial of thesequence, compaction occurs and the clasticgrains can become cemented together to formsedimentary 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 as moundsor pinnacle reefs.

Barrier reef chains which grow in this man-ner may form restricted ocean basins land-ward, in which dolomite and anhydrite areprecipitated by the evaporation of sea water.

When limestones form near shore, there may

be mixing of limestone and eroded clastic ma-terial and in deeper ocean basins limestone andshale 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 toother sediments, limestones tend to fracturewith deformation which increases permeabilityand helps in the dolomitization process.



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Figure A1: Clastic Deposition vs. Carbonate Deposition



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In many parts of the world multiple sequencesof clastic rocks overlie older carbonate se-

quences. Between each of the clastic and car-bonate groups, erosional inconformities arecommon and the nature of deposition withineach group is unique.

A.3 BASIC LOG INTERPRETATION CONCEPTS

Any given rock formation has numerousunique physical properties associated with it.Only those which can be measured and are use-ful will be considered in this course. They are:

a. Φ = Porosity: The void space betweengrains that is generally filled with liq-uids or gases.

b. S w

= Water Saturation: The percent-

age of the pore space filled with water(as opposed to hydrocarbon or air).

c. R = Resistivity: The resistance toelectrical current flow presented by aunit volume of rock.

d. RW

= Water Resistivity: The electrical

resistance of the water filling the pore

space in the rock. This value varieswith water salinity and temperature.e. k = Permeability: The ability of the rock

to pass fluids through it.

Consider the following unit cubes (FigureA2):

Cube A

If the porosity (Φ) is filled with water then,by definition, the water saturation S

W = 100%.

Cube BIf the porosity Φ is 70% filled with water and

30% hydrocarbon, then, the water saturation

70 S

W = % = 70%

70 + 30

and hydrocarbon saturation

Cube “A”:

porosity = Water filled Φ

S W

= 100%

Cube “B”:

porosity = Hydrocarbon and

water in Φ

S W

= 70%

Figure A2



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Shyc

= 1 - Sw = 30%

Therefore the percentage volume of watersaturation

= Φ x Sw

For example: if Φ = 20% and Sw = 70%

Then: 14% of the bulk volume is water and70% of the pore space is water filled.

A.4 RESISTIVITY AS A BASIS FORINTERPRETATION - THE AR-CHIE EQUATION

In the previous section we introduced a num-ber of parameters used to evaluate rock forma-tions. If we could build on the effects of resistivity in conjunction with the other pa-rameters to develop a mathematical relationship,we would have an extremely useful tool for ourwork with potential hydrocarbon zones.

The remainder of this section will be devotedto developing such a formula.

The usefulness of resistivity logging rests onthe fact that:

- water is a conductor (low resistivity)- hydrocarbons and rocks are insulators

(high resistivity)

Consider the following unit cubes (Figure A3):

Cube C The resistivity R

tof the cube will vary with

water resistivity Rw (ie: as R

w increases, R

t in-

creases and vice versa.)

Therefore: Rt α Rw. (1 )

Cube D

Replace 25% of the cube with rock (hence Φ

= 75%) but maintain a constant Rw. Resistivity

Rt increases with decreasing porosity Φ (ie: as

Φ decreases, Rt increases.)

Cube “C” Conditions:

- Constant Current - Porosity = 100%

- S w = 100%

Cube “D” Conditions:


- S w = 100%

Cube “E” Conditions:


- S w = 70%

Figure A3



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Therefore: Rt α 1/ Φ. ( 2 )

Cube E

Replace 30% of remaining porosity Φ withhydrocarbon. Resistivity R

t increases with de-

creasing water saturation Sw

(ie: as Sw de-

creases, Rt increases.)

Therefore: Rt α 1/S

w.( 3 )

By combining the above observations (1,2

and 3), we can say:

1 1

Rtα R

w

x x

Φ Sw

or

Rw

Rtα ( 4 )

Φ Sw

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

Let Sw = 100% (ie: There is no hydrocar-

bon present and the porosity is 100%water filled)

then define Ro = R

t (ie: R

o is the wet resistiv-

ity of the formation for the condition Sw =

100%)

Rw

Roα (5)

Φ

Now let Φ = 1, then Roα R

w

Now let F = constant of proportionalitydefined as Formation Factor.

Therefore: Ro = FR

w

Ro

or F = (6)

Rw

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

1

F 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 was de-termined 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 (sometimes calledtortuosity).

Using the above equations

Recall Ro = FR

w(Equation 6)

aRw

Rt

= Ro = when S

w = 100%

Φm

if Sw

≠ 100%, then

aRw

1

Rtα x

Φm Sw

1

or Rtα R

ox

Sw

Ro

or Swα (8)

Rt

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

Ro

Sw

n

=

Rt

FRw

or Sw

n

=

Rt

aRw

or Sw

n

= ( 9 )Φm R

t

Equation nine forms the Archie Relationshipthat is the basis for all conventional log inter-pretation techniques. Enhancements and re-finements may be applied for the morecomplicated rock types.

The remainder of this course is dedicated tomeasuring, evaluating and using porosity and

resistivity to calculate water saturation andhence hydrocarbon reserves using the conceptsof this equation.

A.5 DEFINITIONS

a) Formation Porosity (Φ)

Defined as the fraction of total volume occu-pied by pores or voids, where:

Pore Volume

Φ = x 100% Total Volume

When the pore space is intergranular it isknown as primary porosity. When the porosityis 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 a

shaly formation, the pore space is called Effec-tive Porosity (Φ

e).



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b) Formation Resistivity (R)

Is defined as the resistance offered by a for-mation to the flow of electrical current. It isexpressed in ohm-metre2 /metre.

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 formationwhen all pore space is filled withconnate water (R

w).

Rw: Is the symbol for the resistivity of

formation (connate) water.

c) Formation Factor (F)

For Resist ivity:

An important relationship exists between theresistivity of a fully water saturated formationand the resistivity of the contained water. Theratio of these two values is called Formation

Resistivity Factor (or more commonly, For-mation 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 Interpre-tation Chart Book is based on several different

F-Φ relationships.

d) Water Saturation: Sw

Is defined as the fraction of pore volumefilled with water where:

Water Filled Pore VolumeS

w = x 100%

Total Pore Volume



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e) Hydrocarbon Saturation (Shy

)

Defined as the fraction of pore volume filledwith hydrocarbons where:

Hydrocarbon Filled Pore VolumeS

hy = x 100%

Total Pore Volume

OR Shy

= 1 - Sw.

f) Clean Formations

The 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 =0.81

φ2

FR =1

φ2

FR =0.62

φ2.15

FR =1

φmm

Vugs orspherical pores

Fractures

This chart gives a variety of formation resistivity factor-to-porosity conversions. The proper choice is best de-

termined 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

0.62

For 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 appropriate

Therefore, from chart, Fr = 280.

Chart Por-1

Figure A4



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h) Key Formulas

FRw

Archie’s Formula: Sw

n =

Rt

where n usually taken as 2

Formation Factor:

Ro

a. From deep resistivity F =

Rw

Rxo

b. From shallow resistivity F =

Rmf

a

c. From porosity F =Φm

i) Key Symbols

BHT - Bottom Hole Temperature in degrees

Celsius.

di - Average Diameter of Invaded Zone.

(Di)

h - Bed Thickness in Metres.

RIDPH

- Resistivity from the Deep Phasor In-

duction.

RIMPH

- Resistivity from the Medium Phasor

Induction.

RSFL

- Resistivity from the Spherically Fo-

cused Log.

Rm - Resistivity of the Mud.R

mf - Resistivity of the Mud Filtrate.

Rmc

- Resistivity of the Mud Cake.

Rw - Resistivity of the Formation Water.

Rwa

- Apparent Resistivity of the Forma-

tion Water.

Rt - Resistivity of the Formation - Un-

contaminated Zone.

Ro - Resistivity of the Formation when

100% water filled.

Rxo

- Resistivity of the Flushed Zone

(Close to Borehole).R

sh - Resistivity of the Shales.

F - Formation Resistivity Factor.

Φ - Porosity in Per Cent.

Sw - Water Saturation, Per Cent of Pore

Space occupied by water in uncon-

taminated zone.

Sxo

- Water Saturation, as above, in

Flushed Zone.

Shc

- Hydrocarbon Saturation as Per Cent

of Pore Space; = (1 - Sw).

K - Coefficient in the SP Formula

SSP - Static Spontaneous Potential - The

Maximum Possible for a particular

Rmf

/ Rw.

PSP - Pseudostatic Spontaneous Potential

- The SP Found in a Thick Shaly

Sand.

k - Permeability in millidarcies. Pore Volume

Φ - Porosity = *100%.

Total Volume

ΦS - Sonic Porosity.

ΦD - Density Porosity.

ΦN - Neutron Porosity.

ΦN + Φ

D

ΦT - Total Porosity ≅ .

2

Φe - Effective Porosity.

Φ2

- Secondary Porosity.

Vsh - Volume of Shale.

Pe - Photoelectric Index.

A complete list of Symbols and Subscripts isincluded in Section J (Miscellaneous)



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A.6 LOG SCALES AND PRESENTATIONSa. Well logs provide a continuous graph of

formation parameters versus depth.

Normal depth scales are:- 1:240 1 metre of log per 240

metres of measured hole depth.Each line is one metre, withheavy lines every 5 metres, andheavier lines every 25 m forease of reading. Depths are in-dicated every 25 metres(Figures A5 and A6).

- 1:600 1 metre of log per 600metres of measured hole depth.

Each line is 5 m, with heavylines every 25m. Depths are in-dicated every 25 metres (FigureA7).

- Other scales are available.These include 1:1200, 1:120,1:48 and 1:5.

- Log grids may be either loga-rithmic (Resistivity logs - Fig-ure A6) or linear (Porosity logs- Figure A5).

b. If a caliper device is present or the log be-ing generated is a type of sonic log, eventmarkers will be placed on each side of thedepth track integrating the quantity of holevolume or transit time recorded.

1) Integrated Hole Volume - Requirescaliper device (Figure A5)

- placed on the left side of thedepth track

- small marks indicate 0.1m3

while large marks represent

1.0m3

.

2) Integrated Cement Volume - Requirescaliper device plus future casing size- placed on the right side of the

depth track when space permits- and if sonic not present

- small marks indicate 0.1m3

while large marks represent

1.0m3

.

3) Integrated Transit Time - Requiressonic tool (Figure A5)

- placed on the right side of thedepth track

- small marks indicate 1 milli-second while large marks rep-resent 10 milliseconds of time.

If the log recorded is via Logging-While-Drilling methods, event markers on bothsides of the depth track (Figure A6) repre-sents the conversion from a time-basedsampling to a depth-based presentation.The markers therefore indicate the number

of data samples per unit depth. In otherwords, the larger the concentration of markers over a depth interval, the greaterthe number of data samples used to makethe log.

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

as well depth, casing depth, mud pa-rameters, maximum temperature, andother comments pertinent to evaluationof log data (Figure A8 and A9).

- Inserts provide such information ascurve scaling, coding, date/time of ac-quisition, data curve first reading pointsand constants pertinent to the loggingrun following the insert. Curve codingon the log data indicates the deepestreading primary measurement (longdashed), to the shallowest reading pri-mary measurement (solid), when two ormore measurements are combined(Figure A10).



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Figure A5: Linear Grid 1/240 Scale



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a) Logarithmic Grid 1/240 Scale

b) 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 A8: Log Heading (page 1)



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



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Figure A10: Log Insert

3.introduction to oh log interpretation

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