Log Interpretation

Introduction to Openhole Logging

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Contents

A1.0 INTRODUCTION TO OPENHOLE LOG INTERPRETATION 1

A.1 USES OF LOGS1

A.2 BASIC PETROLEUM GEOLOGY2

A.3 BASIC LOG INTERPRETATION CONCEPTS4

A.4 RESISTIVITY AS A BASIS FOR INTERPRETATIONTHE ARCHIE EQUATION5

A.5 DEFINITIONS 7

a)Formation Porosity ()8Formation Resistivity (R)8Formation Factor (F)8Water Saturation: Sw 8Hydrocarbons Saturation (Shy)9Clean Formations 9Shaly Formations9Key Formulas 11Key Symbols11

A.6 LOG SCALES AND PRESENTATIONS12


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A1.0 Introduction to Openhole Log

Interpretation


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A.1 USES OF LOGS

A set of logs run on a well will usually mean different things to different people. Let us ex-amine the questions askedand/or answers sought 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 this

well?

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 (in what zone(s))?

What kind of production rate can be expected?

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 individual will possibly use the available data in a different manner. The common approach will be in reading the logs and understanding the various reactions produced by formation characteristics on our logging devices. The factors influencing log reading and the information they provide are what we wish to introduce to you in this course.

deposition is such that crossbedding structures, channel patterns and gradational rock types are common. In areas of freshwater deposition coal beds may occur, indicating non-marine conditions.

After deposition and with deeper burial of the sequence, compaction occurs and the clastic grains can become cemented together to form sedimentary rock.


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A.2 BASIC PETROLEUM GEOLOGY

In order to better understand log responses, we should first review the types of rocks that are found in the boreholes.

Common sedimentary rocks are

sandstone, siltstone, shale, limestone, dolomite and anhydrite

In general, sedimentary rocks are deposited as either clastic sequences containing sand-stone, siltstones and shales or carbonate sequences of limestone, dolomite, anhydrite and shale. (Figure A1).

Clastic Deposition

Clastic rocks are formed from rock fragments and weathered particles of preexisting rocks. These sediments are transported by wind and water and are usually deposited in rivers, lakes and oceans as relatively flat-lying beds. Current and wave action later sorts the sediments such that in high-energy environments coarse-grained sands are deposited and in low energy environments fine-grained silts and clays are deposited. The nature of the

Carbonate Deposition

Carbonate deposition occurs in marine conditions by the precipitation of limestone from organisms as fine particles, shells or massive growths. Limestones are deposited either as flat-lying beds on the ocean floor or as mounds or pinnacle reefs.

Barrier reef chains that grow in this manner may form restricted ocean basins landward, in which dolomite and anhydrite are precipitated by the evaporation of seawater.

When limestones form near shore, there may be mixing of limestone and eroded clastic material. In deeper ocean basins, limestone and shale mixtures are common.

After deposition, later burial may cause dolomitization of the limestone in which the actual composition of the rock is changed to dolomite.

Because of their brittle nature compared with other sediments, limestones tend to fracture with deformation, which increases permeability and 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 sequences of clastic rocks overlie older carbonate sequences. Between each of the clastic and carbonate groups, erosional inconformities are common and the nature of deposition within each group is unique.

A.3 BASIC LOG INTERPRETATION CONCEPTS

Any given rock formation has numerous unique physical properties associated with it. Only those that can be measured and are useful will be considered in this course. They are

a.= porosity: the void space between grains that is generally filled with liquids or gases.Sw = water saturation: the percentage

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

c.R = resistivity: the resistance to electrical current flow presented by a unit volume of rock.RW = water resistivity: the electrical re

sistance of the water filling the pore space in the rock. This value varies with water salinity and temperature.

e.k = permeability: the ability of the rock to pass fluids through it.

Consider the following unit cubes (Figure A2):

Cube A

If the porosity () is filled with water then, by definition, the water saturation SW = 100%.

Cube B

If the porosity is 70% filled with water and 30% hydrocarbons, then, the water saturation

70

SW = % = 70%

70 + 30

and hydrocarbons saturation


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Cube A: porosity = waterfilled SW = 100%

Cube B:porosity = hydrocarbons andwater inSW = 70%


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Figure A2

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S hy = 1 - S w = 30%

Therefore the percentage volume of water saturation

= S

w

For example: if = 20% and Sw = 70%, then

14% of the bulk volume is water and 70% of the pore space is water filled.

A.4 RESISTIVITY AS A BASIS FOR INTERPRETATIONTHE ARCHIE EQUATION

In the previous section we introduced a number of parameters used to evaluate rock formations. If we could build on the effects of resistivity in conjunction with the other parameters to develop a mathematical relation-ship, we would have an extremely useful tool for our work with potential hydrocarbon zones.

The remainder of this section is devoted to developing such a formula.

The usefulness of resistivity logging rests on the facts 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 Rt of the cube will vary with water resistivity Rw (i.e. as Rw increases, Rt in-creases and vice versa).

(1)

Therefore: R. R

tw

Cube D

Replace 25% of the cube with rock (hence= 75%) but maintain a constant Rw. Resistivity

R increases with decreasing porosity (i.e. as

t

decreases, Rt increases).

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Cube C

-Constant Current

-Porosity = 100%

-Sw = 100%

Cube D

-Constant Current

-Porosity = 75%

-Sw = 100%

Cube E

-Constant Current

-Porosity = 75%

-Sw = 70%

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Figure A3

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Therefore: R1/.(2)

t

R w

R

o

(5)

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Now, let = 1, then RoRw .

Now, let F = constant of proportionality defined as the form ation factor.

Therefore: Ro = FRw

Cube E

Replace 30% of remaining porosity with hydrocarbons. Resistivity Rt increases with

decreasing water saturation Sw (i.e. as Sw de-creases, Rt increases).

Therefore: R1/Sw.(3)

t

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

R

o

or F = R

w

(6)

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11 RR

tw

S

w

or

Returning to Equation 5 and introducing porosity as a variable, it is clear that

1 F

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R w

R t(4)

S

w

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

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

w

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

Then, define Ro = Rt (ie: Ro is the wet resistivity of the formation for the condition Sw = 100%):

This is intuitively obvious as the relationship between Ro and Rw is related to that particular

unit cube of rock and its porosity characteristics.

Through empirical measurements, it was determined that

a

F =(7)

m

where

a = constant

m = cementation factor


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The cementation factor m relates to the porosity type and how it will transmit electrical current to the actual rock (also called tortuosity).

Using the above equations

Recall Ro = FRw (Equation 6)

aR

w

when Sw = 100%

if Sw 100%, then

aR1

w

R

t

mS

w

1 or R R

to

S

w

or Sn =

w

aR

w

m R

t

(9)

Equation 9 forms the Archie relationship that is the basis for all conventional log interpretation techniques. Enhancements and refinements may be applied for the more complicated rock types.

The remainder of this course is dedicated to measuring, evaluating and using porosity and resistivity to calculate water saturation and hence hydrocarbons reserves using the concepts of this equation.

A.5 DEFINITIONS

a) Formation Porosity ()

Defined as the fraction of total volume occupied by pores or voids, where

R= R=

to


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or S

w

R

o

R

t

(8)

=

pore volume total volume

100%


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Through laboratory measurements, it was found that this relationship (8) is dependent on the saturation exponent n as

R

t

FR

w

or Sn =

w

When the pore space is intergranular it is known as primary porosity. When the porosity is due to void space created after deposition, (e.g., vugs or fractures in carbonates), the porosity is known as secondary porosity. When shale is present, the pore space occupied by the water in the shale is included with the pore space in the rock to give total porosity (T). If

only the rock pore space is considered in a shaly formation, the pore space is called efective porosity (e).


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b)Formation Resistivity (R)

Defined as the resistance offered by a formation to the flow of electrical current. It is expressed in ohm-meter2/meter.

We use several terms to describe formation resistivity under various circumstances of fluid content.

R: Describes the resistivity of a for

t

mation undisturbed by the drilling process.

R: Describes a special form of Rt. It

o

is the resistivity of a clean formation when all pore space is filled with connate water (Rw).

R: Is the symbol for the resistivity of

w

formation (connate) water.

c)Formation Factor (F)

For Resistivity

An important relationship exists between the resistivity of a fully water saturated formation and the resistivity of the contained water. The ratio of these two values is called formation resistivity factor (or more commonly, formation factor) where:

For Porosity

In a 1942 paper Gus Archie proposed that the relationship between formation factor and porosity could be described by the formula

a

F = m

where

a = empirical constant. m = cementation factor.

Some recommended F and relationships are

0.62

F =(for sands)

2.15

0.81

F =(for sands)

2

1

F =(for carbonates)

2

Chart Por-1 (figure A4) in the Log Interpretation Chart book is based on several different F- relationships.


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R

o

d) Water Saturation (Sw)

F =

R

Defined as the fraction of pore volume filled

with water where

w

F is a constant for the formation under con

sideration. The value of F for any particular

formation depends on:

s =

w

water filled pore volume

100%

total pore volume

- formation porosity - pore distribution - pore size

- pore structure.


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e)Hydrocarbons Saturation (Shy)g) Shaly Formations

Defined as the fraction of pore volume filledThis describes formations where some of the

with hydrocarbons where:formation void space (porosity) is filled with shale.

Shy =

hydrocarbon-filled pore volume

Shale distribution is considered to be: 100%- 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.

total pore volume

orShy = 1 Sw .

f)Clean Formations

The term clean formation refers to those that are shale free.


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Formation Resistivity Factor versus Porosity


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510205010020050010002000500010,000

510205010020050010002000500010,000 FR, formation resistivity factor

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

0.620.81

For Soft Formations: Humble Formula: Fr =or Fr =

2.152

0.62

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

m

EXAMPLE: 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

FR

w

Archies formula: Sw n =

R

o

where n is usually taken as 2

R

w

R

xo

R mf

a


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i) Key Symbols

BHT - bottom hole temperature in degrees Celsius

di- average diameter of invaded zone (Di)

h

RIDPH

RIMPH

RSFL

R

m

R mf R mc

R

w

R wa

-bed thickness in meters

-resistivity from the deep phasor induction

-resistivity from the medium Phasor induction

-resistivity from the Spherically Focused Log

-resistivity of the mud

-resistivity of the mud filtrate

-resistivity of the mudcake

-resistivity of the formation water

-apparent resistivity of the formation

water

Rt-resistivity of the formation (uncontaminated zone)

R- resistivity of the formation when

o100% water filled

-resistivity of the flushed zone

R

xo(close to borehole)

Rsh-resistivity of the shales

F-formation resistivity factor

-porosity in percent

S- water saturation, percent of pore

wspace occupied by water in uncontaminated zone

S

xo

-water saturation, as above, in flushed zone

Shc- hydrocarbons saturation as percent of pore space occupied by water

K- coefficient in the sp formula

SSP- static spontaneous potential - the maximum possible for a particular

Rmf / Rw

PSP - pseudostatic spontaneous potentialthe SP found in a thick shaly sand

k- permeability in millidarcies pore volume

-porosity =100%. total volume

-sonic porosity

-density porosity

-neutron porosity

D N +

-total porosity

2

-effective porosity

-secondary porosity

-volume of shale

-photoelectric index

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


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A.6 LOG SCALES AND PRESENTATIONS

3. Integrated transit timeRequires sonic tool (Figure A5)

-placed on the right side of the depth track

-small marks indicate 1 msec whereas large marks represent 10 msec of time.

If the log is recorded using logging-whiledrilling methods, event markers on both sides of the depth track (Figure A6) represent the conversion from time-based sampling to a depth-based presentation. The markers there-fore indicate the number of data samples per unit depth. In other words, the larger the concentration of markers over a depth interval, the greater the number of data samples used to make 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 comments pertinent to the evaluation of log data (Figures A8 and A9).

-Inserts provide such information as curve scaling, coding, date/time of acquisition, data curve first-reading points and constants pertinent to the logging run fo l-lowing the insert. Curve coding on the log data indicates the deepest reading primary measurement (long dashed) to the shallowest reading primary measurement (solid) when two or more measurements are combined (Figure A10).

a) Well logs provide a continuous graph of formation parameters versus depth.

Normal depth scales are

-1:2401 m of log per 240 m of measured hole depth. Each line is 1 m, with heavy lines every 5 m, and heavier lines every 25 m for ease of reading. Depths are indicated every 25 m (Figures A5 and A6).

-1:6001 m of log per 600 m of measured hole depth. Each line is 5 m, with heavy lines every 25 m. Depths are indicated every 25 m (Figure A7).

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

-Log grids may be either logarithmic (resistivity logsFigure A6) or linear (porosity logsFigure A5).

b)If a caliper device is present or the log being generated is a type of sonic log, event markers are placed on each side of the depth track integrating the quantity of hole volume or transit time recorded.

1.Integrated hole volumerequires caliper device (Figure A5)

-placed on the left side of the

depth track

3

-small marks indicate 0.1 m whereas large marks represent

3

1.0m.

2.Integrated cement volumeRequires

caliper device plus future casing size

-placed on the right side of the depth track when space permits and if sonic not present

3

-small marks indicate 0.1 m while large marks represent

3

1.0m.


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Figure A5: Linear Grid 1/240 Scale


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Logarithmic Grid 1/240 Scale

Data Sample Event Markers for L WD 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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Log Interpretation

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