Balikpapan, 27 November 2012Density Log

Course Objectives

*Understand the basic theory of density log*Able to understand log format ofdensity log*Able to interprete densitylog*Able to determine porosityfrom densitylog

Outline

*Theorytical background*Application*Densitylog format*Densitylog interpretation*Porosity calculation*Conclusions

Theoretical Background

Application

*Porosity log*Mineral identification*Gas detection*Overburden pressure identification*Evaluation of shalysand

Density Log Format

Log Interpretation

Over pressure

identification

Gas

identification

Porosity Calculation

Neutron Log

Course Objectives

*Understand the basic theory of neutron log*Able to understand log format of neutron log*Able to interpreteneutronlog*Able to determine porosityfrom neutronlog

Outline

*Theorytical background*Application*Neutronlog format*Neutronlog interpretation*Porosity calculation*Conclusions

Theorytical Background

* It measures hydrogen concentration in the formation*Hydrogen is mostly in water, oil and gas.*Neutron log can be recorded in cased or open hole*Neutron log will record all of the hydrogen ion

Neutron Source

Two types of neutron sources are used in well logging:

* Chemical Source

* Electronic Source

Americium, berylium

Deutorium, tritium

Neutron Sources Comparison

Chemical (AmBe)

Simple and reliable

Always on

Low neutron output

Low energy (4-5 MeV)

Must be kept in a shield to protect personnel

Electronic (accelerator)

Complex design

Can be pulsed (turned on and off)

High neutron output (eight times more)

High energy (14 MeV)

No measurable radioactivity when turned off

From Ratio to Porosity

Ratio (TNRA)

Poro

sity (

NP

HI)

0.0

0.5

0.4

0.3

0.2

0.1

0 41 2 3

Sandstone

Limestone

Dolomite

Neutron Log Application

As a porosity log

Lithology identification

Gas identification

Neutron Log Format Display

Neutron Log Interpretation

The compensated neutron logging tool does NOT measure porosity. It measures a

hydrogen index. The hydrogen index is related to porosity, but is affected by lithology

and fluid properties.

The measurement is displayed as porosity.

Porosity is usually computed with limestone matrix.

After all borehole corrections, the measurement only represents true formation

porosity in a clean limestone formation filled with fresh water.

Neutron Log Interpretation

Porosity vs. Matrix

Schlumberger Chart Por-13b

Apparent Limestone Porosity (p.u.)

Tru

e P

oro

sity f

or

Ind

icate

d M

atr

ix

20 p.u.

25 p.u.

17 p.u.

20 p.u.

Light Hydrocarbon Effect

Neutron porosity is calibrated assuming that the fluid in the pore

space is fresh water (hydrogen index equal to 1).

Gas has a very low hydrogen index: the amount of hydrogen per unit

volume is much less than in water.

In a gas-

than it would if the same reservoir were filled with water.

What effect does this have on inferred porosity?

Matrix

The Excavation Effect

The two formations sketched below have exactly the same amount of hydrogen.

The neutron log, however, will show different porosities because of the different

amounts of matrix, which also contributes to the slowing down of neutrons.

Computing porosity using only the hydrogen index of the fluid will give a porosity

that is too low.

Gas

Water

Matrix Water