Chap9 Density Lecturenotes

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Chapter 9 -Density logs

Lecture notes for PET 370

Spring 2012

Prepared by: Thomas W. Engler, Ph.D., P.E.



Density Log

• Porosity/Lithology Determination

– Mineral identification in evaporates

– Gas detection (w/ other logs)

– Estimate mechanical properties (w/ sonic)

– Evaluation of shaly sands and complex lithologies

• other

–

Determination of hydrocarbon density – Determination of oil shale yield

– Identify overpressured zones

Uses



Density Log

Logging speed:

15 to 30 fpm

Depth of investigation:

3 to 6”

Tool Schematic

Vertical

Resolution ~ 1 ft



Density Log Principle of Operation

Bassiouni, 1994



Density Log

Density logging is based on measuring the attenuation

of gamma rays caused by compton scattering, (CPS per

energy)

Theory



Density Log

1. Compton scattering depends only on the electron density of the formation(# of electrons/cc)

2. The electron density is related to the bulk density by,

Where for common elements in sedimentary rocks,

But for H2 the ratio is double.

3. To account for variations in Z/A, the tool is calibrated in a freshwater filledlimestone formation.

Where rba is the apparent bulk density.

1883.0e0704.1 ba rr

Calibration

rrA

Z2

be

00.1

A

Z2



Density Log

• r ba r b for most common sedimentary rocks

• For other formations (salt, anhydrite, coal) corrections are necessary.

Calibration



Density Log FDC-Borehole Correction



Density LogTeague (Mckee) Field

La Munyon #76

Caliper

Incorrect

Density

Readings



Density Log

dolofor2.86

lmsfor2.71

ssfor2.65

densitymatrixma

where

f ma

bma

or

f

ma)1(

b

oncontributi

fluid pore

oncontributi

matrix

density bulk

Measured

r

rrrr

rrr

Porosity



Density Log

• Define rf as average density of fluid in pore space

• Depth of investigation of density tool is shallow (invaded zone), thus

• In practice,

Mud Type rmf (gm/cc)oil 0.9

Fresh water 1.0

Salt water 1.1

•In water-bearing zones,

Sxo = 1 and rf = rmf

• In oil-bearing zones, use same rf = rmf Assumes Sxo is large and rh rmf .

h)

xoS1(

mf xoS

f rrr

Porosity



Density Log

The density porosity is usually calculated assuming rf

= 1 gm/cc.Compare this apparent porosity, a, to the true porosity, t , thatcorresponds to a bulk density of 2.1 gm/cc in the following:

(1). a water-bearing sandstone invaded by a mud filtrate of 1.05 gm/cc

density

Porosity-Example

%3.330.165.2

1.265.2

f ma

bma

a

rr

rr

%4.3405.165.2

1.265.2

f ma

bma

t

rr

rr



Density Log



(2). a 0.8 gm/cc oil-bearing sandstone characterized by Sor = 30%.

Porosity-Example

%3.330.165.2

1.265.2

f ma

bmaa

rr

rr

975.0

8.0*3.005.1*7.0

h)

xoS1(

mf xoS

f

rrr

%8.32975.065.2

1.265.2

f ma

bma

t

rr

rr



Density Log



(3). a low pressure gas-bearing sandstone with 30% residual gas

saturation.

Porosity-Example

%3.330.165.2

1.265.2

f ma

bma

a

rr

rr

735.0

0*3.005.1*7.0

h)

xoS1(

mf xoS

f

rrr

%7.28735.065.2

1.265.2

f ma

bma

t

rr

rr



Density Log



(4). a change in matrix density to 2.68 gm/cc.

Porosity-Example

%3.330.165.2

1.265.2

f ma

bma

a

rr

rr

%5.340.168.2

1.268.2

f ma

bma

t

rr

rr



Density Log

Example:

mf = 1.1 gm/cc

ma = 2.71 gm/cc (reference)

A

B

C

Porosity?

rb = 2.81

rb = 2.68

rb = 2.40



Density Log

• Photoelectric absorption depends on both re and the average atomic number

of the formation.

•Two independent parameters, re and U, are measured in a low energy window

LDT - measurement



Density Log

The absorption rate depends on the absorption coefficient per electron (Pe) and re;

1883.0 b

U0704.1

eP

density,electronforSubstitute

3cm

electronse*

electron

barnseP3cm

barns U

r

r

LDT - measurement



Density Log

Advantage:

The Pe curve distinguishes mineralogy regardless of porosity and fluid type inthe pore space.

Common Pe values: quartz 1.81

calcite 5.08

dolomite 3.14

anhydrite 5.05

LDT - measurement



Density Log

LDT –Example

mf = 1.0 gm/cc

Zone Y

Pe = ?

rb = ?

rma = ?

= ?

Zone X

Pe = ?

rb = ?

rma = ?

= ?

Dog Canyon (Wolfcamp) Field



Density LogDog Canyon (Wolfcamp) Field

Blackbird BKR St #2

Pe curve



Density Log

LDT• Detector counts high energy gamma rays (compton scattering) and low

energy region (photoelectric effect)

• Measures both bulk density and Pe

• Pe is strong function of matrix

• Lower statistical variation

•

Better vertical resolution

FDC• Detects only high energy gamma rays

• Measures only bulk density

•

Strong function of porosity, matrix, and fluids

LDT/FDC comparison



Density Log

Overpressured shale

Overpressured

Normal

Pressuregradient



Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook

Series, Vol. 4, (1994)

Chapter 2, Sec 6 – 8

Chapter 8

Schlumberger, Log Interpretation Charts, Houston, TX (1995)

Schlumberger, Log Interpretation and Principles, Houston, TX (1989)

Western Atlas, Log Interpretation Charts, Houston, TX (1992)

Western Atlas, Introduction to Wireline Log Analysis, Houston, TX (1995)

Halliburton, Openhole Log Analysis and Formation Evaluation, Houston, TX (1991)

Halliburton, Log Interpretation Charts, Houston, TX (1991)

Density Log References

Teague (Mckee) Field



Density LogTeague (Mckee) Field

La Munyon #76

Caliper

Pe curve

0 10

Chap9 Density Lecturenotes

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