Chap9 Density Lecturenotes

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 1/25

Chapter 9 -Density logs

Lecture notes for PET 370

Spring 2012

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 2/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 3/25

Density Log

Logging speed:

15 to 30 fpm 

Depth of investigation:

3 to 6” 

Tool Schematic

Vertical

Resolution ~ 1 ft

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 4/25

Density Log Principle of Operation

Bassiouni, 1994

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 5/25

Density Log

Density logging is based on measuring the attenuation

of gamma rays caused by compton scattering, (CPS per

energy)

Theory

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 6/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 7/25

Density Log

• r ba  r b for most common sedimentary rocks

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

Calibration

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 8/25

Density Log FDC-Borehole Correction

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 9/25

Density LogTeague (Mckee) Field

La Munyon #76

Caliper

Incorrect

Density

Readings

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 10/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 11/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 12/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 13/25

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:

(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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 14/25

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:

(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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 15/25

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:

(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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 16/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 17/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 18/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 19/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 20/25

Density Log

LDT  –Example

mf = 1.0 gm/cc

Zone Y

Pe = ?

rb  = ?

rma = ?

= ?

Zone X

Pe = ?

rb  = ?

rma = ?

= ?

Dog Canyon (Wolfcamp) Field

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 21/25

Density LogDog Canyon (Wolfcamp) Field

Blackbird BKR St #2

Pe curve

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 22/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 23/25

Density Log

Overpressured shale

Overpressured

Normal

Pressuregradient

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 24/25

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

8/10/2019 Chap9 Density Lecturenotes

http://slidepdf.com/reader/full/chap9-density-lecturenotes 25/25

Density LogTeague (Mckee) Field

La Munyon #76

Caliper

Pe curve

0 10