CIT-ChE

Well Log Analysis.

Density-log

Contents

1. Introduction

2. Density Log Principle

3. Compensated Density Log Tool

4. Density Porosity

Introduction

• Formation bulk density, ρb is used as the primary porosity indicator. Density is used with other log measurements to determine Lithology and fluid type, and with sonic velocity in seismic data interpretation.

Presentations and Scales

Formation bulk density, ρb, is presented in tracks 2 and 3 on a linear scale. The most frequently used scales for ρb are 2.0 to 3.0 g/cm3 or 1.95 to 2.95 g/ cm3.

Density Log Principal

Density Log is a kind of radioactive logs . It uses a chemical cesium-137 gamma ray source to produces 0.66 Mev gamma rays . These gamma rays can react with formation and some of them will be absorbed , others will be detected by density tools . The gamma ray intensity ( count rates ) or energy spectral can be used to estimate formation density.

Cs Ba + - + 137

55

137

56

Gamma Ray Reaction with Formationelectron

positron

Density log tools use cesium-137 source which produces 0.66Mev gamma rays and record the gamma rays with energy larger than 0.2Mev . This is the range Compton scattering take place(see front figure) .The gamma rays absorption has nearly no dependence on lithology but on formation electron density, e .

Suppose : A is atomic weight , Z is atomic number , the electron density is then related to mass density by :

e = ( )molecule2Z

A

Relationship between Gamma Ray absorption and formation density

The dense formation absorbed more gamma rays than light formation.Density tools are designed to calibrate in fresh-water-filled limestone :

for limestone =2.71g/cm3 , 2Z/A = 0.999 ; for water =1.00g/cm3 , 2Z/A = 1.110 ;(see next

Fig. ) Use e , convert equation ,we can get :

e (lim) =2.7076g/cm3 , e(water) =1.1101g/cm3 .

A linear conversion is used to translate the tool’s measurement of electron density to bulk density:

b = 0.1883 + 1.0704 e

Electron Density(e)Calibration to Bulk Density()

Compensated Density Log Tool

•The figure shows a dual detector CDL tool. The source and detector are push to contact with the borehole wall. The short spaced detector is designed to “look” at mudcake while long spaced detector to search formation. The effect of mudcake can be corrected by experimental diagram .

The depth of investigation of a density tool is quite shallow.

The density is subject to effects from mudcake, rugosity, and invasion.

Environmental Effects

Density Porosity

The density of a mixture is a linear function of the densities of its individual constituents, so it is a simple matter to calculate the porosity of a porous rock .

Suppose the clean formation with fluid-filled pores in the right fig. The bulk density will be :

b =(1 )ma+ f

so we can get :

D =

(ma b )

ma f

For example, in a sandstone formation , where:

ma=2.65 g/cm3

b=2.40 g/cm3, and

f=1.15 g/cm3 (salt mud filtrate)

D =(2.65-2.40)/(2.65-1.15)=0.167

Density porosity can be found by using a calculator or a chart

Shale Effects

Suppose the formation be oil-bearing and with shale.

The bulk density will be : b =(1 Vsh )ma+Vshsh

+ f

So,

=

oil

water

shale

matrix

(ma b ) Vsh(ma sh)

ma b

Photoelectric absorption occurs when gamma rays’ energy are less than 0.2Mev. The amount of gamma rays absorbed by formation are related to formation’s average atomic number Z :

Pe = ( )3.6

Pe is photoelectric absorption index .

Pe is measured by Spectral Density tool (SDL) which can record gamma rays with energy less than 0.2Mev . Next Fig. Shows average Z number of some rocks.

Z

10

Pe measurement and application

Average Z numbers for sand, lime and dolomite

Lithology indicator Pe response to typical minerals

Application of Pe as a lithology indicator

Pe is calibrated by limestone that filled with fresh water. Pe value is almost not affected by formation fluid but sensitive to formation lithology . Pe combined with other log curves can distinguish lithology perfectly .