petrophysict 12 Density

Notes

11

Bulk Density Measurement


Schlumberger 1999

Notes

It uses gamma ray interactions in the Compton Scattering energy range to measure the bulk density of the formation.This interaction is when the incident gamma ray reacts with an electron deflecting it from its path and losing energy in the process.The PEX TLD has three detectors and the LDT two.

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Gamma Ray Physics -density -1

The Density Tools use a chemical gamma ray source and two or three gamma ray detectors.

The number of gamma rays returning to the detector depends on the number of electrons present, the electron density, e.The electron density can be related to the bulk density of the minerals by a simple equation.

e = ( 2Z/A )

Where Z is the number of electrons per atomand A is the atomic weight.

Notes

The LDT tool has two detectors measuring the same density. If there is no mudcake (an impermeable formation) both will read the same. If there is mudcake there will be a slight difference which can be computed and hence the measurement corrected.The Spine and Ribs plot is the graphical representation of the method used.

In the case of the TLD, a forward modelling algorithm is used to compute the formation and mud cake densities and the mud cake thickness.

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Spine and Ribs

The spine represents the line of increasing formation density on the plot of the long spacing count rate versus short spacing count rate.

The presence of mud cake causes a deviation from the line in a predictable manner. Thus a correction can be made to obtain the true density.

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1.9

2.0

2.1

2.2

2.3

2.4

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2.8

2.9

Mud cakewith barite

Mud cakewithoutbarite

IncreasingMud cakeThickness


A

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Short spacing Count Rate

Notes

The presence of a lighter mudcake will make the short spacing read too low, moving the point from A to B. It is corrected by following down the ribs to the spine.In heavier mud cake the shift is in the opposite direction.

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Spine and Ribs

Example:The correct reading is at point A.An increasing mud cake thickness moves the point to B or C depending on whether there is heavy material (barite) in the mud or not.

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1.9

2.0

2.1

2.2

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Mud cakewith barite

Mud cakewithoutbarite



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Short spacing Count Rate

Notes

The standard log from the LDT tool shows both of these curves, the principal curve RHOB and DRHO. The latter is a very good indication of poor hole condition. If the hole is rugous the algorithm will have applied a lot of correction, hence the DRHO will be very active. In some cases the caliper will show a large hole but the DRHO will be flat while in some small holes the opposite will be true.The TLD tool has RHOZ as the principal density output, with DSOZ as the density standoff.

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Density Outputs

The outputs are:RHOZ/RHOB (b), the corrected bulk density.

DRHO (), the correction that has beenapplied to b (LDT only).

RHOZ/RHOB is the main output;

DRHO is a quality control curve (LDT only).HDRA is an equivalent in the TLD.

Notes

The measurement has a very little correction as the major perturbation to the reading ( mudcake ) is automatically taken care of using the two detector system. The only problem affecting the reading is the bad hole conditions which affect all pad tools.

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Borehole Effects

The LDT is a pad tool with collimated source and detectors. It experiences little or no environmental effect.

In large holes, the curvature of the pad versus that of the hole causes a minor error that needs to be corrected.

Notes

Rugosity effects can usually be seen on the DRHO curve, the caliper may be irrelevant.One major cause of problems are turbo-drilled holes, deviated or horizontal wells. The use of a turbine can cause a hole to corkscrew. This shape looks to the tools as a very rugous hole, with, in extreme cases, the log being useless.

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Borehole Effects

Hole rugosity may affect the measurement.

The source and detectors "see" different formations/borehole.

The effect is an erratic and incorrect log.

Notes

This method works better on this type of tool than on the neutron porosity devices as the detectors and sources are collimated and smaller.

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Alpha Processing

As the density tool also uses two detectors it can be Alpha processed in exactly the same way as the CNT.

The resulting log shows a great improvement over the standard output.

Notes

The density of each mineral is unique. The tool is calibrated in limestone , sandstone has a lower density and dolomite is higher; shale varies with the precise clay minerals present.The vertical resolutions of the density measurements is better than the neutron tool. With alpha processing a very high resolution can be obtained.

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Density Parameters

Vertical resolution:

Standard 18"Enhanced 6"

Depth of investigation 6"-9"

Readings in:

Limestone (0pu) 2.71Sandstone (0pu) 2.65Dolomite (0pu) 2.85Anhydrite 2.98Salt 2.03Shale 2.2-2.7Coal 1.5

Notes

The density is often used in a development situation as the porosity tool. This is possible if the matrix density is known.Combination with sonic measurements gives both rock mechanical properties and the acoustic impedance. The latter is used in seismic applications.

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Interpretation/Uses

The density tool is extremely useful as it has high accuracy and exhibits small borehole effects.

Major uses include:Porosity.

Lithology (in combination with the neutron tool).

Mechanical properties (in combination with the sonic tool).

Acoustic properties (in combination with the sonic tool).

Gas identification (in combination with the neutron tool).

Notes

The matrix density ma is known from core analysis or from the neutron density crossplot. The fluid density f , is the density of the mud filtrate. This can be measured on a sample or computed knowing the salinity. In the case of oil base mud it has to be measured.

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Density Porosity

There are two inputs into the porosity equation: the matrix density and the fluid density.

The fluid density is that of the mud filtrate.

( ) += 1mafb

fma

bma

=

Notes

The scales on a neutron - density log often reflect the use to which the log is to be put. In an exploration situation or in a complex lithology the identification of the matrix and then the porosity is the major use. In a development well in a simple ( ,or single ) lithology the porosity is the requirement.The basic scale has the neutron having a total of 60 porosity ( % ) over two tracks encompassing the most common values. In the limestone compatible scale the density is fixed to also cover 60 pu ( this is equivalent to 1.0g/cm3 ) and also to overlay the neutron in limestone. In a sandstone the density will be at a higher porosity ( lower density ) than the neutron, in dolomite it will be the opposite.To change to a sandstone compatible scale the density log is shifted so that its matrix value for sandstone, 2,65g/cm3 coincides with the zero for the neutron porosity.

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Scaling/PorosityThe density tool is usually run with the neutron.To aid quicklook interpretation they are run on

"compatible scales".This means that the scales are set such that for a given lithology the curves overlay.The standard scale is the "limestone compatible" where

the neutron porosity scale is:

To fit this the density log has to have its zero limestone point (2.7 g/cc) on the same position as the neutron porosity zero and the range of the scale has to fit the neutrons 60 porosity units hence the scale is:

Changing to a sandstone compatible scale would put the zero sandstone density, 2.65, over the neutron porosity zero to give:

Notes

The major need for the Volumetric Photoelectric Absorption Index is in formation evaluation equations when a link between the measurements and the formation components such as porosity are required.The Pef alone gives an excellent indication of the lithology. Its major drawback is the effect of barite on the measurement. As the measurement is of gamma rays the dense barite material reduces the amount seen by the tool. However the major effect is the Pe of barite at 267 is far above the normal formation component figure of

Notes

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Volumetrics

Another way of using it is express it in volumetric terms as:

U = Pee

This is called the Volumetric photoelectric absorption index.

This parameter can then be used in a formula for computing the components of the reservoir.

U = Uf + (1 - ) Uma

Notes

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Pef Parameters

Vertical resolution:

Standard 4"

Readings in:

Limestone 5.08Sandstone 1.81Dolomite 3.14Shale 1.8-6Anhydrite 5.05Salt 4.65

petrophysict 12 Density

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