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Presentation on

Density log Muhammad Zubair Idrees

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1.0 INTRODUCTION 1.1 Well logging 3  2.0 THE DENSITY LOG 2.1 Definition 4 2.2 Introduction 5 2.3 Principle 8 2.4 Log presentation scale and units 11 2.5 Tool 14 2.6 Log characteristics 17 2.7 Geological Applications 18 2.8 References 42

Contents

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Definition:The continuous recording of a geophysical parameter along a borehole produces a geophysical well log .

Main objective of well-logging is formation

evaluation. Well-logging is done in most oil wells,

mining exploration wells, and in many water wells.

1.1 WELL LOGGING:

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Definition

Density logging is a well logging tool determining rock bulk density along a wellbore.

2.0 THE DENSITY LOG

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Geologically, bulk density is a function of the density of the minerals forming a rock (i.e. matrix) and the enclosed volume of free fluids (porosity).

Density is one of the most important pieces of data in formation evaluation.

INTRODUCTION

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In the majority of the wells drilled, density is the primary indicator of porosity.

In combination with other measurements, it may also be used to indicate lithology and formation fluid type.

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A radioactive source applied to the borehole wall emits gamma rays into the formation so these gamma rays may be considered as high velocity particles which collide with the electrons in the formation.

At each collision the gamma ray loses some of its energy to the electron, and then continues with diminished energy.

PRINCIPLE

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Compton scattering

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This type of interaction is known as Compton scattering. The scattered gamma rays reaching the detector, at the fixed station from the source, are counted as an indication of formation density. 

The denser the formation, the more electrons are presented, and more energy is lost due to collisions

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The density log is generally plotted on a linear scale of bulk density.

The log is run across track 2 and 3. Most often its scale is between 1.95 and

2.95 g/cm3. The main log is accompanied by a curve

that shows the borehole and mud-cake corrections that have been applied.

LOG PRESENTATION SCALE AND UNITS

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A record of cable tension may also be included, as the density tool tends to stick in poor holes.

A correction curve, is sometimes displayed in track 3 and less frequently in track 2.

The gamma ray and caliper curves usually appear in track 1.

14Figure showing density log

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 The standard density tool has a collimated gamma ray source (usually radio cesium which emits gamma rays, radio cobalt is also used).

It has two detectors (near and far) which allow compensation for bore hole effects when their readings are combined and compared in calculated ratios.

TOOL

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The near detector response is essentially due to borehole influence which, when removed from the far detector response enhance the formation effects.

The most recent density tools use more efficient scintillation detectors which separate high and low energy gamma levels.

Source and detectors are mounted on a plough shaped pad.

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Density Tool

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The tool is run typically as a density-neutron combination along with a gamma ray tool and a caliper.

Its vertical resolution is 33 inches. Depth of investigation is 1.5 inches. The tool can be run in Open hole Cased hole. Borehole fluid of gas or air, water or water

based mud, oil or oil based mud. The logging speed of the tool is 60 feet/minute.

2.4 LOG CHARACTERISTICS:

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 POROSITY CALCULATION:  To calculate porosity from the log derived bulk density

it is necessary to know the density of all the individual materials involved.

By knowing the grain (matrix) density and the fluid density, the equation can be solved that gives

From the summation of fluids and matrix components.  

ρb = Ф x ρf + (1 – Ф) x ρma

  Where ρma = matrix (or grain) density

ρf = fluid density

ρb = bulk density(as measured by the tool hence include porosity and density of grains).

2.7 GEOLOGICAL APPLICATIONS

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When solved for porosity this equation become:

PorosityФ = (ρma - ρb)/(ρma - ρf)

Erroneous porosities may be calculated when the fluid density changes. This is the case when a rock is saturated with gaseous hydrocarbons. In the presence of gas the fluid density drops dramatically. The density log gives too high porosity.

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When oil is present the porosity given by the density log is essentially correct because the density of oil is quite close to that of water. Gas is more mobile and frequently occurs because of large density difference with water.

22Figure showing the effect of gas on density log. In this example gas zone reads about 35%

porosity, it should be 27%

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LITHOLOGY IDENTIFICATION

The densities of the common lithologies are rarely diagnostic since there is too much overlap. Overall, oilfield densities generally measure between 2.0 g/cm3 and 3.0 g/cm3.

The density log is itself a poor indicator of lithology, combined with the neutron log it becomes best qualitative indicator of litholgy.

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Figure showing density ranges of some common lithologies

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The compaction of shales with burial is a well known phenomenon and it can be followed on the density log.

Shale compaction involves a series of textural and compositional changes, resulting in a progressive increase in density.

For example shallow, un-compacted clays have densities around 2.0g/cm3, while at depth, this figure commonly rises to 2.6g/cm3.

SHALE COMPACTION

26Figure showing shale compaction with depth seen on a bulk density log plotted at

a compressed vertical scale

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shale density is often indicative of age. In general, older shales are denser. Paleozoic clays are rare, as are Tertiary

shales. The increase in shale density during

compaction, although essentially due to a decrease in porosity is accompanied by irreversible diagenetic changes.

In the subsurface, a change in compaction trends will indicate a change in age, in other words an unconformity.

SHALE AGE AND UNCONFORMITY

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Figure showing tertiary shale uncomformably overlying cretaceous shale. The abrupt change in density marks the

unconformity.

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Local variations in shale density are more likely due to changes in shale composition.

The increase in density is even more marked when iron carbonate is involved. When organic matter is present, the reverse occurs and the density diminishes,

SHALE COMPOSITION

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Organic matter having a very low density.

An increase in carbonate content is generally accompanied by an increase in shale density.

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Figure showing thin carbonate/siderite cemented horizons

in shale. The intervals may be thin continuous bands

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Bulk density variations in sandstone generally indicate porosity changes.

This is not true when there are changes in grain density.

Overall grain density will change depending on the non-quartz constituents.

Sands are commonly mixed with feldspars (density 2.52 g/cm3), micas (2.65-3.1 g/cm3).

THE DENSITY LOG IN SANDSTONE, COMPOSITION AND DIAGENESIS

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Heavy minerals may also be a constituent (2.7-5.0 g/cm3).

Changes in grain density in sands are gradual and of a moderate order.

Abrupt changes, especially in homogenous beds, often indicate diagenetic or secondary changes.

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Figure showing the effect of muscovite on the bulk density log in micaceous sands. The increase in

density below15m is due to mica content

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Figure showing secondary calcareous cementation in sandstone.

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Density becomes a criterion for lithological identification when it is either abnormally high or abnormally low.

Coals, for example, are identified by very low densities, between 1.2 g/cm3 and 1.8 g/cm3

Pyrite has a very high density between 4.8 g/cm3 and 5.17 g/cm3.

MINERAL IDENTIFICATION

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Figure showing coal, with low density and pyrite with high density, on the bulk density log

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Chemical deposits, because of their purity, may be identified by their densities. Most evaporates tend to give intervals of constant density with little variation. When this occurs, along with densities near the pure mineral values, evaporates are probable.

EVAPORITE IDENTIFICATION

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Figure showing bulk density log over a salt shale series.

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 The presence of organic matter in shales lowers their density.

The normal average matrix density of a mixture of clay minerals is about 2.7g/cm3, while organic matter has densities between 0.50 -1.80g/cm3.

The presence of organic matter therefore has a marked effect on the overall shale bulk density.

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SOURCE ROCK EVALUATION

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This organic matter effect on the density log can be quantified, so that the log can be used to evaluate source rocks.

Difficulties arises when organic matter is mixed with a high density mineral such as pyrite (4.8-5.17g/cm3), Since the density of the pyrite masks the effect of the low density organic matter.

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Figure showing effect of organic matter on the density log.

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Schlumberger, 1998, log interpretation charts; Schlumberger wire-line and testing, SMP-7006, sugar land, Texas.

  Rider, M.H., 1986. The geological

interpretation of well logs, Blacky and Son Limited, Bishopbrigg, Glasgow.175P

  www.Wikipedia.org

2.8 REFERENCES

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Any questions?

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Thank you