Spe Scunpad 1

8/11/2019 Spe Scunpad 1

1/31

Modern logs have more

measurements but the principle is

the same

1. Are the reservoir have

good properties?....

2. Where is

hydrocarbon?.

What these curve mean?


2/31

We have a problem.


3/31


4/31

Petrophysically, to quatify:

1. (Porosity)

2. Sw (Water Saturation)

3. k (Permeability)

4. NetPay (Pay Thickness)

5. h (Height above free water)/contact


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6/31

The simple mechanical caliper

measures a vertical profile of

hole diameter

1. Caliper Logs

CALIPER LOGS

- Applications:

Measure borehole diameter (borehole geometry if multi-arm caliper

tools with 2 or 3 hole diameters measurements 90 or 60 relative to

each other).

Important measurement for drillers: hole geometry, hole/cementvolume.

Hole diameters are an import input parameter for the environmental

correction of petrophysical logs.

- Basic Quality Control:

Perform casing check - should read nominal casing ID.

CALI, C1, C2

Washout: Shale

zone?

Mudcake:

Permeable zone?


7/31


8/31

3. Resistivity and Conductivity Logs

The Resistivity log is a

measurement of a

formations resistivity,

that is its resistance to

the passage of anelectric current.

Schematic illustration of the behaviour of

resistivity logs

Minimum bed resolution under best

conditions (modified from Hartmann, 1975)


9/31

Resistivity Logs

Resistivity was the first log, until the

1950s it was the only log.

In many basins it is still the most common

log measurement.

This familiarity obscures the fact that it is

one of the most difficult measurements to

interpret.


10/31

Resistivity Logs (cont.)

Resistivity tools read deeper than any

other logs.

Several metres for the deepest reading

devices.

The current can be focussed so it

always follows the same path.

Several different depths of investigation.

BUT

Resistivity is a directional property.


11/31

Why is resistivity difficult to interpret?

In anisotropic media (e.g. bedded formation)

the resistivity depends on the current path.

In the well shown opposite the BLUE CURRENT

flows more or less parallel to bedding whereas

the RED CURRENT flows perpendicular.

The RED current, corresponding to a Laterolog

tool can flow in the low resistivity beds.

The BLUE corresponding to an Induction

is forced to flow through all the beds.

What would happen if the well was drilled parallel

to the beds?


12/31

Formation Resistivity

Resistivity varies more than any other property.

0.1 Ohmm for a high porosity sand saturated with saline

formation water.

>1000 000 Ohmm for most evaporites. Electrical Properties can be measured more accurately

than anything else.

Unfortunately

Resistivity is a directional measurement. Different tools can give different readings, different readingsobtained in horizontal and vertical wells.


13/31

How to Read Resistivity Logs

4-decade logarithmicX10 linear

When resistivity exceeds 10 Ohmm

scale switches to 0-100 Ohmm

Because resist ivity

can vary so much

a simple linear scal

is of ten not adequa

X10 Linear scalesare commonly found

in N. America and

E. Europe.

Logarithmic are

used in N. Sea andby W. European

operators.

1 10 100 1000

300 Ohmm

8 Ohmm

16 Ohmm


14/31

Unfocussed Resistivity Tool

Pass Current -I- Between Electrodes A and B.

Measure Voltage -V- Between M and N.

Resistivity is Given by:

R = KV/ I (Ohms Law, K constant for the tool)

Problems:

1. The Current path depends on the relative

resistivity of the mud and the formation.

If Rmud is low most current will flow in

the borehole, if it is high most current

flows in the formation.2. What happens at a bed boundary?


15/31

Focussed Devices.

The problem with Unfocussed devices is the

current path depends on the formation resistivity.

Focussed devices work by forcing the current to

follow the same path no matter what theformation resistivity is.

Two Basic Types: Laterolog and Induction.


16/31

Laterolog

Bucking Current

Measure Current

Current Lines Cannot Cross.

This is exploited in laterologs to forcethe measure current to follow the same

path no matter what the relative

magnitudes of mud and formation

resistivity.


17/31

Dual Laterolog

Bucking CurrentMeasure Current

The Heart of the tool is an array of

electrodes.Current passes from/to the

electrodes labeled Ax and voltages

are measured between the

electrodes labeled Mx. The Bucking

Currents are continuously adjusted

to force the measure currents to flowthe same path through the formation.

By operating at two different

frequencies two different current

paths are created: DEEP and

SHALLOW.


18/31

Induction

1. RF field Produced by Transmitter Coil

2. RF Field Induces an AC Current that f low

around the tool axis.

3. AC current flowing around the tool induces

an RF field that causes a current to flow in the

Receiver Coil .

Insulated Housing

Tx

Rx

A series of Auxiliary Coils focus the current

into a do-nut shape.


19/31

Induction and Laterolog Compared

Laterolog

Low frequency (35Hz)

Needs a conductive path to

the formation.

Conductive Mud.

Good vertical Resolution

Can read up to 100k Ohmm.

6m depth of investigation.

Induction

High Frequency(20kHz)

Works in OBM and air filled

holes

Vertical Resolution poor.

But Phaser and Array

tools are comparable to

laterolog.

Only reads to 100 Ohmm

4-8 m depth of investigation


20/31

Gamma Ray log shows

natural radioactivity.

Spectral gamma ray log

gives abundances of

naturally radioactiveelements, thorium (th),

uranium (U) in ppm and

potassium (k) in %.

a) Average volume from which

radiation are detected

b) Depth of investigation shown to

be dependent on formation

density (Modified from

Hallenberg, 1992)

4. Gamma Ray and Spectral Gamma Ray Logs


21/31

Sonic Log: Applications

Petrophysics and Geology. Porosity Tool.

Lithology (with other tools).

Mineralogy (Coals, evaporites). Over-pressure Detection.

Rock Mechanics.

Seismic.

Well-tie.

Attribute modelling.

5. Sonic or Acoustic Logs


22/31

Measure the time it

takes for sound pulse

to travel between a

transmitter and

receiver, mounted a set

distance away along

the logging tool.

The Principle uses of the sonic log (Conventional, compressional wave

tools)


23/31

Sonic Logs Does not use a radioactive source.

Can work in very poor holes. LWD tools available since mid-1990s

Modern tools can work behind casing.

But need good a good cement bond!

BUT Poor vertical resolution.

Very shallow depth of investigation.

Generally unpredictable response to porosity

actually responds to rock fabric.

The sonic is often the only porosity tool

run above the reservoir interval. It is the

link between geophysics and petrophysics.


24/31

Density Logs

Good vertical resolution.

Predictable variation of porosity with density.

Small environmental effects.

(hole size, mud weight) Make it the preferred porosity tool.

BUT

It does not work in rugose holes.

Shallow Depth of Investigation.

It is less accurate at high density = low porosity

6. Density and Photoelectric Factor Logs


25/31

The logging technique

of the density tool is to

subject the formation to

a bombardment of

medium-high energy

(0.2-2.0 MeV)

collimated (focused)

gamma rays and to

measure their

attenuation between

the tool source and

detectors.

The Principle uses of the density


26/31

Density Logs (Summary)

Predictable response to porosity.

Good Vertical Resolution.

Can give information on lithology.

In-built QC (Delta-Rho)

BUT Shallow depth of investigation

Uses a Radioactive Source.

Needs a smooth borehole wall.

Only Open Hole.

AND

Accuracy Drops with Density (Porosity)

Volume of Investigation changes with Density


27/31

The photoelectric factor (PEF) log

(the Litho-Density log of

Schlumberger) is a continuous

record of the effective photoelectric

absorption cross section index or

Pe of formation.

The Principle uses of the photoelectric factor


28/31

7. Neutron Logs

The photoelectric factor (PEF) log (the Litho-Density

log of Schlumberger) is a continuous record of the

effective photoelectric absorption cross section index

or Pe of formation.

The Principle uses of the neutron log

Depth of investigation of the neutron

tool (modified from Serra, 1979)


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Neutron Logs Works in Cased Hole.

Reasonable Depth ofInvestigation.

Most accurate at lowporosity.

Complements Density log

ALSO

Responds to lithology,hydrocarbon and porosity.

Tool can be tuned to beparticularly sensitive or

insensitive to these.

Difficult Measurement toProperly Understand and

Appreciate. Many Different nuclear

reactions involved.

May have Poor verticalresolution(*)

Uses a RadioactiveSource(*).

Needs a good hole.

May Suffer Significantenvironmental effects(*)

temperature, pressure,salinity etc.

Many different tool typeswith significantly differentresponses.

* - latest tools like the Schlumberger APS are far better in this respect.


30/31

Neutron Tool Principles of Measurement

1. Neutrons emitted from a chemical source.2. Neutrons are slowed down by collisions with

atomic nuclei (Hydrogen is especially

effective).

3. Slow neutrons form a cloud around the tool

4. Some are detected, some are not, some get

absorbed by other atoms (5).

12

2

3

4

5

High porosity

Low porosity

Thermal Neutron Tool


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Thermal Neutron Tool

Fe

1. High Energy Neutron Emission

2. Scattering by Medium Nucleus

3. Scattering by Light Nucleus

4. Scattering by Hydrogen

5a. Capture by Iron

5b. Diffusion and possible detection.

1

2

3

4

5a

5b

0.17c

0.000007c

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