Well logging analysis: methods and interpretation

WELL LOGGING ANALYSISMETHODS AND INTERPRETATION

PETROLEUM GEOLOGY II – WS 2013/14

CRISTIANO ASCOLANI

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Well Logging

“The product of a survey operation, also called a survey, consisting of one or

more curves. Provides a permanent record of one or more physical

measurements as a function of depth in a well bore. Well logs are used to identify

and correlate underground rocks, and to determine the mineralogy and physical

properties of potential reservoir rocks and the nature of the fluids they contain.”

(SPWLA - Society of Petrophysicists and Well Log Analysts, online glossary, last

modified 2010/03/02)

LOGGING AND CORE ANALYSIS - PETROLEUM GEOLOGY II – WS 2013/14

• DownholeSondes lowered into the well bore by a

survey

• On surfaceCore logs, mud sample logs,

hydrocarbon well logs, etc.

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Well Logging

Commonly classified by the kind of energy that is input (active systems) or

received (passive systems):

1. Electric Logging Techniques

• Spontaneus Potential Log

• Resistance Log

• Microlog

• Induction Log

2. Nuclear Radiation Logging

Techniques

• Gamma Ray (Natural Gamma)

Log

• Natural Gamma Spectral Log

• Density or Gamma-Gamma Log

• Neutron Log

• Neutrino Log

3. Acoustic/Seismic Logging

Technique

• Sonic/Acoustic Log

• Acoustic Borehole Imaging Log

• Cross-Hole Seismic Test

• Seismic Tomography

• Borehole Optical Systems

• Borehole Image Processing

• System (BIPS)

4. Other Wireline System

• Borehole Caliper Log

• Directional Surveys

• Borehole Fluid Temperature Log

• Borehole Gravity Log

• Magnetic Log

• Flowmeter Log


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Electric Log

Spontaneus Potential

Log It records the difference in potential

in millivolts between a fixed

electrode at the surface and an

electrode in a borehole• Clays and shales generate one

charge, permeable formations such

as sandstone will generate an

opposite one

• SPs occur when two aqueous

solutions with different ionic

concentrations are placed in contact

(brime-mud, fresh water-mud)

• Hydrocarbon suppression: SP log

response is reduced

Resistivity Log (SP)Characterization of the rock or

sediment in a borehole by measuring

its electrical resistivity.

• Most rock materials: non-conductor

• Enclosed fluids (water based):

conductor

• Hydrocarbon fluids: almost infinitely

resistive


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Electric Log

(CRCLEME, 2002) (Technical Service Center, Bureau of Reclamation, 2002)


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Gamma Ray Log

The simplest radioactive method in geophysical well logging is the natural

gamma log. These logging tools record the level of naturally occurring gamma

ray emissions from the rocks around a borehole.

Different types of rock emit different amounts and different spectra of natural gamma

radiation

• Shales usually emit more gamma rays than other sedimentary rocks: potassium is a

common component in their clay content, and because the cation exchange capacity

of clay causes them to adsorb uranium and thorium.

• Sandstones/carbonate rocks usually do not generate so much gamma rays.

• BE CAREFUL: sometimes non-shales also have elevated levels of gamma radiation:

sandstone can contain uranium mineralization, coal and dolomite may contain

absorbed uranium etc..


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Bulk Density LogRadioactive source and detector

are lowered down the borehole and

the source emits medium-energy

gamma rays into the formation.

Big elements rate: gamma rays rapidly

loose energy

Neutron Porosity LogRadioactive source and detector are

lowered down the borehole and the

source emits neutrons into the

formation.

High H rate: neutrons rapidly loose

energy

𝐵𝑢𝑙𝑘 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝐿𝑜𝑔 ∝1

𝑁𝑒𝑢𝑡𝑟𝑜𝑛 𝐿𝑜𝑔


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Acoustic/Sonic Log

They provide a formation’s interval transit time, designated as ∆t, which is a

measure of a formation’s capacity to transmit seismic waves.

The velocity is calculated by measuring

the travel time from the piezoelectric

transmitter to the receiver

∆𝑡= 𝑡𝑓𝑎𝑟 − 𝑡𝑛𝑒𝑎𝑟

Calculation of porosity: Wyllie’s time

averaged equation

1

𝑉=

∅

𝑉𝑓+1 − ∅

𝑉𝑚𝑎𝑡

(Wikipedia.org)


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Acoustic/Sonic Log

The velocities and corresponding travel times of the rock and matrix are:

(Petrocenter.com)


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Caliper Log

Well logging tool that provides a continuous measurement of the size and shape

of a borehole along its depth.

The caliper tool measures the variation in

borehole diameter as it is withdrawn from

the bottom of the hole, using two or more

articulated arms that push against the

borehole wall

(Wikipedia.org)Multi-finger Caliper, MFC model (Landau

Petroleum Technology Co.).


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Wireline Logging: example

Basic Log (Schlumberger, 2011)


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References

• Oilfield Review Spring 2011: 23, no.1, Schlumberger, 2011

• Oilfield Review Autumn 2012: 24, no.3, Schlumberger, 2012

• In Papp, É. (Editor), 2002, Geophysical and Remote Sensing

Methods for Regolith Exploration, CRCLEME Open File Report 144,

pp 105-115.

• Engineering Geology Field Manual, Second Edition, Volume II; U.S.

Department of the Interior Bureau of Reclamation, 2001

• Wikipedia.org


Well logging analysis: methods and interpretation

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