Well logging

UNIVERSITY OF MYSORE
Department of EARTH SCIENCE
Centre for Advanced Studies in Precambrian Geology
Seminar topic: WELL LOGGING
By:
M AFRIDI AZEEZ
MSc., Applied Geology
University of Mysore
Manasagangothri,Mysore.
Guided by:
Prof, D NAGARAJU
University of Mysore
Manasagangothri,Mysore.

INDEX
• Introduction to Well logging
• Brief explanation of drill hole illustration layer diagram
• Classification of Well logging methods
1. Electrical logging
2. Radioactivity logging
3. Sonic logging
4. Miscellaneous logging
• Logging data interpretation and Application
• Results
• Referance

Well logging
It’s a Geophysical method carried out in subsurface
inside boreholes and in mine workings.
The investigations conducted in drill holes or wells, are
usually termed WELL LOGGING.
The Ultimate objectives in well logging are:
To locate and evaluate hydrocarbon and other mineral
bearing formations encountered in the bore holes.
While in cases they are also used to investigate and
indicate mining exploration and ground water
respectively.
In all drilling operations, an important source of
subsurface information is through cores taken the
holes. These cores give visible evidence of the rocks
and minerals penetrated and permit direct
measurement and evolution.

Bore hole layers illustration diagram
The pressure differential existing between the mud
and the formation fluid causes a continuous
infiltration of mud liquid (filtrate) into the porous
and permeable zones. During this process, The solid
particles of the mud get deposited on the face of
the formations to form what is know as MUD CAKE.
This cake retards further invasion of filtrate which
had already entered the formations, pushed the
original formation fluids (water, oil or gas) away
from the well bore. Thus, the portion of permeable
bed lying immediately behind the borehole wall is
contaminated with mud filtrate and it is known as
the FLUSHED ZONE.
thus, the invasion of the mud filtrate into
permeable beds results in a radial resistivity profile
substantially different from the original one.

Classification of well logging
1. Electrical logging
1. Spontaneous logging
2. Resistivity logging
3. Inductive logging
2. Radioactivity logging
1. Gamma ray log
2. neutron-gamma ray log
3. Formation density log
3. Sonic logging
4. Miscellaneous logging
1. Temperature logging
2. Caliper lagging
3. Dipmeter logging

Electrical logging
All electrical logging operations are carried out in open hole condition i.e., before
the casing is run in and cemented in the hole. In drilling wells for oil or gas, mud is
used to prevent the fluids held under pressure in the rock formations from entering
the borehole and coming up and causing damage.
There are different types of electrical logging on the bases their inputs and the
parameters been measured
1. Spontaneous potential or SP logging
2. Resistivity logging
• Later log
• Micro log
• Micro later log
• Proximity
3. Inductive logging

Spontaneous logging (S P) logging
S P logging records the naturally occurring potential in wells filled with
water base drilling mud. The S P logging in drill holes are mainly due to two
reasons (1) the difference in salt concentration between the water contained
in the formation and the mud filtrate and (2) the filtration of liquids through
the formation into hole or vice versa.
S P logging can be used to locate possible reservoir rocks. The magnitude
of the S P recorded in boreholes varies from a few millivolts to as much as
200 millivolts in rare cases.

Resistivity logging
Resistivity logging in bore holes is comparable to fixed electrode
separation traversing in surface exploration, but run in the vertical
dimension in the hole. The resistivity so could only be the apparent
resistivity where as true resistivity is mud in bore hole, invaded zone
and the adjacent zone, they are calculated by theoretical curves..
Based on the arrangement of the electrodes resistivity logging has
varies previously mentioned types.
They are mostly helpful in hydrocarbon detection.
Where as well logging is further extension of S P logging.

Inductive logging
The parameters measured in the induction log is
conduction. In field practice it’s the inverse of
resistance and the graph is plotted by its data
In induction logging insulated coils are used to energize
the formations with alternation current. In its simplest
form, the inductive logging device consists of a
transmitter coil and a receiver coil both coaxially
supporting on an insulating mandrel and AC source of
constant magnitude and frequency into the transmitter
coil from the oscillator.
The primarily used for logging holes drilled with oil base
mud and empty holes filled with air only. But now days
also used for water bore holes also

Radioactivity logging
Radioactivity logs are based on the nuclear radiations of the
formations and they facilitate interpretation of electrical logs in
detecting possible productive horizons and evaluating their reservoir
properties, these logs can be run either in open or closed holes.
1. GAMMA-RAY Log-
They measure the natural radiations of the formations, in
sedimentary rocks the radioactive elements tends to concentrate in
clay and shale, the gamma-ray log serves to distinguish between
shales and reservoir rock. It is particularly useful for lithology
detection when the S P curve is rounded off.

2. NEUTRON-GAMMA RAY Log –
The neutron-gamma ray logs which measures the radioactivity induced
artificially in the formations by bombarding the rocks with neutrons from a
neutron source, neutron has a mass almost identical with that of the hydrogen
nucleus. Due to this the bombarding neutrons slows down as their is
absorption of neutrons are by mostly by hydrogen. In water and oil since the
hydrogen concentrations per unit volume are about equal the log reflects the
porosity.
3. FORMATION DENSITY Log –
The formation density log is useful for porosity evaluation. The tool uses a
radioactive source applied to the borehole wall in a shielded side wall skid,
thereby irradiating the formations by medium energy gamma-rays. These
gamma-rays undergo scattering as a result of collision with the electrons in the
formations.

Sonic logging
In sonic logging we obtain the acoustic velocities
of various formations in bore hole. In all these
logging unites the time required for a second
wave to travel through two pairs of receivers are
used. The transmitters are pulsed alternatively,
since two receivers are used the travel time in
the mud and mud cake cancel out provided the
hole is essentially uniform in diameter. The
acoustic velocity differs for different materials,
for example the matrix velocity for sandstone is
about 5500m/s for limestone its 7000m/s.
Since the sonic log is partially unaffected by the
type of mud in the bore hole and the invaded
zone, it also serves as a good correlation tool,
furnishing a considerable amount of details.

Miscellaneous logging
1. TEMPERATURE LOGGING
Temperature in the earth generally increases with depth. In drill holes,
temperature are dependant not only on the natural geothermal gradient but
also the circulation of the mud. The temperature logging device utilizes a
temperature-sensitive element which is exposed to well fluids.
2. CALIPER LOGGING
Caliper logging is usually carried out in all the wells, either separately or in
conjunction with other logs. The tool is a sensitive caliper which adjusts to the
size of the boreholes. The log records the variation of borehole diameter with
depth. The data will be useful in quantitative interpretation of well logs as well
as for calculation of the quantity of cement required for cementation casing.

Logging data interpretation
The interpretation of well logging data may be done qualitatively by
inspection of the curves and quantitatively by employing a variety of
formulate. The qualitative interpretation pertains to the definition of
formations of different characteristics, particularly porous and permeable beds
and determination of their boundaries.
For reliable interpretation one has to make sure of all the log data along
with such geological information as in available from the given well and also
from the nearby wells. Knowledge of geology data is very essential for
improving the reliability of the interpretation results.

Applications of Well logging
1. Oil field
Method most preferable – sonic logging and seismic logging
2. Ground water investigation
Method most preferable - electrical logging, specific resistivity logging
3. Investigations in mining field
Method most preferable – electrical logging, magnetic logging, radioactivity logging

Referance
Outlines of geophysical prospecting
a manual for geologist
By: M.B RAMACHANDRA RAO
Formerly chief geophysicist
First publication – 1975
Page number: 343 – 366
images used here are browsed on internet

Well logging

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