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WELL LOGGING
is a continuous record of measurement madeWell login bore hole respond to variation in some physicalproperties of rocks through which the bore hole is drilled.
the study of rock properties and theirIsPetrophysicsinteractions with fluids (gases, liquid hydrocarbons, and
aqueous solutions). Study occurred on core samples.
is the process of usingformation evaluationbore hole measurements to evaluate the characteristicsof subsurface formation
Types of well
1) Vertical
Drilled to aspecific target
Measured Depth = True Vertical depth
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2)Deviated
usually from a platform or
from land to near offshore
Measured depth has to be converted to true
vertical depth
3) Horizontal Drilled to maximize production or minimize problems
such as coning.
Types of boreholes
According to Casing operation
Cased holes
Wireline has developed instruments that can takeformation measurements without interference from the
casing and cement
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the equipment needed for wireline logging consists of thefollowing:-
Up-hole instruments:- (Logging Unit and Rig)
A logging unit :- and an instrument for takingmeasurements. For land rigs, the unit is mounted on a
truck
The unit has a winch and an electrical wireline cable
Down- hole instruments:- (Sondes and cables)The Downhole logging instrument :- called a sonde, is
attached to the end of the cable. The unit also includesequipment to power the sonde and to make a permanent
record of the logSonde normally consists of two main parts:
Sensor: It is an electronically complicated part used forpicking the required property
Cartridge: Surrounding the sensor in the modern tools
and do three functions: * Powering the sensor to be
ON/OFF . * Processing the acquired data (First step of
processing). * Data transmission along cables to the up-
hole instruments.
Open holes
Open hole logs to determine:
Thickness (h) : measured by tool depth-
Porosity (F) : measured by porosity/density tools--Saturation (Sw) : computed from a combination of
porosity, density/lithology & resistivity tool.
According to conductivity of the borehole
Conductive (water base drilling mud)
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Non-conductive boreholes (oil base mud, air drilled orcased holes)
(Radius of investigation) : TheDepth of investigation
distance away from the bore hole that a logging toolcan measure .
: the minimum thicknessVertical resolution
formation that can be distinguished by a tool under
operating conditions).
Depth of investigation and vertical resolution of the
log can vary from millimeters to a few meters.
Ways to check your logs
Calibrations-
-Repeat sections
-Trends and Marker bedsOffset well comparison-
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Resistivities:
Rt - Resistivity of the formation in the virgin zone, ohm-
m
Rxo - Resistivity of the flushed zone, ohm-m
Rm - Resistivity of the mud, ohm-m
Rmc - Resistivity of the mudcake, ohm-m
Rw - Resistivity of 100% formation water, ohm-m
Rmf - Resistivity of 100% mud filtrate , ohm-m
Saturations:
Sxo -Mud filtrate saturation in the flushed zone, %
Sw - Water saturation in the virgin zone, fraction of pore
volume in, %
Diameters:
dh - Diameter of the hole, m
di - Diameter of the flushed zone, m
Dri - Diameter of the flushed and transition zonestogether, m
Thicknesses:
h -Height of the bed being logged , m
hmc - Thickness of the mudcake, m
Well logging tools
. Mechanical tools Caliper log Lithology tools Spontaneous potential log
Gamma Ray log
Resistivity tools MSFL LLS LLD
Porosity tools Density log & LDT Neutron log &
APS Neutron- Density Combination Sonic log
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a-Caliper
A caliper log is a well logging tool that provides a continuous
measurement of the size and shape of a borehole along its depth
The measurements that are recorded can be an important indicator
of cave ins or shale swelling in the borehole.
Indicator of good permeability and porosity zones (reservoir rock)
due to development of mudcake in association with gamma ray log.
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Sp logSP log was
one of the earliest electric logs used in oil industry
SP log is
used to identify impermeable zones such as shale, and
permeable zones such as sand, as well as several otheruses
The SP logis a record of DC voltage differences between the
naturally occurring potential of a movable electrode in
the well bore and the potential of the fixed electrode
located at the surface
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It measured
in millivolts
Electric
currents arising primarily from electrochemical factors
within the bore hole create the SP log response.
Theseelectro chemical factors are brought about by differences
in salinities between mud filtrate (Rmf) and theformation water resistivity (Rw), within permeable bed
Because a
conductive fluid is needed in the bore hole for the SP logto operate, it can't be used in non-conductive (oil based)drilling muds
SP log isused to-detect permeable bed
-detect boundaries of permeable beds
-determine formation water resistivity RW
determine the volume of shale in permeable beds
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GrGamma ray
logs measure natural radioactivity in formations and sand
stones and carbonates have low concentration ofradioactive material. And give low gamma ray readings.
As shale content increase, the gamma ray log responseincrease because of the concentration of radioactive
material in shale.Units :API &PPM.
May beused in calculation of volume of shale
IGR= (GRlog-GRmin)/(GRmax-Grmin)
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Resistivity
Resistivity logs
are electric logs which are used toDetermine HC versus water bearing zones
Because the
rocks matrix or grains are non-conductive the ability ofthe rock to transit a current is almost entirely a function
of water in the bores
Hydrocarbons,like the rocks matrix, are non-conductive;therefore, as the hydrocarbon saturation of the boresincrease, the rock's resistivity also increases.
Unit is "ohm.m"
A geologist, by
knowing a formation's water resistivity (RW), itsporosity (PHI), and a value for cementation exponent
(m), can determine a formation water saturation (SW)from the Archie equation
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Two basics types of logs which are used to measure the formation
resistivity are;
- Induction log
- Electrode log
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Sonic
The sonic log is a porosity log that measures
interval transit time (t) of a compressional sound
wave traveling through one foot of formation.
The sonic log device consists of one or more sound
transmitters and two or more receivers.
Interval transit time (t) in microseconds per footis
the reciprocal of the velocity of a compressional
sound wave in feet per second.
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When time increase the saturation of fluids in rock
increase.
The interval transit time is dependent up on both
lithology and porosity. There for, a formation's
matrix velocity must be known to derive sonic
porosity either by chart or by formula
Phisonic = ((t)log-(t)ma) / ((t)f -(t) ma).
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Neutron
Neutron log is a porosity log that Measures the
hydrogen ions concentration in a formation
Unit MeV or eV "million electron volt"
In clean formations where the porosity is filled
with water or oil, the neutron log measures the
liquid-filled porosity.
Neutrons are created from a chemical source in
the neutron logging tool. The chemical source
may be a mixture ofamericium and beryllium
which will continuously emit neutrons. The
neutrons collide with nuclei of formation's
material result in losing some of its energy.
Because the hydrogen atom is almost equal in
mass to the neutron, maximum energy loss occurs
when neutron collide with hydrogen atom. So, the
maximum amount of energy loss is a function ofhydrogen concentration because hydrogen in a
porous formation, so the energy loss can be
related to the formation's porosity.
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Density
Bulk density increase whit decrease prosity& Porous
rock density increases with increasing water saturation
(compared to dry rock)
The formation density log is a porosity log that measures
electron density of formation.
It can assist the geologist to :1. identify the evaporate minerals
2.Detect gas bearing zone
3.Determine HC density
4.Evaluate shaly-sand reservoir and complex lithology.
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The density logging device consists of a medium energy
gamma ray source that emits gamma rays into a formation.
The gamma ray source is either Cobalt-60 or Cesium-137.
Gamma ray collides with electrons in the formation the
collisions result in a loss of energy from the gamma ray
particle.
Scattered gamma rays - which reach the detector located a
fixed distance from the gamma ray source - are counted as
an indicator of formation density.
The of Compton scattering collisions is a direct function of
the of electrons in a formation (electron density).