well logging equipment and operation -...

Well logging equipment

and operation

edited by G. Pethő & P. Vass

For Petroleum Engineer & Geoengineer MSc Students

Well logging or borehole logging is a geophysical data

acquisition technique on which the borehole geophysics is

based.

A short definition of well logging:

a measurement technique which is used for recording data of

physical quantities, primarily as a function of depth, in order

that the characteristics of rock formations and some

parameters of the boreholes and wells can be determined.

So, a well logging operation is performed in a borehole or a

well and it requires special equipment with measurement

devices.

Well logging or borehole logging

Darwin V. Ellis, Julian M. Singer: Well logging for Earth

scientists

O. Serra: Fundamentals of well-log interpretation

Malcolm Rider: The geological interpretation of well logs

Toby Darling: Well logging and formation evaluation

Crain's Petrophysical Handbook:

https://www.spec2000.net/00-index.htm

WELLOG:

http://www.wellog.com

PetroWiki

http://petrowiki.org/PetroWiki

Some proposed literature

https://www.spec2000.net/00-index.htm

http://www.wellog.com/

http://petrowiki.org/PetroWiki

However, the meaning of well logging is not uniform for the

experts of different specialities.

A geologist usually thinks that it is a mapping technique for

exploring the subsurface.

According to a petrophysicist, it is a tool for evaluating the

hydrocarbon production potential of a reservoir.

A geophysicist considers it as a source of complementary

data for surface seismic analysis.

The opinion of a reservoir engineer is that it provides useful

data for a reservoir simulation and production development.

Well logging

At the beginning, well logging was used for correlating similar

patterns of measured electrical resistivity from one well to

another. By means of the qualitative analysis of well logs the

different reservoirs can be identified and delimited in an

oilfield.

Later, the improvement of measuring techniques and the

development of newer measurement methods resulted in the

evolution of quantitative well log analysis (empirical methods

for the estimation of shale volume, porosity, water saturation

and permeability etc.).

Several empirical relations have been found between the

measured quantities and the reservoir parameters, which are

valid in sedimentary rocks.

Well logging

In these days, two types of well logging technology are used

in the oil industry:

• wireline logging (traditional way of well logging),

• measured while drilling (MWD) technique.

For wireline logging the acquisition of geophysical data is

performed after the drilling string has been run out of the borehole.

During the operation a logging tool (or sonde) is lowered into a

borehole (or a well) by means of a logging cable (wireline).

The logging tool measures the value of some physical quantity.

The position of the tool along the hole can be changed by lowering

or hoisting up the tool.

The measured data is recorded as a function of depth.

The printed or displayed form of the measured data is the log

curve which visually represents the variations of the physical

quantity.

Well logging

Wireline logging

http://www.frontier-cf252.com/oil-well-logging.html

Logging While Drilling is a modern logging technique.

It was developed to continuously collect drilling and

geophysical data from the vicinity of drill bit during the drilling

process.

The downhole measuring instruments (logging tools) are

integrated into the Bottom Hole Assembly (BHA) of the

drilling string.

So, the drilling rig acts as a hoisting equipment to raise and

lower the instruments.

LWD derives from the measurement while drilling (MWD),

technology which was introduced earlier (in 1979, in the

North Sea) to provide (real-time) directional data during the

drilling operation.

Logging while drilling (LWD)

Logging while drilling (LWD)

http://www.aps-tech.com/products/lwd-systems

Wireline logging operations can be divided into two main groups:

• open hole logging,

• cased hole logging.

An open hole logging operation is performed in a portion of a well

after it has been drilled, cleaned, stabilized and the drilling string

has been extracted from the well.

Then the logging tool is being moved along the bare rock sides of

the formation.

The borehole is filled with drilling mud during the logging operation.

The main objectives of open-hole logging are the following:

• lithological identification of the formations,

• determination of the bed boundaries and the reservoir zones,

• estimation of the values of reservoir parameters such as shale

volume, porosity, water saturation and permeability.

Wireline logging

A cased-hole logging operation is executed in a portion of a well

after its casing string has been run in and cemented.

In that case, the logging tool is isolated from the rock formations by

the casing string and the cement sheath.

Cased-hole logging is used to provide additional information from a

well or reservoir that has already been completed.

It can help determine what hampers the flow in a well.

In some cases, a decision must be made to plug and abandon the

well or recomplete it, and the cased-hole log will help identify what

lies beyond the casing of the well.

Cased-hole logging can also be used to evaluate the formation and

its fluid content beyond the casing, as well as to determine the

state of the cement, corrosion and perforation.

Wireline logging

A special branch of the cased hole logging is the so-called

production well logging (PWL) which is performed in wells

under production circumstances to determine the production

parameters of the wells.

The main components of a wireline logging system (from the

borehole to the surface):

• logging tool (or probe or sonde) generally contains a

sensor which is able to measure the value of some

physical quantity,

• armoured cable by which the measuring devices are

lowered and retrieved from the borehole,

• data acquisition system generally mounted on a logging

truck, and collects the measured data transmitted along

the shielded insulated wires in the interior of the cable.

Wireline logging

Schlumberger 1989

Wireline logging system

Logging tool

(probe or sonde)

Surface logging unit

with the data

acquisition system Armoured cable

A logging tool is actually a measurement device applied in

borehole environments.

Several types of logging tools are used in the practice of

wireline logging in order to obtain various information about

the subsurface rock formations and the interior of wells.

Some of them are passive measurement devices, which

means that they can only measure the effects of natural

physical or physicochemical processes (spontaneous

phenomena) taking place under the surface.

Others are active devices because they exert some influence

on the borehole environment by generating some kind of

physical phenomena (eg. electric current, EM field, gamma

ray or elastic waves etc.), and measure the response of the

subsurface medium to the induced effect.

Logging tool or probe

A typical part of a logging tool is a sensor which is able to detect

some physical effect and convert it into electric voltage signal.

The relation between the value of electric voltage and the

magnitude of the detected quantity is determined by the calibration

of the logging tool, and it is the characteristic function of the

sensor.

The measured voltage signal is usually amplified, filtered, digitized

and stored in the tool memory or transmitted to the surface along

the logging cable.

Different terms used for the sensor depending on the

measurement method:

method designation of the sensor

electric measure electrode

electromagnetic receiver coil

nuclear detector

sonic or acoustic receiver


Beside the sensors, the active measurement devices contain one

or more exciting units by which some physical influence can be

exerted on the environment of the logging tool in a controlled way.

The type of influence depends on the applied measurement

method:

method influence designation of the

exiting unit

electric electric current current electrode

electromagnetic EM field transmitter coil

nuclear gamma radiation gamma ray source

neutron radiation neutron source

sonic or acoustic elastic waves transmitter


All the internal parts of a logging tool (sensors, exiting units,

electric circuits and other electro-mechanical parts) are

encapsulated in an antimagnetic stainless steel case called

housing.

The housing isolates the internal parts of the tool from the

borehole and bears the pressure burden.

The housing is generally cylindriform with an outside

diameter of about 4 in. (~ 10 cm) or less.

Due to this relatively small cross-sectional size, logging tools

can pass through boreholes as small as 6 in. in diameter.

Their length varies depending on the number of sensors and

exiting units, the spacing between them and the complexity of

required electronics.


Logging tools

http://www.e-tyhoo.com/Tyhoo/datatemplate/product/p00435.aspx

Some of the logging tools are designed for operating in a

centralized position in the borehole.

This position can be provided by attaching bow-spring

centralizers to the housing of the tool.

Centralized logging tools

http://www.alsglobal.com/

Some measurements require the direct contact of a pad (a

sensor package) or pads with the formation through the

borehole wall.

This requirement can be fulfilled by means of some (2, 4 or 6)

mechanically actuated steel arms or a single mechanically

actuated back-up arm.

These arms are built in the tool, and can be moved by

means of DC electric motors.

While the tool is being lowered, its arms are in closed

positions (so as not to stick in the borehole).

When the tool has reached the bottom of the depth interval to

be logged, the operator extends the arms by means of a

remote control system (the control signals are transmitted

along the logging cable).

Sensor pad

Sensor pads

Darwin V. Ellis, Julian M. Singer: Well logging for Earth Scientists

Hexapod diplog tool (HDT)

http://www.bosondaoil.cn/epview.asp?id=12

Litho density logging tool (LDLT)

http://www.gowellpetro.com/product/litho-density-logging-tool-ldlt.html

A logging tool which is pressed to the borehole wall by a

single back-up arm is in a decentered position inside the

borehole during the measurement.

Each type of logging tool has a measure point.

The measure point is a point on the housing. Its position

along the tool axis depends on the arrangement of the

sensor(s) and exciting unit(s).

The measured value is always assigned to the actual position

of the measure point in the borehole.

As the tool is moving in a borehole, the depth level of its

measure point is also changing.

So, the values measured in different positions are assigned

to different depth levels.


Logging tool string

In practice, logging tools are generally not used alone.

They are usually connected in suitable combinations (each sonde

has a male threaded ends by which it can be driven into the female

end of another one).

This connected combination of logging tools is called logging tool

string. A tool string can be as long as 100 ft (~ 30.5 m).

The most significant advantages of using logging tool strings are

the following:

• saving the rig time (because more characteristics of the rock

formations can be measured simultaneously)

• improved depth correlation of all measured quantity (there is a

common depth reference for all the recorded logging curves)

• facilitation of decision-making at well site (when a quick decision

is needed to make on the further operations).

The components of some commonly used tool strings are the

following:

SP - DLL – GR – MLL - ML –CAL

SP - ATL – GR – CAL – DVL – AZL

SP – GR – CNL – CDL – (ZDL) – CAL

SP: spontaneous potential, DLL: dual laterolog

GR: natural gamma ray, MLL: micro laterolog

ML: micro log (or mini log), CAL: borehole caliper

ATL: acoustic travel-time log, DVL: deviation log

AZL: azimuth log, CNL: compensated neutron log

CDL: compensated density log, ZDL: Z density log (or litho-

density log)

Logging tool string

Logging tool string

http://www-odp.tamu.edu/publications/204_IR/chap_02/c2_f26.htm

But a long and solid tool string can easily get stuck in a wellbore

which is not vertical or full of caverns or wash-outs.

In order to reduce the risk of getting stuck, additional elements can

be built in a tool string.

A knuckle joint allows an angular offset between adjacent

instruments, and a swivel provides independent rotation for the

adjoining portions of a tool string.

Logging tool string

Baker Hughes: Wireline Services Catalog

Each logging tool string has its own reference point.

The reference point of a logging tool string acts as a zero point of a

one dimensional system of reference defined for the logging tool

string.

Before a tool string is being lowered into the hole, its reference

point is aligned with the level of the drill floor.

In this position the depth measurement system is set to zero.

So, the system measures the position of the reference point along

the borehole.

Since the distance between the reference point of a logging tool

string and the measure point of each built-in logging tool is a

known constant, the measured values of the different tools can be

assigned to their own depth values by means of depth shifting.

Since a borehole is not necessarily vertical, the depth determined

by this way is called Measured Depth (MD).

It is not necessarily the same as the True Vertical Depth

(TVD).

Logging tool string

The term wireline refers to the armoured cable which is primarily

used for lowering and pulling up the measurement devices in the

borehole or well.

The logging cable provides a mechanical support for the tool and a

communication channel for data transmission.

Logging cable

O. & L. Serra 2004: Well Logging Data Acquisition and Applications

The cable is wrapped with a two-

layered, galvanized (protection

from corrosion) steel armour.

Both of these layers are formed

by twisted steel wires wound

around the core of the cable.

The wires in the two layers are twisted in opposite directions

to prevent the wires from getting loose.

The steel armour has enough strength to support the tool

weight and provides some more strength to pull on the tool in

case of sticking in the borehole.

The interior of the cable contains one or more shielded (for

electrical noise reduction) insulated wires (teflon is used for

insulation).

These copper conductors provide electrical connection

between the downhole logging tools and the computerized

data acquisition system located on the surface.

The core of the logging cable may also contain an optical

fiber cable, which provides much higher rate of data

transmission.

Logging cable

The communication between the surface equipment and the

downhole devices is duplex:

• the voltage signals of measured data are transmitted from the

tools to the surface,

• electrical power as well as control signals are transmitted from

the surface logging unit to the measurement devices.

Logging cable

http://www.slb.com/~/media/Files/resources/oilfield_review/ors14/win14/2_cables.pdf

For open hole logging

operations, seven-conductor

cables (heptacables) are

generally used (but there are

three- and four-conductor cables

as well).

Single-conductor cables (or

monocables) with smaller

diameter are applied to cased

hole measurements.

A logging tool string is connected to the logging cable by means of

a cable head.

A cable head provides both electrical and mechanical contacts

between the cable and the tool string.

A cable head used in open hole logging (left side).

Cable head

http://www.weatherford.com/en/standa

rd-cable-head

https://www.geoilandgas.com/oilfield/wireline-

technology/wireline-cable-heads

A cable head used in cased hole logging (right side).

The lower end of the logging cable is pulled into the cable head and fixed

to it. A built-in weak point is formed in this part of the logging cable (inside

the cable head). This point has the lowest breaking strength along the

whole cable. The breaking strength of the weak point is a known value.

The weak point allows the cable to be released from the cable head if the

tool string accidentally gets stuck in the borehole and the cable are pulled

too strongly.

This solution provides a controlled way of breaking the cable, and saving

the whole cable from the borehole.

Cable head

https://www.osha.gov/SLTC/etools/oilandgas/servicing/special_services.html

In order to save the logging tool string, a

standard fishing tool must be latched onto the

cable head. This special clamping tool can be

connected to the end of a drill pipe and

conveyed by running a drill pipe string into the

borehole. After tightly fitting the fishing tool to

the cable head, the logging tool string can be

removed from the hole by running out the drill

string.

The logging cable with the logging tool string is moved by means

of a motorized winch on the surface.

The winch is attached to a (winding-)drum and the logging cable is

wound around the drum.

So, the cable is stored on a drum.

Winch

http://ivyzhang918.en.made-in-china.com/product/SBjQJUCEOmWY/China-

Geophysical-Winch-Well-Logging-Winch-Wireline-Winch-Cable-Winch.html

The cable can be lowered or pulled

up by controlling the angular speed

and direction of rotation of the

drum,

A full drum generally carries a cable of a few thousand meters

(generally more than 2000 m).

A winch-man operates the winch by means of a control panel, and

the winch is driven by a motor (generally a hydraulic motor).

Winch

http://earthsky.org/earth/susan-hovorka-on-carbon-capture-and-storage

The direction of the logging cable is changed at two points

between the winch and the borehole by means of two sheaves.

A shave is a grooved wheel inside a pulley. The logging cable fits

into the groove.

Both sheaves are fixed to the drilling rig.While the upper sheave is hanged on the rotary hook, the lower

one is fastened to the rotary table.

Sheaves

http://www.tis-manufacturing.com/products/wireline.php

A logging system set up to a drilling rig

http://www-icdp.icdp-online.org/news/training/Shimabara/Kueck/1_LOG_%20Basics.pdf

The surface equipment of a wireline logging system is the so-

called surface logging unit.

Since it is often required to be transported from one wellsite to

another, it is enclosed into a rugged and transportable cabin.

The cabin is separated into two parts:

• the drawworks area contains the winch assembly,

• the operator’s cab or logging cab houses the control panels of

the winch and other downhole instruments, as well as the

computerized data acquisition and processing system.

The surface logging unit is sometimes called mobile laboratory,

because it can be considered as a laboratory where the results of

the well logging operations are produced.

Surface equipment

The surface logging unit is mounted on a heavy-duty truck or a

skid.

While the first construction is used for logging onshore wells, skid

units are designed for offshore circumstances.

Surface equipment

http://www.kgs.ku.edu/PRS/Ozark/well_1_32.htmlhttps://wrightswell.files.wordpress.com/

2013/12/wwwireline2re.jpg

Some of the logging units are equipped with two drums (dual

drum) for storing different types of cables.

The main winch has a seven-conductor logging cable, and the

smaller winch at the rear generally bears a slim monoconductor

cable for cased hole logging operations.

Surface equipment

Log Interpretation Principles/Application, Schlumberger 1989

Other logging units have a single drum or a split drum. On a split

drum two different cables are spooled separately.

The engine of the logging truck drives the winch by means of a

power take-off system (PTO).

The logging truck also carries the logging tools in its storage bay

during the transportation.

Surface equipment

http://www.naftagas-nfs.rs/en/coiled-turbing-units/hydrarig-hr560

A logging truck provides AC power for the surface instruments,

computers and auxiliary devices by means of a built-in petrol

generator.

The surface logging unit provides DC power for the downhole

tools.

The computerized data acquisition system is installed inside the

logging cab.

It receives, preprocesses and stores the raw measured data

coming from the logging tools.

By means of its interactive software environment, the whole

logging operation can be controlled.

In the case of modern wireline logging systems, the signal

transmission is digital.

It means that the measured voltage signal of a sensor is digitized

inside the logging tool, and the digitized signal is transmitted.

Surface equipment

A telemetry unit placed in the upper part of each tool string collects

the signals (digitally encoded data) from the tools and transmits

them to the surface equipment.

By means of the monitors connected to the computerized system,

the measured data can be displayed and quality controlled

immediately.

Surface equipment

From the continuous recording of measured data, a so-called well

log is produced.

It can be printed on a roll of logging paper by a field printer.

Surface equipment

http://www.isys-group.com/iterra-lite-color/

In order to observe a problem with the movement of a logging

tool string, monitoring the cable tension is continuously

required during the logging operation.

If a logging tool string gets stuck or cannot pass at a point of

the borehole the value of cable tension suddenly changes.

So, the measurement of the cable tension helps in

recognizing the problematic situations.

Cable tension

http://www.stellartech.com/pdf/HL-WellLoggingSystem.pdf

A load cell is a sensor which converts the

load or force acting on it into an electronic

signal.

The sensor is generally covered by a

stainless steel weatherproof housing.

The cable tension measurement device with the load cell can be

inserted between one of the sheave-wheel and the drilling rig (the

rotary hook or the drill floor).

Cable tension


There is another type

of cable tension

measurement device

which is combined

with the depth

measuring assembly.

This construction is

placed before the

drum, and the cable is

threaded through it.

The electronic signal coming from the output of tension device is

transmitted to the winch control panel (located in the logging cab) by

means of a cable.

The weight indicator of the panel displays the value of actual tensional

force loading the wireline cable at the point of the load cell during a

logging operation (in kN or lbf).

The measured cable tension is not only displayed but also recorded

by the surface data acquisition and processing system.

When a tool string is lowered in a borehole, the measured cable

tension is the resultant of the following forces:

• the weight of the logging tool string,

• the weight of the cable lowered into the well,

• frictional forces (which arise as the cable and the downhole tool are

being pulled along the borehole),

• and a Buoyant force coming from the drilling mud column under the

tool string (it compensates some part of the tensional forces).

Cable tension

It is very important to continuously watch the cable tension

during the logging operation, because a sudden and

significant change in the value of cable tension indicates a

problem with the movement of the logging tool string.

Such kind of problems often occur in bad hole conditions.

Mostly the following effects may cause problems:

holding up, differential sticking and key seating.

As a logging tool string is being lowered into a well, the

weight indicator displays a gradually increasing tensional

force normally (because of increasing weight of cable).

A sudden and significant decrease in the cable tension

indicates that the logging tool string has been held up.

Holding up means that the tool string cannot pass through a

certain point or interval of a borehole.

Stuck logging tools

It normally occurs, when the tool string has just reached the

bottom of the hole (which is generally the lowest point of the

interval to be logged).

Therefore, a winch-man always has to lower the logging tool

string very slowly and carefully near the bottom of the hole.

But, holding up may also occur when a constriction, a

blockage, a dog leg, or a ledge (of a harder rock) can be

found in the borehole.

The usual practice in such a situation is pulling up the tool

string and rebuilding the tool in some way (e g. reducing the

length of the tool string or inserting an additional knuckle

joint).

Stuck logging tools

As a logging tool string is being pulled up, the weight

indicator displays a gradually decreasing tensional force in

normal circumstances (because the weight of cable in the

borehole is gradually decreasing as the cable is being

spooled on the drum).

A sudden and significant increase in cable tension indicates

that the tool string has probably got stuck in the hole.

Such a situation may occur when either the cable or the

logging tool string gets in contact with the borehole wall and

becomes embedded.

The differential pressure between the mud column above this

point and the formation keeps the logging tool string in place .

This effect is called differential sticking.

Stuck logging tools

The usual procedure in such a case (differential sticking) is

alternately pulling and slacking the logging cable by means of

the winch.

The cable is permitted to pull up to 90 % of the breaking

strength of the weak point (it is a known value).

By applying this procedure steadily, the tool string is often

managed to rescue.

Stuck logging tools

https://en.wikipedia.org/wiki/Differential_sticking

Another effect which may cause sticking during the upward

movement is the so-called key seating.

Key seating occurs when a groove is cut into the borehole

wall during the drilling process, and it is parallel to the axis of

the hole.

If a logging tool string gets in this small hole (groove) next to

the full-gauge hole, it can easily get stuck.

Stuck logging tools

http://petrowiki.org/Mechanical_pipe_sticking

The formation of a groove in the

borehole wall by the rotation of drill

string with a side force acting on it.

Stuck logging tools

In such a case, the tool string may effectively be blocked at a

certain depth.

Unfortunately, this type of sticking often leads to the break of

the logging cable at its weak point.

In such a case, the tool string may remain in its frozen

position or fall down the bottom of the hole and a saving

action has to be tried to recover it.

Fishing out logging tools

Once, a logging tool string has got stuck, and cannot be recovered by

means of steadily pulling and slacking the cable, one of the following

two options can be selected:

• applying the cut-and-thread technique,

• or breaking the cable at the weak-point (in the cable head).

When the cut-and-thread technique is applied, the cable is cut on the

surface, and threaded through the series of drill pipes which are run

into the borehole.

There is a special fishing head called overshot at the bottom end of the

drill string which enables the drill string to latch on the cable head of

the logging tool string.

After the physical connection between the cable head and the drill

string has been made, the cable is intentionally broken at the weak-

point by using the drilling rig to pull the cable.

Then the logging cable is pulled up from the hole, and the logging tool

string can be recovered by running the drill string out of the hole.

Fishing out logging tools

When the other option is selected, the weak point of the cable

has to be broken by means of the winch.

Then the cable is pulled up and also a drill string with an

overshot (or fishing tool) is run into the borehole.

But in this case, finding the cable head of the logging tool

string is more difficult because the cable does not lead the

drill string towards the cable head.

So the chance of a successful engagement is less.

Fishing out a logging tool string can be an expensive and time

consuming procedure.

When the saving process is very difficult, a sidetrack is drilled

around the unrecovered tools or the given interval is

completely redrilled rather than wasting time and money for

the logging tool string.

For the well log curves, the reference axis of the coordinate

system is the measured depth.

This is the reason why the depth measurement has a very

important role in a logging operation.

In order to determine the depth of a logging tool string, a

calibrated dual-wheel depth measurement device is used.

Depth measurement


The cable is straddled by two measuring wheels.

The movement of the cable revolves the wheels

The measuring wheels are coupled to an encoder which generates

an electric impulse each time after the wheels have been turned a

definite angle.

The length of the cable movement between two impulses

corresponds to the value of the arc length belonging to the

selected angle.

The induced electric impulses are transmitted to the logging data

acquisition computer, which counts them.

Depending on the direction of rotation the value of the arc length is

added to or subtracted from the previous value of the measured

depth after each impulse.

The computer record and displays the actual value of the

measured depth in a digital form.

Depth measurement

In fact, not a vertical depth, but the length of the wireline moving to

and from the borehole is measured in such a way.

The result of the depth measurement is called measured depth (MD)

which is generally different from the true vertical depth (TVD).

Measured depth is displayed on the winch control panel and the

monitor of the logging computer, as well.

For the validation and correction of the depth values measured by the

wheels, magnetic markers are placed at regular intervals (e.g. 25 m or

50 m) along the logging cables.

An additional sensor connected to the depth measurement device is

used to detect the magnetic markers. Its impulses are also collected

by the computer. So, two independent measurements are

implemented.

At each impulse coming from the magnetic markers the measured

depth values are compared. The small difference (generally < 50 cm)

is distributed to the magnetic marker spacing and a correction is made

to the depth values of this interval.

Depth measurement

Additional improvement of the accuracy of measured depth

can be reached by correcting the effects of elastic cable

stretch and temperature.

The expected accuracy of measured depth is 1 m or less

deviation (from the true value) over a 1000 m depth interval.

Logging tool measurements are discontinuously recorded at

some fixed depth increment (e g. 0.100 meters or 6 inches =

0.1524 meters).

High resolution data can be recorded 10 or 20 times more

frequently than the standard sampling rate.

The cable speed measurement can be derived from the

depth measurement, because it is computed as depth

change per unit of time.

Cable speed is also recorded as a function of depth during

the logging operations.

Depth measurement

The depth measurement device (or head) is mounted on the

arm of the spooling system.

The spooling arm can be moved from one side of the drum to

another by the winchman.

In such a way, the cable can be guided between the drum

and the lower sheave.

Depth measurement


Depth measurement

http://www.benchmarkwireline.com/PDF/184_AM3K.manual.2014-08-20.pdf

Most of the logging tool measurements are performed as the tool

string is being pulled up slowly toward the surface.

The advantages of logging in the upward direction are

• a taut cable

• and better depth control.

Logging speed is the speed of cable movement during the logging

operation.

The selection of logging speed depends on the measurement

method and the type of device.

Logging tools which measure statistical processes (nuclear

interactions and gamma radiation) or require mechanical contact

between the sensor and the formation have to be pulled up very

slowly: between 3 m/min and 10 m/min.

Some acoustic and electrical devices can be raised at much

greater speeds during the measurement (15 – 25 m/min).

Logging speed

Recommended maximum logging speeds for some frequently used

measurements:

measurement maximum logging speed (m/min)

spontaneous potential 30

induction log 25

acoustic travel time 18

laterolog 15

microlaterolog 10

microlog 10

neutron logging 9

natural gamma ray 6

density logging 4.5

borehole wall imaging 4.5

Logging speed

The maximum data rate of conventional logging cables is

about 80 kbits/second in the case of analogue transmission.

The change from analogue to digital telemetry (in the 90s)

increased the data rate of conventional 7-conductor wireline

from 80 to 660 kbits/second.

That improvement in data transmission enabled the

development of a newer generation of array and imaging

tools which requires much higher data rate than the previous

generation of logging tools (conventional tools).

By using additional data-compression techniques, as high

data rate can be achieved over a 7-conductor wireline as 5

Mbits/sec.

Data transmission

A logging cable which includes an optical fiber cable is able to

provide data rates above 10 Mbits/sec.

In such a cabel, an optical fiber is used to transmit the data and

electrical conductors are used to transmit power and control

signals.

The disadvantages of fiber-optic cables are the following:

• high cost,

• lower cable strength (sensitive to bending),

• splicing cable (joining two fiber optic cables together) in the field

is problematic.

These problems limited its application to measurements requiring

large bandwidth (e.g. borehole television, borehole seismic etc.)

For the sake of comparison, the typical bandwidth values of LAN

technologies are 10 Mbits/sec, 100 Mbits/sec, 1000 Mbits/sec.

In borehole circumstances, such high data rates have not

implemented yet.

Data transmission

Conveyance methods

https://www.slb.com/~/media/Files/resources/oilfield_review/ors04/aut04/04_advancing_downhole.pdf

Conveyance techniques used for wireline logging:

• conventional wireline,

• pipe-conveyed logging (PCL) (tools are run on drill pipe),

• coiled Tubing (CT) logging (a wireline cable run inside the tube),

• tractor conveyed logging (a downhole tractor is placed to push

or pull the logging tool string).

Conventional wireline

Advantages:

• cost-effective

• fast

Disadvantages:

• gravity dependent (cannot be used in high-angle - deviation >

65°- or horizontal wellbores)

• it has load limitation

Conveyance methods

Pipe-conveyed logging (PLC)

Advantages:

• highly successful

• independent of environment (depth, deviation, extreme borehole

conditions)

Disadvantages:

• slow, uses rig time expensive

• requires rig, drill pipe and associated personnel

Coiled Tubing (CT)

Advantages:

• high success rate

• it can be used in high-angle and horizontal wellbores

• rig not required

• protects the cable from damages

Conveyance methods

Coiled Tubing (CT)

Disadvantages:

• CT logging unit must be mobilized

• requires extra personnel for CT operation

• primarily used in cased-hole logging

• depth measurement can be inaccurate because of stretching

Tractor conveyed logging

Advantages:

fast

it can be used along extended horizontal sections

deviation can exceed even 90°

standard field crew

Disadvantages:

• primarily for cased-hole operation

• not suited for every well

Conveyance methods

Log curve (or simply curve):

a displayed or printed form of a recorded quantity measured by a

logging tool as a function of depth.

Sometimes the measurement is made in fixed positions as a

function of time. In such cases, the index variable of a curve is the

time (not the depth).

A simple log curve assigns only one measured value to each index

value.

An array type log curve, however, assign a one or two dimensional

array of measured values to a single index value. (Typically,

borehole imaging tools and full-waveform acoustic tools record

array type curves.)

Well log (or simply log): a displayed or printed form of several log

curves and data belonging to a logging operation. The visual

components and alfanumeric data are arranged in a well structured

format.

Well log and log curves

Log file: is the basic unit of digital storage and interchange of well

log data.

Most frequently used log file formats in the practice:

Log ASCII Standard (LAS) format was developed by the Canadian

Well Logging Society (http://www.cwls.org/las/) for storing log data

of a single well.

A LAS file contains minimal header information pertaining to the

well and the log curves. The data sets of log curves are arranged

in columns.

Due to the ASCII encoding, the content of a LAS file can be viewed

by means of a simple text editor.

Log files

LAS file

http://www.cwls.org/las/

Digital Log Interchange Standard (DLIS) was introduced by the

American Petroleum Institute.

DLIS is a binary encoded format which separates the different

types of data into different abstract layers.

The layers represents different levels of the abstraction by which

measured data can be organized into logical records (data

structure). The logical records, in turn, can be mapped into

physical records of a storage device (mostly a hard disk drive).

By using DLIS format, data sets of several wells can be stored in a

single file.

WellLogML is an XML based, ASCII encoded format which was

designed for exchanging well log data over networks (Internet and

intranets).

The information content of a WellLogML file is organized

hierarchically into sections.

Log files

Job: means a well logging activity which begins when the service

company arrives at the well site and finishes when it leaves.

Since the whole drilling process of a well is divided into drilling

different portions and well logging measurements are required

after drilling each portion of a well, more than one job is carried

out. For a given well the jobs are identified by their job numbers

(job 1, job 2, …).

Run: means a well logging activity which is connected to a given

type of measurements or measurement combinations (a type of

logging services) within a job. If more than one type of logging

services is applied, the job comprises different runs.

But the run number is not used for differentiating the runs within a

job. It identifies each application of a given type of logging services

in the sequence of jobs belonging to the same well.

So, each type of logging services has an independent numbering

of runs during the whole drilling process.

Definitions of some terms

For example, the resistivity logging of job 1 is identified by run 1

and run 2 is assigned to the resistivity logging of job 2.

If the first application of the radioactive logging occurs during job 2

(so it was not used during job 1), the run of that logging service is

identified by run 1. The next run of the radioactive logging (run 2)

will be performed during a later job (probably during job 3).

Trip: means the logging activities which begins when a logging tool

or tool string is inserted into the borehole and finishes when it is

removed from the borehole.

Normally, a single trip is enough to implement a given logging

service (that is a run of a logging service within a job).

When some technical problem occurs with the logging tool string

during the measurement, an additional trip may be required for

producing correct logging curves.

A trip belongs to a run and is identified by a trip number.


Pass: means a continuous data recording process during a trip. It

begins when data recording is started and finishes when it is

stopped. So, the pass is the unit of the effective measuring

operation, which does not include the movement of the downhole

tools without data recording in the well or borehole.

A pass is defined within a trip and identified by a pass number.

Regularly, two different passes are completed during the same trip.

Main pass is the data recording process of the entire interval to be

logged. The log curves recorded during a main pass are displayed

in the main section of the well log.

Repeat pass is an additional data recording process which

recovers not the entire but some part of the interval to be logged.

The log curves recorded during a repeat pass are displayed in the

repeat section of the well log.


Since the logging curves of the main and repeat passes have a

partially common depth interval, the main and repeat versions of

the logging curves can be compared to each other.

In fact, the repeat pass is aimed at quality controlling the result of

the main pass.

Well fitting log curves of the main and repeat sections indicate that

the results are correct.

The minimal logging interval of a repeat section is 50 m within the

interval of a main section. Generally, the lower or the upper part of

the main section is repeated.


Well name: Well 1

First interval to be logged: 0 - 500 m

Job 1

Run 1: DLL-DVL-AZL-GR-SP-MLL-CAL (resistivity tool string)

Trip 1:

Pass 1: (450 - 500 m) repeat pass

Pass 2: (0 - 500 m) main pass

Second interval to be logged: 450 – 1400 m

Job 2

Run 2: DLL-DVL-AZL-GR-SP-MLL-CAL (resistivity tool string )

Trip 1:

Pass 1: (1350 -1400 m) repeat pass

Pass 2: (450 -1400 m) main pass

Run 1: CDL-CNL-CAL-GR (radioactive tool string)

Trip 1:

Pass 1: (1350-1400 m) tool failure

Trip 2:

Pass 1: (1350 -1400 m) repeat pass

Pass 2: (450 -1400 m) main pass

Example

Log sections most frequently used in a well log:

• header (contains data about the well and the

logging operation)

• tool string configuration (a sketch and data

about the tool string)

• upper scale (the scales of log curves)

• log section for presenting the main pass in a

lower depth resolution (e.g. 1:1000)

• lower scale (the same as the upper scale)

• miscellaneous

• log section for presenting the main pass in a

higher depth resolution (eg. 1:200)

• log section for presenting the repeat pass in

a higher depth resolution (eg. 1:200)

• trailer

The structure of a printed well log

https://wiki.ppdm.org/Well_Logs_Reference_guide#Definitions

Depth scales used for printed logs

English

1 : 1200 1" = 100 ft

1 : 600 1" = 50 ft

1 : 240 1" = 12 ft

1 : 120 1” = 10 ft

1 : 48 1" = 4 ft

1 : 24 1" = 2 ft

Metric

1:2000 1 cm = 20 m

1:1000 1 cm = 10 m

for correlation and overview

1:500 1 cm = 5 m

1:200 1 cm = 2 m

for a detailed presentation

1:100 1 cm = 1 m

1:50 1 cm = 50 cmDifferent depth scales

provide the presentation of

log curves with different

depth resolutions on the well

logs.

A typical (well) log header

Malcolm Rider: The Geological Interpretation of Well Logs

The log section of most field logs contains three parallel tracks.

A track is a long rectangular area with a grid. A track is the place of

displaying one or more log curves.

An additional narrow column containing the divisions of depth is also

inserted between track 1 and track 2. Tracks 2 and 3 are adjoining.

The vertical division of the grid lines depends on the depth scale but it is

always linear.

The horizontal division of the grid in a track can be linear or logarithmic

with 4 cycles (represents 4 orders of magnitude). The scales of the log

curves are fitted to the horizontal divisions.

Track 1 always has a linear grid with 10 divisions along the horizontal

direction.

Tracks 2 and 3 may have

• a common logarithmic grid,

• a common linear grid with 20 divisions along the horizontal direction,

• or a hybrid of logarithmic grid in track 2 and linear grid in track 3.

Standard formats of a log section


Malcolm Rider: The Geological Interpretation of Well Logs


Darwin V. Ellis, Julian M. Singer: Well logging for Earth Scientists

Before starting a wireline logging job, the drilled portion of a

borehole must be cleaned, stabilized and the drill string must be

extracted.

Next the wireline logging system is set up to a drilling rig

Then the first logging tool string is assembled, attached to the

logging cable head and checked.

The depth measurement system is set to zero when the reference

point of the tool string is positioned at the level of the drill floor.

The tool string is lowered into the hole to the bottom depth of the

interval to be logged (in most cases, the bottom of the drilled

section).

Most of the logging tool measurements are recorded while the tool

string is being pulled upwards.

The logging cable attached to the tool string provides a mechanical

support for the tool and a two-way communication channel for data

transmission.

Summary of a well logging operation (1)

The cable is wound around a motorized drum and it is guided

manually during a logging process (by using a spooling system).

The typical pulling speed varies between 180 m/h (~590,6 ft/min)

and 1500 m/h (4921,3 ft/h). Its actual value depends on the type of

measurements.

The surface data acquisition and processing system not only

receives but also pre-processes (e.g. depth-matches) and stores

the readings of logging tools built in the tool string.

Well depths are measured with a calibrated measuring wheel

system. Cable speed and cable tension are also recorded and

checked during the whole operation.

While a logging job with a simple logging program may take only a

few hours, performing a wider range of open-hole logging

operations may require 2 or 3 days (logging with each tool string

takes a few hours to complete).


Before and after each trip of a logging tool string, a field calibration

process is performed on the logging tools.

During the calibration the influence of the environment on a logging

device is controlled by means of a suitable calibration standard

(calibrator). The value of the physical quantity represented by a

calibrator is precisely known.

The applied calibration standard depends on the type of

measurement. It can be another measurement device of known

accuracy, a device generating the quantity to be measured (e.g. a

voltage for electric measurements), or a physical artefact (e.g.

steel rings with known diameters for borehole caliper

measurements).

The value measured by the logging device under the test and the

known value of the calibrator are compared.

If no significant difference is observed between these values, the

logging tool can be considered as reliable.


If the logging tool does not measure the expected value of the

calibration standard within a specified margin of error, the tool

cannot be used for the measurement, or its recorded logging curve

cannot be accepted.

The measured logging curves and other data are real-time

displayed on a monitor.

A well log is edited from the continuous recording of the data, and

paper prints are made in the field.

After the last logging tool string has been pulled up to the surface,

the wireline logging system is taken down and the logging crew

leaves the drill site.

The drilling program, in turn, continues with the next step.


well logging equipment and operation -...

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