Cables and Skates—
Improving the Weakest Links
The cable is perhaps the most vital component for wireline logging; without it,
wireline measurements and wireline logs are not possible. High logging tensions
inherent in complex trajectories and ultradeep wells have exposed weaknesses in
conventional cable designs. However, engineers have introduced new technologies
and designs that improve wireline operations in ultradeep wells and are addressing
other weak links in logging components by developing new downhole and
surface hardware.
Chris Babin
New Orleans, Louisiana, USA
Serko Sarian
Sugar Land, Texas
Oilfield Review Winter 2014/2015: 26, no. 4.
Copyright © 2015 Schlumberger.
MDT, MSCT, Saturn, SureLOC, TuffLINE and WellSKATE are
marks of Schlumberger.
Teflon and Tefzel are registered trademarks of E. I. du Pont
de Nemours and Company.
PEEK is a trademark of Victrex USA, Inc.
The wireline cable lends its name to a major
segment of the oil and gas service industry.
Schlumberger’s history as a technology corporation finds its origin in being the world’s first well
logging company. That first log was acquired in
1927 using tools attached to a cable and lowered
into a well in the Alsace region of France. The
simple cable used then was a crude precursor to
the wireline cables in use today. Modern logging
> Fishing for cable. If the wireline cable is
unintentionally broken, the drilling rig must first
fish the cable from the hole before retrieving the
logging tools. This process can be difficult and
time-consuming. The drilling roustabout is using
a cutting torch to remove the tangled mass of
cable from the grapple before fishing more cable
from the well.
18
cables serve a crucial role as conduits for electrical power sent from the logging unit to downhole
tools, and they link surface equipment with
downhole sensors, usually by way of telemetric
data exchange. For most E&P operators, and perhaps even service providers, little thought is
given to the cable—until a failure occurs. Then
the importance of the logging cable and the role
it fills in data acquisition become all too obvious.
When problems arise, field and office personnel
might view the logging cable as the weakest link
in a chain.
Some wireline cable weaknesses are inherent,
resulting from physical limitations; these engineering-based limits are well documented and
exceeding them comes with recognized risk. The
traditional heptacable—so named because seven
insulated copper wires are located in the center of
the cable—is rated for breaking strength and safe
working loads (SWLs). Other limitations may be
less well-known to oilfield operators, and some
limitations are consequences of poor operating
technique. Conditions that are out of the control
of the logging operator can also result in cable
damage and failure.
The trend toward ultradeep well depths has
brought to light design weaknesses that were
rarely a problem in the past. Recent deep drilling
activity has produced wells that exceed 11,000 m
[36,000 ft]. In these wells, the maximum cable
tension at the surface during logging is more than
double that routinely encountered in shallower
Oilfield Review
wells. Cables deployed in deep and geometrically
complex wells have high tension because of
heavier logging combinations, the greater weight
of longer cables and higher cable friction. The
logging tensions encountered in these ultradeep
and complex wells magnify systemic weaknesses
and have resulted in cable-related incidents not
commonly observed in the past. Because most of
these operations occur in deepwater wells, the
cost of failure is greatly amplified compared with
lost-time costs in land-based operations.
Wireline operations–related weaknesses that
are not specific to the cable can also threaten the
logging process. If a toolstring fails to reach an
objective zone, data cannot be acquired. These
data are used by engineers, geologists and petro-
Winter 2014/2015
physicists to understand the hydrocarbon production potential of both well and reservoir, and the
opportunity to obtain these data for a particular
well may be lost forever if logging operations fail.
In addition, a logging tool stuck downhole while
attempting to acquire data creates a major concern for both service companies and operators.
Another potential weakness in wireline logging operations is a component that is designed to
fail, or at least break on command. The connection between the cable and the logging tools is the
logging head. A weakpoint in the head is designed
to have a lower breaking strength than that of the
logging cable. The weakpoint allows controlled
release of the tools without breaking the cable.
When a logging toolstring becomes stuck in a well,
the drilling crew traditionally cuts the cable, runs
drillpipe over the cable and down to the tools and
latches onto them. After receiving indications of
tool engagement with the drillpipe, the drilling
crew uses the rig to pull on the cable and intentionally break the weakpoint. The logging crew
can then retrieve the freed cable, and the drilling
crew recovers the downhole tools using the pulling power of the drilling rig.
An unintentional release of the cable from
the downhole tools, either by a broken cable or
an accidentally broken weakpoint, is one of the
worst cable-related failures. Broken cable still
attached to the tools must first be fished out of
the well before the tools can be retrieved, a process that can take days (previous page). Failure
19
Outer armor
Inner armor
Jacket
Conductor wires
> Traditional heptacable design. The seven-conductor heptacable is the standard cable for openhole
wireline logging. In these cables, the wires of the outer armor layer are usually larger diameter than
those of the inner layer. The outer layer is wound in the opposite direction to that of the inner layer to
maintain a dynamic torque balance and counter the tendency to unwind. The outer armor layer carries
more tension than the inner and thus has a higher inherent torque. The cable core consists of the
jacket, conductor wires and filler material. The insulated conductor wires are protected by a
semiconducting jacket.
to retrieve logging tools is an expensive conseCables are available in a variety of configuraquence in itself; however, the cost of sidetracking tions, compositions and styles. Most are fit-foraround unrecovered tools and redrilling intervals purpose; for example, small-diameter, singlemay far exceed the cost of lost tools.
conductor monocables are used for production
Recent engineering efforts have addressed services in cased wells. Their small crosscable design weaknesses, produced high-strength sectional area makes them better suited than
rig-up accessories, provided more powerful log- large-diameter cables for pressure operations.
ging units and led to the design of downhole Compared with monocables, heptacables offer
hardware that complement higher strength higher strength, can handle more electrical power
cables. Software developers have also developed for downhole tools and have higher data transfer
a program that helps logging engineers under- rates. Heptacables are available in a range of
stand downhole cable conditions and safely diameters. Slickline cables may be referred to as
retrieve tools to the surface.1
wireline cables, but these specialty cables are
3
This article describes proprietary Oilfield
innovations
Reviewsolid wire and have no internal conductor.
in logging cable design that increaseWINTER
the oper14/15 The heptacable is the standard for openhole
Cable
Fig 2 logging (above). Traditional heptacables comating range and margins of safety for
wireline
ORWINT
14/15prise
CBL an
2 outer armor layer of steel wires and an
operations. New and modified auxiliary
hardware
augment the use of these new cables. Case studies inner armor layer of steel wires wound around a
from deepwater operations in the Mediterranean core. The core has an outer semiconducting
Sea, West Africa, China, the Gulf of Mexico and jacket that contains a spiral band of six conducthe Gulf of Thailand demonstrate the application tors, filler material, an inner semiconducting
of these new technologies and designs.
jacket and a single, insulated center conductor.
The jacket protects the inner conductor wires,
which are coated with a material such as polyproLogging Cable Primer
Wireline logging implies acquisition of downhole pylene, Teflon or Tefzel (ethylene tetrafluoroethdata via tools that are attached to a cable—a ylene resin) insulation.4
wireline—and lowered into a well. The cable
The outer armor layer of a standard 0.46-in.
conveys power and control commands from a [1.17-cm] cable is a band of 24 steel wires
surface logging unit and provides real-time, two- wrapped in one direction covering a band of 24
way communication between the unit and down- thinner inner wires wrapped in the opposite
hole tools. The surface logging unit records and direction; the two layers balance the tension and
processes data from which petrophysical logs torque of the cable. Standard armor wires are
are generated.2
manufactured from high-strength galvanized
20
improved plow steel (GIPS). To build higher
strength cables, design engineers replace standard GIPS armor wires with wires made from
stronger metal.5
Manufacturers rate cables for temperature
and tension limits. The maximum temperature
for cables made with polypropylene insulating
material is 150°C [300°F]; cables with Tefzelcoated insulation may have ratings above 288°C
[550°F]. Ratings may be quoted for one hour of
use; for continuous operations of longer duration,
cables carry lower ratings.
A new 0.46-in. diameter cable made with GIPS
has a breaking strength of 16,700 lbf [74.3 kN]
and an SWL of 8,345 lbf [37.1 kN].6 Although
GIPS cables were the standard for many years
and still are in many areas of the world,
Schlumberger operations typically rely on cables
made with higher strength steel armor wires that
significantly raise the breaking strength and the
SWL. A common 0.46-in. diameter cable used for
openhole logging today has a breaking strength of
19,410 lbf [86.3 kN] and an SWL of 9,705 lbf
[43.2 kN].
To lower tools on a cable, logging units use a
winch attached to a drum on which cable is
spooled (next page, top). A full drum may carry
several thousand meters of cable. Standard practice is to spool cables onto the drum with an
applied tension of 1,000 lbf [4.48 kN]. This tension facilitates spooling cable onto the drum.
During normal logging operations, cable tension is measured at the logging unit. When tools
are in a well, the tension includes the weight of
the logging tool, the weight of the cable spooled
into the well and frictional forces that result as
the cable and tools are pulled along the wellbore.
Buoyancy forces from the drilling mud offset
some of the tension.
As the cable is spooled onto the drum during
and after logging, the tension will almost always
exceed the original 1,000-lbf spooling tension. In
normal operations, underlying rows of cable are
not at risk of damage from this higher tension
because the maximum allowable tension is not
sufficient to mechanically damage the cable. This
remains true providing the winch operator spools
the cable properly and does not allow the cable to
overlap itself, which can cause mechanical damage to the cable. Logging crews carefully align
the logging unit during setup to ensure that the
cable can be properly spooled.
Standard cables can, however, be damaged in
deep and ultradeep well logging operations—wells
with depths in excess of 6,100 m [20,000 ft]—
even when properly spooled because the normal
Oilfield Review
cable tension in these wells is sufficient to crush
underlying cable. High logging tension may also
occur at shallower depths in S-shaped wells
because of increased frictional forces acting on
the cable. Schlumberger defines high-tension
logging operations as those with surface tensions
exceeding 8,000 lbf [35.6 kN]. High logging tension poses a risk of crushing logging cable on the
drum and facilitates other types of failures.
A newly manufactured cable has substantial
torque imbalance, and it takes time for the armor
layers to relieve the torque, stretch and reposition themselves. During the first descent of a new
cable, cable tension creates an unequal load distribution between the inner and outer armor layers; however, the layers can move independently
of one another, and cable rotation during operations should balance the torque and tension differences. The process of balancing cable torque
in a new cable is referred to as seasoning.
If the outer layer unwinds, an outer-armor
distortion in the form of a birdcage develops
(right). This condition results in tension that is
no longer carried by the full cable but by the
1. For more on logging tool conveyance: Billingham M,
El-Toukhy AM, Hashem MK, Hassaan M, Lorente M,
Sheiretov T and Loth M: “Conveyance—Down and
Out in the Oil Field,” Oilfield Review 23, no. 2
(Summer 2011): 18–31.
2. For more on basic logging operations: Andersen MA:
“Discovering the Secrets of the Earth,” Oilfield Review 23,
no. 1 (Spring 2011): 59–60.
3. For more on slickline services: Billingham M,
Chatelet V, Murchie S, Cox M and Paulsen WB:
“Slickline Signaling a Change,” Oilfield Review 23, no. 4
(Winter 2011/2012): 16–25.
4. Schlumberger manufacturing often uses the following
naming convention for classifying cables: X-YYZ AAA,
in which X is the number of conductors, YY is the cable
diameter in 1/100 in., Z refers to construction components
and AAA refers to the armor. A standard-issue cable
for routine logging at temperatures less than 150°C
[300°F] is the 7-46P GIPS, which is a seven-conductor,
0.46-in. cable with polypropylene-coated conductors (P)
and galvanized improved plow steel (GIPS) armor
wires. The 7-48A SUS cable is a seven-conductor,
0.48-in. diameter cable that has Teflon-coated conductors
and Tefzel jacketing material (A) and superultrastrength
(SUS) cable armor wires. This cable is suited for use in
high-tension and high-temperature operations. Tefzel
polymer is a fluorine-based plastic with high corrosion
resistance and strength over a wide temperature range.
5. For more on the development of high-strength cables:
Alden M, Arif F, Billingham M, Grønnerød N, Harvey S,
Richards ME and West C: “Advancing Downhole
Conveyance,” Oilfield Review 16, no. 3
(Autumn 2004): 30–43.
6. Breaking strength values are quoted for a new cable and
do not account for wear, age and mechanical damage,
which can significantly reduce a cable’s rating. The
breaking strength is measured with either both cable
ends free or both ends fixed. Ends-free testing, which
allows the cable to rotate when tension is applied, is
representative of downhole conditions. The 7-46P GIPS
cable breaking strength for ends fixed is 16,700 lbf
[74.3 kN]. The SWL may be quoted as half the breaking
strength, which provides a factor of safety of two. An
alternate method of determining SWL for special
high-strength cables is 62% of the ends-fixed
breaking strength.
Winter 2014/2015
> Full cable drum. Cable is spooled onto a drum that is attached to a winch.
The wireline crew runs the logging tools in and out of the well using the winch.
1 in.
Outer armor wires
Inner armor wires
Outer armor wires
Inner armor wires
Oilfield Review
WINTER 14/15
Cable Fig 3
ORWINT 14/15 CBL 3
> Logging cable birdcage. The inner and outer layers of a torque-balanced
logging cable share the tension load. If the torque becomes unbalanced,
the outer layer tends to unwind and separate from the inner layer, which
allows a birdcage to form (top). When a birdcaged cable is stressed, the
inner armor layer bears the majority of the load and breaks first. Stress is
then rapidly transferred to the outer armor wires, which also break. The
broken cable (bottom) shows evidence of a sudden tensile break of the
inner layer; the elongated and nonuniform nature of the broken outer layer
wires is evidence of its unwinding before the cable broke.
21
smaller inner armor layer, which greatly reduces
cable breaking strength. A birdcage is often
caused by sudden changes in the cable tension
such as can occur when a stuck tool comes free at
high tension. Rapid tension cycling, or yo-yoing,
which consists of repeatedly increasing and
releasing cable tension, can cause a birdcage to
form. In addition, yo-yoing can create loops in the
cable when torqued cable bends back upon itself
or when the cable tension is slacked off. Loops
cause cable kinks and knots when tension is
reapplied to the cable; kinks and knots significantly reduce the cable SWL.
Cold flow is compression-induced cable
deformation. The term describes the low-temperature extrusion of core material from the middle
of a cable. When a cable is spooled onto a drum at
high tension and stored in that condition, permanent deformation and damage to the core material occurs over time. Compression causes the
inner armor to squeeze the core, damaging the
jacket material and displacing the insulation covering the conductor wires (below). As the core
material of the compressed cable extrudes, the
inner conductor wires may eventually short out
against the cable armor. Cold flow may also occur
when torque in inner armor wires constricts the
core and reduces the jacket diameter.
Shorted conductor
to inner armor wire
High-strength
wireline cable
Dual-drum capstan
Logging unit
Depth, ft
Dual-drum capstan
10,000
Weakpoint
20,000
> Dual-drum capstan. To prevent cable crushing, drum damage and cold
flow in high-tension logging, logging crews place a capstan between the rig
floor and the logging unit. The capstan (inset) consists of two large,
hydraulically powered grooved wheels that have several wraps of cables
around them. Cable on the rig side of the capstan is under high tension;
cable on the drum side is maintained at a lower tension for spooling onto
the winch drum. Capstans extend the useful life of logging cables, although
they present operational risks: Maintaining the proper tension balance is
difficult, and synchronizing the capstan speed with that of the logging
winch can be problematic. They are also additional pieces of equipment
that must be mobilized to remote offshore locations.
The dual-drum capstan, introduced in the
1970s, relieves cable tension that occurs while
spooling the cable onto the drum (above).7
Although the capstan eliminates tensioninduced cold flow for cable on the drum, it can
increase cable torque, which may be the more
damaging phenomenon.
> Core crushing and cold flow. Cold flow can
occur after extended storage of a cable on a drum
under high tension. This effect is characterized by
flattening of the cable and separation of the
armor strands. Over time, the polymer of the core
may plastically deform, which can eventually lead
to the copper conductor wire shorting to the
armor wires or to each other. A cable with
shorted conductors must be pulled from service
and may not be repairable.
22
well depth in deepwater operations reached
10,700 m [35,000 ft], and utradeep wells have
pushed the limits of traditional cable design.8
Normal logging tensions of 15,000 lbf [66.7 kN]
have occurred in some wells—the combined
effects of cable weight, long and heavy toolstrings
and frictional forces.
Ultradeepwater wells that have high logging
cable tensions were first encountered in the
Tension or Torque
ReviewGulf of Mexico and then the North Sea but are
In the past 35 years, the well depthsOilfield
attainable
14/15now common offshore Brazil, Africa, India and
by offshore rigs have increased moreWINTER
than 75%
Cable Fig 6
(next page, top). Deepwater rigs areORWINT
now capa14/15Asia.
CBLGulf
6 of Mexico operations routinely experible of drilling to 12,200 m [40,000 ft] in 3,050-m ence cable tension above 13,000 lbf [57.8 kN],
[10,000-ft] water depth. As of 2012, the maximum and 10,000 lbf [44.5 kN] is not uncommon else7. For more on the dual-drum capstan tension relief system:
Alden et al, reference 5.
8. Sarian S, Varkey J, Protasov V and Turner J:
“Polymer-Locked, Crush-Free Wireline Composite
Cables Reduce Tool Sticking and HSE Risk in Emerging
Deepwater Reservoirs,” paper SPE 164762, presented at
the North Africa Technical Conference and Exhibition,
Cairo, April 15–17, 2013.
9. For more on high-tension cable maintenance: Alden et al,
reference 5.
Oilfield Review
40,000
35,000
True vertical depth, ft
where in the world. These are normal logging
tensions; tool sticking can subject cables to
higher short-term loads.
Extreme conditions have required service
companies to rethink cable technologies. Service
companies first produced high-strength and
ultrahigh-strength cables by upgrading the armor
wire material. The breaking strength of some of
these cables exceeded the pulling capabilities
of older-generation logging winches. Capstan
tension relief systems, for example, were limited to 15,000 lbf of differential load capacity.
Unfortunately, stronger cables did not resolve all
the problems of ultradeep well logging. Cable and
drum failures, which had not previously arisen,
began to occur as the forces exerted on logging
systems stressed them beyond their original
design specifications (below). To address these
concerns, Schlumberger engineers took a close
look at logging in deep wells. They studied cable
structure and traced the root causes of premature cable failure.
Traditional cable designs have two layers of
steel armor wires that are wound in opposite
directions to maintain torque balance. The armor
wires are the mechanical strength element of the
cable. The two layers, which are free to move
independently of each other, share the tensile
load; they rotate and stretch under load—
although not always equally. The wires of the
outer armor layer are typically of larger diameter
than the wires of the inner layer.
Design engineers found that cable torque
increases proportionally with tension; torque
accumulates with each descent and with tension
cycling. Devices that bend the cable, such as the
cable drum and the sheaves that direct the cable
30,000
25,000
20,000
15,000
10,000
1980
1985
1990
1995
2000
2005
2010
Year
> Increasing Gulf of Mexico maximum well depths. From 1980 until almost 2000, the maximum true
vertical depth recorded for offshore oil and gas wells was less than 25,000 ft [7,600 m]. Soon after,
deep­water Gulf of Mexico maximum well depth ramped up to 30,000 ft [9,145 m] and exceeded
35,000 ft [10,670 m] in 2009.
into the well, act as torque barriers and increase
torque accumulation in the logging cable. Torque
also accumulates at the logging winch when the
cable is spooled onto the drum.
When a tool is stuck downhole, or if the cable
is not free to rotate, the torque can become
unbalanced. If the tension is repeatedly cycled,
the outer layer of armor begins to unwind and
lose contact with the inner layer. The inner layer
tightens, constricting the core. If the outer layer
unwinds, the inner layer may become the only
strength element, compromising the SWL for the
cable, which may cause the cable to break at
what should be a reasonable logging tension. This
scenario became all too common in the early days
of ultradeep well logging.
In addition to breakage, crushing and cold
flow became common in cables used for logging
ultradeep wells. Cables spooled under high tension require tension and torque relief.9 Cable
maintenance to relieve stored tension and torque
is performed onshore with special spooling
equipment. For most deepwater offshore operations, which are located far from land, performing these tasks in a timely manner is difficult
because of logistics.
Traditional Logging Drum
High-Strength Drum
Flange force up to 2 million lbfOilfield Review
WINTER 14/15
Cable Fig 7
ORWINT 14/15 CBL 7
Pressure on drum core
up to 10,700 psi
> Cable spooling forces. Logging cables are spooled onto empty traditional drums (left) with an applied tension of 1,000 lbf. During logging operations, the
tension can be much higher, which causes the spooled cable to exert large forces on the drum (middle). For example, with 10,000 lbf of cable tension, the
drum flange may experience outward forces of up to 8,900 kN [2 million lbf], and the combined forces of tension and cable weight can generate drum core
pressures of up to 74 MPa [10,700 psi]. Cable drums used for logging with standard and high-tension cables in shallower wells do not experience sustained
forces of these magnitudes. After drum failures during high-tension operations exposed drum design weaknesses, engineers developed higher-rated
drums (right) that also have greater cable-carrying capacity than do traditional drums.
Winter 2014/2015
23
Crush-resistant core
Outer armor
Outer polymer
layer
Inner armor
PEEK-coated
conductor wires
Crush-resistant
filler material
PEEK-coated
core
> TuffLINE cable cross section. Engineers designed the TuffLINE 18000 cable with a crush-resistant
core (left) that is double extruded; a thin PEEK shield covers the core and individual conductor wires.
Polymer encapsulation (right, black) locks all the armor wires together as well as to the core, which
eliminates birdcaging and helps maintain the cable’s torque balance. The reduced number and
diameter of outer armor wires result in decreased overall cable weight and drag compared with that
of other cable designs, which translates to lower downhole logging tension.
The proprietary polymer in the TuffLINE
New Cable Designs
The solution to both torque imbalance and cable core fills the void space between the conmechanical damage seemed simple: build a ductor wires and is also extruded between armor
crush-free cable using armor wires that are layers. This process creates a cable that is almost
torque balanced, locked together and locked to impervious to crushing and deformation. Adding
the core. After several years of development and to the strength of the cable are the double- and
much trial and error, Schlumberger engineers triple-extruded conductor wires, which include a
introduced the TuffLINE 18000 torque-balanced layer of PEEK polymer.10
composite wireline cable. The first of its kind,
The SWL of the TuffLINE cable is 18,000 lbf
this heptacable has several features that other [80 kN]; the ends-fixed breaking strength is
logging cables lack.
28,000 lbf [125 kN] and the ends-free breaking
A proprietary polymer composition, which is strength is 27,000 lbf [120 kN]. These limits
applied in a unique extrusion process, fills the exceed the pulling power of offshore logging
space between the inner armor and the cable core units. In the event the toolstring becomes stuck,
as well as between the armor layers (above). The the drilling rig may be used to pull the cable with
polymer layer locks the armor wires into place and a T-bar attachment.11 The TuffLINE cable diamedoes not allow them to unwind, which eliminates ter is 0.50 in. [1.27 cm], which is larger than the
birdcaging. This design allows the cable to be standard 0.46-in. logging cable but similar in
Oilfield Review
repeatedly cycled without fear of the cable
break-14/15
diameter to that of other high-strength and ultraWINTER
ing below its SWL. No cable seasoning isCable
required,
Fig 9 high-strength cables.
ORWINT
improving operational efficiency compared
with14/15 CBL
The 9outer armor layer is composed of wires of
that for operations that rely on conventional log- smaller diameter than those of the inner layer.
ging cable designs.
These smaller wires reduce the weight per unit
Heaviest Toolstrings
Real-Time Tension at 4,600 m [15,092 ft],
12 1/4-in. Borehole
Acoustic and imaging tools
9,700 lbf, includes drag from calipers and centralizers
Pressure and sampling tools
9,400 lbf at 4,500 m [14,765 ft]
Triple-combo tools
8,150 lbf with caliper open
> Logging at the limits in deepwater West Africa. In deep water off the coast
of West Africa, an S-shaped well profile resulted in high tension at TD. In
the 12 1/4-in. section, the three heaviest logging strings had tensions at TD
of 9,700 lbf [43 kN]; 9,400 lbf [42 kN] at 4,500 m and 8,150 lbf [36 kN]. The
operator rightly anticipated logging tensions in excess of 10,700 lbf for the
deeper 8 1/2-in. section.
24
length of the cable in air to 416 lbf/1,000 ft
[6.07 kN/1,000 m], which is less than that of the
smaller diameter superultrahigh-strength logging cable (424 lbf/1,000 ft [6.18 kN/1,000 m])
that is frequently used in deepwater operations.
The outer armor wires are held apart from one
another by the polymer layer, reducing the sliding friction of the cable, which in turn reduces
cable tension.
A recent deepwater exploration well in the
eastern Mediterranean Sea targeted a zone
around 5,000 m [16,400 ft]. The original plan
called for a vertical well, but stuck pipe in a shallower section resulted in a sidetrack and a well
deviation of 35° from vertical. High cable tension
encountered during a previous logging run plus
model predictions resulted in a bottomholetension projection in excess of 10,000 lbf.
The remote location precluded mobilization of a
capstan on short notice. Alternatives included
multiple descents with short toolstrings or drillpipe-conveyed logging, which would have added
five days for logging.
A TuffLINE cable was mobilized to the wellsite from the North Sea and installed in the
existing surface equipment. Run 1 included six
openhole logging tools, but hole conditions prevented the long toolstring from reaching TD. The
toolstring was shortened, and TD was successfully reached in Runs 2 and 3. Formation pressure measurements and sampling during Run 4
and rotary sidewall coring during Run 5 were
completed without incident. As predicted in the
modeling program, four of the five logging runs
encountered sustained logging tension exceeding
10,000 lbf. Multiple short-duration pulls of
16,000 lbf [71 kN] were made while logging, each
of which freed the stuck tools and allowed logging to continue.
The operator saved five days of rig time compared with the number of days that would have
been required for pipe-conveyed logging. An
additional day of rig time was saved because the
TuffLINE cable required no seasoning prior to
logging. Although cable tension exceeded
10,000 lbf, and no capstan was used, the logging
crew observed no cold flow damage or crushing
during postjob examination. In addition, despite
multiple tension cycles to 16,000 lbf, no torquerelated cable birdcages were observed.
In a deepwater offshore West Africa environment, Total E&P drilled an S-shaped ultradeep
well.12 The anticipated logging tension was in
excess of 10,700 lbf [47.6 kN] (left). Future field
Oilfield Review
Winter 2014/2015
90
10,000
TuffLINE cable ascending
TuffLINE cable descending
High-strength cable, predicted
Well deviation
80
70
7,500
50
5,000
40
Deviation, degree
60
Tension, lbf
development depended on acquiring a comprehensive set of wireline petrophysical data. A
traditional wireline logging suite was planned,
and advanced measurements from nuclear
magnetic resonance, acoustic logging and imaging tools along with rotary sidewall coring and
fluid sampling were included in the evaluation
program. Using the MDT modular formation
dynamics tester to acquire uncontaminated
representative samples was crucial for engineers to determine fluid properties and identify
compartmentalization.
Normal logging tension—the weight of the
toolstring while moving up the hole—includes
tool weight, cable weight and frictional forces
minus buoyancy forces. In the event of tool sticking, the logging operator increases tension with
the winch up to a maximum safe pull to overcome
sticking forces. The maximum safe pull tension is
normally the SWL of the cable. If a mechanical
weakpoint is used in the logging head, its rating,
minus a factor of safety, may limit the maximum
tension. Maximum safe pull values may be further reduced if they exceed any system capacity
such as limitations of the logging unit, cable
drum and rig-up equipment.
The well was drilled in 2,500 m [8,200 ft] of
water, and well depth was in excess of 5,000 m.
The initial 17 1/2-in. section was S-shaped with
greater than 20° deviation. The 12 1/4-in. and
8 1/2-in. sections were vertical. Because the cable
tension in the 12 1/4-in. section was slightly less
than 10,000 lbf, which is the limit for logging
without a capstan, operations could be performed with high-tension logging and rig-up
equipment that included a 7-48A SUS cable.13
The predicted tension of the 8 1/2-in. section was
greater than 11,000 lbf [48.9 kN], and the hightension equipment previously deployed would
now require the use of a capstan.
Schlumberger engineers and the operator
considered four options:
•deploy and install a capstan; availability was
questionable and the rig logistics were
problematic.
•use drillpipe-conveyed logging; estimated additional rig time was four days, which would cost
an additional US$ 5 million.
•make multiple trips with short toolstrings;
each trip would take from 12 to 18 hours.
Assuming no pipe trips were required between
logging runs, multiple trips would add a minimum of three days to the program.
•deploy the TuffLINE cable, which could be used
with the high-tension rig-up equipment already
on location without adding significant risk and
would not require the use of a capstan.
30
20
2,500
10
0
0
0
2,000
4,000
Depth, m
> Cable tension while logging an S-shaped well. Wireline engineers used the Well Conveyance Planner
software to predict surface cable tensions (black) and to plot a record of logging tension while
descending (blue) and ascending (red) in the 8 1/2-in. portion of a well. For the high-strength logging
cable used to log the 12 1/4-in. section, the planner extrapolated a surface cable tension of 10,700 lbf
with tools at TD. The actual maximum cable tension was only 9,400 lbf—less than predicted—because
of the TuffLINE cable’s lower drag coefficient and lower unit weight compared with the other cable.
The trajectory of the S-shaped well (green) changes from vertical at about 3,000 m [9,800 ft] measured
depth to a maximum angle of about 24° before it returns to near vertical around 4,000 m [13,130 ft]. In
the vicinity of the deviated section, cable tension decreases while the toolstring is descending and
increases while it is ascending. This phenomenon is caused by higher frictional forces on the cable
through the deviated section.
The operator decided on the TuffLINE cable string was deployed than the one used in the
option, and a drum of cable was flown in from a 12 1/4-in. section (above). The reduction in cable
neighboring country. In all, eight descents were tension was attributed to an 18% reduction in the
performed. Although the cable was new, no sea- drag coefficient through the S-shaped portion of
soning was required and stretch was negligible. the well and the reduced weight of the TuffLINE
Standard operating procedure when a capstan is cable compared with that of the conventional
not used in high-tension operations is to swap high-tension cable.
cables after six descents, which helps avoid
Although the TuffLINE cable was previously
torque- and tension-related Oilfield
damageReview
and cold unused, it was able to deliver repeatable depth
WINTER
14/15
flow. Limiting descents with the
TuffLINE
cable accuracy. Traditional logging cables stretch durCable Fig 11
was not required; the same cable
was
used
for
all 11ing seasoning, which can cause depth repeatabilORWINT 14/15 CBL
eight descents.
ity problems that are exacerbated in deep and
The logging crew observed that during the ultradeep wells. As a result, logs are often depth
job, the logging tension did not reach the pre- adjusted to correct for these discrepancies. After
dicted 10,700 lbf. The maximum tension was only multiple descents, the TuffLINE cable exhibited
9,400 lbf [42 kN], even though a heavier tool- negligible stretch. The logging crew saw evidence
10.PEEK (polyether ether ketone) polymer is a highperformance, high-temperature thermoplastic used in
engineering applications.
11.A T-bar is a device that is clamped onto a logging cable
near the rig floor; it allows the drilling rig elevators
to be used to apply direct tension. Using the elevators
bypasses the logging unit, upper sheave and
lower sheave.
12.Sarian S, Varkey J, Protasov V, Montesinos J, Ventura D
and Greusard D: “In a Challenging West Africa
Deepwater Well, Polymer-Locked, Crush-Free Wireline
Composite Cables Help Save Four Days of Rig Time for
TOTAL E&P CI While Avoiding Tool Sticking and
Reducing HSE Risk,” paper 28, presented at the 18th
Annual Offshore West Africa Conference, Accra, Ghana,
January 21–23, 2014.
13.The SUS denotes superultrastrength cable.
25
of the depth accuracy between runs of the MSCT
mechanical sidewall coring tool and the Saturn
3D radial probe tool (left). They observed the
imprint of a hole left by a drilled sidewall core on
the packer element of the probe tool that was
within 6 cm [2.4 in.] of the set depth. The two
tests were taken at a depth greater than 5,000 m.
> Negligible cable stretch. Conventional heptacable designs can result in new-cable stretch of up to
several meters during initial descents because of torque-induced seasoning effects. The TuffLINE
cable requires no seasoning or special treatment. The depth accuracy of this cable was evident from
two logging runs with a new cable. A Saturn tool, which uses a packer made of soft material that
allows it to conform to the borehole wall, was run after a rotary sidewall coring tool. One of the Saturn
packer set depths coincided with a sampling point taken with the coring tool. When the Saturn tool
was retrieved to the surface, the packer element (left) retained an imprint of the hole made by the
rotary sidewall core bit. A core bit is placed on the packer near the imprint for reference (right). At
approximately 5,000-m MD, less than 6-cm difference occurred between the two logging runs.
Tri-Roller
Tri-Roller
Dual-Wheel
Dual-Wheel Roller
Roller
More than New Cables
Two major challenges encountered by logging
crews are rugose holes and logging high-angle
wells in which gravity alone may be insufficient
to deliver tools to TD. Logging crews have successfully logged wells with deviations up to 70°
without resorting to drillpipe-conveyed toolstrings or wireline tractors; however, some of the
successes in getting downhole in high-angle wells
may be attributed to chance.
Oil wells rarely have smooth bores between
the bottom of casing and TD. Washouts frequently
occur when the formation around the wellbore—
brittle shale sections or unconsolidated sand
intervals—breaks out and enlarges the wellbore.
Consolidated, permeable formations are less
likely to wash out, and the borehole through
these sections is usually in gauge—the same
diameter as the drill bit. A large washout above
Roller
Roller Bottom
Bottom Nose
Nose
Oilfield Review
WINTER 14/15
Cable Fig 12
ORWINT 14/15 CBL 12
> New accessory hardware. Engineers designed the WellSKATE family of auxiliary conveyance equipment to facilitate logging operations. These lowfriction and low-contact devices help logging tools reach TD and also reduce sticking while logging.
26
Oilfield Review
Winter 2014/2015
90
350
Relative bearing
Well deviation
80
300
250
60
187.5° average relative bearing
50
200
40
150
30
33° average deviation
Relative bearing, degree
70
Deviation, degree
an in-gauge section may form a ledge, which can
cause logging tools to stop, or sit down. After sitting down on a ledge, tools may have difficulty
realigning with the wellbore and proceeding
downward. If the toolstring cannot be coaxed
downhole, crucial logging data may be lost.
Reaching TD is not the end of the logging
journey. The toolstring can differentially stick to
the wellbore wall while tools are being logged out
or retrieved from a well. Differential sticking is a
problem encountered most often while tools are
being pulled out of the well, usually while logging
at slow speeds. This condition results when the
hydrostatic pressure of the mud column exceeds
the pore pressure of the formation, especially in
zones that have been depleted by production or
when heavy mud weights are used to control the
well. The mud pushes the logging tools or the logging cable against the permeable underpressured
zone, causing them to stick.
The logging operator can increase the logging
tension to pull the tools free, but the resulting
stick-slip movement greatly reduces log data quality. During the process of release, data may not be
acquired, or the quality may be severely degraded.
In the worst case, logging tools or the cable may
become stuck to the wellbore wall, and cable tension alone is not sufficient to pull the tool free.
Tools must then be fished out using drillpipe.
Schlumberger design engineers examined
available solutions for enhancing logging operations and facilitating getting tools to TD along
with solutions for retrieving stuck tools. Based on
the results of their study of existing auxiliary
equipment, they developed two families of products: WellSKATE low-friction well conveyance
accessories and the SureLOC electronically controlled cable release device. The WellSKATE
accessories are a variety of friction reducers,
standoffs, wheeled rollers, flexible connections
and bottom noses designed to keep the tools moving downward or to reduce sticking when moving
upward (previous page, bottom). The SureLOC
system is a controlled release weakpoint.
Low-friction accessories include low-contact
standoffs, low-friction standoffs and inline rollers. These devices include dual-wheel and triroller wheels that are bolted on the outside of the
tools. The wheels are designed to prevent the full
toolstring from having direct contact with the
wellbore, which reduces sticking and friction.
For operations such as formation fluid and pressure sampling or mechanical sidewall coring that
require the toolstring to remain in place for
100
20
50
10
0
0
10
20
30
40
50
60
70
80
0
MDT formation tester stations
> Helping an MDT toolstring reach the objective. An operator needed fluid samples from a deepwater
West Africa well. The MDT tool was conveyed on wireline with tri-wheel and dual-wheel WellSKATE
rollers in a 12 1/4-in. well that had a 33° deviation (green). The offset design of the dual-wheel roller
helped orient the MDT probe downward as indicated by the relative bearing (blue) showing approximately
187.5° throughout the 79 stations attempted. A relative bearing measurement of 0° points up. This optimal
positioning resulted in only one lost seal during all tests; similar nearby wells logged without the
WellSKATE rollers have averaged in excess of 30% lost seals.
extended periods of time, the rolling wheels easBased on model assumptions that the full
ily break free from the formation when the tool length of the tool would be in contact with the
moves off sampling points.
wellbore, the expected normal cable tension at
A roller bottom nose, designed to replace tra- the surface would be in excess of 10,000 lbf.
ditional flexible hole finders, moves freely should However, the friction reduction from WellSKATE
a tool sit down on a ledge. When tool weight is accessories resulted in a maximum cable tension
applied, the bottom nose can realign the tool of only 8,500 lbf [37.8 kN].
with the wellbore.
In addition to reducing the normal tension, the
In China, WellSKATE rollers were used on a orienting effects of the WellSKATE dual rollers
large MDT toolstring accessing a target reservoir helped maintain an optimal downward position for
at 18,045 ft [5,500 m] in a well that had 70° devia- setting the MDT probe (above). Whereas the operOilfield
Review ator typically experienced a 30% rate of seal failure
tion. Because of the rollers, the drag
coefficient
WINTER 14/15
in nearby wells, only one seal was lost in 79 staof the toolstring was reduced fromCable
0.43 Fig
to 0.17.
14
tions14attempted—a less than 1.3% failure rate—
The new hardware made a logging operation
posORWINT 14/15 CBL
sible on wireline that otherwise may have when the WellSKATE hardware was used.
required drillpipe conveyance.
For a comparable operation offshore Sometimes Tools Stick
West Africa, in a well that had 33° deviation, One objective of the TuffLINE cable designers was
WellSKATE rollers helped an MDT toolstring to provide a cable that reduced the number of
reach a target zone and then provide better time-consuming fishing operations. Sometimes,
efficiency than similar operations performed despite the best cable designs, tools become stuck
without the WellSKATE rollers. During MDT tool downhole. When this occurs, the logging crew usuoperations, the maximum pressure differential ally cuts the cable, and the drilling crew strips
was 2,400 psi [16.5 MPa], and the stationary time over the cable with drillpipe. They use a grapple
for a single set was limited by the operator to attached to the end of drillpipe to latch onto the
eight hours.
27
Electrical connection for
control release command
Upper head connection
High-strength release spring
Lower head connection
Prepackaged release bobbin
> Controlled release weakpoint. Rated to withstand 12,000 lbf of direct tension, the SureLOC weakpoint (left) is the strength element in the logging head.
The upper and lower head connections (right) snap into the head between the cable and the tools. Should logging tools become stuck, the engineer
sends software commands and power via the electrical connection to release the bobbin (bottom right). After the bobbin releases, a high-strength
spring (top right) forcefully separates the cable from the head.
logging tools. After the crew confirms engagement
of the tools, the weakpoint is broken, the cable is
retrieved, and the rig crew pulls out the pipe with
the logging tools attached. This operation is
referred to as cut-and-thread fishing.
Before breaking the weakpoint, an operator
may elect to acquire data while pulling the tools
from the hole with the drillpipe. This much longer operation is referred to as logging-while-fishing (LWF). If the weakpoint cannot be broken, or
the operator elects to maintain cable-to-tool contact while retrieving the toolstring, a reverse cutand-thread may be performed, in which the cable
is cut and reattached after each stand of drillpipe
is pulled from the well.14
Two types of weakpoints are used for wireline
logging: mechanical and controlled release.
Mechanical weakpoints have long been the standard hardware for wireline logging. The logging
engineer determines a weakpoint strength such
that the weakpoint will break before the cable
breaks. The weakpoint value is determined using
the SWL for the cable minus the weight of the
logging tools.
If a tool is differentially stuck, the tool weight
and frictional forces acting on the tool no longer
act on the weakpoint. The only considerations for
determining the maximum cable tension that can
be applied at the surface without breaking the
weakpoint are the cable weight in mud and frictional forces acting on the cable.
28
The margin of error is small for selecting a
proper mechanical weakpoint in the case of
heavy toolstrings; the selected weakpoint may be
optimal only at the deepest point in the well. In
some scenarios, such as in S-shaped wells or
when the cable is differentially stuck, the tension
from the surface does not effectively reach the
stuck tool, and breaking the weakpoint may be
impossible without exceeding the SWL of the
cable. For these reasons, after extra- and ultrastrength logging cables were introduced, electrically controlled weakpoints became more
common as a method of freeing the cable from
the logging tools.
Controlled release weakpoints are designed
Oilfield Review
to withstand a tension that exceeds the SWL of
WINTER 14/15
the cable. The
SureLOC 12000
release system has
Cable
Fig 15
an SWL of 12,000 lbf
ORWINT [53.4 kN]
14/15 CBLand
15a significantly
higher breaking strength. The operator can apply
direct tension to the logging string up to the SWL
of the cable without fear of breaking the weakpoint (above).
For example, the weakpoint in the head of a
logging toolstring with a 10,000-lbf surface tension while logging up experiences only the effective weight of the toolstring below it. Because the
SWL of the TuffLINE cable is 18,000 lbf, the operator can apply an additional 8,000 lbf over the
normal surface logging tension in an attempt to
free the toolstring without parting the cable or
unintentionally breaking the weakpoint.
Schlumberger design engineers have developed a 12,000-lbf and an 8,000-lbf version of the
SureLOC cable release. This new design replaces
both mechanical weakpoints and previous generation electrically controlled release devices
(ECRDs).15 The original ECRD, rated for 8,000 lbf,
is activated by applying current from the surface.
It uses no software control for actuation. The
ECRD can be activated only when no tension is
applied; this condition may not be possible if the
cable above the toolstring becomes stuck.
The SureLOC device is activated by the logging
engineer using software commands combined
with applied electrical power. The zero-tension
condition required to activate the ECRD is not
necessary for use of the SureLOC release. In a
well in the Gulf of Mexico, a SureLOC device was
successfully actuated with 2,300 lbf [10.2 kN] of
residual head tension.
In a high-pressure, high-temperature field
in the Gulf of Thailand, an operator used the
SureLOC 12000 device to overcome problems
previously experienced with controlled release
weakpoints.16 Wireline crews logging in offset
wells encountered frequent tool-sticking problems; existing weakpoints and controlled release
devices were found to be unreliable on multiple
fishing operations. In 2011, the wireline logging of five wells was canceled because of perceived weaknesses in mechanical and controlled
release weakpoint designs. After implementing
the SureLOC device, which increased the limits
Oilfield Review
Well Conveyance Planner
–4,000
0
Maximum safe pull on cable (measured at tension device)
2,000
4,000
20,000
North, ft
80
+
6,000
8,000
Deviation
60
Up
10,000
40
20
Tension of cable on the drum
Planned
+
Average layer tension
0
0
5,000
Depth, ft
10,000
10,000
20,000
30,000
Distance from tool head, ft
0
0
Well Profile
Spooled
5,000
Down
0
+
10,000
Tension, lbf
East, ft
–2,000
Wellbore deviation, degree
2,000
2,000
Surface-measured tension, lbf
–2,000
0
0
TVD, ft
–4,000
15,000
Logging Tension
Optimized
Conveyance Package
Drum Force
High-strength
wireline cable
100
Logging unit
+
Depth, ft
10,000
Weakpoint
Relative operational risk, %
Dual-drum capstan
80
Potential
risk
60
40
Potential
risk
20
20,000
Residual
risk
Residual
risk
0
High-strength cable and capstan
Wireline Setup
TuffLINE 18000 cable and OSU-PA
Relative Operational Risk
> The Well Conveyance Planner software. The planner software provides a graphical interface (top) so that engineers can visualize well profiles, monitor
and predict logging tension, report on drum forces and determine optimal conveyance solutions. Planner data can be generated for routine operations and
for high-tension wells. Using relevant well data such as borehole geometry, mud properties and downhole temperature and pressure and combining
toolstring parameters, system components and rig-up hardware (bottom left), the software creates operational risk reports (bottom right) that can identify
weaknesses in the system and offer alternative solutions that may reduce risk.
for safe tension, the operator reduced the total
number of fishing jobs while improving the operational efficiency when fishing was required. The
operator estimated it saved several million US
dollars and was able to acquire full sets of logging data.
Fishing Flowchart
To complement new equipment and assist logging operators, Schlumberger engineers developed software that models forces encountered
while logging. The Well Conveyance Planner software analyzes well information such as borehole
geometry, logging tool parameters, cable limita-
Winter 2014/2015
tions, mud conditions and downhole temperature
and pressure. It also helps identify weaker components in the system (above). The program predicts maximum sustained tension and maximum
allowable instantaneous
tension for pulling free;
Oilfield Review
pulling capabilities
are
continuously updated
WINTER 14/15
while loggingCable
operations
are
Fig 16 in progress. Operator
ORWINT
14/15 CBL
16 software to
limitations can
be entered
in the
ensure compliance with policies that may be specific to the well, field or operation.
The planner can help the logging engineer
visualize well conditions and track changes in
tension conditions. It generates an operational
risk diagram for various tool and cable scenarios. Deviated and extended-reach wells can be
modeled, and tension for complex logging situations can be predicted in advance.
14.The reverse cut-and-thread technique is similar to
traditional cut-and-thread fishing. The drillpipe is run in
and attached to the tools, but while being pulled out of
the hole with the drillpipe, the cable is reconnected for
each stand, and the well is logged in short sections as
the pipe is slowly retrieved. Because it is a timeconsuming operation, this method is usually performed
only over zones of interest.
15.For more on the original ECRD system: Alden et al,
reference 5.
16.Surapakpinyo K, Hanchalay C, Fundytus N, Ford R,
Pakdee S, Sarian S, Battula A and Nery N: “High Tension
Electrically Controlled Release Device Improves
Reliability of Stuck Tool Release in the Gulf of Thailand,”
paper SPE 168281, presented at the SPE Intervention
and Coiled Tubing Association, The Woodlands, Texas,
USA, March 25–26, 2014.
29
Fishing Flowchart
Wireline openhole
fishing operation
Perform
LWF
operations?
Yes
Criteria
acceptable
for LWF?
Perform fishing
risk assessment
No
Cable
stuck?
Yes
LWF
Engage fish
No
Pull out of hole
(POOH) cable
Yes
Convert to LWF
No
Stuck tool
LWF finished
Radiation
sources?
Yes
Legal or client
requirements?
No
Yes
Client grants
exemption?
No
Release
weakpoint
No
POOH cable
Yes
Perform fishing
risk assessment
POOH pipe
End
Criteria
acceptable
for open-ended
fishing?
Perform fishing
risk assessment
No
Determine if standard
or reverse cut and
thread will be used
Yes
Perform openended fishing
Release
weakpoint
Reverse cut
and thread?
Engage fish
POOH pipe
End
Open-ended
fishing
5
Yes
Reverse
cut and thread
4
Free the cable
Engage fish
Engage fish
2
Logging
while 5
fishing
4
3
2
1
Standard
cut and thread
Free the cable
3
1
1 0 1
No
2
3
4
5
Cut-andthread
fishing
POOH cable
Client agrees
to release
weakpoint?
2
3
No
4
Release
weakpoint
POOH pipe and
cable to casing shoe
Yes
POOH pipe
End
POOH pipe
and cable
5
Reverse cutand-thread
fishing
End
> Fishing flowchart. A fishing flowchart is integrated into the Well Conveyance Planner software. By following a well-defined systematic process, the
flowchart helps engineers plan the fishing operation should a toolstring become stuck in a well. The software also plots weighted risk factors (colored circle)
to predict fishing success and possible nonproductive time (NPT). The ranking results are numerical (gray quadrilateral): A higher number indicates less
likelihood of failure. The risk levels are shaded from lowest (blue) to highest (dark red). In this example from a deepwater offshore well, the best option is
open-ended fishing. This type of analysis led engineers to reconsider
traditional
Oilfield
Reviewcut-and-thread methods for fishing in ultradeepwater wells.
WINTER 14/15
Cable Fig 17A
ORWINT 14/15 CBL 17A
30
Oilfield Review
Fishing Efficiency
Failures by Fishing Technique
50
4%
Number of events
40
11%
30
27%
20
58%
10
0
2006
2007
2008
2009
2010
2011
Year
Total fishing jobs
Jobs that have failures while fishing
Fishing jobs that have recorded NPT
Cut and thread
Reverse cut and
thread
Open ended
Other
> Fishing efficiency and failure analysis. Schlumberger logging engineers working in deepwater offshore environments analyzed fishing operations over a
six-year span (left). Data from fishing jobs that have fishing failures and NPT were further broken down by the fishing method used (right). Cut-and-thread
operations, both traditional and reverse cut and thread, accounted for 85% of the failures. Open-ended fishing was responsible for only 11% of the failures.
A fishing flowchart is included in the planner,
which the logging engineer can access before
tools become stuck in the well. The flowchart
helps engineers identify areas of concern, especially on deepwater floating rigs, where excessive
surface tension and complex rig-ups add to the
risks associated with traditional cut-and-thread
fishing operations.
The use of high-tension cables and controlled
weakpoints has led Schlumberger offshore operations personnel, along with some operators, to
reassess the choice of the cut-and-thread method
when fishing for logging tools. The fishing decision flowchart identified a lower risk methodology for fishing logging tools from deep and
ultradeep wells (previous page).
For shallow wells, the cut-and-thread technique is time efficient and is usually the best fishing option. For ultradeep wells constructed in
deep water, the hourly rig cost while fishing must
be factored into the analysis for choosing a fishing method. In addition, complex rig-ups and
high-tension cable conditions add personnel
Winter 2014/2015
risks that are rarely a factor when fishing in shallower wells.
In a recent study conducted by Schlumberger
offshore operations personnel, engineers examined fishing data from 2006 to 2011 (above). The
data revealed that although 88% of all fishing
operations were performed successfully, 34% of
those operations recorded NPT. Cut-and-thread
operations accounted for 85% of the NPT fishing
events. Controlled weakpoint release followed by
open-ended fishing for logging tools accounted
Oilfield Review
for 11% of NPT
events.14/15
Not only were fewer NPT
WINTER
events associated
with
Cable Fig 18open-ended fishing than
ORWINToperations,
14/15 CBLbut
18 the success
with cut-and-thread
rate was the same for both techniques. In addition, the open-ended technique was deemed
more efficient, more cost effective and even more
reliable than traditional cut-and-thread and
reverse cut-and-thread methods.
One justification for traditional cut-andthread operations is uncertainty associated with
breaking mechanical weakpoints and past unreliability of controlled release devices. The reliability of the SureLOC controlled release weakpoint
has eliminated that concern.
Safety is another consideration for not using
traditional cut-and-thread fishing. During cutand-thread operations, for each connection of
the drillpipe, the cable is tensioned to approximately the same value as when the tools became
stuck while logging. Maintaining and repeatedly
tensioning the cable to the extreme cable tensions encountered while logging ultradeep wells
put personnel at greater risk should any part of
the system fail during fishing. Sheave wheels, tiedown chains, slings and logging units are all part
of the system, and their exposure to high-tension
cycles increases the risk of component failure.
Following the fishing study, Schlumberger
engineers working in the Gulf of Mexico on deepwater, high-tension wells began recommending
the open-ended fishing technique. Moving away
from traditional cut-and-thread fishing represented a major shift in methodology because cutand-thread fishing had been considered the only
reliable method for retrieving tools. In two years
of using the open-ended technique, offshore
31
Fishing Success
3%
Fishing Time per Operation
< 1%
120
Fishing time, h
100
80
60
40
97%
20
0
First trip
Second trip
Third trip
Open ended
Cut and
thread
Reverse cut
and thread
> Open-ended fishing success. Following a study of fishing methods, offshore logging crews began recommending open-ended fishing rather than the
cut-and-thread method for ultradeep wells. The success rate for tool recovery over the two-year period of study was 100% (left), with 97% recovery on the
first trip in the hole with drillpipe. The efficiency of open-ended fishing in deepwater drilling is further reflected in the time per operation compared with
traditional and reverse cut-and-thread methods (right).
operations had a 100% recovery rate for tools
(above). The average fishing time for open-ended
fishing attempts was less than 20 hours. The average time for cut-and-thread operations was
nearly 60 hours; reverse cut-and-thread average
was almost 120 hours.
Engineers now recommend the open-ended
fishing method for deepwater logging. Operators
may be reluctant to change to this method
because cut-and-thread fishing is entrenched in
the industry; in addition, fishing for tools that
contain radioactive sources may be controlled
by local regulations that require the use of the
cut-and-thread technique.
Offshore Upgrades
Two heavy-duty modular offshore logging units
are now available to take advantage of the higher
rated TuffLINE cables and SureLOC weakpoints.
The standard Schlumberger OSU-F offshore logging unit, which was designed in the 1970s, is
rated for 8,000 lbf of logging tension. The new
OSU-PA offshore logging unit is capable of pulling
20,000 lbf and is available with a high-strength
logging drum that can hold 11,000 m [36,000 ft]
of TuffLINE logging cable (next page).
32
The OSU-PA has a Det Norske Veritas (DNV)
rating for continuous logging tension up to
16,000 lbf using a full drum of cable.17 If conditions warrant higher short-term tension such as
for stick prevention, the unit is certified for an
instantaneous pull of up to 18,000 lbf without a
capstan. The modular unit is composed of four
parts: a diesel power pack, a logging cabin, a
hydraulic winch and a lifting beam. The lifting
beam has a DNV lifting certification.
Oilfield
Review
The three
main
modules—power pack, cabin
WINTER 14/15
and winch—can
be installed as one piece or
Cable
separately
and Fig
are19
connected by hydraulic and
ORWINT 14/15 CBL 19
electric control cables. This modular flexibility is
incorporated to improve safety and footprint
restrictions. In high–surface tension operations,
the winch operator can be located in the cabin
away from the winch module.
The similarly equipped and rated OSU-PB is a
Conformité Européenne- (CE-) marked offshore
unit.18 The OSU-PA operates with a clean-air diesel power pack; the OSU-PB uses an electrohydraulic power pack. The OSU-PB has also been
approved for Zone 2 atmosphères explosibles
(ATEX) operations.19
A dual-drum tension-relief capstan system
that has a higher rating than that of previous versions is available and can be synchronized and
controlled directly from the OSU-PA or the
OSU-PB. This new design is rated for an SWL
of 24,000-lbf [106.8-kN] tension, 30,000-lbf
[133.4-kN] maximum tension and winch speeds
of up to 30,000 ft/h [9,150 m/h]. A TuffLINE cable
17.Det Norske Veritas (DNV) is an international rating and
classification organization. In 2013, DNV merged with
Germanischer Lloyd (GL) to form DNV GL. For more on
DNV certifications that cover logging units and lifting
equipment: “Standard for Certification Number 2.22,”
Det Norske Veritas AS (June 2013), https://exchange.
dnv.com/publishing/stdcert/2013-06/Standard2-22.pdf
and “Standard for Certification Number 2.7-1,”
https://exchange.dnv.com/publishing/stdcert/2008-11/
Standard2-7-1.pdf (accessed November 3, 2014).
18.The CE marking declares that a product meets the
requirements of applicable Conformité Européenne (CE),
or European Conformity, directives.
19.ATEX is the name commonly given to the two European
Commission directives for controlling explosive
atmospheres. For more on ATEX directives related to
offshore operations: “Directive 94/9/EC on Equipment
and Protective Systems Intended for Use in Potentially
Explosive Atmospheres (ATEX),” European Commission
Enterprise and Industry, http://ec.europa.eu/enterprise/
sectors/mechanical/documents/legislation/atex/
(accessed October 6, 2014).
Oilfield Review
> The OSU-PA offshore logging unit. This newly designed unit is DNV certified to 16,000-lbf tension. Shown in its lifting cage, the modular unit comprises a
POSU clean-diesel power pack (left), a COSU logging cabin (middle) and a WOSU logging winch (right). The OSU-PA is capstan compatible.
on a high-tension drum used with an OSU-PA
allows continuous logging tension up to 16,000 lbf.
The capstan is recommended, however, when
predicted normal surface tension exceeds
13,000 lbf.
Forward Thinking
When almost all wells were vertical, unless deviated by accident or from downhole circumstances, traditional logging tools and cables were
suited for the job of acquiring petrophysical data.
Today, the percentage of horizontal and highangle wells has increased, and vertical wells have
become the exception in many regions. Highangle and horizontal wells are more likely to be
Winter 2014/2015
logged with LWD equipment than on wireline.
But LWD tools often have lower temperature and
pressure ratings than wireline tools have, and
some measurements must rely on wireline conveyance for acquisition.
Evaluating deep and ultradeep wells requires
the use of wireline cables for data acquisition.
Innovative engineering designs are making these
wireline operations feasible and adding margins
of safety that were not previously possible.
The future of the drilling industry is focused
on what has been, until recently, inaccessible
resources. Deepwater drillers and operators have
equipment to reach those prizes. By eliminating
weak links in the wireline system, logging companies can safely and more effectively follow them
with crucial wireline logging tools. The ultimate
goal is to deliver tools downhole that acquire
data to help operators better understand their
fields and discoveries.
—TS
33