drilling with casing promises major benefits.docx

8/14/2019 Drilling with casing promises major benefits.docx

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Drilling with casing promises major

benefits

Bob Tessari, Garret MadellTesco Drilling Technology

Calgary

Tommy WarrenTesco Drilling Technology

Tulsa

A wire line winch is used to run

and retrieve the BHA through

the casing. It is driven by a high-torque motor attached directlyto the winch axle (Fig. 3).

This fully hydraulic powered drilling rig with a telescoping mast wasutilized for Tesco's first casing drilling tests (Fig. 4).

A casing drilling system developed by Tesco Drilling Technology aims toreduce costs, improve drilling efficiency, and minimize hole problems by

casing the well as it is drilled.

The system has been under development for 3 years, and extensive testing hasdemonstrated that it can function effectively in both vertical and directional

wells.

The technology uses unique rig and downhole equipment that functions as anintegrated drilling system where standard oil field casing is used to transmitmechanical and hydraulic energy to the drill bit. A wire line retrievable

drilling assembly that is latched into the casing eliminates the need for

tripping with a conventional drillstring.

Casing drilling system
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The casing drilling system described in this article takes a bold step away

from the traditional method of drilling oil and gas wells. In the conventionaldrilling process, a drillstring consisting of drill collars and pipe is used to

apply mechanical energy (rotary power and axial load) to the bit as well as toprovide a hydraulic conduit for the drilling fluid.

The drillstring is tripped out of the hole each time the bit or bottom-holeassembly (BHA) needs to be changed or when TD is reached. Casing is then

run to provide permanent access to the well bore.

The fundamental premise behind developing a casing drilling system is thatwell costs can be reduced if the casing is installed as the well is drilled. A

redesign of surface rig equipment and downhole systems is required to

achieve this objective.

Cost savings can then result through the elimination of purchasing, handling,inspecting, transporting, and tripping the drillstring while reducing hole

problems that are associated with tripping. In addition, significant savings canbe gained through a reduction of rig equipment needs and operating costs.

Although modest savings may be achieved by reducing drillstring tripping andhandling times in trouble-free wells, savings incurred through a reduction of

hole problems can become far more substantial. There are many situations

where problems such as lost circulation, well-control incidents, and borehole

stability problems can be directly attributed to tripping the drillstring.

In other cases, it is difficult to run the casing after the drillstring is tripped out

because of poor borehole quality. Some of these problems caused by boreholestability issues are directly attributed to drillstring vibrations.1The casingdrilling system can reduce these incidents by eliminating tripping operations

and providing a drillstring that is less prone to vibrations.

Downhole system

The casing-drilling process eliminates the conventional drillstring by using thecasing itself as the means of transmitting hydraulic and mechanical energy to

the bit(Fig. 1) [44,572 bytes].A wire line-retrievable BHA attached to the

bottom of the casing drills a well bore of adequate size to allow the casing tobe advanced freely.

The BHA is attached to the bottom of the casing by a landing assembly so thata wire line unit can be used to retrieve and replace it without needing to trip

pipe out of the well(Fig. 2) [50,470 bytes].
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The BHA consists of a pilot bit and underreamer that are sized to pass through

the "drill-casing." Thus, it becomes possible to drill a hole while providingadequate clearance for the drill-casing and subsequent cementing.

For directional applications, the BHA includes a bent-housing downhole

motor and measurement-while-drilling (MWD) tool. Other equipment, such aslogging-while-drilling (LWD) or coring equipment, may also be run to

perform almost any operation that can be conducted with a conventional

drillstring.

The BHA is run below a landing assembly to transport it into the well andmate up with a special casing shoe joint. Spring-loaded dogs located on the

landing assembly engage a no-go groove on the casing shoe.

This positions the assembly so that positive-locking axial keys extend into aprofile to transmit compressive (bit weight) and tensional drilling loads fromthe drilling assembly to the casing. A torque anchor mates with recesses in the

casing shoe to provide rotation and torque transfer from the casing to theBHA.

Seals located on the landing assembly incorporate upward and downward-facing pressure cups that prevent flow around the BHA landing assembly

while drilling. A bypass system allows drilling fluid to be circulated,

preventing well swabbing and casing sticking when running or retrieving the

BHA.

A drilling shoe positioned on the bottom of the casing is dressed with either

(polycrystalline diamond compact) PDC cutters or tungsten carbide chips,ensuring a full gauge hole is obtained ahead of the casing. The drilling shoe isalso designed to facilitate retrieval of the BHA back into the casing as it is

pulled.

Setting-and-retrieving tools

A custom-designed, wire line setting-and-retrieving tool is used to install andremove the BHA. A swivel prevents the rotational twist of the braided wire

line so that the casing can be rotated during wire line operations.

An emergency shear sub, activated when line tension reaches 20,000 lb,

provides for a straight-pull emergency release of the latch in case tight-holeconditions warrant a disconnect. The design and testing of a pump-down

pressure set tool is in progress and an electric retrieving concept is beingconsidered for future application.

Casing drilling rig


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Casing drilling can be implemented either with a specially developed drilling

rig or by a conventional rig modified for casing drilling. To date, the systemhas been used only with a rig designed specifically to prove the entire system

and to maximize the efficiencies of casing drilling.

Special equipment is needed to handle casing in a drilling mode and to handlethe wire line retrievable BHA. A top drive must be used to rotate the casing.In addition, a split crown and split traveling blocks facilitate effective wire

line access to the top of the casing through a wire line blowout preventor

(BOP).

A large wire line unit is needed that is sufficient to run and pull the BHA

efficiently. Pipe-handling tools for the casing are also required.

Tesco has built three rigs that are hybrid casing-drilling and conventional-drilling rigs. A fourth is under construction, exclusively designed for use withthe casing-drilling process.2

In addition to the rig requirements mentioned above, these rigs include other

features that allow the entire drilling process to be implemented moreeffectively. The rigs are designed with hydraulic power units for the mud

pump, drawworks, top drive, and wire line unit, reducing equipment weight

while taking advantage of the company's top-drive design.

All this equipment is operated under computer control through programmablelogic controller interfaces that minimize the potential for human operator

error, optimize equipment performance, reduce manpower requirements, and

facilitate data acquisition.

Fig. 3 shows a wire line unit developed to provide a larger load capacity-maximum pull of 42,000 lb with 5/8-in. single conductor braided wire-than is

available from most commercial units.

The wire line unit is installed as an integral part of the rig and is located

adjacent to the main drawworks. The hydraulic power and computer controlallows the driller to function as the wire line operator from the driller's control

room.

The hydraulic drive on the drawworks is very similar to that shown for the

wire line unit and acts as a brake that allows the drillstring to be advancedwith the drawworks under power rather than with the power unit disengaged

by a clutch.

This facilitates building a robust automatic driller and is the first step in

developing an adaptive control system for bit advancement that responds to


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dynamic conditions related to vibrations, casing/borehole contact,

bit/formation interaction, mud -motor loading, and other sources of dynamicinstabilities.

The initial commercial applications of the casing drilling technology are

anticipated to be applied to relatively low-cost land wells. These wells aredrilled rapidly, with frequent rig moves; consequently, the mobilization costscan be high.

The hybrid casing drilling rigs are designed in modular packages that can be

moved and rigged-up quickly with the use of standard tractors and oil fieldpickers. The casing-drilling rig currently under construction is mounted on a

trailer and designed with a load distribution so that it can be moved in Canada

under almost year-round conditions.

A number of features of the casing-drilling process allow for a lighter rigdesign. The casing is picked up as single joints from the pipe rack. Thus, it

does not need the monkey board, setback area, and heavy derrick associatedwith a conventional rig.

The shorter and smaller derrick reduces wind load considerations, allowingfor the construction of a lighter derrick and substructure.

Smaller mud pumps can also be used because larger casing IDs significantly

reduce the friction loss, as compared to conventional drill pipe and collars.Furthermore, the larger OD of the casing allows adequate annular velocity to

be achieved with lower flow rates than otherwise achieved with drill pipe.

The overall result is that the rig:

Is not as heavy to move Requires less capital investment Uses less power Requires a smaller crew.

Development and testing program

The casing-drilling system has undergone full-scale field trials in two wellsdrilled in Tesco's Calgary yard (Fig. 4). A total of 9,100 ft were drilled with

95/8, 75/8, 7, 51/2, and 41/2-in. sizes of casing used to simulate both vertical

and directional wells(Fig. 5) [51,243 bytes].

These tests proved the fundamental casing drilling concept was sound, butalso identified a number of items that require further attention.
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The first test well consisted of a vertical leg drilled to 3,040 ft using 95/8,

75/8, and 51/2-in. casing strings. Afterward, the well was plugged back, and adirectional leg was drilled with 51/2-in. casing to an inclination of 86.3

The well was plugged back again, open-hole sidetracked, and a second

directional leg was drilled to an inclination of 60 with a 45 azimuthal turnincluded in the build. These two directional legs were used to beginunderstanding the issues related to producing a workable casing drilling

system for directional wells.4The second test well was drilled as a

continuation of the directional work. The first leg was drilled with 7-in. casingout to 91 with an average build rate of 8/100 m. This was followed by an S-

turn sidetrack, including a vertical section drilled with 41/2-in. casing to adepth of 3,020 ft(Fig. 6) [154,502 bytes].

Casing connections

The primary requirements for a suitable casing drilling connection are that it

withstand the torsional, axial, and bending loads experienced while drilling aswell as to allow uninhibited passage of wire line running tools.

In addition, the connection needs to exhibit satisfactory operationalcharacteristics including ease of rig handling, repeat make up, and the

maintenance of adequate pressure integrity after drilling.

Several hole and casing sizes with various combinations of connectionspecifications were used in the test program. Flush OD and ID, premium

integral connections were selected for the first test well based on fatigue tests

managed by Lone Star Technologies Inc. The connections performedreasonably well but were replaced because of operational and economicconsiderations once the vertical leg of the first well was completed.

The second selection of connections (CNV-BTC) utilized a buttress threadprofile that incorporated an internal load ring for torsional and sealing

capability. Both standard weight and special clearance coupling profiles, with

and without beveled external shoulders, were run.

Couplings were installed on the mill ends with a special thread compound that

prevented further make up during the drilling process or loosening whilebreaking out connections. The pin ends were lightly bead blasted and coated

with molybdenum di-sulfide to resist galling upon make up of the buttress

interference fit thread.

The CNV-BTC connection was used for all subsequent testing and performed

quite well. One casing coupling failure occurred on the 7-in. casing in the

horizontal leg of the second test well.
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A classic fatigue failure occurred in a thread root at the end of the pin in a

special clearance K55 CNV-BTC coupling after rotating through a 15/100 ftdogleg. Inspection of the remainder of the casing couplings did not reveal any

additional cracks, but even a single failure is not acceptable.

Reducing dogleg severity and using a stronger-grade, standard-thicknesscoupling should reduce the stress and improve the fatigue resistance.

Bit and underreamer performance

Drilling with casing and wire line-retrievable tools requires a cutting structuresmall enough to pass through the casing while retaining the ability to drill a

hole larger than the OD of the drill casing. This is accomplished by using anunderreamer positioned behind a smaller diameter pilot bit.

Tests were run with both roller cone and PDC pilot bits. Both two and threearm commercially available PDC underreamers were run. A few runs were

made with a PDC drilling shoe on the casing so that an underreamer was not

needed.

No testing was performed in rocks hard enough to preclude the use of PDC

bits, but a retractable roller-cone bit may be suitable for such applications

(OGJ, Mar. 8, 1999, pp. 51-56). Further development and application ofspecialty underreamer and cutting structure designs are also being addressed.

A conventional drilling assembly consisting of a mill-tooth bit, drill collars,

and drill pipe was used for a drill rate comparison during the first vertical welltest.2 Penetration rates with the casing-drilling BHAs were comparable tothose of the conventional BHA.

BHA running and retrieval

The BHA running-and-retrieving system has gone through several iterations

during the testing program, resulting in a more effective system for vertical

well applications. The major challenges that have been overcome include:

1. A functionally reliable bypass system2. A proper fit of the running tools and drilling assembly in the casing3. A reliable full bore positive stop landing system4. Effective seals that ensure debris is not pumped around the tool.

There is also a certain amount of operator experience needed to reliably

operate the system, even when the wire line mechanical systems functionproperly.


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A pump-down, setting-and-retrieval system is required for high-angle

directional well applications. In the directional testing program to date, thecasing has been pulled back until the shoe was situated at a sufficiently low

inclination (about 30) for wire line operations to be successful. A pump-down system has been designed but has not yet been tested.

Formation evaluation

Unless the casing is pulled back above the zone of interest for logging,continuous casing of the well as drilling proceeds prohibits open-hole logging

with conventional wire line logging tools.

Alternately, cased-hole logs can be run inside the casing or the well may becontinuously logged while drilling. In most cases, the casing drilling system

will be run in development drilling situations where the formation evaluationprogram can be designed to complement the casing drilling requirements.

Cutting samples were collected without difficulties in the testing program and

actually improved because of the faster bottoms-up time. A conventional core

barrel was run below the drill-lock assembly, and a 25-ft core was cut byrotating the casing and the core barrel. The core was recovered completelywith wire line, reducing coring times as compared to conventional methods.

Cementing and drilling out

The casing does not have a float collar to catch the cement displacement plugs

once the casing setting depth is reached. The lead wiper plug is simplypumped out the end of the casing, but the top displacement plug must land andlatch into the casing, serving as a float.

Several different plug designs were used in pursuit of a plug that wouldreliably lock into place, hold pressure both from above and below, and theneasily drill out without leaving debris in the hole.

The first two objectives were achieved with a plug constructed fromcomposite material, leaving little debris in the hole. The result with the

composite plug was quite encouraging, but more work still needs to be doneon the plug and drill-out procedures.

Balanced cement plugs for sidetracking were set with no difficulty. Typically,

each plug, about 330 ft in length, was pumped into place with water ahead andbehind. The drill casing was then hoisted above each plug followed by reversecirculation and plug top confirmation using a cement quality log.

Directional capability


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Four directional legs were drilled with the casing drilling system during the

test program.4 Fig. 6 shows the directional programs used on the two pathsdrilled in the second well. The directional assemblies generally consisted of a

bit, underreamer, bent housing mud motor, MWD, flex monels, and drill-latchassembly. The directional legs were drilled with the 7 and 51/2-in. casing

strings.

The build rates were somewhat more aggressive than expected for aconventional drilling assembly, and a high dogleg section was created near the

top of one of the directional legs.

No difficulties were experienced with directional control, and the well could

be turned to the target with no more effort than with a conventional steerable

motor. Exercises in building angle, simultaneous build and turning, and

openhole sidetracking were successful.

At higher inclinations, normal variations of penetration rates between sliding

and rotating were seen, but no excessive difficulty was experienced slidingeven out to 90 in the second well. One unexpected result was that the

directional assembly built angle in the rotating mode more than expected,

requiring less sliding at high inclination than anticipated as compared toconventional steerable-motor assembly performance.

A few operational issues had to be worked out in regards to the ease of which

ledges were created in the hole as the undereamer rotated off bottom-such asoccurs when pumping up MWD surveys.

This caused the casing to hang up several times on the first two directionallegs before the cause of the problem was identified. Overall, the casingdrilling system functioned well in directional tests, even when casing drilling

with a 7-in. string at relatively high build rates.

Mud and hydraulics program

The test wells were drilled with water-based muds using various polymeradditives for viscosity and clay stabilization. In the first vertical well, the

annular velocities were about 120-130 fpm while in the second well, the

annular velocities were closer to 300 fpm.

No hole-cleaning problems were encountered at lower velocities nor holeerosion at higher velocities. Some problems were experienced with bit balling

in the first well. However, this was attributed to the lower flow rates anddesign of the particular bits and underreamers.

Potential cost savings


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Experience with the casing-drilling test wells suggests cost savings in the 10-

15% range for trouble-free wells when casing drilling can be used acrossintervals. For onshore wells in relatively fast drilling areas, incorporating

these savings with the savings that may be realized from more-efficient rigmoves and the elimination of hole problems related to tripping the drillstring

could provide an overall savings of up to 30%.

In offshore applications where the day rates are much higher, the mosteffective applications may occur through time savings that accompany

advantages of drilling larger strings of casing into place. In addition, thistechnology may avoid hole stability problems that occur during tripping

operations. Finally, the possibility exists where an entire string(s) of casingmay be eliminated.

Acknowledgment

The authors wish to acknowledge the technical contributions of Tesco

Divisions of Gris Gun, Mainline Hydraulics, and Kelon Electric. Extensivetechnical and operational inputs were provided by Ryan Energy Technologies,

Reeves Wire line, and Computalog Ltd. as well as numerous other service and

supply companies.

References

1. Santos, Helio, Placido, J.C.R., and Wolter, Claudio, "Consequences andRelevance of Drillstring Vibration on Well bore Stability," SPE/IADCpaper 52820, presented at the SPE/IADC Drilling Conference,

Amsterdam, Mar. 9-11, 1999.2. Laurent, Michael, Angman, Per, and Oveson, Dale, "A New Generation

Drilling Rig: Hydraulic Powered and Computer Controlled," paper No.

99-120, presented at the CADE/CAODC Spring Drilling Conference,Apr. 7-8, 1999.

3. Tessari, R.M., and Madell, Garret, "Casing Drilling-A RevolutionaryApproach to Reducing Well Costs," SPE/IADC paper 52789, presented

at the SPE/IADC Drilling Conference, Amsterdam, Mar. 9-11, 1999.4. Tessari, R.M., Madell, Garret, and Warren, T.M., "Casing Drilling-A

Revolutionary Approach to Reducing Well Costs," paper No. 99-121Presented at the CADE/CAODC Spring Drilling Conference, Apr. 7-8,

1999.

The Authors

Bob Tessaribegan his oil field career in 1964 working on drillingrigs for Parker Drilling in Canada. Five years later, he attended the

University of Alberta, receiving a BS in chemical engineering.


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Tessari spent the next 8 years working overseas as a drilling engineer for

ARCO in Indonesia and later as Amoco Egypt's drilling superintendent for itsGulf of Suez offshore operations.

In 1982, Tessari joined a small drilling company in Canada that designed,

manufactured, and operated six land rigs. While managing this company,Tessari formed Tesco Drilling Technology in 1986 to do research anddevelopment on new drilling tools and processes.

Tessari is a member of the Society of Petroleum Engineers and the Canadian

Association of Drilling Engineers and the president and CEO of Tesco Corp.

Garret Madellis the engineering manager for Tesco's casingdrilling division. He graduated with a BS in petroleum engineering

in 1979 and has over 20 years of drilling experience.

Madell has been involved both domestically and internationally inplanning, engineering, supervision, and coordination of all types of wells fromheavy oil to critical sour land wells to high-pressure offshore wells with

floating, jack up, or platform rigs.

His most recent work has been to coordinate the research and development forthe casing drilling.

Tommy Warrenis director, research and development, for Tesco

Drilling Technology. He has more than 25 years' experience inworking with drilling technology development in the areas of bit

mechanics, directional drilling, and drilling systems.

Warren was the 1997 recipient of the SPE Drilling Engineering Award and iscurrently serving as the chairman of the 1999 SPE Annual Technical

Conference and Exhibition. He is also an SPE Distinguished Lecturer on thetopic of Rotary Steerable Systems. Warren holds BS and MS degrees in

mineral engineering from the University of Alabama.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.


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