Drilling with casing promises major benefits

Bob Tessari, Garret Madell
Tesco Drilling Technology
Calgary
Tommy Warren
Tesco 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 directly to the winch axle (Fig. 3).

This fully hydraulic powered drilling rig with a telescoping mast was utilized for Tesco's first casing drilling tests (Fig. 4).
A casing drilling system developed by Tesco Drilling Technology aims to reduce 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 has demonstrated that it can function effectively in both vertical and directional wells.

The technology uses unique rig and downhole equipment that functions as an integrated drilling system where standard oil field casing is used to transmit mechanical 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

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 conventional drilling 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 to provide a hydraulic conduit for the drilling fluid.

The drillstring is tripped out of the hole each time the bit or bottom-hole assembly (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 that well 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 can be gained through a reduction of rig equipment needs and operating costs.

Although modest savings may be achieved by reducing drillstring tripping and handling 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 borehole stability issues are directly attributed to drillstring vibrations.¹ The casing drilling 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 the casing 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 to be advanced freely.

The BHA is attached to the bottom of the casing by a landing assembly so that a 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].

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 providing adequate 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 as logging-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 and mate 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 a profile to transmit compressive (bit weight) and tensional drilling loads from the 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 the BHA.

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 is also 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 and remove 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-hole conditions warrant a disconnect. The design and testing of a pump-down pressure set tool is in progress and an electric retrieving concept is being considered for future application.

Casing drilling rig

Casing drilling can be implemented either with a specially developed drilling rig or by a conventional rig modified for casing drilling. To date, the system has 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 handle the 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 with the 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 more effectively. 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 programmable logic 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 control allows 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 advanced with 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 dynamic conditions related to vibrations, casing/borehole contact, bit/formation interaction, mud -motor loading, and other sources of dynamic instabilities.

The initial commercial applications of the casing drilling technology are anticipated to be applied to relatively low-cost land wells. These wells are drilled rapidly, with frequent rig moves; consequently, the mobilization costs can 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 field pickers. 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 rig design. 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 associated with a conventional rig.

The shorter and smaller derrick reduces wind load considerations, allowing for 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 wells drilled 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, but also identified a number of items that require further attention.

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 a directional 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 turn included in the build. These two directional legs were used to begin understanding the issues related to producing a workable casing drilling system for directional wells.⁴ The second test well was drilled as a continuation of the directional work. The first leg was drilled with 7-in. casing out 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 a depth 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 as well as to allow uninhibited passage of wire line running tools.

In addition, the connection needs to exhibit satisfactory operational characteristics 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 connection specifications 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 performed reasonably well but were replaced because of operational and economic considerations once the vertical leg of the first well was completed.

The second selection of connections (CNV-BTC) utilized a buttress thread profile 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 while breaking 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.

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 ft dogleg. 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-thickness coupling should reduce the stress and improve the fatigue resistance.

Bit and underreamer performance

Drilling with casing and wire line-retrievable tools requires a cutting structure small 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 an underreamer positioned behind a smaller diameter pilot bit.

Tests were run with both roller cone and PDC pilot bits. Both two and three arm 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 of specialty 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 well test.2 Penetration rates with the casing-drilling BHAs were comparable to those 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:

A functionally reliable bypass system
A proper fit of the running tools and drilling assembly in the casing
A reliable full bore positive stop landing system
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 function properly.

A pump-down, setting-and-retrieval system is required for high-angle directional well applications. In the directional testing program to date, the casing 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 be continuously logged while drilling. In most cases, the casing drilling system will be run in development drilling situations where the formation evaluation program 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 by rotating the casing and the core barrel. The core was recovered completely with 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 simply pumped out the end of the casing, but the top displacement plug must land and latch into the casing, serving as a float.

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

The first two objectives were achieved with a plug constructed from composite material, leaving little debris in the hole. The result with the composite plug was quite encouraging, but more work still needs to be done on 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 and behind. The drill casing was then hoisted above each plug followed by reverse circulation and plug top confirmation using a cement quality log.

Directional capability

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 paths drilled in the second well. The directional assemblies generally consisted of a bit, underreamer, bent housing mud motor, MWD, flex monels, and drill-latch assembly. The directional legs were drilled with the 7 and 51/2-in. casing strings.

The build rates were somewhat more aggressive than expected for a conventional 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 sliding even 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 to conventional 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 as occurs when pumping up MWD surveys.

This caused the casing to hang up several times on the first two directional legs before the cause of the problem was identified. Overall, the casing drilling 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 polymer additives 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 hole erosion at higher velocities. Some problems were experienced with bit balling in the first well. However, this was attributed to the lower flow rates and design of the particular bits and underreamers.

Potential cost savings

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 across intervals. For onshore wells in relatively fast drilling areas, incorporating these savings with the savings that may be realized from more-efficient rig moves 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 most effective applications may occur through time savings that accompany advantages of drilling larger strings of casing into place. In addition, this technology may avoid hole stability problems that occur during tripping operations. Finally, the possibility exists where an entire string(s) of casing may be eliminated.

Acknowledgment

The authors wish to acknowledge the technical contributions of Tesco Divisions of Gris Gun, Mainline Hydraulics, and Kelon Electric. Extensive technical 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

Santos, Helio, Placido, J.C.R., and Wolter, Claudio, "Consequences and Relevance of Drillstring Vibration on Well bore Stability," SPE/IADC paper 52820, presented at the SPE/IADC Drilling Conference, Amsterdam, Mar. 9-11, 1999.
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.
Tessari, R.M., and Madell, Garret, "Casing Drilling-A Revolutionary Approach to Reducing Well Costs," SPE/IADC paper 52789, presented at the SPE/IADC Drilling Conference, Amsterdam, Mar. 9-11, 1999.
Tessari, R.M., Madell, Garret, and Warren, T.M., "Casing Drilling-A Revolutionary Approach to Reducing Well Costs," paper No. 99-121 Presented at the CADE/CAODC Spring Drilling Conference, Apr. 7-8, 1999.

The Authors

Bob Tessari began his oil field career in 1964 working on drilling rigs for Parker Drilling in Canada. Five years later, he attended the University of Alberta, receiving a BS in chemical engineering. 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 its Gulf 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 and development 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 Madell is the engineering manager for Tesco's casing drilling 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 in planning, engineering, supervision, and coordination of all types of wells from heavy 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 for the casing drilling.

Tommy Warren is director, research and development, for Tesco Drilling Technology. He has more than 25 years' experience in working 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 is currently serving as the chairman of the 1999 SPE Annual Technical Conference and Exhibition. He is also an SPE Distinguished Lecturer on the topic of Rotary Steerable Systems. Warren holds BS and MS degrees in mineral engineering from the University of Alabama.