UK report £600 million market spur for coiled tubing development team

Helix co-venturers plan comprehensive design improvements for under-utilised workover method

Frank Taylor
Taylor Associates

Present systems for drilling horizontal/lateral step-outs from existing wells are merely adaptations of conventional full-size drilling technology. Little attempt has been made to fully use the potential opportunities and advantages of coiled tubing.

Coiled tubing allows re-entry into existing wells through production tubing under pressure and drilling at or under balance without the use of a rig. Well recompletions at low cost become possible using coiled tubing to provide high productivity lateral drains as a very normal operation for many existing wells.

Some estimates put the number of known wells worldwide at 90,000 that could benefit from this type of workover, with a market put at between £100 million and £600 million over the next two decades. However, what the oil industry currently needs is a system that offers:

A quality, high speed communications path (minimum 100 bits/sec), to and from the downhole assembly and the surface. Present mud pulse technology offers speeds up to 10 bits/sec which is inadequate for the purpose.
An orientation tool controlled from the surface capable of dynamically adjusting bit face position.
A reliable downhole motor in small sizes that improves on the relatively short field life currently available. This motor must allow the use of fluids such as foams for drilling underbalance.
A traction or thruster system that will allow reasonable weight to be applied to the bit without risk of helical buckling and downhole lock-up. This system should control the reactive torque from the downhole motor to reduce CT fatigue and help stabilise the drillstring.
A surveying system for real time downhole control and steering of the wellbore. Data acquisition electronics interfaced with the telemetry to allow simultaneous gamma ray formation characterization and magnetometers/accelerometers that confirm system position and operation. At the same time, it would be desirable to measure annular pressure near the bit to control drilling at or near balance.

Integrated design
approach

Project Helix is novel in that it brings together three small companies in Scotland working to achieve an integrated system to fulfill these needs: Astec Developments, Stirling Design and TSL Technology. The project has been a long time in the gestation phase, but this year Phase 1 has at last begun with the financial backing of Scottish Enterprise, the Offshore Supplies Office, and three oil companies. GEC-Marconi is acting as a sub-contractor in the provision of an integrated coiled tubing drilling system.

While joint industry projects in drilling have been quite common, this is the first time so many companies have combined together to produce an integrated package to address an industry need.

Stagralan, as the joint venture is known, is owned in equal shares by the three co-venturers and Scottish Enterprise. The Marine Technology Directorate has been contracted to manage the contracts for the work packages and to appoint a project coordinator, responsible for coordinating and monitoring progress as well as providing regular updating for the Steering Committee on which are represented the project sponsors.

Funding for the project at a budget of £625,000 is already secured, mainly from Scottish Enterprise, three oil companies, Stagralan, and the OSO. Phase 1 aims to deliver the engineering design, finalised cost estimates, and a focused plan to complete the project.

After Phase 1 the project is seeking further support from the European Community THERMIE programme to complete the balance of the project at an estimated cost of £0.9 million. The work comprises:

Manufacture and assembly of two trial systems.
Equipment testing.
Drilling three onshore wells.
Complete analysis of system performance.

Independent oil company Pentex will be providing three land wells for the demonstration; Transocean Drilling will provide the use of a coiled tubing rig; and Tristar will provide engineering services and a dry hole for an initial system field test.

Although the programme is ambitious, it should be emphasized that several components have already been proven individually:
(1) The orientation sub has already worked downhole in a form controlled hydraulically from the surface. It is scheduled to be run again next month.
(2) A single stage roller vane motor and bearings have been surface tested for 100 hours in sand laden fluids with negligible wear.
(3) Acoustic transmission through 4,500 ft of coiled tubing has al ready been demonstrated at the IDDTC in Aberdeen.

Many details of the technologies complement and enhance one another, for instance the hollow shaft of the downhole motor permits placing the sensors nearer to the bit than has been previously possible. The features of the variable bent sub are applicable to traditional BHAs, but in the integrated Helix system complement the sophisticated traction system under development.

Roller traction, vane motor

Astec Developments is working on the roller traction system (RTS) and the roller vane motor. The roller traction system works on the principle that rolling elements offset from the centre line of tool rotation in contact with the borehole wall will, if given skew angle, follow a helical path on the circumference of the hole with a linear component along the axes of the hole.

This linear component can be arranged to be in either direction for a given direction of tool rotation. It translates into a tractive force that is dependent on the input torque, roller loading (side force), and the coefficient of friction between the rollers and the borehole wall. The rate at which the tool can move forward is dependent on the speed of rotation, the roller skew angle, and the borehole diameter.

The RTS will form part of the Helix drilling assembly and will use assembly weight and flexure to provide side loading to the rolling elements. A proportion of the power output of the drilling motor thus provides a tractive force on the coiled tubing and also applies weight to the bit.

The coiled tubing roller vane motor will be based on extensive development work carried out on a larger size proof of concept motor. This work demonstrated clearly the ability of the roller vane concept to provide a motor capable of working without the need for elastomers in the rotor stator assembly. Tests were run on oil and water-based mud with up to 2.5% sand in suspension. The motor worked at an overall efficiency of close to 50%, only 10% less than predicted.

Tests were also run successfully powering the motor on compressed air. The wear experienced in over 100 hours of testing was within acceptable limits. Recent work on motor parts subject to wear have shown bearing surface wear of less than 0.001 in. after 100 hours of on-load testing using oil-based mud with 5% silica sand in suspension.

The 2-7/8 in. diameter coiled tubing motor is expected to be approximately 4 ft long, including its bearing package. Previous test work indicates a power output of 15-20 HP at 600 rpm. This motor is a three-stage machine with each stage fed in parallel. The stages are displaced 120 from each other to provide a smooth output torque and with the elimination of elastomers, the motor can be powered by a variety of aggressive fluids including nitrogen foam. It is hoped to achieve a service life of 100 hours between services.

Orientation,
downhole actuator

Stirling Design International is working on the orientation system and the downhole actuator. The orientation system has been designed to work with conventional drilling assemblies whilst providing all the required features of the Helix bottom hole assembly for the consortium.

The operation is very simple. It consists of a piston incorporating a keyway which runs along a mandrel surface on the upper sub, to locate a key. On the lower end of the piston is cut a helical slot, in which is located a helical key, attached to a mandrel of the lower sub.

Application of hydraulic pressure to either side of the piston causes it to move up or down the mandrel, proportional to the volume of hydraulic fluid pumped into the chamber. The piston cannot rotate as it is keyed to the upper mandrel. Wherever it is positioned along the mandrel corresponds to a unique position of the helical key in the helical slot; hence the lower sub is rotated in either a clockwise or anti-clockwise direction corresponding to the movement direction of the piston in its chamber.

When no further position change is required, inlets to either side of the piston are closed, hydraulically locking the piston in place. Hence orientation of the tool remains fixed until a further orientation change of the bit face is required.

The tool has already been built in a form designed to be controlled hydraulically from the surface. Primary requirement of the orientation tool is for it to function dynamically while drilling ahead with optimum weight and torque on the bit. Other desirable features are the ability to rotate either clockwise or anti-clockwise in selected small angular increments of change.

The tool has been extensively bench tested, cycled with 2,500 lb weight on bit and 250 ft-lbs reactive torque with a peak stall torque of 1,000 ft-lbs through the entire 360 of the tool cycle. The first field trial took place in the Hemswell No 2 well last summer, with exemplary tool performance. What is now needed is the ability to communicate instructions to the tool from the surface and have them implemented downhole through the actuator.

Further development is now planned to produce a surface commanded downhole hydraulic power source which will obtain its energy from the kinetic energy of the drilling fluid flow. Commands will be transmitted via the through metal telemetry system.

It is intended to bench test a breadboarded system of the elements at typical pumping rates to observe cycle times for energy generation storage and recharge under varying operating conditions, thereby confirming system capabilities before starting the expensive exercise of packaging into a small and compact downhole system.

The proposed actuator will allow the orienting tool to be used with any coiled tubing reel without the hydraulic control lines of the present system. Additionally, it will provide the actuating power for the on/off variable bent sub, which has been included in the system to enable the directional driller to modify build rates while the toolstring is still in the well.

It will allow final adjustments to be made downhole, so saving on a trip for BHA changeout. Additionally, it will allow the toolstring to be run through the narrowest part of the hole - the tubing - yet will also enable the desired bend angle to be selected once clear of the tailpipe or minimum clearance restriction.

Instrumentation
and control

TSL Technology is working on the instrumentation and control sub and the sensor sub. The system's non-magnetic drill collar will house a shock-resistant instrumentation and electrical power sub. In the prototypes, electrical power for all functions will come from batteries. Consideration will be given to provision of downhole power generation in production systems.

The instrumentation will comprise a processor and buffer memory with an expandable number of data acquisition channels. The electronics will process and compress data for transmission to surface, buffering it until the telemetry has completed the transfer.

It will also receive commands from the surface such as optimising data sampling in regions of interest or operating the orientation tool or variable bend. To perform this latter function, TSL is developing an electrically operated miniature hydraulic valve for Stirling's actuator sub.

The system's steering dynamics will be studied in order to develop control algorithms for drilling smooth hole trajectories autonomously. This should lead to improved reach and generally better drilling performance. The downhole processor will operate the control loop. However, overall azimuth and well plan information will remain under drilling engineer supervision at all times.

To assist the engineer, the surface logging software will provide graphical displays of the drilling operation as it proceeds, making full use of the high telemetry rate. Safe underbalance drilling requires real time information on bottom hole annulus pressure. A suitable sensor will be provided.

Slim bottom hole assemblies are flexible and with the articulated joints it becomes important to measure tool face and inclination as close to the bit as possible. In the Helix systems, data will be recorded from additional accelerometers placed in a separate sub approximately three metres from the bit. This data will form part of the real time control system: the engineer and system will know where the bit is now as well as where it has been. The sub will house a natural gamma ray sensor to give a lithology curve.

Through-metal telemetry

The objective is to provide a fast and reliable communications link with the bottom hole assembly. Present mud pulse technology allows up to 10 bits/sec one way communications. Two way communications via wireline provides good data rates, but introduces other complications.

Downhole steering becomes possible by monitoring toolface orientation via near bit sensors. The less tortuous hole drilled allows longer lateral departures. MWD data will become available for CT drilling offering advances in the drilling process and in safely drilling at or near balance.

A new processing technique has been developed which increases the potential rate to about 600 bits/sec. The Helix project will be well satisfied with 100 bits/sec to achieve its aims. Cost-effective implementation of this technology has been made possible by the availability of powerful digital signal processing chips. Tests on representative structures have given excellent results.

The downhole part of the telemetry system comprises three units:

Transmit and receive transducers
Electronics software processing package
Downhole power supply.

The electronics section is controlled by a digital signal processing chip which codes and decodes the messages.

Conclusion

The developments proposed above are each a standalone advance in coiled tubing drilling, but knit together in Helix offer a total greater than the sum of the individual parts. Much of the proposed work is not research at all but development or technology transfer from one industry to another.

Testing of system elements will be carried out incrementally so that each advance becomes available commercially as soon as possible. The project is now seeking additional support from industry for work after Phase 1. Supporters will be repaid from licence royalties over the five years following successful commercialisation.

Editor's note:This is an edited version of a paper given earlier this year at the Drilling & Well Completion Technology Conference in Aberdeen, organised by IIR, London.