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The variety of operations for which coiled tubing is being considered is greater than ever before. Where coiled tubing traditionally was mainly used in well intervention & servicing operations, it is now seriously being considered for many well side-track drilling programmes.
(Published on October 2008 – JEC Magazine #44)
Here is the solution proposed by Airborne Composite Tubulars B.V., based on a feasibility study.
As rig rates increase and reservoirs get smaller, increasing the rate of penetration, reducing total drilling time and increasing accuracy are essential in bringing total drilling cost down whilst maximizing recovery. Downhole pressures and temperature in real time, force (including torque) and vibration, borehole stability, circulation rate and cementing are factors that must be taken into account if these two goals are to be achieved.
Airborne Composite Tubulars proposes to apply thermoplastic composite coiled tubing (CCT) with integrated electrical conductors and optical fibres as opposed to a combination of steel coiled tubing and E-line or multi-conductor wire line. This alternative presents unique advantages. The integration of power conductors and optical fibres provides the possibility for real-time acquisition of petro-physical and drilling dynamics data. The absence of a cable inside the coil simplifies operating and makes it possible to proceed with cementing operations without first having to remove the cable or change the coil. In addition, the lower friction increases the circulation rate, reducing the required number of wiper trips, and the pressure requirements at surface are lower. The mechanical properties of composites in the coiled tubing application offer a number of other opportunities in terms of weight, fatigue, chemical resistance (e.g. PEEK versus H2S), and corrosion resistance. Moreover, they allow special tubing designs such as tapered tubing and/or bespoke strength and stiffness characteristics along the tubing.
Using composite coiled tubing does affect the operation. It allows for new possibilities but also puts additional requirements. The most important consideration is that the coil is more flexible than steel. This results in longer coil life, but also in earlier lock-up in highly deviated wells when a compressive force is applied to the CCT downhole. The CCT has lower friction levels and much lower weight in mud than steel, however, resulting in lower pull out of hole forces and lower loading on the coil. In contrast to steel, the mechanical tubing strength characteristics can be designed to fit for purpose.
The industry has seen developments in the area of CCT in the past decade, although these developments were based on a thermoset matrix. Airborne has selected the thermoplastic matrix system because it offers higher allowable strain, greater toughness, and superior impact resistance and rapid-gas-decompression characteristics. It also allows welded connections. The company developed the production technology required for manufacturing thermoplastic tubulars, in house. A production test-bed was constructed and used to qualify the production techniques. The first production line has been designed in concept, and the detailed design phase has commenced.
CCT has many advantages but is significantly more costly than steel. Airborne is working towards proving the extended fatigue life of a coil in the projects in which it is currently engaged. Still, it is expected that evaluating and considering composites in coiled tubing for a specific application will require taking the full operation – i.e. total service life, impact on drilling performance and impact on operational aspects such as equipment, space usage, offshore cranes, etc. – into consideration.