The development of highly deviated to horizontal wells has raised the problem of using downhole measurement tools where such tools can no longer be run in under the simple effect of gravity. Transmission of a force from the surface, whether thrust or traction, to a logging tool at a depth of several thousand metres can be achieved by semirigid stems. Such stems made of composite materials — glass or carbon fibres embedded in a resin — may also include a way of transmitting data. As compared to steel, the combination of materials such as fibres and resin provides advantageous mechanical properties with regard to traction strength, flexibility and lightness. Moreover, the continuous manufacturing technique — pultrusion — is well suited both for producing long single-unit lengths and for inserting any line such as a monoconductor, multiconductor or coaxial electric conductor.
So far, several lengths of semirigid stems, including end fittings, have been produced, reaching more than 200 metres of single-unit length and showing that the extension to several thousand metres raises no technical problem. The principal parameters of the composite material such as the Young's modulus, the ultimates stress and strain have been measured during bending, tension and compression tests. Dynamic tests in tension and bending have also been performed.
The use of a semirigid stem may be considered by itself or inside a coiled tubing:
When used by itself to push or pull a tool in a highly deviated borehole, the integrated electric conductor ensures two-way data transmission;
When used inside coiled tubing, it preserves all the inherent advantages of the coiled tubing (implementation, high-pressure hydraulic conduit). More hover, the semirigid stem is fully compatible with winding/unwinding cycles, and it protects the electric line from any damage.