Compact downhole tractor provides improved horizontal well mobility

Aug. 1, 2000
Smedvig is developing a new downhole tractor which it believes offers significantly improved performance and a wider area of application than existing models.
Prototype of the downhole tractor under development by Smedvig and AaTechnology.
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Smedvig is developing a new downhole tractor which it believes offers significantly improved performance and a wider area of application than existing models. Over the next few months, the tractor will undergo extensive testing and qualification, and the company hopes to have it out in the market by the second quarter of next year.

Downhole tractors offer advantages over conventional wireline technology for performing various tasks in horizontal well sections. It is therefore no great surprise that the number of tractor runs appears to be on the increase, or that quite a large number of models have come onto the market in a relatively short period, says Dag Oluf Nessa, Business Development Manager for Smedvig's well services department. However, the technology has yet to achieve a sufficient level of reliability and user-friendliness, for which reason some operators have proved reluctant so far to take it on board.

Nessa claims the Smedvig tractor should advance the technology in several respects. It is both smaller and lighter than existing models. It is about one meter in length and weighs about 50 kg, compared with existing models which are four-five meters in length and weigh 150 kg or more.

The compactness of the Smedvig tractor is beneficial given the height restrictions that often apply in the rigup areas where tool strings are made up. It also makes the tractor easy to transport - by helicopter, for example.

The Smedvig tractor also has double the push/pull strength of existing models, and its gripping force increases with increased load, Nessa says. Most existing models incorporate hydraulics, and the electro-hydraulic combination entails a significant power loss. The Smedvig tractor is purely electro-mechanical, which according to Nessa, minimizes the power loss between the motor and the wheels. Because it is purely electro-mechanical, it is likely to suffer less failure and require less maintenance than models incorporating complicated hydraulic and electrical systems.

Two-directional system

Unlike existing models, the tractor can move in both directions. The body of the tractor expands to fit the diameter of the hole. The wheels are located around its whole circumference and engage with the wall at more or less a 90 degree angle, which allows the tractor to be driven in both directions. This means it is not subject to a limitation suffered by existing models - they have to be pulled back to the surface on a wire cable. The distance they penetrate into the horizontal section is limited by the strength of the cable and cable head.

The Smedvig tractor does not require extensive specially trained personnel and special equipment to operate it or integrate it with the other well servicing equipment.

The design of the tractor was developed by AaTechnology, a small startup company based near Bergen, with which Smedvig has entered into an agreement. A prototype has been built which has confirmed the machine's mobility and power characteristics, but some design changes are being made to ensure it will work in different pressure and temperature regimes in oil and gas well environments.

The tractor will undergo an extensive program of in-house testing followed by final qualification in live well scenarios by independent bodies such as Rogaland Research. A number of oil companies in Norway have shown great interest in the tractor, which according to Nessa, also has the potential for application in new areas: in vertical wells for tasks such as scale cleaning; in coiled tubing/ slim-hole drilling; and in subsea wells.