The tractor is powered by fluid pumped in two parallel tubes from the Snorre A platform to the field’s subsea production system and thereafter into its subsea wells.
At this point, the tubes intersect so that the flow can be controlled in the desired direction from the platform control room.
The subsea template is equipped with 20 sets of points, which are used to guide the tractor down the right tube in the right well. In addition, the wells are equipped with stations.
Production control valves have also been positioned in various formations in some of the wells.
Dedicated key
Before being launched from Snorre A, the downhole tool is fitted with a special key which matches a specific lock profile in the relevant well.
The key presses against the tube wall throughout the excursion, and stops the tractor as soon as it reaches the right lock.
The tractor can carry downhole tools for a range of different operations. These include reservoir management devices which close off poorly-producing zones and open new ones to optimize flow.
Snorre A also has a tool kit for repairing downhole safety valves as well as equipment to install pressure and temperature sensors.
Once a job has been completed, the pumping flow in the piping system is reversed to drive the tractor with its tool assemblage back to the platform.
Benefit
Although the installation of this technology is expensive, Snorre A has benefited by having the cheapest and most cost-effective solution for light intervention in subsea wells on the NCS.
Shutting down and re-starting a wellstream in the reservoir can be done by a six-strong 'tractor driver' team in about 24 hours.
This costs roughly USD 30 000 per day (NOK 200 000 per day), while traditional well intervention methods deployed from mobile rigs can easily reach about USD 430 000 or NOK 3 million per day.
An average of four well operations are carried out annually on Snorre A with the aid of the well tractor system, which helps to maintain a high level of subsea output.
The subsea production system on Snorre currently accounts for 25 per cent of its total production.
Facts
The technology for pumping equipment down a well dates back to the late 1950s.
The system used on Snorre A is the result of a collaboration between Rogaland Research in Stavanger, Triangle Technology, Halliburton and the field’s various operators.
With pump, tank, manifold and sluice piping, the well tractor facilities form an integrated part of the field’s processing installations.
The subsea unit comprises an advanced 20-slot template, and two independent tubes descend to the production packer in each well. An NPU manifold controls the direction of downhole flow.
