Intervention expert Stephen Stragiotti is a big fan of Well Tractors, which his company helped to develop nearly a decade ago and are now used extensively throughout the Norwegian Continental Shelf and worldwide. "The Well Tractor was first field tested in March 1996 in Gullfaks well number B33. It was first developed by Welltec in Denmark with Statoil," Stragiotti recalled.

Maritime Well Services in Norway has also been supplying the technology.

Since then at least 1,062 miles (1,700 km) of tractor-travel distance has been clocked up by the operator.
The well tractor is an electro-mechanical system developed as horizontal and highly deviated wells became more common-place over the last decade. Originally the technology was developed to deploy downhole tools without relying on gravity and at lower cost than coiled tubing (CT). Their use allows conveyance of isolation plugs, perforating guns and wireline logging tools. They are deployed by cable lowered into the hole, and they are fitted with retractable drive wheels - up to 20 - that are hydraulically powered to allow them to drive along horizontal or deviated well bores to their destination, pushing a train of equipment sometimes up to 5 miles (8 km) to the end of the well bore. Until recently the greatest depth achieved with a tractor was offshore Canada on the Hibernia platform in well B16-36 where perforating guns have been pushed out 30,644 ft (9,340 meters) Then, in September, one of the technology suppliers established a new tractor record in the Gulf of Mexico. The first run was with a cement logging tool taken to 30,668 ft (9,350 m) for ChevronTexaco working with Halliburton on the Petronius development which broke Welltec's own previous record by 30 ft (10 m). On a second run a Halliburton cement plug went to 30,602 ft (9,330 m).

Tractors are usually deployed when well bore deviation exceeds 60° to 65° from vertical. "We have gone as far as 70?," Stragiotti, discipline leader for wireline well intervention at Statoil, said.

Since its inception and initial field trials in 1996, Statoil rapidly increased its use of downhole tractors.
Now, the technology has moved on to facilitate wireline drilling. "We recently completed an operation where our technology was used to drill barium sulphate scale from inside of a well bore on the end of electric wireline," said Steve French, executive vice president of Welltec.

That application involved a 3 1/8-in. diameter tool with a milling face on the end. The technology has also shrunk in size. Tools as short as 16 ft (4.9 m) can be supplied. "When we started in 1996 the tools were as long as 20 ft (6.4 m)," French said.

"In 1996, we had six well tractor operations," said Stragiotti. In 2002, the number was 71, and in 2003, 129.

He said Statoil saved US $72.40 million on its well intervention costs in 2003 using tractors.
"A well tractor is one third of the price CT. That cost saving is quite huge when you have that number of operations.

"The alternative way into a horizontal well is by CT which is much more expensive and takes more time," he said. "For interventions on a subsea completed well, a CT-equipped vessel or rig is necessary and requires much more planning."

For Statoil with a large number of mature wells to service and an ongoing desire to extend oil recovery on mature fields on the Norwegian Continental Shelf, Stragiotti said a tractor is an attractive proposition because it allows intervention to take place at an economic level. Tractors can be used to set isolation plugs in non-productive zones that have watered out, so remaining oil pockets can be tapped by re-perforating after isolating the water.

"We can go for the marginal areas because the tractor is more cost effective than CT," Stragiotti noted. Costs for a CT service provider are saved. "The other part of the cost-equation is reduced rig time, and you can initiate a tractor intervention far sooner because it is simpler and you can get it started much quicker. You can get new zones on production faster and CT can take quite a long time to plan - there is much more planning involved."

Rapid and easy deployment of a tractors is demonstrated by a comparison with the number of CT operations conducted by Statoil: It performed 30 CT jobs in 2003, and 129 tractor operations.
Statoil's Statfjord field has been one of the big beneficiaries of the technology. Sixty operations were carried out there in 2003. "Each target there is quite small. We perforate a new interval and produce it for 1 to 3 months," Stragiotti said. "These are such small pockets of oil but it gives us
the incentive to produce marginal resources of oil.

"Statoil was the first to use this technology on a large scale and to promote the technology," he added.
While the technology is now well established, Statoil wants to extend its use by using longer and stronger cables to extend deployment range. "One of the limiting factors is the strength of the cable - because you have to pull it back to surface," Stragiotti said. With a 26,240-ft (8,000-m) deployment, a lot of cable has to be pulled back to surface.

Also various other operations have been added to the basic tool, i.e. the introduction of the CT tractor for extending the reach of CT, for shifting sliding sleeves, milling, stimulation, fishing and other mechanical work, plus side track drilling with CT.

Welltec dates evolution of the technology from 1987 when Jorgen Hallundbaek, the company's owner and founder, first thought about a downhole transport system for horizontal well intervention. He researched the concept at Denmark's technical university. The first field trials took place in 1996 with the first commercial offering of the tool in 1997. In December 1998, Welltec marked its first perforation job with a miniaturized 21/8-in. tool. Later supporting offices and services facilities were established to supply a global market in the United Kingdom, Norway, Houston, Calgary, Singapore and Kuala Lumpur, then more recently in Alaska and Texas.

Various applications of the system are now offered, sometimes combined with CT to extend the reach of CT in horizontal well bores with a 7,000 lbs pull force recorded. The CT tractor has also been used to extend the reach of CT in openhole formations beyond the point of CT buckling for acid stimulation. So far, 5,000 ft (1,524 m) of extension has been achieved pulling 2-in. diameter coil in open hole. Another specialized application involves a specially designed tool that can apply up to 10 tonnes of force down hole while being deployed on the end of electric wireline. This is used to actuate downhole completion valves. Other variations involve manipulation of sliding sleeves in intelligent completions.