The downhole tractor is a recent application to the well intervention discipline and was developed as an enabling tool to access deep, long horizontal wells which became popular from the early 1990's. Until tractors came to the market coil tubing was the main conveyance method to access these new 'designer' type wells. Ironically the first tractor developed was run on coiled tubing, yet tractor technology has now replaced coiled tubing in many applications as a primary method of well intervention.
Further development was rapid with the main target being wireline conveyed, in particular electric line. To date there are now several key companies involved in the tractor market offering many services including logging, milling and cleaning and solids removal.
Principle of Operation:
The tractor unit comprises 3 main sections - a wheeled section, a hydraulic module and an electronic module. The system is run in hole on electric line until 'lock up' is reached (i.e the depth at which point the assembly will no longer move under gravity) and then the tractor is powered up electrically through the wireline. The motor powers the hydraulic pump supplying pressure to the wheeled section which extends the wheels outwards. The wheels start rotating thus gripping the tubing wall and moving the assembly downwards until target depth is reached.
Many different tools can be run on tractors, applications include:
- Well logging and perforating services
- Cleaning, milling and sand bailing operations
- Manipulation work - opening / closing sliding sleeves, control valves, setting / pulling packers and bridge plugs
Useful data can be located - here